SE440152B - SET UP AND NAVIGATION RECEIVERS TO DERIVATE NAVIGATION AMENDMENT HEALTH INFORMATION - Google Patents

Description

15 20 25 30 35 78107111- “1 There are currently eight multi-station transmitter chains for LORAN-C for operation in 1980. This new navigation system will result in a possible decommissioning of the previous navigation system LORAN-A.

LORAN-C is a hyperbolic radio navigation system that operates at pulsed low radio frequency (100 kHz). LORAN-C radio navigation systems use three or more synchronized ground stations, each of which transmits radio pulse chains, which at their respective transmission starts have a fixed time relationship to each other. The first station to be broadcast is designated as the main station while the other stations are designated as secondary stations. The pulse chains transmitted by each of the main station and the secondary stations consist of a series of pulses, each pulse having an exact envelope shape, with each pulse chain transmitted at a constant and precise repetition rate and with each pulse time separated from a subsequent pulse with a fixed and precise time interval. In addition, the pulse station transmissions of the secondary stations are delayed by a sufficient time after the pulse station transmissions of the main station to ensure that the time taken for their arrival at reception equipment throughout the area of operation of the special LORAN-C system will follow the reception of the pulse chain from the main station.

Since the pulse series transmitted by the main and secondary stations consist of pulses of electromagnetic energy propagating at a constant speed, the time difference in the arrival of pulses from a main station and a secondary station will represent the difference in length of the transmission paths. from these stations to the reception equipment for the LORAN-C system.

Focus for all points on a LORAN-C map representing a constant distance difference between a main and a secondary station and indicated by a fixed time difference in terms of the arrival of their pulse chains with the carrier frequency 100 kHz describes a hyperbola. The LORAN-C navigation system enables a navigator to take advantage of this hyperbolic connection and accurately determine its position using a LORAN-C map. By using a reasonably low frequency, for example 100 kHz, which is characterized by low attenuation, and by measuring the time difference between the reception and signals from main 781071lr-1 and secondary stations, the modern LOFAN-C system provides positional accuracy in the position of the equipment that lays within 60 meters and with a repeatability within 15 meters.

The theory and operation of the LORAN-C radio navigation system have been described in detail in an article by W.P. Frantz, W. Dean and R.L, Frank entitled "A Precision Multi-Purpose Radio Navigation System", 1957 [.R.E. Convention Record, Part 8, p. 79. The theory and operation of the LORAN-C radio navigation system have also been described in a pamphlet 10 issued by the Department of Transportation, United States Coast Guard, No. CG-462 and dated August 197H, entitled "LORAN-C User Handbook".

The LORAN-C system of the type described in the said article and the pamphlet and now used is a pulse type system, where the energy is radiated by the main station and by each secondary station in the form of pulse trains which include a number of accurate shaped and timed bursts of radio frequency energy, as already indicated. All secondary stations each emit pulse chains consisting of eight discrete time-separated pulses 20 and all main stations emit the same eight discrete time-distinct pulses but also emit an identifying ninth pulse which is carefully separated from the first eight pulses. Each pulse in the pulse chains transmitted by main and secondary stations has a carrier frequency of 100 kHz, so that it can be distinguished from the much higher carrier frequency used in the previous LORAN-A system.

The discrete pulses emitted: of each main and Xx) (of each secondary LORAN-C transmitter are characterized by a very precise difference of 1000 microseconds between adjacent pulses. Each given point on the carefully formed cnvc1 «ppcn for each pulse) is also separated by exactly 1000 microseconds from the corresponding point on the envelope of a previous or subsequent pulse within the eight-pulse pulse chain.To ensure such an accurate time accuracy, each main and second station stations are controlled fl By on ~~ wium frequency standard clock and the clocks in the main and secondary stations are synchronized with each other.

As already stated, LORAN-C equipment was used to measure the time difference in the arrival of the series of pulses 781071lv1 10 15 20 25 30 from a main station measuring the series of pulses from a selected secondary station, both stations being within a given LORAN-C chain. This arrival time difference, which is measured, was used together with special maps with hyperbole information regarding the arrival time difference printed on it. These maps are standard hydrographic maps prepared by the United States Coast Guard and the hyperbolic curves printed on them for each secondary station are marked with information indicating the arrival time difference. The arrival time difference between series of pulses received from a main station and selected from the associated secondary stations must be measured accurately before the navigator can locate the hyperbola on the map that represents the measured time difference. By using the arrival time difference information between a main station and two or more secondary stations, two or more corresponding hyperbolas can be located on the map and their common intersection point accurately identifies the location of the LORAN-C receiver. It is clear that any inaccuracies in measuring the time difference between the arrivals of signals from main and secondary stations lead to determination errors.

There are other hyperbolic navigation systems in operation around the world and they are similar to LORAN-C and a professional can easily adapt the new receiver specified here for these. There is a LORAN-D system, which was used by the United States military forces, as well as the already mentioned LORAN-A system. There are also systems called DECCA, DELRAC, OMEGA, CYTAC, GEE and the French WEB, all of which operate in different sections of radio frequency spectrum and achieve different degrees of accuracy.

The LORAN-C receiving equipment currently in use is comparatively considerable in size and is heavy and requires comparatively large effects. Furthermore, the current LORAN-C-nwttagarc are comparatively expensive and are therefore only available on board larger vessels and aircraft. Due to the cost, size, weight and power requirements of existing LORAN-C receiving equipment, such equipment is not in general use on board small aircraft, fishing boats or pleasure boats. In addition, such LORAN-C receiving equipment requires: hk'nmw fl muMc {h'íLwm 10 15 20 25 30 35 H0 7810714-1 5 for heating and providing information indicating time difference measurements. The current LORAN-C equipment is further complicated and has many controls and an operator must have some training in the use of the equipment.

There is therefore a need in this area for a new LORAN-C receiver which has light weight, has few controls and is therefore easy to handle even by untrained persons and also requires little electrical power and is comparatively cheap fl.

Such equipment will therefore meet the needs of those who do not currently have any LORAN-C receiving equipment.

With the means set forth in the characterizing part of claim 1, the method according to the present invention and much of the complicated and costly automatic acquisition and tracking are eliminated by means of dedicated circuits in LORAN-C navigation receivers trained according to the prior art.

With the means stated in the characterizing part of claim 8, a small, light and inexpensive receiver is provided which uses comparatively low electrical power.

Four thumbwheel switches on the LORAN-C equipment were used by the operator to enter group repeat rate information for a LORAN-C chain covering the area in which the LORAN-C equipment operates. This information entered through the thumbwheel switches is used by an internal microprocessor to locate the signals from main and secondary stations within the selected LORAN-C chain.

The receiver according to the invention receives all signals which occur within a small bandwidth, which has its center at the operating frequency of 100 lHz for the LORAN-C system.

A digital register coupled to logic circuitry was then used to continuously check all received signals to search for the unique pulse paths transmitted by the main and secondary stations. The microprocessor present in the new LORAN-C equipment analyzes all output signals from the register and logic circuitry to indicate that signals from main or secondary stations have been received to first determine if they match the group selection rate of the Selected The LORAN-C chain and ddrelLer generate ~ II histograms for the arrival time of signals from the secondary stations. Since the equipment has approximately located and receives the pulse trains from the selected main and secondary stations, the microprocessor causes other circuitry to operate with fine tuning.

In the fine-tuning method, the microprocessor renders the equipment ineffective in analyzing signals other than those received within 35 microseconds in terms of the approximate arrival time of the signals from the secondary stations determined geKm1 use of the histogram. The microprocessor also puts other equipment into operation to analyze the phase for each pulse and to locate the zero period of the third period for each received pulse. In the event that the third period zero pass for a pulse is not located at the approximate time indicated by the microprocessor, an analysis circuit device for the microprocessor indicates whether 10 microseconds should be added or subtracted from the approximate arrival time, and then the analysis process is repeated. This analysis process and the shift of the approximate search point are repeated until the zero period of the third period in the desired pulse in the selected main and secondary LaLion pulse paths is located. Using an accurately crystal-controlled clock interval in the new equipment, the microprocess performs accurate arrival time difference measurements between the time of arrival of signals from the main station in the selected pulse train and the arrival of pulse trains from the secondary stations. The equipment operator uses other thumbwheel switches to indicate two secondary stations, whose information regarding the arrival time difference is to be presented visually. The operator of the LORAN-C equipment uses these two readings together with a LORAN-C hydrographic map to locate the physical position of the navigation equipment on the earth's surface.

In an alternative embodiment of the invention, a front panel keyboard may be used in place of the thumbwheel switch and the microprocessor may be programmed to perform other functions which may include but are not limited to use as a computer. Other possible uses are limited only by the memory capacity provided in the microprocessor or in an auxiliary memory, which is added to the microprocessor in a well known manner, and by the fan1aní of the rounding cone. 10 20 25 30 78107114-'1 The operator handling the new LORAN-C navigation receiver can quickly and easily calibrate the receiver's main oscillator unlike any previous receiver. To perform this, the operator places the equipment in a calibration mode, where the output of the oscillator is compared with the group repeat information (GRI) entered via the thumbwheel switches. The presentation was used to indicate to the operator whether the equipment is in calibration mode or requires a simple setting by the operator.

The new LORAN-C navigation receiver should be better understood through the detailed description that follows and is to be taken in conjunction with the accompanying drawings. -Pig, 1 is a general block diagram of the present LORAN-C navigation receiver. -Pig. 2 shows the shape of each pulse in the pulse trains transmitted by all LORAN-C main and secondary stations.

-Fig. 3 graphically shows the pulse trains transmitted by main and secondary stations within a LORAN-C chain. -Fig. 4 shows a portion of a LORAN-C navigation map. -Pig. 5, 6 and 7 are detailed block diagrams of the present navigation receiver.

-Pig. Fig. 8 is a detailed block diagram of the fast disk register shown in Fig. 5. Fig. 9 shows the manner in which Figs. 5, 6 and 6 are to be arranged in relation to each other when reading the detailed description.

To understand the general or detailed mode of operation of the new LORAN-C receiver, it is best to first seek to understand the nature of the signals transmitted by LORAN-C stations and received by the new receiver. Representations of these signals are shown in Figures 2 and 3 and will now be discussed.

All main and secondary stations transmit pulse groups, as already indicated, with a specific group repetition interval, which is defined as shown in Fig. 3. Each pulse has a carrier frequency of 100 kHz and has a carefully selected shape, For each LORAN-C chain select a group (GRI) of sufficient length to, as shown in Fig. 2, call interval time for the transmission of the pulse trains from the main station and each associated secondary station plus the time between the transmission of each pulse train from the main station, so that signals received fpgn two or several stations within the chain never overlap to contain each other when received anywhere within the coverage area of the LORAN-C chain. Each station transmits a pulse train with eight or nine pulses per GRI, as shown in Fig. 3. The main station pulse train consists of eight pulses, each pulse being shaped as shown in Fig. 2, with each of the eight pulses separated by exactly 1000 microseconds, and with a ninth pulse separated by exactly 2000 microseconds from the eighth pulse. The pulse train for each of the secondary stations X, Y and Z contains eight pulses formed as shown in Fig. 2, and each of the eight pulses is separated by exactly 1000 microseconds. The image representation of the pulses transmitted by the main station and the three secondary stations X, Y and Z associated with the main station and shown in Fig. 3 indicates that the pulse trains never overlap and that all are received within the group repetition interval. Pig. 3 also shows a representative arrival time difference in terms of the pulse trains from each of the secondary stations in relation to the main station. These arrival time differences have been denoted by TX, TY and TZ and make up the time differences that the new recipient measures.

It is clear that the arrival time difference between the reception of the pulse train from the main station and the pulse trains from the secondary stations X, Y and Z will vary depending on the location of the LORAN-C receiving equipment within the coverage area of a LORAN-C chain. In addition, the signal strength of the received signals from the main and secondary stations will vary depending on the position of the receiving equipment, as indicated by the different heights of the representative pulse lines shown in Fig. 3.

The delayed or separated ninth pulse from each main station not only identifies the pulse train coming from a main station but the ninth pulse is also turned on and off by the Coast Guard according to a "flash" code well known to those skilled in the art to indicate particular faulty secondary stations in a LORAN -C-chain. These "blink" codes are published by the Coast Guard on LURAN-P ~ Pav1uru4.

When the LORAN-C system was installed during World War II, carrier phase coding was used as a military security method, but after the war, when the need for military security ceased, the phase coding was called for the abolition of space wave 10 15 20 25 30 7a1ó ? 1Å-1 distortion. Upon cancellation of space travel distortion, the phase of the pulses with the carrier frequency 100 kHz changes from the main station and the secondary stations in a LORAN-C chain to correct for space wave interference in a manner well known to those skilled in the art. Space waves echo the transmitted pulses, which are reflected back to earth from the ionosphere. Such space waves can arrive at the LORAN ~ C receiver anywhere between 35 microseconds and 1000 microseconds after the ground wave for the same pulse is received. In the case of 35 microseconds, the space path will overlap its ground wave, while in the case of 1000 microseconds the space wave will overlap the ground path of the following pulse. In both cases, the received space wave signal shows distortion in the form of fading and pulse shape changes and both of these can cause position errors.

In addition, a space wave can be received with higher levels than a ground wave.

To prevent space waves with a significant delay from affecting time difference measurements, the phase of the carrier is changed by 100 kHz at selected pulses within a pulse train in accordance with a predetermined pattern. These phase code patterns are published by the Coast Guard on the LORAN-C maps.

The exact pulse envelope shape of each of the pulses transmitted by all the main and secondary station tones is also carefully selected to aid in measuring the exact arrival time difference between a pulse train from a main station and a pulse train from a secondary station, as is well known. by the person skilled in the art.

In order to make accurate time difference measurements, one way according to the prior art is to superimpose corresponding pulse envelopes for pulses from a main station and a selected secondary station. Another method also used here is the detection of a specific zero crossing of the carrier with the frequency 100 kHz for pulses from main and secondary stations.

Now that it has been stated how the signals transmitted by main and secondary stations in LORAN-C networks are handled and how they are used for navigation purposes, it is easier to understand the operation of the new LORAN-C receiver which will now be written.

Fig. 1 shows a general block structure for the new LORAN-C navigation equipment. The filter and for [ÜruIärkar ~ I and the antenna 2 are of normal construction for the grouse> if used in all LORAN-C ~ receivers and are permanently tuned to 10 '15 20 25 -30 ”7810714-1 10 a center frequency of 100 kHz, which is the operating frequency of all LORAN-C transmitting stations. The filter 1 has a passband width of 20 kHz. Received signals are fed to a fast shift register 3 and a zero crossing detector 6.

The input signal to the zero-pass detector 6 is amplitude-limited first so that each period of each pulse is represented by a binary one and each negative half-period is represented by a binary zero. The leading or positive edge of each binary one corresponds exactly to the positive steepness of each sine wave that each pulse comprises. Detector 6 is thus a positive zero detector detector. As will be described in more detail later, the logic circuit 16 also provides an input to the zero crossing detector 6, but this has not been shown in Fig. 1, thereby setting a 10 microsecond window and it is only within this window that the front the edge of each binary one can be detected. The end result is that only the positive zero pass in the third period within each pulse in the pulse trains transmitted by each LORAN-C station is detected and an output signal is provided from the detector 6.

It can be seen that the latch circuit 5 has inputs from the zero crossing detector 6 and the logic circuit H. The clock counter 7 is a crystal controlled clock which runs continuously when the new LORAN-C receiver is in operation. The count present in the counter 7 at the moment when the zero crossing detector 6 indicates a positive zero crossing at the third period is stored in the latch circuit 5 and its contents are then fed to the multiplexer 8, which is a time division multiplexer used to multiplex the many conductors from the logic circuit 16, the latch circuit 5, the clock counter 7 and the thumbwheel switches 11, 61 and 52 to the microprocessor 9. The count in the latch circuit 5 indicates for the microprocessor 9 the time when each positive zero crossing is detected.

The fast shift register 3 has a filter at its input for receiving the output signal from the filter 1 within a narrower band with a width of 5 kHz which has its center at the road wave frequency 100 kHz.

The input signal to the register 3 is also amplitude-limited, so that a pulse train with 1s and 0s is produced as an input signal to the writing register, where shifting takes place at a rate of 100 kHz. Due to the shift frequency of 100 kHz, only pulse steps from 10 15 20 25 30 35 7810714-1 11 main and secondary stations in the LORAN-C system will result in output signals from each of the individual steps in the shift register included in the fast shift register. 3. Logic circuits in register 3 are used to analyze the contents of the shift register contained in register 3 to determine firstly whether the signals represent a pulse train from a LORAN-C station and secondly to determine whether the pulse train comes from a main or a secondary station and finally to indicate the special phase coding of those received from a LORAN-C station. The logic circuit H, including a latch circuit and a circuit for storing information from needles, registers the register 3 which indicates whether a pulse train is coming from a main or secondary station and for further indicating the transmitted phase code. This information stored in the latch circuit 4 of the logic circuit 4 is fed to the microprocessor 9 via the multiplexer 8 for use in the processing of received LORAN-C signals. At the same time as the information is stored in the latch circuit within the logic circuit U, there is an output signal from the circuit N which allows the latch circuit 5 to store the count in the clock counter 7, and this indicates the time of occurrence. It is to be noted that the clock counter 7 also emits an input signal to the multiplexer 8, so that the microprocessor 9 can keep track of a continuous running time indicated by the recurring course of the counter 7.

The output signals from the thumbwheel switches 11 constitute inputs to the multiplexer 8 and the operator of the new LORAN-C equipment can introduce the group repeat rate of a selected LORAN-C chain into the microprocessor 9. The group repeat rate is also called the group repeat interval (GRI). In alternative embodiments of the present invention, the thumbwheel switches 11 may be replaced by a keyboard that the operator may use to access the microprocessor 9 to perform many things, including performing navigation calculations.

According to the different types of information entered into the microprocessor 9 via the multiplexer 8 from the circuits already described, the microprocessor 9 determines if and when signals received via the filter 1 come from the main and secondary stations of the selected LOPAN-C chain. When the microprocessor has located the signals from the selected main station, which is done by matching the input signal in the form of the GRI number via the four thumbwheel switches 11 and the arrival time difference between each pulse train transmitted by the selected main station, the microprocessor 9 to similarly locate the corresponding secondary station signals. To locate the secondary stations, the microprocessor 9 generates a histogram through arrival time information for arbitrary and all secondary station signals stored in twenty batches generated by the microprocessor in its own memory between the arrival of two arbitrary consecutive main station pulse trains. When signals from the secondary stations of the selected LORAN-C chain are determined by secondary station signal counting numbers which appear in the histogram bins at the same rate as the GRI of the selected LORAN-C chain, the microprocessor 9 performs a finer search by generating shorter time histogram bins. Each of the histogram compartments where the count times for the arrival times of the signals of the secondary stations in question are stored is subdivided by the microprocessor 9 into one hundred smaller time-section histogram compartments to more precisely determine the arrival time of the pulse trains from the selected LORAN-C chain secondary stations. Each of these smaller histogram compartments or sections stores arithmetic numbers corresponding to the time of reception of signals received within consecutive periods of twelve microseconds. In this way, the microprocessor 9 accurately determines the arrival time of pulse trains from the main and secondary stations of the selected LORAN-C chain within periods of twelve microseconds.

When the microprocessor 9 determines the particular histogram time sections of twelve microseconds within which secondary station signals are received, the microprocessor causes an operating time signal which causes the equipment to work fine-tuning using the logic circuit 16 to accurately find the positive zero passages during the third period. within each pulse from the selected main and secondary station pulse trains. To perform this function, the approximate arrival times of successively received pulses are entered in the main and secondary station pulse trains in succession in the latch circuit 15 and the contents therein are fed to the comparator 14. The comparator 14 compares the contents of the latch circuit 15 with the contents of the clock counter 7. , and when there is a match, the comparator provides an output signal to the logic circuit 16. The time entered in the latch circuit 15 is in fact a time calculated to be 3% microseconds before the arrival time. The output of the comparator ih to the logic circuit 16 is used to store three time signals therein, with these obtained from the microprocessor 9. These three time signals represent bars appearing 2.5 microseconds, 12.0 microseconds and 30.0 microseconds after the output of the comparator ln. At the end of these three timed sequences, a received pulse is checked for phase coding. The phase coding information is able to accurately locate the zero crossing within the third period in each pulse in the pulse trains from the main and secondary stations.

In the event that the signal characteristics already described are not received immediately before and at fixed points during a pulse, the microprocessor 9 knows that there is an error in the time calculated and placed in the latch circuit 15 and the microprocessor 9 will then either increase or reduce the estimated time for subsequent pulse trains by 10 microseconds and the new calculated time value is placed in the latch circuit 15. The logic circuit 16 again analyzes incoming signals at the points already mentioned. This process of addition or decrease by 10 microseconds to the estimated time is repeated until the microprocessor 9 carefully locates the third positive zero pass within each pulse in the pulse trains lowered by each main and secondary station in the selected LORAN-C. chain, and then determines whether the received pulse paths come from a main or a secondary station and further determines the particular space-wave phase code transmitted by each of the stations.

Since the microprocessor 9 in operation together with the other circuits in the LORAN-C receiver intended here has located and read to the pulse trains transmitted by the main and secondary stations in the selected LORAN-C chain and has provided the measurement of the the desired arrival time difference required for LORAN-C operations, the microprocessor 9 causes a visual indication to occur via the display device 12 for the equipment operator. The output information is plotted on a LORAN-C hydrographic map in a well known manner to locate the physical location of the LORAN-C receiver.

The operation of the new LORAN-C equipment will now be described in more detail. Fig. 2 shows the shape or waveform of each pulse transmitted by both main and secondary stations in a LORAN-C system. The waveform of the pulse has been carefully chosen to facilitate the detection of the zero period of the third period in a manner well known to those skilled in the art.

One method known to those skilled in the art is to take the first derivative of the curve represented by the envelope of the pulse shown in Fig. 2, and this first derivative clearly indicates a point 25 microseconds from the beginning of the pulse. The next zero crossing that follows this indication is the desired zero crossing within the third period of the carrier frequency. Similar to the known method just described, the new LORAN-C receiver detects the third zero crossing within each pulse from the main station and each secondary station. The precise arrival time difference measured using a LORAN-C receiver is measured by the zero crossing of this third period in the fifth pulse from the main train pulse train and the zero crossing of the third carrier period in the fifth pulse from the manually selected secondary station.

Fig. 3 shows a representation of the nine-pulse and eight-pulse signals transmitted by a main station and by the secondary stations in a LORAN-C chain. The narrow vertical bars each represent a pulse shape of the type shown in Fig. 2. The heights of the vertical bars represent the relative signal strength of the pulses received by a LORAN-C receiver. It can be seen that the signal strength of the pulses from the main station and each of the secondary stations is not identical.

Fig. 3 shows that the group repetition interval (GRI) is indicated as the period between the first pulses in two consecutive main station pulse trains within a given LORAN-C chain. This information is available on standard LORAN-C hydrographic maps and was used to calibrate the oscillator in the new LORAN-C receiver which will be described in detail later.

In a manner well known to those skilled in the art, the LORAN-C equipment was used to measure the time difference in arrival between the pulse train from a main station and the pulse trains from two or several secondary stations as 781071¿æ-1 15 belongs to the main station. This arrival difference is shown in Fig. 3 as TX, TY and TZ. Fig. H shows a representative image of a LORAN-C hydrographic map. This map shows three sets of arcuate curves, where each set of curves has a five-digit number as its designation and this number is followed by one of the numbers X, Y or Z.

The numbers correspond directly to the information regarding arrival differences denoted by TX, TY and TZ and shown in Fig. 3 and measured by means of a LORAN-C receiver. In Fig. U, the particular secondary station to which a set of cup-shaped curves belongs has been indicated by the addition of X, Y or Z after the number on the curve.

LORAN-C maps show land areas, for example island 80 in Fig. H. An operator of the new LORAN-C receiver, located on board a ship 81 near island 80, will measure information regarding the arrival time difference between the main station and at least two of the three secondary stations included in the LORAN-C chain. When measuring the secondary station X, the operator will measure 379000 on the new LORAN-C receiver. As shown in Fig. 4, the position line (LUP) 379,000 is shown passing through the vessel 81. Similarly, the operator will measure information relating to the time change difference with respect to the secondary station Y and the receiver will then enter the number 699 800. The position line (LOP) for this receiver reading also passes through the vessel 81. If the operator of the LORAN-C receiver measures information indicating the arrival time difference with respect to the secondary station Z, the reading will indicate 493500 and this position line also passes through the vessel 81.

The operator can therefore carefully fix the position of the vessel 81 on the LOFAN-C map. Due to this location information on the map shown in Fig. H, the ship 81 can, for example, be navigated towards the port 82 on the island 80.

It is to be noted that the sample on a LORAN-C map shown in Fig. U shows only five digits on each position line with the new LORAN-C receiver according to the present invention having six digits. The lowest or sixth digit is used for interpolation between two position lines on the LORAN-C map in a manner well known to those skilled in the art. In the simple example now given, the ship 81 is located exactly on three position lines and therefore no interpolation is required for positioning a position line between those shown on the map in Fig. U. It is therefore worth noting that the six-digit numbers obtained using this equipment each include an extra zero added to the end of the five-digit position line numbers shown on the LORAN-C maps. A sixth digit that is not zero in the receiver would require interpolation between the position lines on the map.

Pig. 5, 6 and 7 show a detailed block diagram of the new LORAN-C receiver which will now be described in detail. Pig. 5, 6 and 7 are to be arranged in the manner indicated in Fig. 9 so that the following description is to be understood in the best way.

LORAN-C signals are received by means of a conventionally constructed antenna 2 and a conventionally constructed filter and preamplifier 1, as is well known to those skilled in the art. Interference caused by different radio signals and signals from other navigation systems will be mainly eliminated by the filter 1, which has a fire width of 20 kHz and is centered at 100 kHz, with a transverse decrease at both ends of the band. The filter 1, which is of a conventional construction used in many LORAN-C receivers, will not be described in more detail. In the same way, the choice of the antenna 2 and / or the construction thereof is also well known to a person skilled in the art and is not stated here in detail so as not to make the description laden with details which are well known to the person skilled in the art and which could also divert attention. from the understanding of the invention. The output signal from the filter 1 is unmodulated and is applied to a limiter 17 in the zero-crossing detector 6 and to a bandpass filter 19 with a bandwidth of 5 kHz.

When the new LORAN-C equipment is initially put into operation, it works with eüïgrowning methods where it only tries to generally locate the pulse trains from main and secondary stations in the selected chain. This function takes place by means of a fast shift register 3, as will be described.

The filter 19 has a bandwidth of 5 kHz and is centered on the 100 kHz carrier frequency of the LORAN-C signals and causes most false signals to be rejected. the hUFAN-U signals and a few false signals pass through the filter 13 10 15 20 25 30 '7810714-1 17 to the limiter 20. The limiter 20 demodulates and strongly limits the incoming signals, so that only a chain of binary 1s constitutes an output signal. from the limiter. Each of the binary pulses in the output of the limiter 20 corresponds to a false signal pulse or to each of the pulses in the pulse trains from main and secondary stations. These pulses are applied to a fast register 3, which is shown in block form in Fig. 5 but is shown in detail in Fig. 8 and will be described in more detail later in this description.

The fast-shift register 3 consists of ten series-connected shift registers, all of which are clock-controlled or shifted with the same period as the pulses from main and secondary stations in the LORAN-C system are received, as well as logic gates. This is a period of 1000 microseconds, as shown in Fig. 3. These ten shift registers store a window time sample of the received signals which are analyzed to determine if the signal stored in the shift registers represents a pulse train from a main or secondary station in the LORAN-C system. Due to the clock control, the sample will move in time. The logic gates connected to different stages in the shift registers provide output signals which are used to analyze the signals temporarily stored in the register to determine if the received signals are from a main or a secondary station and if the received signals have what the United States Coast Guard calls group repetition interval A or B phase coding. These phase codes are well known to those skilled in the art. When the fast shift register 3 has determined that a pulse train has been received from a main or secondary station, the logic gates, which will be described in more detail later, will impose an output signal on one of the conductors MA, MB, SA or SB to indicate the signals. comes from a main or a secondary station as well as the special phase coding of the signal.

A signal indication indicating that the received signals come from either a main or a secondary static is stored in the latch circuit 21. In addition, the latter output signal from the register 3 is applied via the OR gate 22 to the R / S flip-flop? placing this flip-flop in its extended position with its 1-output signal high to indicate that a pulse train from either a main or secondary station has been received. The R / 5 flip-flop 7810714--1 g1o 15 20 25 30 35 18 The R / S flip-flop 23 1 output signal is applied via the OR gate ZH to the latch circuit 5. In particular, this output signal is transmitted from the flip-flop 23 to the clock input CK on the latch circuit 5 and causes the latch circuit to store the contents of the BCD counter 26 (in the clock counter 7) at the time it is determined that the signal has been received from the main or secondary station, as indicated by the signal at the input CK. The count that is stored indicates the actual time when the pulse train was received. As already briefly stated, the contents of the latch circuit 5 will be fed to the multiplexer 8 in Fig. 6 and then inserted into the microprocessor 9.

The multiplexer 8 in Fig. 6 is required to input signals into the microprocessor 9 in Fig. 7 due to the limited number of inputs on the microprocessor and the large number of conductors via which signals must be applied to the microprocessor.

The multiplexer 8 accomplishes this by using time division multiplexing operations. Multiplexers in the form of integrated circuits are available in the market, but the multiplexer can also be provided by a plurality of logic AND gates with two inputs, one input of each gate being connected to the conductors on which the signals to be multiplexed are present, while the second input on each gate is connected to a clock counter arrangement which causes individual or groups of logic gates to have the other inputs sequentially fed in a cyclic manner. In the present embodiment, the multiplexer 8 comprises those of the type Texas Instrument T174151.

From Fig. 6 it can be seen that there are input signals to the multiplexer 8 from the logic circuit 4, the latch circuit 5, the clock receiver 7, the thumbwheel switches 11, 61 and 62, the logic circuit 16 and the microprocessor 9. The input signals to the multiplexer 8 from the microprocessor 9 on conductors 40 can be used to control the operation of the multiplexer 8.

The contents of the BCD counter 26, stored in the latch circuit 5 in response to the reception of a pulse train from a main or secondary station, are fed via the multiplexer 8 to the microprocessor 9 and indicate to the microprocessor the time for receiving a valid pulse train from a main or secondary station. or secondary station.

After the microprocessor 9 has received the contents of the latch circuit 5 via the multiplexer 8, indicating the time for receiving a pulse train from a main or secondary station, the microprocessor outputs a signal to the latch circuit 21 to reset this - the LATCH RESET signal - to erase the information stored therein in preparation for storing a subsequent indication of a main or secondary station. The LATCH RESET signal is also passed via the OR gate 80 to the flip-flop 23 to reset it.

Since signals constituting input signals to the microprocessor 9 from the latch circuit 5 represent the reception of signals from main and secondary stations from more than one LORAN-C station chain, the microprocessor 9 requires an input signal from the operator in charge of the equipment to turning of the thumbwheel switches 11 indicate the particular LORAN-CI chain of interest. The operator first consults a LORAN-C hydrographic map published by U.S. Pat. Coast Guard and thereby receives the group repeat interval (GRI) for the LORAN-C station chain of interest. Using the four thumbwheel switches 11, the operator sets the repeat rate or GRI.

As already described, the latch circuit 5 is used to store the count present in the BCD counter 26 each time a pulse train from a main or secondary station is detected by the fast shift register 3. At the same time, the information stored in the latch circuit 21 is also applied to the microprocessor 9. via the multiplexer 8 to indicate that the signal comes from a main or secondary station as well as its phase coding.

In the aforementioned rough search, the microprocessor 9 analyzes the main and secondary station information entered L via the latch circuit 5 to determine which indications represent signals from the selected LORAN-C chain.

The microprocessor 9 stores the time for the signal reception of the pulse trains from all main and secondary stations as indicated by the counts stored in the latch circuit 5 until the microprocessor has definitively located and read to the selected stations and can therefore calculate the time for the arrival of subsequent pulse trains. from there.

The microprocessor has been programmed to generate ten batches, each corresponding to one of twenty consecutive time periods, each having a duration of approximately 7810714-1 10 15 20 25 30 20 twelve hundred microseconds. The count stored in the latch circuit 5, when the logic circuit U indicates that a pulse train has received from a main or secondary station, causes a count to be stored internally in the microprocessor 9 in the corresponding one of the twenty compartments. The microprocessor 9 has been programmed to store the count numbers in these twenty compartments, which form a histogram, to determine which compartments contain count numbers indicating receipt of main and secondary station pulse trains with the correct GRI.

When the microprocessor 9 consistently receives signals from the main station in the selected LORAN-C chain, it causes a front panel lamp designated "M" to turn on to indicate that the receiver has been read to the correct main station signals. As the microprocessor 9 locates each secondary station associated with the selected LORAN-C chain, it causes the corresponding front panel lamps "51", "52", "53" and "S4" to turn on as each secondary station is locked to the. This indicates to the operator which secondary stations can be accepted for use in LORAN-C measurements. The microprocessor 9 then takes only those of the twenty histogram bins in which the main and secondary station signals of the selected chain are stored and subdivides each of these bins into one hundred bins, which correspond to successive »time sections with a duration of twelve microseconds each. .

The process now described is repeated for the shorter duration histogram batches generated in the internal memory of the microprocessor 9 to more precisely determine the arrival or reception time of the pulse trains from the secondary stations in the selected LORAN-C chain.

Once this histogram processing has been performed to determine the time for receiving main and secondary station pulse trains with an accuracy within twelve microseconds, the microprocessor 9 generates an operating time signal, which causes the equipment to switch from the coarse search mode to the fine-tuning mode. will be described later.

To place the equipment in the fine-tuning work network, the microprocessor 9 emits a signal at the output designated ROUGH SEARCH INACTIVE. The latter signal is transmitted via the OR gate 80 to the reset input R of the flip-flop 23 and thereby signals from the register 3 are prevented from being applied to position wall H to place the flip-flop 23 in the extended or 1 position. Microprocessor H 10 20 25 30 35 7810714-'1 21 also imposes a signal on its output FINANCING ACTIVITY for the equipment to be placed in the fine-tuning mode, where the arrival time of subsequently received signals is carefully determined and a reading is indicated on the presentation device 12.

In particular, the fine-tuning operation signal is applied to the comparator 1H shown in Fig. 7 to make it effective.

One of the two input signals to the comparator 1H consists of the output signal from the BCD counter 26 at 7 o'clock and this signal appears on the conductor REAL TIME. The second input signal to the comparator 1a is a number stored in the latch circuit 15 and this number is calculated by the microprocessor 9 as now described. When the microprocessor 9 determines the arrival time of the pulse trains from the main and secondary stations of the selected chain by the sample search mode and then switches to the fine search mode, it calculates the arrival time for subsequent pulse trains from main and secondary stations from secondary or fine histograms (with duration 12 microseconds). Using these fine histograms, the microprocessor 9 actually calculates a time which is 35 microseconds before the expected arrival time of a subsequent main or secondary pulse train and enters this information in the latch circuit 15 via the conductor PREVIOUSLY under control by a signal generated by another microprocessor. on the control steam. The comparator 1U compares the signal from 7 o'clock with the number sonlfhuß stored in the latch circuit 15, and when a correspondence is obtained between these two digital numbers, an output signal is obtained from the comparator 14 which sets the flip-flop in the logic circuit 16 to the exhibited or 1 position. The output signal of the flip-flop 30 is applied to the reset input R of the counter 31 and to one of the two inputs on the OR gate 32. Since the flip-flop 30 is in the displayed position, its output signal is high and it is passed via the OR gate 32 to the set-up input of the flip-flop S and the rocker 33 will hereby be brought into the exposed position with the 1-output high.

The high 1-output signal from the flip-flop 30 is applied to the reset input R of the counter 31 and causes this counter to be reset. After resetting, the counter counts 31 to 8 and then stops counting and causes its output TC to become high. The TC output signal of the counter 31 is applied to the reset input R of the counter clock n 34 and the counter 34 is thereby rendered inactive to count when the counter 31 has been counted to 8 and is thereby no longer effective to count. This is because the flip-flop 30, by being placed in its exposed position with its 1-output high, makes the counter 31 effective to count by resetting it, thereby making its TC output zero, thereby removing the signal to the counter 34 'reset input R.

The counter 34, which is reset, is thereby made effective to count in response to the signal with the frequency 1 MHz which constitutes the input signal to its clock input CK. The counter 3H differs from the counter 31 in that it counts to its maximum count of 10000 and then resets itself to count to 10000 again and again. Due to the repeated count of the counter 34 to 10000, the same output TC even has an ignal which occurs at a rate of 1000 microseconds due to the division which the counter 3H produces in the case of the signal appearing at the input CK with the frequency 1 MHz. The counter 34 will therefore provide output signals at the same rate as each of the pulses received in the pulse trains from the main and secondary stations. The 3% TC output of the counter is applied to the second input of the OR gate 32 and is also applied to the clock input CK of the counter 31. This causes the count in the counter 31 to increase by one each time the counter 34 counts to 10,000. At the end of 8000 microseconds, the counter 31 has reached its full count and its output TC is high and causes it to be applied to the reset input R of the counter 34 to the counter 34 is reset and stops counting. The counter 31 does not cause to be reset until the flip-flop 30 has been reset and has its 1 output low. This occurs when the output signal TC becomes high, and when this signal is applied to the reset input R of the coat of arms 30, the flip-flop will be reset or reset. This removes the high input signal to the reset input R of the counter 31 and leaves the counter with its full count and with its output TC high.

One of the purposes of the timing function provided by the counters 31 and 3% is to check the phase coding of the pulse trains received from the selected main and secondary stations. When the microprocessor 9 changes the receiver to the fine tuning mode, the microprocessor will in parallel enter the phase coding for the first eight pulses in the main and secondary station pulse trains in the selected LORAN-C chain in the logic circuit. 16 parallel-serial converter 35. The converter 35 is a common shift register of a type well known to those skilled in the art which can be supplied with signals in parallel and then shift these out in series to thereby effect the parallel-serial conversion. As is well known to those skilled in the art, each of the pulses in the pulse trains received from main and secondary stations has a special phase coding. This phase coding is stored in the microprocessor 9 and is selected by information entered into the equipment by the operator using the thumbwheel switches 11. It is clear that the clock input signal CK to the converter 35 is the same as the 10Û0 microsecond signal from the counter BH output.

The contents of the converter 35 will thus be shifted in series from its output Q at a rate of 1000 microseconds. It is to be noted that the output Q of the converter 35 is connected to one of the two inputs of the EXCLUSIVE OR gate 36 of the zero crossing detector 6. This gate 36 acts as an inverter in this case, and this is done in a manner which is well known to circuit designers. When a particular of the pulses in the pulse trains received from a main or secondary station has a positive phase, there is no signal or a zero on the output Q of the converter 35, if the phase codes match. The result is that each radio frequency period in the particular pulse, which is severely limited by the limiter 17, will pass directly through the gate 36 to the flip-flop 37 without being changed. Upon the expected reception of each particular pulse in the pulse trains from the main and secondary stations having a negative phase, the converter 35 will have shifted its contents so that the output Q is high or one. This high input signal is applied to the second input of gate 36 and causes gate 36 to invert the phase of the pulse output signal from limiter 17. This means that the signal constituting input signal to detector 6 is effectively shifted 180 °, thereby eliminating the negative phase coding applied to it. special heart rate. This is done in order to obtain an output signal from the EXCLUSIVE OR gate 36 to put the rocker 37 in the set position exactly at the beginning of each pulse in the pulse trains from the main and secondary stations.

The flip-flop 37 in the detector 6 is exposed with its 1-output high in the manner already described, thereby causing the latch circuit 5 '7810714-'1 10 15 20 25 30 ace 24 to store the contents of the counter 26 at this particular time. The microprocessor 9 will thereby receive a time indication of the beginning of each radio frequency period for each of the pulses, and this information is used to perform the required measurements of arrival time differences which form the basis of the LORAN-C system. The flip-flop 37 is reset before the start of the first period in a subsequent pulse received from a main or secondary station, and this is done by the already described LATCH RETURN signal.

The microprocessor 9 determines the expected arrival time of the positive zero pass of the third period for each of the pulses in the next pulse train to be received from the selected main and secondary stations. The microprocessor 9 then subtracts 35 microseconds from this time, thereby creating a time which should occur five microseconds before the start of the first radio frequency period in each pulse in the pulse trains of the main and secondary stations. This time, which occurs 5 microseconds before the start of each pulse in the pulse trains, constitutes an output signal from the microprocessor 9 on its output conductor PAST and constitutes an input signal to the latch circuit 15 under control of its control input by signals from the microprocessor. The contents of the latch circuit 15 are fed to the comparator 14, which is activated by the microprocessor feeding its input E when the equipment is placed in the fine-tuning mode.

It is to be noted that the comparator 1H also receives an input signal denoted REAL TIME and constitutes the locking output signal from the BCD counter 26 in the clock counter 7 in Fig. 5. When there is a match between the two input signals to the comparator 14, an output signal is obtained therefrom which puts the flip-flop 30 in the logic circuit 16 in the set position, the 1 output of the same becoming high. As already mentioned, the counters 31 and 34 will hereby be made operative to start the counting, as already described. The 1 output of the rocker 30 is also connected via an OR gate 32 to the position input S of the rocker 33 in order for the latter to be exhibited with the 1 output high. As can be seen from Fig. 6, the 1 output of the flip-flop 33 is connected to the reset inputs of the counters 38, 39 and 41 and to the clock input CK of the flip-flop 42, all of which are present in the logic circuit 16. The purpose of the latter elements is to help the microprocessor 9 to 7810714-1 analyze each received pulse in the pulse trains from the main and secondary stations to accurately determine the arrival time of the third period positive zero pass in each pulse.

It can be seen that the clock inputs CK on each of the counters 38, 39 and H1 are driven by a clock signal on the conductor CLK.

The source of this clock signal is the 10 MHz clock H5 in the clock counter 7 shown in Fig. 5.

The flip-flop 33, by exhibiting its 1-signal, supplies the reset input R to each of the counters 38, 39 and 41, thereby resetting these counters and allowing these counters to begin counting. As shown in Fig. 5, the counter 88 has been designated as a 30 microsecond counter. This means that it counts and produces an output signal on its output TC 30 microseconds after this counter has been made to count. In the same way, the counter 39 has an output signal at its output TC 2.5 microseconds after it has been made effective to count. The counter 41 has an output signal at its output TC 12.5 microseconds after it has been made effective to count. Thus, 2.5 microseconds after the comparator 1a has brought the flip-flop 30 to the set position, thereby bringing the flip-flop 33 to the set position, there is an output signal from the counter 39 to the clock input CK on the flip-flop H3 in the logic circuit 16. The output of the counter 39 TC remains high until the supply of the reset input R ceases. In the same way, 12.5 microseconds after the counter 41 has been activated by resetting the same, there will be an output signal from the same to the flip-flop input CK of the flip-flop. The flip-flop 43 is a type D flip-flop and it stores any signal present at its D input when its clock input CK is fed. It is to be noted that the D input of the flip-flop 43, as well as the D-inputs of the flip-flops 42 and 44, receive an input signal from the output of the EXCLUSIVE OR gate 36 in the zero-pass detector 6 in Fig. 5. The output signal of the gate 36 is a square wave pulse which corresponds to each radio frequency period in each pulse in the pulse trains received from the main and secondary stations in the LORAN-C system, and this output signal is also inverted to find out the phase coding in the manner already described.

The counter 39 will run out of time and cause the clock input CK of the flip-flop H3 to become high at a time which is 32.5 '7810714-1 10 151 20 25' '30 35 26 microseconds before the expected arrival of the positive third period zero crossing in each pulse. It should be noted that this point 32.5 microseconds occurs 2.5 microseconds before the first period of each pulse. At this time, only noise to be received by the LORAN-C equipment, and more specifically only noise with a frequency that falls within the bandwidth of the filter 1 to 10 kHz. Statistically, noise powder applied to the flip-flop 43 D-input will occur as often as they do not. The counter 39, which feeds the flip-flop input CK of the flip-flop 43, will thus cause this flip-flop to store either zeros or ones on a proportionally equal basis, if the microprocessor 9 has carefully determined the positive zero crossing of the third period and the output of the counter 39 occurs before the start of each pulse. The Q output of the flip-flop 43, like the Q-outputs of the flip-flops 42 and 44, is connected via the multiplexer to the microprocessor 9, as shown in Figs. 6 and 7. The microprocessor 9 receives and stores the output signal from the flip-flop 43 for a total of 2000 samples and is programmed to take the mean value of these samples obtained from the flip-flop 43. There will be approximately as many ones as zeros received therefrom, if the input signal to the flip-flop 43 D input is received before any pulse in the pulse trains from the main and secondary stations.

The counter 41 completes its count 12.5 microseconds after it is activated by the output of the comparator 14, as already described. The output signal from the counter 41 occurs 7.5 microseconds after the start of the first period of each pulse in the pulse stages, if the microprocessor 9 has carefully determined the position of the positive zero passage of the third period in each pulse. This time will occur at the midpoint of the negative period of the first radio frequency period in each pulse. The moment the counter 41 feeds the clock input CK on the flip-flop 44, the D-input of this flip-flop will receive a zero signal from the gate 36. The result is that the Q output of the flip-flop 44 will also be zero and this zero signal will be fed to the microprocessor 9 via the multiplexer 8, as already described. The microprocessor 9 will also store the output signal from the flip-flop 44 for 10,000 samples, with one per pulse, and is programmed to take the average value of these samples to determine if they are predominantly zeros for representing 7810714-1 27 a negative half-life.

In the event that the microprocessor 9 does not initially accurately determine the position of the positive period of the third period for each pulse in the pulse trains from the main and secondary stations, and this will usually happen when the microprocessor 9 initially switches LORAN-C equipment to the fine tuning mode, the outputs from flip-flops 43 and 44 will not be as described immediately before.

When the estimated time is too long, the sample points clocked into the flip-flops 43 and 44 by the respective counters 39 and 41 will both appear during each pulse in the pulse stages. As a result, the averages provided by the microprocessor 9 for the flip-flops 43 and 44 will result in positive or negative averages and will not result in an average value of zero. In response to this condition, the microprocessor 9 subtracts 10 microseconds from the estimated arrival time and the described sequence is repeated. When the estimated time is too short, the averages of the stored samples at the points 2.5 microseconds and 12.5 microseconds will both be zero and the microprocessor 9 will then add 10 microseconds to the estimated arrival time. This recalculation and repetition of the circuit operation just described is repeated until the output signal from the flip-flop 43 results in a zero average value to the microprocessor 9 and the output signal from the flip-flop 44 results in a negative average value. As the microprocessor 9 gets closer to the exact arrival time, the microprocessor can add or subtract less than 10 microseconds to the calculated time to determine the exact value of the estimated arrival time.

The counter 38, which is also made operative to count upon receiving the output signal from the comparator 14 via the flip-flop 33, counts to time a period of 30 microseconds and at the end of this an output signal is provided at the output TC of the counter. The output signal from the output TC of the counter 38 is fed to the reset input R of the flip-flop 37 in the zero-crossing detector 6 and is also fed to the reset input R of the flip-flop 33. The flip-flop 37 will hereby be reset, with its 1-output low, immediately before receiving the third period zero crossing for each pulse received in the pulse trains from the main and secondary stations of the selected LORAN-C chain. The very limited output signal from the limiter í7 which occurs immediately after the flip-flop 37 is reset is a response to the positive zero pass of the third period for each pulse.

As a result, the 1-output of the flip-flop 37 will be high in direct correspondence with the leading edge of the severely limited output signal in the form of a square wave pulse obtained from the limiter 17 and corresponding to the positive zero pass of the third period. As already described, this is caused by the count number constituting the contents of the BCD counter 25 being clocked into the latch circuit 5 and indicating the exact time of receipt of the positive zero passages of the third period in each pulse in the pulse stages. This information is applied via the multiplexer 8 to the microprocessor 9, as already stated, during the processing. In response to this information, the microprocessor 9 can perform the desired measurements of arrival time differences required in the LORAN-C equipment. Since the arrival time difference measurements took place; the microprocessor 9 produces the relevant output signals on its presentation output signal conductors, which carry input signals to the presentation device 12.

The output signals from the microprocessor 9 to the presentation device 12 are applied to the digital presentation units in question therein. The digital presentation unit 51 is used to visually present the arrival time difference information for a selected secondary station, and the digital presentation unit 52 is used to visually present the arrival time difference information for a second selected secondary station. The inputs to these digital display units are encoded and decoded appropriately by anode drivers 46 and 47, anode drivers RS and decoder drivers 40 and 50 for driving, respectively. digital presentation units 51 and 52.

These presentation units together with their associated decoding and driving circuit devices are well known to those skilled in the art and are commercially available. In the present embodiment of the invention, the display units 51 and 52 are fluorescent display units of the Itron FG612A1 type, but they may also be light emitting diode display units or those having liquid crystals or some other form of visual display units. 10 15 20 25 30 35 7810714-'1 29 In order to select the secondary stations, which are to be presented by the units 31 and 32 in terms of their arrival time differences, the thumbwheel switches 61 and 52 have been arranged. The switch 61 is located in the room with respect to the presentation unit 51 and one of the numbers "1" to "4" is selected by means of this switch to indicate to the microprocessor 9 the information to be presented. Similarly, switch 62 is associated with display unit 52 and is used by the equipment operator to indicate the particular arrival time difference of the secondary station to be presented through display unit 52. Switch 11 has no details but consists of eight individual switches of the type shown. is indicated by the switch 61 in Fig. 7. The action of the inhibited thumbwheels causes speech to appear in a window and the connections of the switch indicate the selected number.

A signal-to-noise pushbutton 62 'is also located on the front panel of the equipment and a depression of it by the operator causes the presentation given by the display units 51 and 52 to be replaced by a value of the signal-to-noise ratio for the same secondary stations indicated by the mode of the corresponding switches 61 and 62. The microprocessor 9 is programmed to calculate the signal-to-noise ratio values to be presented and responds to the action of the button 62 'to change the presentation through the display units 51 and 52. To control the signal-to-noise ratio, the microprocessor 9 stores fourteen thousand samples of the first negative half-periods of each pulse, as indicated by the counter already described 41. It is easy to see that pure noise would cause the detection of seven thousand negative half-periods while a perfect signal would lead to the detection of fourteen thousand negative half-periods . Consequently, numbers between seven thousand and fourteen thousand will indicate the signal-to-noise ratio, and this ratio becomes higher when the number of detected negative half-periods increases against the sample number fourteen thousand. It is the numbers between seven thousand and fourteen thousand that will be presented through the presentation units 51 and 52, when the signal noise button 62 'on the front panel is actuated.

It is easy to see that the microprocessor 9 can be programmed to present numbers between 0 and 100 corresponding to the range from seven thousand to fourteen thousand by using a simple interpolation algorithm. Other speech schemes can also be used to indicate the signal-to-noise ratio.

Although what has now been stated is at present considered to be the preferred embodiment of the invention, it is merely an illustrative example and the rapid changes in technology will make various changes and modifications apparent to those skilled in the art and may be made without departing from the scope of the invention.

Thus, for example, programming can be added to the microprocessor and the keyboard can be used as an input device and the presentation device as an output device for performing calculations of all kinds, and the presentation device can also be used to provide a digital clock with information regarding day, date or other information. According to another modification, the microprocessor can provide navigation instructions via the presentation device.

Claims (9)

10 15 20 25 30 35 7810714-1 31 PATENT REQUIREMENTS A method of deriving location information for navigation purposes by performing measurements of the time period between the receipt of periodic signals in pulse train form from a selected main transmitter station and selected from a plurality of secondary transmitter stations associated with the main station in a navigation system, each signal pulse train being stored. when it is received for analysis of its design to determine whether it has been received from a main or secondary station, and an indication of the time of reception of each pulse train is stored together with an indication of whether it comes from a main or secondary station, which stored indications are used to identify the selected main station and the associated secondary stations, and on the other hand the arrival time of signals received from the selected main station and the secondary stations after determining which periodic signals are received therefrom is indicated. of that each signal that received at the estimated time of arrival from the selected main station and the secondary stations are analyzed to determine if the estimated time of arrival is correct and to modify the estimated arrival times, if necessary, to locate a specific point in each one of the periodic signals, the time difference in the arrival between the specific point of each of the periodic signals received from the main station and each of the selected secondary stations being measured and in that an output signal is provided in response to the time difference measurement to be used for navigation purposes. 2. A method according to claim 1, characterized in that the selection of the main station and associated secondary stations as well as the selection of the secondary stations to be used for performing the time period measurements are done manually. 3. A method according to claim 2, characterized in that the storage of signal pulse trains comprises the storage of polarity properties with respect to each of the discrete points on the periodically received signals, the signal analysis being performed with respect to the nolarity properties. 7310714-1 32 4. The method of claim 1, wherein the step of indicating the time of receiving said signals comprises analyzing said stored indications for the selected main station and associated secondary stations to determine the mean value of the arrival time of the signal pulse trains. A method according to claim 1, characterized by the further step of checking phasing for each pulse in the signal pulse stages received from each of the selected main stations and associated secondary stations against stored phase code information to determine if the received the pulse train from each of the main and secondary stations consists of a space wave reflected from the ionosphere, such pulse trains not being taken into account. 15 A method according to claim 5, characterized by the further steps of removing the phase coding from the pulses in the pulse trains received from the selected main station and to the associated secondary stations before storing the indication of the time of receipt of each pulse train to obtain accurate measurements of the time period between the reception of signals from the selected main station and each of the selected secondary stations. A method according to claim 4, characterized in that the step of storing the polarity properties is performed for a plurality of samples and comprises a step of providing an indication of the number of times a particular polarity occurs in said plurality of samples from each of the secondary stations to provide an indication of the signal-to-noise ratio of the signals received from the latter 30 stations. 8. Navigation receivers for providing navigation information by receiving and measuring the arrival time differences of coded radio signals received from a plurality of navigation transmitters arranged in groups consisting of a main transmitter station and a plurality of secondary transmitter stations and the radio signals transmitted by each of the main and secondary stations comprises a series of pulses in a pulse train, the pulse train transmitted by a main station being different from the pulse train transmitted by a secondary station to separate the two the station types, and including means for selecting a group of transmitters to be used for measuring arrival time differences of received pulse trains, and first logic means (3, H) for determining when received signals are correctly coded to indicate that they come from the navigation transmitters. and providing output data thereon, and processor means (9) for storing and analyzing output data data. from the first logic means for determining when received radio signals come from transmitters in a group of transmitters as selective underuse of the selector means (11), and other logic means (16) operable and operating together with the processor means (9) after determining that the received radio signals come from the selected group of transmitters to indicate the time of reception of subsequently received radio signals and then analyze the latter radio signals to thereby locate a specific point on the latter radio signals which the processor means use to accurately measure the arrival time differences of the radio signals from individual transmitters in the selected group of transmitters and provide a visual output signal (12) of said measurements to provide navigation information, characterized in that the measurements of the signal arrival time differences are arranged to always take place between the arrival time of the signals from a main station and selected secondary stations, the first logic means comprising on the one hand a multi-stage shift register (3), which is used to store pulses in the pulse stages when they are received from the main and secondary stations and on the other hand third logic means (4) connected to separate stages in the shift register for analyzing a stored pulse train to determine whether it is coming from a main or a secondary station and provide a questionable indication of the analysis to the processor means (9). Navigation receiver according to claim 8 in the case that individual pulses in the transmitted pulse stages are phase-coded, with phase codes for different main and secondary stations stored in the processor means (9), characterized _vs1ov14-1 10 15 20 25 30 34 that a phase code correction apparatus (35, 36, 37) has been provided, in which the phase codes of the signals subsequently received from main and secondary transmitters in the selected group of transmitters constitute input signals to the correction apparatus and were used to remove the phase coding. from the signals received in the following before the received radio signals are used to perform the measurements of the arrival time differences. The transmitters in each group of navigation transmitters all transmit Navigation receivers according to claim U for their pulse trains with a predetermined repetition rate which is specific to each group, characterized in that the selector means (11) are manually operable to indicate the repetition rate of a selected group of transmitters for the processor means (9), which use said indication of the rate to first identify signals received from the selected transmitter group and then to calculate the subsequent time for the reception of the radio signals from the selected group of transmitters. 11. characterized by, on the one hand, a presentation device (12) which is a Navigation receiver according to claim 10, capable of cooperating with the processor means (9) in order to provide a visual presentation of the measurements of the signal arrival time differences used for navigation, and on the one hand, means for specifying for the processor means special of the secondary stations for which the measurements of signal arrival time differences are to take place. 12. characterized in that the processor means (9) is arranged that the Navigation receiver according to claim 9, can generate an operating time signal after the calculation of the time for receiving radio signals subsequently received from a selected group of navigation transmitters and other logic means (16). (28, 29, H1) are operable by the operating time signal to generate time signals which cause samples of any received signals to be input signals to the processor means which store a plurality of such samples and provide amplitude average values thereof for analysis to determine whether the calculated time for signal reception is correct for locating the specific point of said radio signals from which the measurements of signal arrival time differences are made to derive navigation information. The case in which the processor means (9) generates an operating time Navigation receiver according to claim 12 for the signal after calculating the time for receiving radio signals received in the following, characterized in that the means for generating time signals comprise first time means. (38) operable by operating signals to cause a first sample of arbitrary signals received by the present navigation receiver to be input to the processor means (9), which stores a plurality of samples of the first signals and provides the amplitude mean value thereof, wherein this average value becomes zero when the signal samples are taken outside said pulses, and in addition second time means (29) set in operation by operating time signal are arranged to cause a second sample of arbitrary received signals to become input signal to the processor means (3), which stores a several other samples and provides- amplitude average one of them, the second sample average being different from zero when the second sample Lagus during the reception of an arbitrary one of said pulses, and on the other hand third timing means (41) are arranged to provide, put into operation by said operating time signal, a third time signal, which causes a third sample of arbitrary signals to become an input signal to the processor means (9), which stores a plurality of the third samples and produces the amplitude value thereof, the third sample average being different from zero when the third sample has taken during any of the said pulses, and the processor means is arranged to determine from the first, second and third samples whether the calculated time for the arrival of the radio signals is correct, the processor means (9) being arranged to revise the calculated time until predetermined pulse parameters are located whereby the specific point can be located to perform said measurements of the signal arrival time difference. 7310714-1 10 15 36 lä. Navigation receiver according to claim 8, characterized in that the processor means (9) is arranged to store the results of the analysis of the radio signals by means of the second logic means (16) for a plurality of samples and in that the processor means is programmed to derive information regarding the signal-to-noise ratio from the stored majority of samples. 15. A navigational receiver according to claim 1H, characterized in that the processor means (9) is arranged to store the analysis samples as binary zeros and ones at input signals in the form of pure noise to the navigation receiver, resulting in an equal number of binary zeros and ones from said navigation receiver. plurality of samples, and pure signals from the inputs of the navigation transmitters to the receiver, resulting in only binary zeros being obtained from said plurality of samples, and further reacting to the number of zeros and ones in said plurality of samples so that the processor means causes a visual output to be emitted to indicate the signal-to-noise ratio. / T