FR2694096A1 - Error correction for global positioning system with fixed transmitter - using narrow bandwidth radio frequency navigation channel to transmit cyclically digital correction messages of satellite distance deviations - Google Patents

Abstract

The system's satellites are liable to cause occasional errors in the positioning of boats. With each message, data as transmitted of small distance deviations of all the satellites and the important distance deviations of at least one satellite. In each cycle, the number of messages equals the number of satellites. Each message is lightly weighted to the group data and heavily weighted to the single satellite. The transmission band, 285.3 to 285.7 kHz allows propagation over the sea with low levels of attenuation and atmospheric noise. The bit rate is therefore low and each message is limited to 50 bits in 10 seconds using 2.5 Hz two-phase modulation. USE/ADVANTAGE - Radio navigation. Low bandwidth allows favourable waveband.

Description

 The invention relates to a radionavigation method, in particular for the radiolocation of boats, from radionavigation signals emitted by a certain number of satellites forming part of a radionavigation system such as the system called GPS, and emitting signals containing errors likely to cause positioning errors of user vessels and at least one signal from a fixed station such as a shore station, adapted to establish data for correcting said errors, these data being transmitted in the form digital modulation of a radio carrier transmitted by said station.

 A process of this type is already known. This method has the disadvantage that the band of radio signals required for the transmission of the correction data is relatively wide. However, a very favorable frequency range for the transmission of radioelectric signals allowing propagation of signals over seawater with a relatively low attenuation, and in which the atmospheric noise is clearly lower than for other frequencies is already assigned to the maritime users, such as the band extending from 283.5 KHz to 315 KHz.

 This favorable frequency band is prohibited for correction data systems because of their wide bands necessary for the transmission of this data. Having to be thus worked in areas of the less favorable frequency spectrum, fixed stations must transmit at comparatively very high power. Another consequence of this prohibition is that the known method does not allow the use of the equipment of fixed stations already installed and adapted to operate in the aforementioned frequency range assigned to maritime users.

 The object of the present invention is to provide a method which allows correction data to be transmitted within the aforementioned favorable band.

 To achieve this goal, the width of the modulation band is kept narrow by transmitting with a message all the correction data information only the data relating to the relatively small distance deviations and only part of the information relating to the large deviations and by transmitting the other parts of this information of large deviations successively with the following messages, so that after a number of predetermined messages forming a cycle which is repeated, all of the correction data have been transmitted, the number of said messages of each cycle being chosen so as to be able to limit the width of the modulation band to a value allowing it to be placed in the spaces available in a frequency band where the radioelectric signals propagate over seawater with relatively attenuation low and a low level of atmospheric noise, such as the fr band accordingly radio-electric from 285.3 to 285.7 kHz allocated to public service radio navigation.

 According to an advantageous characteristic of the invention, the correction data relating to deviations from short distances from all the satellites whose radio navigation signals are received by the boat are transmitted with each message and the correction data relating to deviations from relatively large distances d 'at least one of said satellites.

 According to another characteristic of the invention, each message comprises the correction data relating to the deviations of the large distances from one of said satellites, the number of messages in each cycle being identical to the number of satellites used.

 According to yet another advantageous characteristic of the invention, each message whose data is transmitted in digital form includes the least significant of the data for correcting the deviations in the distances of all the satellites and the most significant of the data for correcting the deviations of the distances from one of said satellites.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the appended schematic drawings given solely by way of example illustrating a method of realization of the invention and in which:
- Figure 1 illustrates the frequency spectrum used in a preferred embodiment of the method according to the present invention; and
- Figure 2 illustrates in the form of a block diagram the phase modulation of the signal carrying the correction data of a fixed station, according to the method of the invention.

The radionavigation method which is especially designed for maritime users such as boats, makes it possible to establish the position of a user, from radio signals emitted by a certain number of satellites of a radionavigation system such as GPS system (Global
Position System) which includes 18 (or 21) satellites and from specific signals from a fixed transmitting station, for example a ground station. Since the radio navigation signals emitted by the satellites are liable to contain errors which cause positioning errors of the user, said fixed station is adapted to note the differences in distances called pseudo-distances produced by the aforementioned errors which are radio navigation signals between each satellite and the position of the station, because the station knows precisely the trajectory of each satellite, it is able to establish for each satellite the distance errors implied by the signal emitted by the satellite, with respect to its exact position. The fixed station communicates distance errors to the user as correction data.

 As will be described in more detail below, the method according to the invention is designed to be implemented on existing transmitting station installations, advantageously by the radio navigation system of 18 transmitters, known under the name "RANA". The RANA system uses a 400 Hz band extending between 285.3 and 285.7 KHz. This narrow band is part of the frequency range particularly allocated to maritime users between 283.5 KHz and 315 KHz, which has particularly favorable properties insofar as in this range the radioelectric signal propagates over seawater with an attenuation compared to the inverse of the distance, which is low (7 db at 500 km), the atmospheric noise being relatively low.

 In the RANA system, in the aforementioned 400 Hz band, each of the transmitters transmits for 30 seconds, every 10 minutes, sequential signals FO, F1 of respectively 285,400 KHz and 285,600 KHz and continuously a permanent frequency. The stations of the RANA chain are numbered from 1 to 19 and the permanent frequency respectively transmitted is equal to 285,400 + (station number + 6) x 6.25 Hz. Figure 1 shows the frequency spectrum used in the allocated band. The hatched block A illustrates the permanent frequencies of the stations bearing the numbers 1 to 9, while the hatched block B represents the stations bearing the numbers 1 1 to 19. The line Fc between the two blocks A and B represents the central frequency which n is not used.

The central frequency is 285,500 KHz and the permanent frequencies range between 285,440 and 285,560 KHz.

 The method according to the invention uses the permanent signals of frequency close to the central frequency of 285.500 KHz to transmit the correction data to the user, that is to say to the boat which wishes to determine its position. These data are transmitted in digital form. To this end, the permanent signal of a RANA transmitter of number close to the unused central frequency is advantageously modulated in phase, by a low frequency signal of 150 Hz. The signal 150 is phase-contrasted (bi-phase) Hz by the binary digital message to be transmitted.

 Figure 2 schematically illustrates this modulation. In the example shown, the carrier frequency is that of station number 9 transmitting the permanent signal of 285493.75 Hz. The signal is modulated in phase in 1 by the 150 Hz signal produced by a generator 2 on which a modulator acts. 3 of Oln; which receives the binary digital data. The phase modulation index is fixed at + 450 (m = 0.78), which creates two lateral lines at the carrier signal, which have a relative amplitude of 0.4. As shown in FIG. 1, the frequencies of these two lateral lines, bearing the references RL, are centered in the free emission bands extending respectively from 285,300 to 285,400 and from 285,600 to 285,700 KHz. The side lines of order 2, inherent in the phase modulation, are weak enough to be neglected.

 Thus for a typical installation using a 20 W power transmitter, with an antenna efficiency of 15%, one obtains for a central line of 3 W, side lines at 150 Hz of at 0.5 W x 2, the lines higher order lateral being less than 50 mW.

 The maximum bi-phase modulation frequency is chosen at 2.5 Hz, which allows, on the one hand, to transmit a logic signal at 5 bits per second, and, on the other hand, to make the interference of the two-phase modulation on the neighboring permanent carrier frequency lines, and on the sequential frequency lines 285,400 KHz and 285,600 KHz.

 To make the method involving the transmission of correction data sufficiently effective, it has been found that the correction must be transmitted at the minimum rate of 1 message per 10 seconds. The method according to the invention achieves this objective by transmitting at the rate of 5 bits per second, a message consisting of 50 bits.

To this end, each 50-bit message transmitted every 10 seconds has the following structure for a process using 5 satellites and involving the presence of five receiving channels in the boat's receiving device:
- bit 1: it indicates the two-phase sign. The phase of the bi-phase signal received at the start of the 10 seconds GPS from the satellite received by the boat is sufficient to serve as a reference.

 - bit 2 to 7: low weight of the corrections for channel n0 1.

 - bit 8 to 13: low weight of the corrections for channel n "2.

 - bit 14 to 19: low weight of the corrections for channel n "3.

 - bit 20 to 25: low weight of the corrections for channel n "4.

 - bit 26 to 31: least significant of the corrections for channel n "5.

 - bit 32 to 36: transmission health for the 5 satellites indicating whether the signal from the corresponding satellite can be retained or not. The state of each of the bits is an individual indication of the presence or absence of the satellite in question. It will be considered to be absent, for example if the signal-to-noise ratio is less than a reference value. Logical states 1 and 2 for example for bit 32 indicate whether correction data from satellites 1 have been transmitted or not.

 - bit 37 to 41: number of one of the satellites, that is to say of the satellite whose most significant distance correction is transmitted in the message.

 - bit 42 to 45: most significant of the distance correction of the satellite carrying the transmitted number.

 - bit 46 to 50: parity checks.

 As is clear from the above, with each message lasting 10 seconds, the fixed station transmits to the user the least significant corrections for the 5 satellites as well as an individual presence / absence message and the most significant ones distance correction of one of the 5 satellites, identified by the number also transmitted in the message. By transmitting with the successive messages the most significant of a satellite, the user receives every 50 seconds the most significant corrections of the 5 satellites. This mode of transmission ensures a quantification of the distance with a value of 0.51 m for a maximum distance correction value of + 260 m. A correction speed of 16 m / 10 seconds is obtained, a parity check of 5 bits for 45 message bits and a first acquisition time of 60 seconds for a number of satellite corrections equal to 5.

 The receiver is able to establish the correspondence between the various satellites and the reception channels, from the association of the satellite whose number and most significant of the distance correction are transmitted to the channel reserved for information concerning this satellite. . For this purpose, each 10 second period of a message is allocated a reception channel.

Thus the message received for example in the third period of time, for example between the 21st and the 30th seconds of an hour, the information from the satellite whose number and most significant signals are transmitted are received in channel n "3 and the structure of the messages is organized in such a way that the 3rd low-weight segment of message correction concerns this satellite. The method according to the invention therefore allows the receiver to establish the allocation of channels to the satellites and to reconstruct the set of correction information for each satellite by correctly associating the least significant and the most significant.

 The method according to the invention has the considerable advantages that thanks to the narrow modulation band the data transmission can be carried out in the radio band from 285.3 KHz to 285.7 KHz which is used by the RANA radio navigation system. . In this band the signals are transmitted with a weak attenuation in sea water and the atmospheric noise remains much lower than that noted for the lower frequencies. In addition, the invention makes it possible to use an already installed system of transmitters, that is to say the RANA system. The use of these transmitters does not require any modification of the structure of these transmitters. It is only necessary to add a simple module capable of executing the modulation operation indicated by the diagram in FIG. 2. The implementation of the method according to the invention also has the advantage that the transmitted power can be relatively low, d 'especially since the narrowness of the band used favors the extraction of the signal in the rough.

 The invention which provides the numerous and important advantages which have just been indicated is based on the observation that it is sufficient for accuracy requirements of maritime users to transmit with a message only the correction data (low weight of the corrections for all satellites and most significant corrections for a single satellite), which makes it possible to detect small variations during each 10-second transmission phase, and to transmit data relating to larger position variations only every 50 seconds .

Claims (7)

 1. A radionavigation method, in particular for the radiolocation of boats, from radionavigation signals emitted by a certain number of satellites forming part of a radionavigation system such as systems called GPS and emitting signals which may contain errors liable to '' cause positioning errors of user vessels, and at least one signal from a fixed station such as a shore station, suitable for establishing correction data for said errors, these data being transmitted by said signal under digital form by modulation of a radio carrier transmitted by said station, characterized in that the width of the modulation band of said carrier is kept narrow by transmitting with a message all the information of correction data only relatively small distance deviation data and only part of the information relating to significant deviations and by transmitting the other parts of this information of significant deviations successively with the following messages, so that after a number of predetermined messages forming a cycle which is repeated, all of the correction data has been transmitted, the number of bits of each message being chosen so as to be able to limit the width of the modulation band to a value making it possible to place it in the spaces available in a frequency band where the radioelectric signals propagate on water from sea with relatively low attenuation and a low level of atmospheric noise, such as the radio frequency band from 285.3 to 285.7 KHz allocated to the public service for radionavigation.  2. Method according to claim 1, characterized in that one transmits with each message the correction data relating to the deviations of the short distances of all the satellites whose radio navigation signals are received by the boat and the correction data relating to the relatively large distance deviations of at least one of said satellites.  3. Method according to claim 2, characterized in that each message comprises the correction data relating to the deviations of the large distances from one of said satellites, the number of messages in each cycle being identical to the number of satellites used.  4. Method according to claim 3, characterized in that each message, the data of which is transmitted in digital form, includes the least significant of the data for correcting distance differences of the set of satellites and the most significant data of data for correcting differences distance from one of said satellites.  5. Method according to one of the preceding claims, characterized in that each message further comprises a reference bit, for each satellite a bit indicating whether the signal transmitted by this satellite can be used, the number of the satellite including the most significant distance correction is transmitted in the message and parity check bits.  6. Method according to one of the preceding claims, characterized in that the message is transmitted to the carrier signal by phase modulation by a low frequency such as a frequency of 150 Hz modulated by phase opposition by the digital message according to a frequency such as a frequency of 2.5 Hz.  7. Method according to claim 6, characterized in that each message comprises 50 bits and is transmitted for 10 seconds.