r12 - 26 Aug 2010 - 04:13:53 - LukeWestonYou are here: TWiki > LunarNumbat Web > CurrentDevelTasks > LNTaskRadarAltimeter

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.

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.

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

I | Attachment | Action | Size | Date | Who | Comment |
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gif | avnrb.gif | manage | 43.0 K | 24 Aug 2010 - 12:58 | LukeWeston | |

png | system-flow.png | manage | 24.8 K | 22 Sep 2009 - 08:39 | LeeBegg | System diagram |

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