“Africa and remote sensing are not for sissies”

South African Synthetic Aperture Radar II

 

Receiver

The receiver unit can be thought of as the reverse process of the transmitter. Although this is the case theoretically, practically we have to make quite a few modifications.

The received signals (echo’s) from the targets will be extremely low in power, depending on several factors: target RCS, range, interference etc. The frequency band of interest needs to be filtered and then passed through a low noise amplifier (LNA). The receiver follows a two stage down-conversion process using the same LO’s as in the transmitter, this is to maintain coherence (see block diagram below).

The additional two stages of the receiver unit that are of interest to us are the Sensitivity Time Control (STC) and the Manual Gain Control (MGC).

STC

The STC is essentially an electronically controlled attenuator/ gain block that varies with time. It is able to attenuate the strong returns from close by targets and amplify the returns from the longer range scatterers. A curve with the variation of gain with ground range is preloaded onto the radar controller unit (RCU) which can in turn control the STC via a DAC.

MGC

The MGC is a 3 bit digital attenuator followed by gain blocks. It is used to boost the signal of signals with a low return to improve the signal to noise ratio. It can also be switched to attenuate the high power returns to prevent the signal from being driven into compression.

The final output signal at 158 MHz is then split into two channels (I and Q). It is then inserted into the sampling unit. The sampling unit makes use of direct if sampling (input signal is base-banded and filtered digitally), sampling the I and Q channels at 180 degrees out of phase. The sampling frequency is 210 MHz, which is 5% more than the minimum sampling rate of 200 MHz (Nyquist rate).

The credit for the design and construction of the Receiver is given to Ajmal Mohungoo, who completed a Masters of Science in Electrical Engineering from the University of Cape Town in 2004.

Block diagram of receiver chain