Signal processing

Details Signal processing Signal processing

2003-02-05

2004-12-21

Sotomayor, John B. (Department: 3662)

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

Clutter elimination

C342S160000, C342S161000, C342S162000, C342S195000

Reexamination Certificate

active

06833808

ABSTRACT:

This application is the U.S. national phase of international application PCT/GB01/03507, filed in English on Aug. 3, 2001 which designated the U.S. PCT/GB01/03507 claims priority to GB Application No. 0019825.9. The entire contents of these applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for signal processing. More particularly, but not exclusively it relates to a method and apparatus for digital signal processing where the signal is, typically, a radar signal.
Current maritime and aviation radar systems operate on the principal of outputting a train of radio frequency pulses in a given field of view, typically 8-16 pulses with an angular spread of 3-4°, and receiving the beam returned from a target, typically a ship or an aircraft, waves or the littoral environment. The delay of a returned pulse from the time of transmission is a measure of the distance of the object from the transmitting ship.
Doppler radar is a form of radar where an output signal is modulated onto a higher ‘carrier’ frequency local oscillator (LO) signal, the output signal frequency typically being around 5 MHz and the LO signal frequency around 3 GHz. The returned signal is demodulated by decoupling the LO frequency therefrom.
The amplitude and phase angle rotation rate of the demodulated reflected signal at a given distance, range cell, are respectively indicative of the radar cross-section of the target and also the relative radial velocity of the target with respect to the transmitting ship. For example, if an object is stationary all of the returned pulses will have the same amplitude and phase angle in the range cell containing the object and subtraction of successive returned pulses will return a near zero value for this range cell. The returns from these stationary objects are known as clutter and are particularly prevalent in littoral regions. A typical system has a moving target indicator cancellation figure, i.e. the noise level after two pulses are subtracted from each other, of −60 dB.
However, if the object is moving with a radial component of velocity relative to the transmitter, the phase of the reflected pulses (emitted at different times) within a range cell will vary, the rate of change of the phase between pulses being equal to the Doppler frequency of the object. Thus, the distance, and relative velocity of the object can be ascertained.
However, the LO does not produce an idealised, pure, single frequency. The LO will typically produce a generally symmetric distribution of frequencies about the central frequency which decrease monotonically away from the central frequency (for example, in the case of a local oscillator with a −20 dB point at 1 kHz, a target with 1 kHz Doppler shift relative to stationary clutter must have a reflected intensity (radar cross-section) greater than 1% of the centre peak, clutter, intensity in order to be detected). The spread of the frequency distribution is dependent upon the type and quality of LO used.
The LO also has instabilities associated with it which introduce phase noise into the transmitted pulses, a typical oscillator has a correlation time of approximately 50s. There is a drift in the LO frequency between the transmission and reception of a pulse which can give the impression that clutter is moving.
Where the LO phase drift at successive times is cumulative rather than simply random, it will be nearly constant for a few successive delays (i.e. through successive range bins) after a given transmitted pulse. The phase drift at the same delay after the next pulse will be different, but again it will vary very little over a few successive delays.
The clutter returns can be very large, for example a 3-4° width beam is 1 km wide 20 km from the transmitter and therefore “sees” a large amount of clutter. Target returns may be as low as 10
−8
of the clutter returns.
Maritime radar require significant power outputs, typically in the kW to MW ranges, in order to obtain the desired detection range. This necessitates the amplification of the modulated signal. The amplifiers used are, for example travelling wave tubes (TWT), not very efficient at maintaining the signal frequency unchanged during amplification and this introduces a yet further phase drift between the LO output and the returned signal.
Use of very long coherent dwells with many pulses can provide a reduction in the processed phase noise, but this is not a practical option for an multifunctional radar (MFR) operating in surveillance, due to time budget limitations. Phase noise is therefore expected to be a key limitation to target detection in clutter.
Another important feature of some radars (e.g. airbourne, ground vehicle, or maritime) is that they are frequency agile. The ability to switch between transmitted frequencies can be important, for example where there are many ships in a flotilla and interference between radar transceivers is clearly undesirable. Frequency agility is also important for other reasons such as non-cooperative target recognition (NCTR), multipath detection arrangements and target reflection cross-section detectability (RCS).
The provision of frequency agility does however introduce phase noise into the system as it requires a versatile LO which can not be optimised over a broad range of frequencies as for good radar sensitivity virtually all circuits must be synchronised to the LO). Thus, the independence of the detected signal from the LO frequency can be established.
Mechanical vibrations introduce phase noise into the detected signal as they result in relative motion between the transmitter and the receiver. The cost of reducing phase noise by mechanical, or electronic hardware, means is very large and increases the price of low phase noise radar systems significantly.
SUMMARY OF THE INVENTION
It is an aim of some embodiments of the present invention to provide a method of signal processing that reduces the phase noise present in processed radar signals.
It is a further aim of the present invention to provide a method of signal processing that ameliorates at least one of the above problems.
It is still further aim of the present invention to provide signal processing means that reduce the phase noise present in processed radar signals.
It is a yet further aim of the present invention to provide signal processing means that ameliorate at least one of the above problems.
U.S. Pat. No. 4,137,533 discloses a method of discriminating a target radar signal from background clutter.
It will be appreciated in principle, and scope of protection sought, that any reference made herein to ‘radar’ encompasses electromagnetic radiation of any frequency and that any reference made herein to geographically distinct transmitters and receivers does not necessarily relate to a large separation, e.g. km, it can relate to only a few metres.
It will be appreciated that any reference herein to phase drift or phase noise relates to any variation in phase due to instabilities, electronic or physical, within the radar system.
According to a first aspect of the present invention there is provided a method of discriminating a time variable target radar signal from a background including the steps of:
I) acquiring range variable returns from a series of radar pulses having variable amplitude and phase;
II) sampling the returns to produce a set of ranged signals attributable to a set of range cells, the ranged signals having variable amplitude and phase for each range cell;
characterised by the steps of:
III) obtaining an estimate of the variation in phase drift between the transmission and reception of the ranged signals for each range cell;
IV) producing a smooth function representative of the variation in phase of the ranged signals between nearby (e.g. successive) range cells in each of the returns;
V) modifying the acquired range variable returns with respect to the function representative of the variation in phase between nearby range cells in order to obtain a corrected value for the ampl

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