GB2315628A

GB2315628A - Radar altimeter

Info

Publication number: GB2315628A
Application number: GB8609449A
Authority: GB
Inventors: Nigel Charles Trueman Coote; Christopher John Alder
Original assignee: GEC Marconi Ltd; Plessey Co Ltd; Marconi Co Ltd
Current assignee: Plessey Co Ltd; BAE Systems Electronics Ltd
Priority date: 1986-04-17
Filing date: 1986-04-17
Publication date: 1998-02-04
Anticipated expiration: 2006-04-17
Also published as: GB8609449D0; GB2315628B; SE8800464D0; FR2763135A1

Abstract

A radar system for determining altitude comprises a pulsed radar transmitter 12 and receiver 22, a tracking system for tracking echo return pulses, and a control circuit 18 for adjusting, as an approximately matched pair, both the transmitted pulse width and the receiver bandwidth as functions of a search/track height. The tracking system is adapted to be responsive to leading edge portions of the radar echo return pulses.

Description

RADAR SYSTEMS The present invention relates to radar systems for measuring height above ground (i.e. altimeters).

It is becoming increasingly important to obtain the maximum possible height capability for altimeters, with limited transmit power, by making the system fully adaptive to height.

For pulsed altimeters adaptivity with height is especially critical since a system with fixed pulse width etc would have a signal to noise ratio (S/N) varying with height, h, proportional to h-3. The resultant maximum height capability would be very restricted.

If the pulse width T is made proportional to height, but the receiver bandwidth B is held constant, the peak signal to noise of ground echoes is proportional to h-2. Whilst this signal to noise ratio is greater than 1, it can be shown that the signal pulse tracking error is proportional to

1/(B fm/N).

If the bandwidth B is increased, the S/N ratio varies as

and so the tracking errors vary proportional to

Thus a (mismatched) receiver system in which the product BT is much greater than 1 usually gives the best tracking accuracy. This is the form of altimeter that arises naturally for systems in which the pulse width is increased proportional to height, but the receiver bandwidth is held constant; such a form is appropriate when the maximum height is still moderate, and the specified limit on tracking error does not increase with height.

If it is desired to increase the maximum height capability still further however it is necessary to cater for the situation in which the peak S/N ratio is negative on a single pulse basis, and many pulses are integrated in the tracking loop etc. Height accuracy is not improved in this situation by having a wider receive bandwidth than for matched filtering (BT approximately equal to 1).

Use of an approximately matched filter in the receiver which maximises the peak S/N ratio on a single pulse basis for a point target also enables more efficient integration both within pulse, for an extended target such as the ground (lower effective sampling loss) and over many pulses via a tracking loop etc (lower detector loss). At large heights the limiting factors on radar system performance may be the ability of the altimeter to lock onto ground echoes initially (detection/acquisition procedure) and, subsequently, to verify with reasonable data freshness, that lock has been maintained during apparent tracking. The acquisition and maintenance of lock depends directly on the peak S/N ratio, optimised by use of matched filtering in the receiver. A fully adaptive system in which the pulse width is proportional to height and the receiver bandwidth is approximately matched to the pulse width is therefore most appropriate for pulsed altimeters intended to have a large maximum height capability, and for which the specified limits on tracking error increase as a fixed percentage of height. The peak S/N ratio for such a fully adaptive altimeter system varies only as h-1.

The use of (approximately) matched filtering for the range tracking of isolated targets, such as aircraft, has been proposed previously. The tracking system in such situations however is normally of the "split gate type, designed to balance the leading and trailing halves of the echo pulses and so track the target centroid. The alternative, for isolated targets, is leading edge type tracking, in which normally derivative type tracking is used on both leading and trailing edges of the echo (separately), whose average approximate target centre. For some of the reasons given previously approximately matched filtering (with associated centroid, split gate tracking) is to be preferred at negative S/N ratios also for isolated targets.

The altimeter application differs from that with isolated targets in two respects. Firstly it is desirable for altimeters designed to achieve percentage type tracking accuracy to adapt the pulse width to search/track height, so that matched filtering implies also adaption of receiver bandwidth. Secondly the tracking system for an altimeter is necessarily based on the leading edge of the echo train, even for approximately matched filtering, since it is desired to track the nearest ground echo: the total echo train to each pulse may be quite long, due to echoes from ground sources further away, so that split gate, centroid, tracking is not appropriate.

According to the present invention there is provided a radar system for determining altitude comprising a pulsed radar transmitter and receiver, and a tracking system for tracking echo return pulses, wherein the radar system is provided with a control circuit for adjusting, as an approximately matched pair, both the transmitted pulse width and the receiver bandwidth as functions of a search/track height, and wherein the tracking system is adapted to be responsive to leading edge portions of the radar echo return pulses.

In one embodiment the control circuit is adapted in operation to adjust the transmitted pulse width in direct proportion to the search/track height whilst the receiver has a filter the bandwidth of which is adjusted in operation in inverse proportion to the search/track height.

In one embodiment the control circuit includes a search/track height circuit adapted to store and update data relating to the search/track height and to provide height dependent signals to the transmitter for adjusting the transmitted pulse width, a sampling circuit coupled to the receiver filter for obtaining noise and signal samples of the filtered echo pulse train, a signal comparison circuit coupled to the sampling circuit for receiving data on the noise and signal samples, for comparing the two with a bias factor and for feeding a feedback signal along a feedback line to the search/track height circuit, and a processor coupled to the sampling circuit and the search/track height circuit for calculating when a tracking lock condition has been achieved.

In a preferred embodiment the bias factor is height dependent, the processor being coupled to the signal comparison circuit for providing data on the bias factor.

The present invention will be described further by way of example with reference to the accompanying drawing which illustrates in schematic block form a radar system in accordance with one embodiment of the present invention.

The radar system comprises a tracking system which utilises two samples points in a filtered echo return pulse, at different time delays from transmission of each pulse. The delay for one of the sample points a signal sample point, defines the current track or search height, h say, for the altimeter, after correction for known biasses. The other sample point, a noise sample point, precedes the signal sample point by an interval sufficient to ensure that the noise signal sample contains no ground echoes, provided the signal sample tracks, or precedes, the nearest ground echoes. This interval is normally a fixed fraction, such as 10%, of the total signal sample delay. The receiver is provided with means of comparing the magnitudes of the two samples, either directly, or indirectly by means of thresholds (digital system). The equilibrium situation for the tracking loop is that the signal sample exceeds the noise sample by a predetermined, possible height dependent, amount or multiple, on average. Departures from the equilibrium situation generates a feedback signal to alter the signal sample time delay, or equivalently the current search/track height h, for subsequent pulses. The search/track height h is then used to adjust a number of parameters, including (i) The pulse width T, which is proportional to h in a fully adaptive system.

(ii) The receiver bandwidth B, which is inversely proportional to h in a fully adaptive system.

(iii) The bias in the comparison between signal and noise samples.

(iv) The time delays for the signal and noise samples.

For all purposes except (iv) the adjustments may normally be made at a rate much less than the pulse repetition rate (prf), since fine tuning to the exact value of h is not required, and indeed some further filtering may be desirable before height output is made to external devices.

The above described system would give a mean tracking point which lies at a displacement along the leading edge of the echo train depending on the peak S/N of the echo appropriate to local terrain reflectivity, height above ground, aircraft altitude etc. This leads to uncompensated systematic height errors for the altimeter of up to +12 pulse width, which, for the fully adaptive system, is a fixed percentage of true height. A refinement of the above system which is the subject of a co-pending patent application is to incorporate gain control (AGC) on transmit power designed to hold constant under varying conditions the peak S/N of the echo train. This reduces systematic errors, since the mean tracking point should then always be at approximately the same point on the leading edge.

Referring more particularly to the drawing a clock 2 is provided which when in operation transmits a signal along a line 4 to a timing circuit 6 for controlling the pulse start at the radar prf (possibly including jitter about the mean prf). Timing signals from the timing circuit 6 are fed along lines 8 and 10 to a transmitter 12 and a sampling circuit 14 respectively.

The transmitter 12 is adapted to transmit a pulse width T variable in dependence upon a signal received along a line 16 from a circuit 18. The circuit 18 serves to store and update the current search/track height, h, and provides height dependent signals along the line 16 to the transmitter 12, for adjusting the transmitted pulse width T, and along a line 20 to a receiver filter 22 to enable the bandwidth, B, of the receiver filter 22 to be adjusted as a function of the current search/track height, h. The pulse width, T, and the bandwidth, B, are approximately matched and in a preferred embodiment the pulse width, T, is controlled in direct proportion to the height, h, whilst the bandwidth, B, is controlled in a manner inversely proportional to the height, h.

The transmitter power may, in one embodiment, be controlled by an AGC (automatic gain control) loop, not shown, the signals from which would be fed along a line 24 from a processor 26.

The output of the receiver filter 22 is fed along a line 28 to the sampling circuit 14 which also receives data on the search/track height, h, from the circuit 18 along a line 30. The sampling circuit 14 serves to obtain noise, signal and possibly other samples of the filtered echo train at time delays from pulse start dependent on the current search/track height h. The noise and signal samples are fed to a comparison circuit 32 via lines 34 and 36 respectively, and are also fed to the processor 26 via respective lines 38 and 40. Other samples are fed via a line 42 to the processor 26.

The comparison circuit 32 compares the noise and signal samples, with bias (possibly height dependent) factors received from the processor 26 along a line 44, to generate a feedback signal to the circuit 18. The feedback signal is fed along a line 46 via a height feedback correction circuit 48 to the circuit 18, the circuit 48 being coupled to receive signals from the processor 26 via a line 50.

Data as to the current search/track height, h, is fed back to the processor 26 along a line 52. The processor 26 serving, for example, to determine whether lock has been obtained, to provide further filtering on the height, h, before display to an external device (not shown) coupled to an output line 54, and to adjust height dependent parameters.

It is to be understood that the drawing only illustrates the essential elements of the embodiment. For example mixers used to translate between RF and baseband frequencies are not shown.

Although the present invention has been described with respect to a particular embodiment, it should be understood that modifications may be effected within the scope of the invention.

Claims

CLAIMS:

1. A radar system for determining altitude comprising a pulsed radar transmitter and receiver, and a tracking system for tracking echo return pulses, wherein the radar system is provided with a control circuit for adjusting, as an approximately matched pair, both the transmitted pulse width and the receiver bandwidth as functions of a search/track height, and wherein the tracking system is adapted to be responsive to leading edge portions of the radar echo return pulses.

2. A radar system as claimed in claim 1 wherein the control circuit is adapted in operation to adjust the transmitted pulse width in direct proportion to the search/track height whilst the receiver has a filter the bandwidth of which is controlled in inverse proportion to the search/track height.

3. A radar system as claimed in claim 1 or claim 2 wherein the control circuit includes a search/track height circuit adapted to store and update data relating to the search/track height and to provide height dependent signals to the transmitter for adjusting the transmitted pulse width, a sampling circuit coupled to the receiver filter for obtaining noise and signal samples of the filtered echo pulse train, a signal comparison circuit coupled to the sampling circuit for receiving data on the noise and signal samples, for comparing of those samples using a bias factor and for feeding a feedback signal along a feedback line to the search/track height circuit, and a processor coupled to the sampling circuit and to the search/track height circuit for calculating when a lock condition has been achieved and updating the bias factor.

4. A radar system as claimed in claim 3 wherein the bias factor is height dependent, the processor being coupled to the signal comparison circuit for providing data on the bias factor.

5. A radar altimeter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

5. A radar system substantially as hereinbefore described with reference to and as illustrated in the accomanying drawing.

Amendments to the claims have been filed as follows CLAIMS:- 1. A radar altimeter comprising a control circuit, a pulsed radar transmitter and receiver, and a tracking system for tracking echo return pulses, in which the control circuit is adapted in operation to adjust the transmitted pulse width in direct proportion to the search/track height whilst the receiver has a filter the bandwidth of which is controlled by the said control circuit in inverse proportion to the search/track height, and in which the control circuit includes a search/track height circuit adapted to store and update data relating to the search/track height and to provide height dependent signals to the transmitter for adjusting the transmitted pulse width, a sampling circuit coupled to the receiver filter for obtaining noise and signal samples of the filtered echo pulse train, a signal comparison circuit coupled to the sampling circuit for receiving data on the noise and signal samples for comparing those samples using a bias factor and for feeding a feedback signal along a feedback line to the search/track height circuit, and a processor coupled to the sampling circuit and to the search/track height circuit for calculating when a tracking lock condition has been achieved and updating the bias factor, and wherein the tracking system is adapted to be responsive to leading edge portions of the radar echo return pulses.

2. A radar altimeter as claimed in Claim 1 wherein the bias factor is height dependent, the processor being coupled to the signal comparison circuit for providing data on the bias factor.

3. A radar altimeter as claimed in Claim 1 or 2, in which the transmitter power is controlled by an automatic gain control loop, the signals from which are fed along a line from the processor.

4. A radar altimeter as claimed in any one of Claim 1 to 3, in which the processor has an output line providing a signal to an external height indicator device.