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True heading from GPS

here

Impressive!

Administrator
Shoreham EGKA, United Kingdom

I am not sure how they achieve it on a base of 39 cm, but measuring coordinates from two DGPS receivers placed, say, at the opposite wingtips would indeed allow for an easy calculation of heading with a sufficient accuracy.

LKBU (near Prague), Czech Republic

These things have been around for ships quite a while, e.g. this one: http://www.tnlcom.gr/el/compasses/simrad_mx575_sat.html Cost is around 500$. I don’t know if they work well on fast moving vehicles like aircraft. The working principle is differential GPS based on carrier-phase measurements which is mostly used for (near) stationary applications like surveying. “Naked” GPS modules capable of that can be bought for less than 50$ a piece, so with two of them and a microcontroller board one could start tinkering…

EDDS - Stuttgart

I am sure I must be missing something, but as usual it is not clear what. Surely every cheapo GPS receiver calculates heading and ground speed by triangulating between the current position, the previous position, and the time between them? The result is offered in the mininum NMEA record, the GPRMC – and has never belied me.

What more does this device offer?

EBZH Kiewit, Belgium

The difference is that your single receiver can give you your track, but not where your aircraft nose is pointed (i.e. heading).

Jan_Olieslagers wrote:

Surely every cheapo GPS receiver calculates heading and ground speed by triangulating between the current position, the previous position, and the time between them?

In order to do that, your GPS receiver must be moving. But a typical GPS receiver, even if SBAS capable, has a positioning accuracy in the order of 2-5 metres at best. For an aircraft which moves at a multiple of that every second and is going along a straight line most of the time, these precision constraints still result in good track and speed outputs. But anything which moves slowly or not at all will get erratic track/heading and speed readouts.

This device here (and similar ones) use two GPS receivers on a fixed baseline – in this case an unusually short one. By directly comparing the raw data outputs of these receivers – and most importantly the phase difference in the carrier signal – the relative position of the two receivers can be determined with sub-centimeter accuracy. From this and the knon baseline, heading, tilt angle and speed can be computed with high precision even in the stationary case. And also in environments where magnetic headings are difficult to get (ships made of steel or steel structures of buildings and even some airport aprons containing a lot of steel reinforcements).

Last Edited by what_next at 16 Jun 15:35
EDDS - Stuttgart

I would be interested to know the refresh rate. We use something similar in the calibration environment to add mathematical corrections to aircraft position error (i.e. if our calibration antennae aren’t ideally aligned due to roll/yaw). Anything less than 10Hz produces junk – we have to assure a 20mm 3D positional accuracy.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Dave_Phillips wrote:

I would be interested to know the refresh rate.

The link to the marine unit I posted above (Simrad MX575) gives a refresh rate for heading of 1 to 20 Hz. The naked GPS receiver modules usually have 20Hz refresh rate, but some of that will be lost in processing and averaging when the GPS reception is not optimal. Therefore the wide range of refresh rates.

EDDS - Stuttgart

Jan_Olieslagers wrote:

Surely every cheapo GPS receiver calculates heading and ground speed by triangulating between the current position, the previous position, and the time between them? The result is offered in the mininum NMEA record, the GPRMC – and has never belied me.

It calculates track in that way — not heading!

ESKC (Uppsala/Sundbro), Sweden

Yes, that is very true. As long as one uses GPS for strategical purposes, i.e. navigation, what’s the issue? And that is what GPS was meant for, after all. For tactical issues, such as getting out of an immediate difficulty, there are other solutions around. While I admire the ingenuity of the offer, I cannot as yet see any added practical value in it. And I naturally distrust the use of resources, however freely available, for purposes they were never meant for. But I have a reputation for being slow in the uptake…

Last Edited by at 16 Jun 16:47
EBZH Kiewit, Belgium
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