Synthetic aperture radar

Details Synthetic aperture radar Synthetic aperture radar

1989-12-05

1990-10-16

Tarcza, Thomas H.

Communications: directive radio wave systems and devices (e.g.,

Synthetic aperture radar

G01S 902

Patent

active

049638777

DESCRIPTION:

BRIEF SUMMARY
This invention relates to synthetic aperture radar, and more particularly to a synthetic aperture radar system arranged to counteract imaging errors due to radar platform divergence from a linear track.
Synthetic aperture radar (SAR) is a known technique for increasing radar resolution. Unlike conventional radars employing a reflecting dish antenna to focus radiation, SAR involves a fixed radar system on a moving platform such as an aircraft or a satellite. Normally, although not necessarily, the radar antenna emits radiation in a direction broadside to the platform track. The radar beam typically has a fan-shaped profile. As scattering objects enter the beam, they begin to reflect signals back to the radar system. They continue to produce radar returns until movement of the radar platform along its track takes them out of the beam. At any time along track, the radar system is accordingly receiving signals from all scattering objects within the beam, and can locate each scatterer in the range or across track dimension from pulse time of flight.
A typical aircraft-borne SAR system would be used to monitor a target area at a distance in the order of 50 Km from the aircraft track. The target depth or range variation in the across track dimension is typically 9 Km. Each radar pulse produces a multiplicity of returns which are separated in the time or across track range dimension. The returns are divided by time gating into a large number of range gates. 6,000 range gates and a target depth of 9 Km for example correspond to a range resolution of 1.5 meters and a time gating interval of 10 nanoseconds, the time taken for radiation to travel 3 meters.
The range gating procedure provides no information on location of scatterers within the radar beam in the along-track or azimuth dimension. All scatterers within each range gate contribute to the instantaneous signal at the SAR antenna. In effect the antenna performs a vector summation of amplitude contributions received at any instant. It detects only the resultant of these contributions to provide what is referred to as a raw data value for each range gate.
In order to improve resolution in the azimuth dimension, SAR uses a number (.about.500) of successive raw data values for each range gate. This is equivalent to analysing the history of scatterers as they pass through the beam. When a scatterer enters or first becomes illuminated by the radar beam, the aircraft radar platform has a positive component of velocity towards it. This component falls to zero when the scatterer is broadside on, and then becomes increasingly negative as the scatterer recedes relative to the aircraft. The frequency of the radar return from a scatterer is accordingly Doppler shifted to a degree which varies as it passes through the radar beam. In order to produce a focussed image of a scatterer based on its scattering history as it passes through the beam, its signals require relative adjustment to correspond to a single Doppler value. Conveniently, this value is arranged to be zero so that scatterers are focussed at the broadside on or zero Doppler position relative to the aircraft. To achieve this, raw SAR data values are multiplied by a respective phase factor taken from a correlation reference function e.sup.i.alpha.x.spsp.2, where .alpha. is a constant and x is a position-in-beam variable; x varies from a maximum positive value at the trailing edge of the radar beam aft of the aircraft, to zero broadside on and to a maximum negative value at the beam leading edge. The multiplied values are summed to provide a single focussed image point. This procedure is carried out for each range gate over successive sets of raw data values, eg values 1 to 500, 2 to 501, 3 to 502. . . r to 499+r, to produce a focussed image in azimuth for each range gate. Mathematically it is expressed as follows;- ##EQU1## where Y.sub.n =nth focussed image point amplitude having in-phase and quadrature components P.sub.n and Q.sub.n ; reference function e.sup.i.alpha.x.spsp.2, where x=(1/2L-k).delta.x and .delta.

REFERENCES:
patent: 4034370 (1977-07-01), Mims
patent: 4084158 (1978-04-01), Slawsby
patent: 4692765 (1987-09-01), Politis et al.

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