GB2256765A

GB2256765A - Imaging apparatus

Info

Publication number: GB2256765A
Application number: GB8926854A
Authority: GB
Inventors: Anthony Currie
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-11-28
Filing date: 1989-11-28
Publication date: 1992-12-16
Anticipated expiration: 2009-11-28
Also published as: GB2256765B; GB8926854D0

Abstract

The earth's surface 2 is imaged (Figs. 1-3) by means of a synthetic aperture radar (SAR) system carried by an orbiting satellite 1. A previously transmitted radar pulse is scattered by feature 9 and received along at least two received beams 7a and 7b producing a plurality of samples of data per transmitted pulse. This allows the use of a lower pulse repetition frequency, PRF than would be possible with a conventional system allowing a wider swath 5 to be imaged whilst still satisfying the Nyquist criterion and maintaining spatial resolution in the azimuth direction. The beams from the future may either diverge to the satellite, as shown, or returns from a set of features corresponding to the plurality of returned beams may converge to the satellite (Fig. 7). Returned signals corresponding to each feature are combined, either at the satellite, or at a ground station. The system may be used for missile detection. Sound beams may be used for ultrasound (e.g. foetus or seabed) mapping. <IMAGE>

Description

Synthetic Aperture Imaqinq Apparatus This invention relates to synthetic aperture imaging apparatus particularly but not exclusively for imaging the earth's surface from an orbiting satellite.

In the use of such apparatus, pulses of electromagnetic radiation are transmitted to the ground where they are scattered and reflected back to the satellite. The way in which the incident radiation is scattered is determined by the topography and radar reflectivity of the illuminated surface. Hence by suitably processing the received scattered radiation at the satellite, an image can be formed of the surface topography and radar reflectivity.

Relative movement between satellite and the surface enables a large radar aperture to be synthesised. Apparatus using this effect is called synthetic aperture radar or SAR.

The pulses of transmitted radiation illuminate a portion of the earth's surface which is called the swath or radar footprint. The swath extends in two orthogonal directions, an azimuth direction and a range direction, as shown in Figure 1. The azimuth direction is parallel to the direction of relative motion and the range is the width of the swath. Although the ground directly underneath the satellite may be imaged, it is usual for SAR systems to be used to scan obliquely sideways.

Since the satellite is moving relative to the surface, the frequency of radiation received at the satellite varies as the satellite approaches, passes over and then travels away from the scatterer due to the Doppler effect. Thus the returns spread over a band of frequencies, which in turn means that for good spatial resolution in the azimuth direction the receiver must be able to receive a large frequency bandwidth. According to sampling theory, in order to avoid spectral aliasing, the Nyquist criterion must be satisfied; that is to say, the frequency at which samples are taken must at least be equal to the bandwidth. Hence the rate at which the pulses are transmitted, the pulse repetition frequency or PRF, must be made at least equal to the bandwidth.

Unfortunately, although a high PRF enables good azimuth resolution to be achieved it has undesirable consequences for ambiguities in the range direction. The scattered radiation is received during the inter-pulse time. If a high PRF is used, the time between pulses for receiving radiation is reduced and also the number of transmission intervals during any length of time is increased. An example of a radar footprint produced by an SAR having a high PRF is shown in Figure 2, where the footprint is made up of a number of narrow width swaths separated by intervals a corresponding to the transmission period, during which no imaging information is received. The intervals a are often referred to as "blind" regions because no useful information is received from them.

A further problem exists with a high PRF because pulses from previous cycles return from distant scatterers during the receive period of subsequent cycles, producing an image of a more distant scatterer superimposed on closer detail.

This means that imaged features in the third closest swath S3 to the satellite in Figure 2 are superimposed on features imaged from the second closest swath S2 because the pulse used to image the closest swath S1 returns from more distant scatterers in the third swath S3 during the receiving period of the subsequent cycle.

Two approaches have in the past been adopted in an attempt to solve these problems.

The first approach is to produce a number of receive beams which are spatially separate in the range direction having different elevations, each receive beam being arranged to receive information from one swath only. Although this approach reduces the problem of receiving returns from more distant swaths, it does not overcome the problem of the "blind" regions in the intervals. Further, in order to produce the required separate receive beams, the antenna must be carefully designed as the number of intervals sets a requirement for an antenna size and shape which can not easily be met.

The second approach is called scan SAR, in which a receive beam is switched between swaths during the receive periods between pulses. However this degrades the spatial resolution in the azimuth direction because the receive period is shared between swaths.

The present invention arose in an attempt to provide improved SAR imaging apparatus.

According to a first aspect of the invention there is provided synthetic aperture imaging apparatus for imaging a surface from a platform moving relative to the surface comprising: transmitting means for transmitting a pulse of radiation to the surface; receiving means for receiving the radiation, after it has been scattered from the surface, along at least two receive beams spaced apart in a direction parallel to the direction of relative motion; and means for combining signals representative of the received radiation from the two beams to produce an image of the surface.

The receive beams may be spaced apart by having separate phase centres and they may be parallel, converging or diverging, the phase centre being the actual or notional origin of the beam. In another embodiment the beams may have a common phase centre but be angularly displaced relative to one another.

By displacing the receive beams relative to each other along the direction of motion (or azimuth direction) each scatterer is sampled a number of times corresponding to the number of beams. Thus two beams would produce twice the number of samples and three beams produce three times the number of samples than would be possible using a system employing one beam. By employing the invention the number of samples is increased, and thus the PRF may accordingly be reduced whilst still meeting the Nyquist criterion for maintaining spatial resolution in the azimuth direction. By reducing the PRF, the width of the swath in the range direction is increased, to the extent that one wide swath covers an area of the surface that in conventional systems would be covered by three or four narrow swaths with three intervals.Since the intervals have been eliminated, a complete image of the area of the surface can be produced and hence the need for multiple elevation beams is eliminated.

In a first embodiment of the invention the receive beams have phase centres displaced relative to each other along the direction of relative motion.

Preferably, the separations of the phase centres are equal to the receive antennas length in the azimuth direction divided by the number of receive beams.

Preferably, the beams are substantially coincident at the surface. Thus scattered radiation is received simultaneously producing two sets of samples which can be combined by interleaving one set with the other to give a sequential output.

In a second embodiment of the invention the receive beams have substantially the same phase centre.

Preferably, the receive beams are contiguous in a direction parallel to the direction of relative motion. The beams will sweep a scatterer on the surface successively, that is, first one beam will pass over the scatterer and then the next, producing a successive output and because the beams are contiguous the coverage will be the maximum possible.

Preferably, where the beams have the same phase centre, the means for combining comprises compensation means to compensate for relative Doppler shifts of the beams.

Preferably, the compensation means shifts the frequency of the signals received by at least one of the beams up or down. This allows the full bandwidth to be regained by combination of the shifted signals with the unshifted signal.

Preferably, the receive beams cover substantially the same area of the surface as the transmit beam. This makes the most efficient use of the available transmitter power in that all the area illuminated by a pulse of radiation is imaged.

Preferably, the receive beams are generated by a phased array antenna. Although this is preferable it is also possible to generate the receive beams by using antennas dedicated to each beam.

Although for imaging from satellites it is preferable for the radiation used to be electromagnetic radiation, in some embodiments acoustic radiation may be preferable. It is envisaged that such embodiments may include the imaging of the seabed from a ship or the imaging of a foetus in a womb.

According to a second aspect of the invention a method of imaging a surface from a platform moving relative to the surface comprises the steps of: transmitting a pulse of radiation to the surface; receiving radiation after it has been scattered from the surface along at least two receive beams spaced apart in a direction parallel to the direction of relative motion; and combining the received signals to produce the image.

Specific embodiments of the invention are now described by way of example only with reference to the accompanying drawings in which: Figure 3 shows a satellite for imaging the earth's surface and the transmission of a pulse of radiation in accordance with a first embodiment of the invention; Figure 4 shows the satellite of Figure 3 receiving radiation scattered by features of the surface; Figure 5 is an explanatory diagram relating to the operation of apparatus in accordance with the invention; Figure 6 shows a satellite imaging the earth's surface in accordance with a second embodiment of the invention; Figure 7 shows the satellite of Figure 6 receiving scattered radiation; and Figure 8 and Figure 9 show power spectra of scattered radiation.

With reference to Figure 3, a satellite 1 travels at a velocity V relative to the earth's surface 2.

A beam 3 of transmitted radiation having a phase centre, or origin, 4 is radiated by an active phased array antenna on board the satellite 1. The beam 3 is directed obliquely to the surface 2 producing a swath 5.

The radiation is transmitted as a series of pulses produced at a PRF which is low enough to generate a relatively wide swath 5, which in this case extends in the range direction for about 100 km. As the satellite 1 continues in its orbit it illuminates a strip 6 on the surface 2.

After transmission of a pulse, as illustrated in Figure 3, receive beams 7a and 7b are generated having their phase centres 4 and 8 at a separation of 6 metres in the azimuth direction, as shown in Figure 4. The receive beam 7a is identical in shape to the transmit beam 3 and also has the same phase centre 4. Although the receive beams 7a and 7b have different phase centres they are coincident at the earth's surface 2, that is to say, they cover the same area.

A feature 9 on the surface 2 and within the swath 5 scatters the transmitted pulse and some of the scattered radiation travels back to the satellite 1 along the receive beams 7a, 7b. Each transmitted pulse therefore produces two sets of data samples, one set of data samples per receive beam.

By making the separation of the phase centres 4 and 8 equal to the distance travelled by the satellite 1 between transmitted pulses, it is possible to obtain two signal histories which can be combined to obtain a single history sampled at a rate which satisfies the Nyquist criterion.

Typical data sets produced by the receive beams 7a and 7b are shown in the graphs of Figure 5 which are plots of frequency against time, fd being the peak Doppler frequency.

The Doppler frequency shift is positive when the scatterer is in front of the satellite 1, zero when the scatterer is broadside of the satellite and negative when the scatterer is behind the satellite 1. The circles and crosses represent the samples received by the beams 7a and 7b respectively, graph (a) being the output derived from beam 7a and graph (b) being the output derived from beam 7b. It can be seen that because of the separation of the phase centres the outputs can be easily combined to give the number of samples required to satisfy the Nyquist criterion as shown in graph (c), which is a plot of the combined output, thereby allowing the PRF to be half that required in a conventional system. The combined output is then transmitted to a base station on the earth.

In a second embodiment of the invention, shown in Figure 6, a satellite 1, moving relative to a surface 2, transmits a series of pulses of radiation to the surface 2 in a transmit beam 10 illuminating a swath 11. In the time between transmission of the pulses three receive beams 12, 13 and 14 are generated by a active phased array antenna as shown in Figure 7. They originate from a common phase centre 15 but are angularly spaced apart in the azimuth direction.

The beams 12, 13 and 14 are contiguous in the azimuth direction at the surface 2 and span the swath 11 illuminated by the transmit beam 10.

As the satellite 1 moves relative to the surface 2 it therefore illuminates a strip 16, generating a large synthetic aperture. The radiation returned from scatterers on the surface 2 is used to produce an image of the strip 16.

After transmission of a pulse a feature on the surface 2 is swept by the beams 12, 13 and 14 in succession because of the relative motion of the satellite 1. The power spectra produced by receive beams 12, 13 and 14 are shown in figures 8(a), (b), (c) respectively. Each plot has a frequency which will vary from +fd to -fd because of the Doppler effect, as the scatterer enters and leaves the beam. It should be noted that the centre frequency, that is the frequency when the scatterer is broadside to each beam will not be same for each beam because of their different orientations. However, the orientations of the receive beams 12, 13 and 14 are known and therefore the true Doppler centre frequency for each beam can be determined. The frequency of the signal produced by beam 12 is then downwardly shifted. The signal produced by beam 13 is maintained at the same frequency whilst the frequency of the signal produced by beam 14 is upwardly shifted by the known Doppler centre frequencies.

After the frequency shifting has been carried out the signals are combined to give a full bandwidth bf as shown in Figure 9 before transmission of the combined output to a base station on the earth.

The beams 12, 13 and 14 each cover a bandwidth approximately one third of the bandwidth of the transmit beam that is to say the swath 11 has been split in the azimuth direction into three allowing the PRF to be approximately one third of that required by conventional systems whilst still satisfying the Nyquist criterion for each beam.

In other embodiments of the invention the number of receive beams may be increased allowing an even lower PRF to be used, increasing the swath width further without sacrificing spatial resolution. Although the combination of the received signals has been described as taking place at the satellite in some embodiments it may be more appropriate to carry out the combining of received signals at a base station on the earth.

Although the invention has been described in relation to imaging the earth's surface from a satellite, it will be appreciated that the present invention may be used for imaging from aircraft, or by using ultrasound, mapping the seabed from a ship, for example. It should also be noted that the invention may have application in situations where the platform is stationery and the surface to be imaged is moving relative to the platform.

Although it is envisaged that the invention will be most useful in mapping the topography of a surface it may be used in applications where any differences between an imaged surface at one time and another time is detected, for example for detecting missiles in flight.

Claims

1. Synthetic aperture imaging apparatus for imaging a surface from a platform moving relative to the surface comprising: transmitting means for transmitting a pulse of radiation to the surface; receiving means for receiving the radiation, after it has been scattered from the surface, along at least two receive beams spaced apart in a direction parallel to the direction of relative motion; and means for combining signals representative of the received radiation from the two beams to produce an image of the surface.

2. Apparatus as claimed in claim 1 wherein the receive beams have phase centres spaced apart in a direction parallel to the direction of relative motion.

3. Apparatus as claimed in claim 2 wherein the separations of the phase centres are equal to the receive antennas length in azimuth direction divided by the number of receive beams.

4. Apparatus as claimed in claim 2 or 3 wherein the beams are substantially coincident at the surface.

5. Apparatus as claimed in claim 2, 3 or 4 wherein the means for combining the signals interleaves the signals.

6. Apparatus as claimed in claim 1 wherein the receive beams have substantially the same phase centre.

7. Apparatus as claimed in claim 6 wherein the receive beams are contiguous, in a direction parallel to the direction of relative motion, at the surface.

8. Apparatus as claimed in claims 6 or 7 wherein the means for combining comprises compensation means to compensate for relative Doppler shifts of the beams.

9. Apparatus as claimed in claim 8 wherein the compensation means shifts the frequency of at least one of the beams up or down.

10. Apparatus as claimed in any preceding claim wherein the receive beams cover substantially the same area of the surface as the transmit beam.

11. Apparatus as claimed in any preceding claim wherein the receive beams are generated by a phased array antenna.

12. Apparatus as claimed in any preceding claim wherein the radiation is electromagnetic radiation.

13. Apparatus as claimed in any of claims 1 to 11 wherein the radiation is acoustic radiation.

14. Apparatus as herein described with reference to and as illustrated by Figures 3 to 5 or Figures 6 to 9 of the accompanying drawings.

15. A method of imaging a surface from a platform moving relative to the surface comprising the steps of: transmitting a pulse of radiation to the surface; receiving radiation after it has been scattered from the surface along at least two receive beams spaced apart in a direction parallel to the direction of relative motion; and combining the received signals to produce the image.

16. A method of imaging substantially as herein described with reference to and as illustrated by, Figures 3 to 5 or Figures 6 to 9 of the accompanying drawings.