Radar Altimeter

White Label Space have proposed to used a radar altimeter based on SDR (Software Defined Radio) principles.

The design is based on the radar altimeter on the Moon Impact Probe (MIP) from the Chandrayaan-1 moon orbiter of the Indian Space Agency (ISRO).

MIP radar altimeter key parameters:

• Frequency: C-Band, 4.3 GHz
• Output power: 1 W, continuous wave frequency modulated
• Sweep: 100 MHz (+-50 MHz)
• Sweep period: 100 Hz
• Output sample rate: 100 samples/sec
• Maximum range: 5 km
• Accuracy: 2m when range < 150m, 3% otherwise

Analysis of MIP radar altimeter:

• FFT: 200 times/sec, 64 point
• Max beat frequency: ~670 kHz
• Max sampling rate: ~1300 kHz
• Sampling rate decreases as range decreases, down to ~40 kHz?

Note: WLS could not find any vendor who could provide hardware 'out of the box' that has the range required.

Calculate radar parameter support

A git repository at http://github.com/llnz/lnradardesign/tree/master has code that performs the calculations for the parameters of the radar. They are still being added to as more useful calculations are discovered.

Continuous Wave Frequency Modulated Radar System description

CWFM radar is a very smart way of creating a radar. It reduces the peak power demand, and also gets both range and speed information.

First a ramp generator creates the up and down linear frequency sweeps, in a triangle pattern (sweep up, then down, repeat). This frequency is shifted up to the radar frequency and transmitted at the target. The bounce back is mixed with the sent signal, and the result is an amplitude beating signal. A low pass filter then removes the high frequency radar signal, giving just the beating signal. This signal is then digitised by an analogue to digital converter (ADC). Two sets of samples are taken, one for the up sweep, and one for the down sweep. Each set of samples are feed into a Fast Fourier Transform (FFT), and the peak (power) frequency is found for each. The difference between the up sweep frequency and the down sweep frequency is used to calculate the relative speed. The average of the frequencies is used to calculate the distance.

Below is a diagram that describes at a high level the main components of the radar system.

The FFT and calculation stages are done in software, and the ramp generation can be done in software.