Wednesday, December 1, 2010

Pulse-Doppler Radar

A Pulse-Doppler radar is a radar system which is capable of detecting target location such as bearing, range, and altitude. It also measures the radial velocity, using the Doppler effect to determine the relative velocity of objects. Radio frequency energy pulses that return from the target are processed to measure the frequency shift between carrier cycles in each pulse and the original transmitted frequency. To achieve this, the transmitter frequency source must have very good phase stability and the system is said to be coherent.

The Pulse-Doppler radar is based on the fact that targets moving with a nonzero radial velocity will introduce a frequency shift between the transmitter master oscillator and the carrier component in the returned echoes. This is because the signal is subject to Doppler shift, so echoes from closing targets (moving toward the transmitter) will show an apparent increase in frequency and echoes from opening targets (moving away) will show an apparent decrease in frequency. Target velocity can be estimated by determining the average frequency shift of carrier cycles within a pulse packet. This is typically done by means of a 1D fast Fourier transform or using the autocorrelation technique. The transform is performed independently for each sample volume, using data received at the same range from all pulses within a packet or group of pulses. In older systems, a bank of analogue filters were used.

The radial velocity of the target can easily be calculated based on knowledge of the radar frequency, speed of light, pulse repetition frequency and average phase frequency shift.

It is essential that the received echoes are coherent with the carrier signal, at least during the time it takes for all echoes to return and be processed, in order for Pulse-Doppler radar to work at all. To achieve this, a number of techniques are employed, the most common being that the transmitter signal is derived from a highly stable oscillator (the COHO) and the received signal is demodulated using an equally stable local oscillator (the STALO), which is phase locked to it. Doppler shift may then be accurately resolved by comparing the frequency components of the returned echo with the frequency components of the transmitted signal.

A fundamental problem associated with Pulse-Doppler radar is velocity ambiguity, since Doppler Shifts crossing the next line in the frequency spectrum will be aliased. This problem can, however, be alleviated by increasing the PRF, which increases the spacing between adjacent lines in the transmitted spectrum allowing greater shifts before aliasing occurs. For military radars intended to detect high speed closing targets, it is common for PRFs of several hundred kilohertz to be employed.