4 ms·
Radar for an altimeter since by it's very nature it needs to measure distance and not just speed is going to be frequency modulated. Unmodulated/continuous freq
by anfilt 5y ago
Radar for an altimeter since by it's very nature it needs to measure distance and not just speed is going to be frequency modulated. Unmodulated/continuous frequency radar can only measure speed. When you start processing the small amount of doppler frequency shift from reflections with a changing frequency things get tricky especially when one has have multi-path return (aka it's reflecting off multiple objects) This will lead two or more reflections with roughly the same frequency, but they have a phase shift. That's the kind of shift I am talking about here. The more noise you have the more difficult it is to separate these reflections when processing these signals.
Edit: I will add the a fair amount of interference by multi path return can be mitigated by making the transmitter and receiver shielded from other directions and designed to be highly directional. This also is generally not to hard to do for an altimeter since the datatum one is measuring after all is just elevation over a small area, compared to something like radar for topography that wants to cover a large area.
- Dylan16807 5y agoOkay, sure, you frequency modulate your output. But the frequencies you get back should be almost identical, right? That doesn't explain why something 200MHz away would interfere.
- anfilt 5y agoYes, they will be very close in frequency. If you have multi-path return you will have two more or more frequencies with different phases. The problem is not so much this alone. All transmitters will have spurious emissions of some level and also your going to have higher order harmonics of the root frequency that will be outside the allocated spectrum. The higher the transmit power of a transmitter the more higher order harmonics you will see above the noise floor. At least for low power transmitters you don't have to worry about more than 2nd order harmonic. Although if you have a powerful enough transmitter like a cell tower 3rd order harmonics can be a problem. Spurious noise is going to depend a lot the design of the transmitter and amplifiers and filters. So how much noise leaks into this bad used for aircraft altimeters is gonna depend alot on the quality of the equipment cellar providers use. Noise can effect radar more easily than normal communication transmissions.