White Label Space have proposed to used a radar altimeter based on the design of the radar altimeter on the Moon Impact Probe (MIP) from the Chandrayaan-1
moon orbiter of the Indian Space Research Agency (ISRO)
We are also interested in the possible application of software-defined radio (SDR) principles to the design of this altimeter.
Here is a description of the basic technical characteristics of the Chandryaan MIP altimeter (from here
"The C-band radar altimeter will measure the altitude of the probe in the final descent phase from ~5 km till impact. The radar makes use of an FM-CW type transmitter with the centre and modulation frequencies of 4.3 GHz and 100 Hz respectively, and a transmitted output power of 1 W (CW) with a frequency deviation of ±50 MHz. The receiver has a sensitivity of –78 dBm. A data rate of 5 Kbits per second with an update rate of 100 measurements per second are envisaged. The antennae system has been designed to have a gain of +10 dB and a DSP processor has been used for data processing. The altimeter would have an accuracy of 2 m for heights measured up to 150 m and 3% of the measured height for the range between 150 m and 5 km. A basic field evaluation of the altimeter has been carried out using aircraft sorties and by using an onboard GPS as reference."
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 radar hardware 'out of the box' that has the range required.
Calculations of 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
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 a triangle wave, sweeping up and down. This analog ramp waveform is used to control a voltage-controlled oscillator (VCO), which generates a high-frequency microwave signal which is swept backwards and forwards in frequency. (Terrestrial C-band radar altimeters typically operate centred around 4.3 GHz, with an allowed bandwidth of 200 MHz. Therefore, we typically sweep from 4.2 to 4.4 GHz.) This signal is then transmitted at the target. The reflected signal is delayed relative to the transmitted frequency - and therefore its frequency is shifted, and the magnitude of that shift is proportional to altitude. The received signal is mixed with the transmitted signal, and the result is a beat frequency which is equal to the difference between the frequencies of the two signals. A low pass filter then removes the high frequency radar signal, giving just the beat frequency. This signal is then digitised by an analog 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 fed 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. It is also possible that we might perform the ramp generation in software.
Matjaž Vidmar's 4.3 GHz radar altimeters
Slovenian engineer Matjaž Vidmar has designed and built a C-band (4.3 GHz) radar altimeter intended for use on small aircraft - and he has released the designs as "open source"!
(NOTE: Vidmar has put all the details up on a public webpage - but I don't know if he has specified any licensing arrangements.)
First-generation radar altimeter (analog signal processing, PIC microcontroller)
Second-generation radar altimeter (digital signal processing, ARM7 microcontroller)
Plus the board designs and source
This design (the ARM7-based design) is apparently capable of down to 15 cm
maximum resolution, and operation at altitudes between 0 and 5000 feet (1524 m).
Block diagram of Matjaž Vidmar's ARM DSP based 4.3 GHz vertical navigation radar: see http://lea.hamradio.si/~s53mv/avnr/adesign.html