[amsat-bb] Re: inquiry about homebrew az-el systems
8p6sm at anjo.com
Fri Mar 8 11:22:22 PST 2013
On 03/08/2013 01:46 PM, Phil Karn wrote:
> On 03/08/2013 12:08 AM, Gus wrote:
>> Still, it sure sounds interesting! What do you think it would cost to
>> put one together?
> Dunno. I'd have to build one.
Willing to have a go at it? I'd contribute towards parts for a prototype...
> Also dunno why you would need two IMUs.
Because I foolishly thought to compare data from the antenna and the
base, to get pointing angles. Only after posting did I realize that one
IMU would give antenna position data in the earth frame of reference
(not the vehicle frame of reference).
> Platform acceleration (not mere motion) might be a problem but I'd
> have to think about how to compensate for it.
Are you familiar with the UAV Dev Board? They do all manner of clever
tricks and don't even have a magnetometer!
> Other than that, the only thing I'm concerned about is RFI from the
> transmitter getting into the sensor. You could simply not read it when
Won't the IMU work in a Faraday cage? Yes, but power has to get in and
sensor data has to get out, so RF will still be a problem. What about
auto-sensing the RF and delaying the output from the IMU or telling the
CPU not to read them? Could be a problem for big-mouthed rag-chewers
like myself. Also, in a Field Day type environment with several nearby
transmitters operating, your tracker could be offline for an entire
pass. Of course! Fibre optic control cable! Obvious, isn't it? :-)
> A GPS will still be almost mandatory for both satellite antennas and
> telescopes for accurate time and location. This is needed not only for
> the pointing calculations but also to look up magnetic declination and
> inclination to interpret the magnetometer data. Then the magnetometer
> and accelerometer together give you a 3-axis orientation in space
> without calibration, assuming you don't have anything nearby to
> distort the earth's magnetic field.
I've got a couple uBlox 5's around here somewhere...
73, de Gus 8P6SM
Barbados, the easternmost isle.
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