Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1998-09-02
2000-06-06
Pihulic, Daniel T.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
342137, G01S 1322, G01S 1324
Patent
active
06072420&
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an ultra wide band coherent radar system assessing the Doppler shift of the returned radar echoes, which radar system optimises the radio interference discrimination capability. The invention has arisen during the design of low frequency ultra wide band--UWB--SAR radars. It is, however, possible to apply the invention in UWB radar systems working at other frequencies and of other design, as long as they are coherent radar systems assessing the Doppler shift of the radar echo, which is obvious to a man skilled in radar technology. The invention will in the following be described based on the mentioned low frequency UWB SAR concept.
DESCRIPTION OF THE RELATED ART
The term ultra wide band--UWB--implies that the radar operates with a radar signal relative bandwidth of the order of one octave or larger. Thus range resolution will be of wavelength order. Low frequency is understood in the sense that the frequency is below the microwave frequencies mostly used by radars. Since frequencies below the microwave region are adopted generally for telecommunication, the problem of interference with telecommunication signals arises. The invention more specifically lays down a radar design in which the pattern of radar signal transmission and reception has the specific purpose of allowing cohabitation between radar operation and ongoing telecommunication services in the same frequency band.
The primary conceived embodiment of the invention is in a radar aircraft moving above the earth surface for the purpose of obtaining wavelength resolution radar images of the ground. The images are obtained by a signal processing technique known as synthetic aperture radar--SAR. The requirement of wavelength resolution is in this context considered a merit in its own, reducing the SAR image noise independently of resolution. Thus one may improve SAR image quality at a given resolution by reducing the frequency thus diminishing the ratio between spatial resolution and wavelength. Therefore shifting frequencies down into the telecommunication band is of interest.
While UWB signals provides wavelength range resolution, a similarly high cross range resolution calls for a radar antenna which collects signals over a wide aspect angle interval (beamwidths in the regime 30.degree.-130.degree.). 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 in the CARABAS system for instance.
The fundamental limitation of microwave radar performance is set by thermal noise with which the radar signal has to compete. For low frequency radars, operating in all but extremely remote areas of the world, performance is not limited by thermal noise but by man-made radio frequency interference--RFI--white will be highly abundant below 1 GHz. The statistical character of RFI is entirely different from the Gaussian statistics of thermal noise.
Radio traffic signals (in which term here also television broadcast signals are incorporated), which are the cause of the RFI, invariably appear with bandwidths much smaller than the bandwidth used by the radar. For pure radio signals the bandwidth may be as narrow as 1 kHz. Television signals are much more broad band but are still narrow band compared to the radar bandwidth.
At frequencies above the short wave bands, i.e. above say 20 MHz, the radio traffic signals normally stem from nearby and sometimes line-of-sight radio transmitters. The received radar signal, which is the echo from the ground of the preceding transmit radar signal, is typically many orders of magnitude weaker than these radio signals. Therefore the bands used by radio traffic are not usable for the radar, though only some percentage of the radar band would actually be occupied by RFI and thus unusable. However in order to enable the void frequencies to be fully exploited the requirements on radar reception linear dynamic range are extreme.
SUMMARY OF THE I
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