On 2018-12-13, Jan Panteltje  wrote:

> Dennis Lee Bieber
>
>> On Wed, 12 Dec 2018 22:36:49 -0000, "NY"  declaimed the
>> following:
>>
>>> That's interesting. I'd have thought that with a good aerial and no
>>> obstructions so you can see lots of satellites you should be able to
>>> get very good results.
>>
>>        For ground speed, not air speed... Having a 200kt ground speed
>> with a 100kt tail wind means an air speed of only 100kt.

I encountered the opposite on a recent flight: air speed 100kt,
ground speed 50kt.

> Yes true, but pilots are well aware of wind speed and direction, meteo.
> Simple addition is not too much for them..

Or even vector addition, using the good old E6B that dates from WWII.
(No batteries to go dead, either.)

> So A GPS SOG (speed over ground) can be used to verify pitot tube
> type sensors.

More or less.  Weather forecasting isn't perfect...

> I have done some research or better say experiments myself making
> ultrasonic air speed sensors,basically for boating, some I posted
> about in sci.electronics.design.
> These days you can buy those for maybe a hundred Euro or so.
>
> All those methods combined is the LEAST that should be mounted
> in a many millions dollar worth of plane.

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.

>>> My phone varies between about 20m and 5m radius of error for lateral
>>> movement. Not sure how accurate it is for altitude.

I have an ancient Garmin 48 handheld GPS which usually gets altitude
within a couple of hundred feet.

>>         I believe convention is to assume vertical error is 150% of lateral.
>> Your phone is also most likely using just the C/A signal; airlines may be
>> able to get decryption keys allowing use of the precision signal. I don't
>> even know if any consumer level GPS is able to make use of dual frequency
>> signals yet -- the precision signal is sent over two (or now, maybe four)
>> frequencies, allowing the receiver to correct for ionospheric delays. C/A
>> traditionally was sent on one frequency only.
>
> 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

> 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.

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.

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