SAR radar system

Details SAR radar system SAR radar system

1998-03-20

1999-10-19

Pihulic, Daniel T.

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

Synthetic aperture radar

342179, G01S 1390, G01S 7295, G01S 1302

Patent

active

059696621

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a SAR radar system. Such systems concern a SAR radar carried by a vehicle moving above the surface of the Earth or another planet and comprises means to obtain radar wavelength resolution images of the surface. The technique known as synthetic aperture radar--SAR--is well-known though until recently the achieved resolution has been much coarser than the wavelength. By ultra wide band, UWB, is here understood the use of a radar signal with a relative bandwidth of more than one octave. Such a large bandwidth will result in range resolution of wavelength order. A similarly high azimuth resolution calls for a radar antenna which collects signals over a wide aspect angle interval (in the regime 30.degree.-120.degree.). The antenna may either be steerable to illuminate one and the same patch on the ground as the radar vehicle passes by this patch, so called spotlight SAR, or the antenna beam may be sufficiently wide to continuously illuminate the full adopted aspect angle interval, so called strip map SAR. The combination of large relative bandwidth and a wide aspect interval for data collection enables the wavelength limit for resolution to be approached, as has been successfully demonstrated for instance in the CARABAS system, Swedish patent 8406007-8 (456 117), European patent 86900306.1 (202 320) and U.S. Pat. No. 4,866,446 and 4,965,582 all hereby incorporated by reference.
The vehicle carrying the radar may be an aircraft, an unmanned aerial vehicle, a so-called UAV, or a satellite. The radar may also be installed onboard a space vehicle for the purpose of exploring the surface of other planets.


SUMMARY OF THE INVENTION

The strip map SAR mode is particularly suitable for wide area surveillance. Wavelength resolution in wide area strip map SAR imaging requires radar frequencies below 1 GHz, in order that the amount of data would not be overwhelmingly large. The attainable resolution will be of the order of a meter.
Strip map SAR imaging is a steady-state process producing a constant flow of radar raw data. An obvious requirement is that SAR processing of the data collected must occur at a pace matching the data collection rate. This requirement will henceforward be referred to as real-time computational capacity. The demand for efficient methods for real-time processing of wavelength resolution strip map SAR data is obvious and is the specific objective of the invention.
The cited CARABAS radar system may serve as an illustrative example of the requirements on wavelength resolution strip map SAR processing. CARABAS is programmable to operate with a variety of parameter settings, but the following particular choice can be considered typical:


______________________________________ Radar frequency band 20-70 MHz Aspect angle interval 130.degree. Resolution azimuth .times. range 1.5 .times. 3 m Groundspeed 100 m/s Slant range swath interval 12-20 km Receive duty factor 57% Step bandwidth 2.2 MHz Number of steps 23 Step dwell time 525 .mu.s PRF 84 Hz Receiver dunamic range 14 bits Receiver sampling frequency 5 Hz Receiver output rate 70 Mbits/s Mean data rate 41 Mbits/s ______________________________________
The radar signal thus covers nearly two octaves of bandwidth and is transmitted in a 130.degree. broad beam to one side of the radar vehicle. The entire transmission sequence is split up into 23 frequency steps for which the signal is transmitted entirely before the transmission of the next frequency step. Each step covers 2.2 MHz of bandwidth, which enables data to be sampled at a rate of 5 MHz. The moderate sampling rate allows 14 bits of AD dynamic range. This is important in order that the radar receiver would not be saturated by man-made radio frequency interference, RFI, which is highly abundant below 1 GHz.
Radio traffic signals, which are the cause of the RFI, invariably appear as concentrated spectral peaks for which bandwidths may be as narrow as 1 kHz. If the radar receiver is linear, these peaks

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