Charlie Gibbs wrote
>On 2018-12-13, Jan Panteltje  wrote:
>> So A GPS SOG (speed over ground) can be used to verify pitot tube
>> type sensors.
>
>More or less.  Weather forecasting isn't perfect...


Of course, but detecting a defective pitot tube and using SOG and wind to land
instead
is a good emergency strategy.


>Oh, I'm sure it's all there.  Tying it all together in the Flight
>Management System with properly designed (and documented!) software
>is another matter.

Oh I dunno, been writing software now for ages, and designing stuff.
It is not that hard.



>> Yes, for absolute position.
>> These days however units with GPS, GLONASS, Beidu are just 25 $.
>> Put 3 in a box and you can perhaps get smaller errors, Galileo is
>> coming too, if not already here.
>
>That's old hat.  The standard today is WAAS:
>
>https://en.wikipedia.org/wiki/Wide_area_Augmentation_System

WAAS is GPS only AFAIK.
We need the other systems too.
Likely for example China and Russia have their own by now?
So on a flight from the US to say China the system needs to support both - or
soon both, systems.
Oh and Europe will not stay behind.



>> All is relative, here I fly my Hubsan drone with 100% GPS controlled
>> auto-pilot to a few centimeter accuracy:
>>  https://www.youtube.com/watch?v=bI_0mjwlvNw
>>
>> asm code is here:
>>  http://panteltje.com/panteltje/quadcopter/index.html
>>
>> I mean even for a pressure based altimeter you need to calibrate at ground.
>> Maybe it is not so hard to (for example via radio) calibrate the on board
>> GPS relative to one on the landing strip, basically what I do here.
>> Over short time scales the GPS position does not normally wander that much.
>> The math is very simple, see the C source code for composing the flight
>> path on same page:
>>  fly_waypoints-0.4.tgz
>
>You might be thinking about differential GPS, where a receiver on the ground
>compares its position as reported by GPS to its known (accurately surveyed)
>position and transmits a correction signal to nearby receivers.  The
>difference between this and WAAS is that WAAS uses a handful of receivers
>scattered all over North America, while DGPS is designed as a short-range
>solution for a specific location.

Yes that is differential GPS, in my case the GPS in the remote is the ground
station
and the GPS in the drone the other one.
They call that 'follow me' mode, follow the remote, is what it does,



>A friend recently retired from Nav Canada, where his job was calibrating
>ground-based aeronautical navigation aids.  With a properly set up DGPS
>unit, he was able to measure positions with 1cm accuracy.

Right.


There is also altitude radar, this guy build his own:
 http://lea.hamradio.si/~s53mv/vnr/theory.html
 http://lea.hamradio.si/~s53mv/avnr/adesign.html

Do I see a raspi opportunity there ?


His explanation of GPS and GLONASS is the best I have found so far, including
the decoding hardware,
he did that way before you could buy those 25 dollar GPS thingies,
 http://lea.hamradio.si/~s53mv/navsats/theory.html

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