“Africa and remote sensing are not for sissies”
South African Synthetic Aperture Radar II
Radar Digital Unit
The RDU consists of 3 sub-systems:
· The digital pulse generator (DPG)
· The sampling unit (SU)
· The timing unit (TU)
The DPG is a dual channel, 150 MSPS, 14 bit DAC conforming to the single PMC form factor. This unit generates the I and Q channels for up conversion by the transmitter unit. The waveforms are clocked externally by a local oscillator (150 MHz) generated by the FDU. It is triggered every pulse repetition interval (PRI) by the timing unit (TU).
The DPG is supplied by Parsec in Pretoria in the form of the PM488.
The SU is a dual channel, 105 MSPS, 14 bit ADC also conforming to the PMC form factor. The SU employs its channels to sample the IF directly. As with the DPG the SU is clocked externally by the FDU (105 MHz), and triggered every PRI by the TU. The dual channels of the SU sample 180 degrees out of phase, which essentially provides a net sampling rate of 210MHz.
The SU is supplied by Parsec in Pretoria in the form of the PM480.
The TU provides all the system triggers required by the radar system, namely:
· Pre-pulse trigger: notifying the pre-amplifier and the DPG that the transmitter is about to “burst” out a waveform
· Transmit trigger: notifying the DPG to output the chirp waveform
· Sampling Trigger: to notify the sampling unit to start sampling the IF signal from the receiver
The RCU controls the operation of the TU. The PRI of the unit is dependant on the speed of the airplane i.e. the faster the airplane is moving, the shorter the PRI. The RCU receives the aircrafts state information from a GPS unit and an Inertial Measurement Unit (IMU).
The credit for the design and construction of the RDU is given to Justin Webster, who completed a Masters of Science in Electrical Engineering from the University of Cape Town in 2004.