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Jul 19, 2013, 07:38 PM
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Radar notes

Radar is probably going to be the dominant navigation method for personal drones, whether indoor or outdoor. It was how the skycrane hovered over Mars without GPS. Automotive demands are greatly lowering the cost, but the famous MIT course made it famous.

You still can't buy a solid state module which spits out a 3D radar image. Those are still reserved for the rich & famous. You have to get RF components & build your own front end.

Modern radar uses a frequency modulated continuous wave. It tunes the transmitted carrier wave up & down in frequency. Then it multiplies the transmitted waveform at any instant by the reflected waveform (mixing) at the same instant. The product is a sine wave at the difference in frequencies (the IF output).

The reflected waveform's frequency is always a delayed copy of the transmitted frequency, allowing the distance & doppler shifted velocity of the target to be deduced from the difference in frequencies. A 2nd ping is required without modulating the carrier frequency to separate the distance from velocity.

2 receivers can be used to determine the direction of motion. The mixed output frequency (IF output) can be deduced by a microcontroller ADC.

Accuracy depends on a very rapidly sweeping carrier frequency with a very smooth slope. The MIT radar uses an XR-2206 function generator to modulate the carrier frequency. The transmitter is a voltage controlled oscillator. A crusty old microcontroller PWM won't do the job. The most expensive parts are the RF components, not the function generator.

Hobbyists have not succeeded in making phased array rader to get a 2D image. The MIT course relies on shifting the radar horizontally to get a 2D image, but it requires a very narrow beam width. Their tin can antennas have a very narrow beam width.

Synthetic aperture radar is not the same as radio interferometry or phased array.

For horizontal position of a drone, you need to do some antenna design to get a narrow beam & mount it on a rotating platform on the order of 1200rpm. It would construct a map of the surroundings, but only at 1 altitude.

Another option is just measuring velocity in 4 directions with 4 sensors of wide beams, then integrating like optical flow. It might be possible with only 3 sensors. It would just need doppler mode, no function generator, but a very high speed ADC to get accurate velocity.

It might be possible to detect horizontal velocity with 2 sensors looking down & slightly off nadir. That's how the sky crane seemed to do it. It's described in a $31 paper. The skycrane will probably be the blueprint for all radar guided drones.

The altitude radar would always use the full function generator & doppler mode. There are lots of problems which the goog searches don't cover & which require experimentation.

Then of course, the IF output consists of 2 signals: I & Q. That's because there are actually 2 carrier waves: a sin & cos (quadrature modulation). I/cos/in phase is 1 carrier wave. Q/sin/out of phase is the other carrier wave.

The 2 carrier waves are really an easier way of visualizing what's really happening, which is modulation of the amplitude & phase of a single carrier wave to get more bandwidth. The I/cos part is related to the amplitude. The Q/sin part is related to the phase. All your AM, FM, FSK, PSK can be described with I & Q.

Only the I part is needed for radar, but the Q part can theoretically improve accuracy. It requires playing around with real parts. The MIT project doesn't to quadrature modulation & no radar front ends have been integrated on a single chip like 2.4Ghz radios. Perhaps the MIT design can be miniaturized enough to do the job.
Last edited by Jack Crossfire; Jul 19, 2013 at 08:16 PM.
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Jul 19, 2013, 08:41 PM
DFC~ We Do Flyin' Right
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Autonomous robotic plane flies indoors at MIT (2 min 48 sec)
Aug 22, 2013, 05:06 PM
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ground-based RADAR for hover?

Really interesting. RADAR does seem to be not in the consciousness of people building hobbyist sensing systems. This sounds a little like the Microwave landing system that can be used to land planes.

Jack, how easily do you think a low-cost ground-based RADAR sensing station could be built in order to allow a multi-rotor aircraft to hover/loiter directly above it?

-Dave M.
Aug 22, 2013, 11:56 PM
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You probably could make a tracker out of a brushless gimbal & cantenna radar for $500.

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