GB2463774A - Radar system for detecting and analysing weather systems - Google Patents

Abstract

A radar apparatus is disclosed comprising a transmitter, a receiver, and a means for processing reflected signals, in which the transmitter produces a plurality of pulses, each pulse having a pulse width, and a time interval between each pair of pulses. The pulse width of each pulse produced may be of a different magnitude from the previous pulse and the time interval between any two consecutive pulses may be varied in a single pulse repetition frequency PRF cycle to enable more than one type of measurement to be made by the apparatus.

Description

Improved Weather Radar This invention relates to radar and particularly its use in the detection and analysis of weather systems.

In order to accurately predict and map weather patterns it is necessary to identify various parameters, including wind velocity and the amount of precipitation present in a given volume of air. Radar is one of the principal tools for gathering physical data relating to weather systems.

Meteorological precipitation covers all forms of water from ice to rain and mixtures of the two. Precipitation and water droplet size distribution can be estimated by measuring the reflectivity of a weather system being subjected to a radar pulse. The speed of movement of the system can be calculated by carrying out Doppler shift measurements on the system.

Radars used by meteorological organisations commonly employ both of these techniques to gather data, sweeping across a given azimuth and at a given elevation, and running preset scan patterns. Typically they must repeat such measurements every five minutes to provide near-continuous information on how weather systems develop.

In general a weather radar system would comprise a transmitter, a receiver and some data processing means. Most systems use a magnetron type transmitter which are cheaper and sufficiently effective, although a Klystron transmitter may also be used.

Transmitters emit a series of pulses with many such pulses emitted during a second.

The pulse is characterised by its Pulse width which is a measure of the length (in time) for which each individual pulse is emitted and the pulse repetition interval (PRI), which is a measure of the time interval between pulse emissions. Transmitters are often characterised in terms of a pulse repetition frequency (PRF) which is the inverse of the PRI The received reflected signal may be treated in different ways to obtain different data.

The reflectivity of the signal may be measured to give an indication of water droplet distribution. Doppler shift between two consecutive pulses reveals the radial velocity of the reflecting object, and the time between transmission and receipt reveals the distance of the weather system being tracked.

In a magnetron system it is advantageous if the system keeps within its maximum mean power limit. This requirement means that the combinations of pulse width and pulse repetition frequency (PRF) are restricted. Known transmitters will transmit a fixed pulse width and PRF, although some vary the repetition frequency after a sequence of pulses at a given PRF to a series at a different PRF.

Some examples are

a) Fixed Pulse Width and PRF (e.g. 2.Ops, 300 Hz) b) Fixed Pulse Width and Staggered Dual PRF (e.g. 0.5ps, 900/1200 Hz) c) Fixed Pulse Width and Interleaved Dual PRF (e.g. 1.0 jis, 800/1200 Hz) d) Fixed Pulse Width and Interleaved Triple PRF (e.g. 2.0 jis, 303,325,379 Hz) The conventional way such Pulse Width PRF options are used in Weather Radar Systems is to schedule a sequence of plan-position indicator (PPI) Scans. Each scan would provide data for different purposes-in the example above, (a) would be used for long range reflectivity scans, (b) would be used for shorter range Doppler! reflectivity scans, (c) would be used for shorter range Doppler! reflectivity scans, (d) would be used for long range Doppler! reflectivity scans.

Clearly, operating such a system is time intensive and since each set of data scans must be repeated for various elevations, arid data is required to be refreshed every five minutes, it would be advantageous to be able to obtain a more comprehensive set of data from each scan.

Although this is not critical for gathering reflectivity data, where it is desirable to gather refractivity data it is important to maintain a constant temperature at the magnetron to preserve the reliability of the measurements. For this reason, in certain circumstances it is useful to keep the mean power of the transmitter fairly constant.

It is possible to employ more transmitters! receiver combinations but this adds an additional cost that may outweigh the perceived operational benefits.

Similarly, it is possible to recover Doppler data from a long range reflectivity scan as in example (d) above but the unambiguous velocities using this method are very small and therefore difficult to extract using unfolding techniques.

Radar systems also exist that work with a dual transmission frequency, but these require the received signals to be treated by separate receivers which adds expense.

Therefore it would be advantageous to run a system that could make more than one type measurement in each PPI scan without building more receivers, and without resorting to intense computational treatment.

Accordingly, the current invention provides for a radar apparatus comprising a transmitter, a receiver and a means for processing reflected signals, in which the transmitter produces a plurality of pulses, each pulse having a pulse width, and a time interval between successive pulses characterised in that both the magnitude of the pulse width of a pulse and the time interval between any two consecutive pulses may be varied in a single PRF cycle.

Such an invention allows for a PRF repeat cycle' to be created, which may repeat many times in a single PPI scan. The creation of the PRF repeat cycle now permits dual purpose scanning thereby enhancing the operational effectiveness of the radar system.

The transmitter is set up to run in a mode that can be triggered to emit pulses at a set pulse width, so that operators can predetermine the most appropriate sequence of trigger signals to give the most suitable PRF repeat cycle for the meteorological conditions to be analysed. A pre programmed scan sequence can be programmed to repeat over a PRF cycle to optimise the search. Although this invention focuses on triggering a series of pulses of a known strength and separated by a set time interval, the invention is not limited to triggered pulses. Indeed, any means for emitting predetermined pulse with and pulse frequency PPI scans could be used.

Since the PRF Repeat cycle can now be optimised for a given system, the data received from one PP[ scan could be used to quickly alter the PRF cycle so that where different criteria are required or more information is needed, say for a quickly evolving, or unstable or otherwise dynamic system, the PP[ scan can be quickly changed, although in normal applications related to gathering weather data, such a more of operation is seldom likely to de used.

By careful selection of the Pulse width and PRF, the mean transmitter power can be kept constant or near constant, and this in turn keeps the temperature of the Magnetron constant or near constant. The added certainty of accuracy in doing so enables data relating to refractivity and phase recovery, which might currently be discarded, to be collected.

For low elevation PPI scans where ground clutter exists, such clutter now forms part of a stable radar picture. The main variable affecting the recovered phase is the humidity between the ground clutter and the transmitter! receiver. By being able to exploit the refractivity and phase recovery, it is possible to develop a low altitude humidity map. Such exploitation is assisted by maintaining the constant power output.

The system will now be described with reference to the following drawings Figure 1 Shows a general set up of a weather radar system according to the invention Figure 2 Shows a Pulse Repetition Cycle Figure 3 Shows a second Pulse Repetition Cycle Figure 4 Shows a third Pulse Repetition Cycle Figure 5 Shows a fourth Pulse Repetition Cycle Figure 1 shows the set up of a radar system. The transmitter (10) is configured to emit pulses as triggered according to a predetermined sequence by a processor (20). In this case the transmitter is a bespoke unit constructed by Communications and Power Industries of Paolo Alto of California, USA, although any transmitter with similar functionality would be suitable. The receiver (30) and the processing equipment are normal off the shelf systems that would be familiar to anyone skilled in the art.

The Transmitter (10) is a magnetron based transmitter having an output (12) and a composite trigger input (14). The composite trigger is a pulsed logic control signal from the Processor (20) to the transmitter where the width of the composite trigger pulse determines the length of the transmitter's output pulse and the pulse repetition frequency of the composite trigger pulse determines the PRF of the Transmitter's output pulse. A Transmitter sample coupler (16) provides an input back to the Processor (20) having an input (22) at a Transmitter sample digitizer.

The reflected signals from the antenna system are relayed into the Receiver 30 and thence input into the signal digitizer (24) of the Processor (20). Finally the processed data is output (26) to a suitable communications link for treatment.

By using the above system a skilled operative can pre programme a set of PRF repeat cycles to collect data relating to more than one parameter in one PPI scan.

Figures 2 to 5 show different means of operating the system as set out in figure 1 with different PRF repeat cycles.

In figure 2 the system sends a series of pulses at 2 is Pulse width with a PRI ofl/300s which are measuring reflectivity typically to a range of 250 km. In between two shorter pulses are used to perform pulse pair Doppler processing. In this example the PRI between the Doppler pairs is varied but not their pulse width. The interval between the shorter pulses is varied in order to produce data with different unambiguous velocities, permitting velocity enfolding which increases the effective unambiguous velocity.

In figure 3 a similar PRF repeat cycle is used, but here as the PRF between pulse pairs to be used for Doppler processing increases, the pulse width is decreased. In this example, the mean power of the magnetron remains unaffected enabling more accurate results.

In figure 4 a similar PRF repeat cycle is used but the second pulse of each Doppler pulse pair is a 2us pulse to give a near constant mean power than in other cases.

In figure 5 the apparatus is operated at a constant pulse width with a varying PRF. In this mode of operation the invention delivers a more integrated energy profile with better radar effectiveness.

Claims (11)

1. Claims 1. A radar apparatus comprising a transmitter, a receiver and a means for processing reflected signals, in which the transmitter produces a plurality of pulses, each pulse having a pulse width, and a time interval between successive pulses characterised in that both the magnitude of the pulse width of a pulse and the time interval between any two consecutive pulses may be varied in a single PRF cycle.
2. 2. An apparatus as claimed in claim 1 in which the radar is configured to detect meteorological conditions.
3. 3. An apparatus as claimed in claim 2 having a Pulse Repetition Frequency (PRF) repeat cycle in which the PRF repeat cycle comprises pairs of short interval pulses for determining Doppler shift separated by one or more pulses ones for capturing reflectivity data
4. 4. An apparatus as claimed in any preceding claim in which the transmitter comprises a Magnetron.
5. 5. An apparatus as claimed in any preceding claim in which the transmitter is a Klystron transmitter
6. 6. A method of capturing a variety of signal data from a single PPI radar scan comprising the steps of; Triggering a succession of pulses from a transmitter, each pulse having a pulse width and a time interval between successive pulses, Detecting a reflected signal from the pulse, calculating the reflectivity of any target causing said reflection, characterised in that both the magnitude of the pulse width of a pulse and the time interval between any two consecutive pulses may be varied in a single PRF cycle.
7. 7. A method as claimed in claim 6 in which there are triggered a first pair of pulses separated by a first time interval, a third pulse separated by a second time interval and a second pair of pulses separated by a third interval.
8. 8. A method as claimed in claim 7 in which the each of the first pair of pulses and the second pair of pulses have the same pulse width and the first and third time intervals are different
9. 9. A method as claimed in claim 7 in which the both of the first pair of pulses and the both of the second pair of pulses have the same pulse width, the pulse width of the first pair o pulses being different from the pulse width of the second pair of pulses and the first and third time intervals are of different magnitudes.
10. 10. A method as claimed in claim 7 in which each pair of pulses has an earlier pulse and a later pulse in which the earlier pulse and the later pulse are of different pulse widths
11. 11. A method as claimed in claim 7 in which the every pulse has the same pulse width but the time intervals between the first pulse pair and the third pulse and the second pulse pair are all different from one another.