[amsat-bb] Re: inquiry about homebrew az-el systems

Gus 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 
> transmitting.

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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