How does a GPS know your heading?
 

When I fly with a portable GPS how does it know my heading when it's not slaved to the compass/di and I haven't input information on the winds aloft?

Apologises in advance if there's a blindingly obvious answer.
TIM

TRUE heading. It knows the position, then 1 second later it knows the new position. Draw a line between them and that is your TRUE heading.....Then it repeats every second or so and updates the "heading" numbers on the screen.

Pixies and magic, maybe with a bit of witchcraft thrown in for good measure.

Or maybe what EA says, but I think pixies are much more believable to be honest compared to how GPS works.

Good answer... And if you want to demonstrate it, try to rotate your GPS without changing position, you'll see that the heading will not change at all. What the GPS knows is the direction of your movement.

Algebra
 

Smart software using simple maths calculations derived from the position info of the gps works it out. if you imagine a chess board and the gps places you at white kings rook or A1 the next position fix is at white king 4 or D4, then your track, assuming the board is north south, would be appx 050, you can work it out exactly if you know the dimensions of the squares etc. If you stand still with a portable it will fluctuate in heading and speed. Start moving in a straight line and you will soon have heading and speed indications. So the unit needs to be moving to give you this info. Hope that helps PG.

OK, I own up, I've never flown with a GPS. I may be being simple here but I always thought that was the track.

It doesn't. (There, is that blindingly obvious enough? :) )

GPS does not know your heading :ooh:

As above, it knows your Track. Whilst it could work out your Track from successive positions, in fact a USB / Serial GPS output includes "Track" as part of the Data stream.

The other aircraft onboard systems do know your heading (and IAS/TAS), but not your track (and GS).

Hence why when you combine a GPS with, say a digital instrument system, you now also can get Wind info...

NoD

I am one of those GPS pilots. I love em. The GPS won't know your heading it will only know your track over the ground. ie in a strong crosswind there will be a marked difference between track over the gound and your heading on the compass. The plus is with this you don't have to do wind corrections on the ground, as you can just follow your heading before making wind corrections on the GPS as it won't get it's heading(sorry ground track) upset by any winds. I normally look at the difference between the GPS gound track and the DI to see what the actual wind is.This difference in track and heading is exactly the same as the difference between airspeed and ground speed displayed by the GPS.

My previous aircraft had an EFIS with the GPS built in, this could do both as it had an external compass connected to it as well as a GPS head. Interestingly because it also had the airspeed built in it could work out winds aloft and display these on the screen as well.

Hope this helps.

Steve

Many thanks for the replies...all helpfull,
TIM

Had a pilot come to me complaining that the compass and ASI in his aircraft were in error.
I asked how he knew this, his reply "I have checked it against the shiney new GPS I bought at the weekend". It quickly became obvious that he did not understand the difference between what was displayed on the cockpit instruments and what was displayed on the GPS.

It was quite scary how many PPL were using GPS for navigation but have no understanding of what is displayed on the screen.

What do winds aloft have to do with heading ? The aircaft has whatever heading it (or it's commander) likes. You may be flying south in a 200 knot headwind and you're actually moving north. Your heading is south.

I'd also offer the slight correction that a GPS is not figuring heading on changes in position, but it is looking at the velocities. Mathematically it is a better solution. GPS engines calculate not only position but also North and East velocities, then it's a simple bit of trig to get the direction you are traveling.

-- IFMU

IFMU, but how does the GPS calculate the velocities in the first place? That has to be done using position differences, not?

The only other way that I can see would theoretically work is to work out the velocities by Doppler shift but with satellites literally all over the place I don't think that would work.

I find it disappointing and frankly astonishing that there is such confusion in some peoples' minds between track (TRK) and heading (HDG). A similar confusion exists in some peoples' heads between TRK and desired track (DTK) - the notion being that if DTK = TRK, then they are on the planned track..... Of course that isn't true, they're simply parallel to the planned track.

DTK - the planned track between waypoints.
TRK - the actual track at the moment of observation
BRG - the track between position at the moment of observation and the next planned waypoint.
HDG - the direction in which the aircraft's nose is pointing. CANNOT be measured directly by GPS.
XTK - the off-track distance at the moment of observation
TKE - the track angle error between TRK and DTK

If DTK=BRG, you're on your planned track to the next waypoint (assuming you've passed the previous waypoint).

If TRK=BRG, you're on a the direct track from where you are to the next waypoint

If TRK=DTK, you're flying parallel to your planned track and will not necessarily reach the next desired waypoint.

Is it any wonder there are airspace busts resulting from GPS users ignorance of their systems?

GPS is a superb, reliable and flexible system. But it cannot be used correctly by people who don't understand the capabilities of the system and the meaning of the displayed values.

This is not true for all GPSs! My Garmin Colorado has a magnetic compass built in, so it can show heading and track information. Sadly not an aviation device so no nav maps nor a wind aloft calculator!

Surely .... it can only show a heading relative to its own fore and aft axis, can it not ? If it isn't bolted along the fore and aft axis of the aircraft it won't show the aircraft heading, only the track made good.

Of course, it could have compass needle, which would point to magnetic North, but that's not the same.

Educate me.

Basically it receives a time signal and location of a number of satellites. With this information a little trig can calculate position.

Do this 2 or 3 times calculate distance moved over a period of time and you have enough information for track and ground speed.

Of course your magic GPS does this in split seconds much quicker than I can type.

How Does GPS Work?

"What do winds aloft have to do with heading ?"

If you know your track (GPS) and you know the winds aloft....you can deduce your heading.

"I'd also offer the slight correction that a GPS is not figuring heading on changes in position, but it is looking at the velocities. Mathematically it is a better solution"

Interesting.
Is there anyway you could explain that without me having to have Graduate Physics/Maths.

TIM