GPS Rhumb Line or Great Circle Track?

[Milt]

GPS Questions

What is the heading indicated by a GPS receiver to reach a waypoint ? Is it along a Rhumb Line or a Great Circle track?

What is the precise length of the Nautical Mile used by the GPS receiver algorithms?

Where is the datum used to indicate the position of an airfield? Is it the centre of the main runway or some other arbitrary point.?

What is the datum used for the formal postion of a city or town?

What are the recommended procedure using a GPS receiver to swing your aircraft's or ship's compass?

Come in Jepperson.

[OzExpat]

1. Great circle.
2. 1852 metres.
3. ARP
4. I believe the convention used to be the main or principal Post Office, but that might not be the case these days.
5. Huh???? :eek: :uhoh:

[India Four Two]

Milt and OEP,

5. The answer is close to home :)

http://rrp.casa.gov.au/drafts/DRAFTac043-17(0)_Nov04.pdf

See Section 11 Air Swings

.

[Old Smokey]

Ain't PPrune marvellous! A reply from Calgary, to a question from Canberra, with information promulgated/printed only a few miles away, and most amazing of all, a Huh? from OzExpat!!!!!!

Boy oh boy OzExpat, I'm going to come up with a new fountain of knowledge if you do that again:ok: (I didn't know the answer either):}

Regards (only kiddin OEP),

Old Smokey

[OzExpat]

My thanks to India Four Two - as well as amazement! There's gotta be a story behind how a contributor in Canada can spot an AC from CASA for the benefit of a bloke in Canberra - and all the rest of us!

That was certainly a first for me Smokey. Not the sort of thing that I'd ever come across, hence the reaction. Obviously... :O And, yes, I've now saved the AC for future reference - I trust that you've done the same? :}

[India Four Two]

OEP,
Not much of a story really. I had remembered reading about using GPS for compass swinging either in aircraft or yachts and so a quick Google search produced a whole list of candidates, but I thought the CASA pdf was most appropriate under the circumstances ;)

I thought I might try this sometime, but it occurs to me that a wind calculation and a Track to Heading correction is going to be required during the post-flight data reduction - at least for the slow aircraft that I fly (mainly gliders and towplanes), unless it is a very calm day!

[OzExpat]

Thanks for that IFT, you've succeeded in putting both me and me mate OS to shame! I need to think about that heading correction for drift but at first glance I think you're right. GPS always considers a track not a heading.

Now for the real question that's been bugging me since this topic arose. Why would anyone want, or need, to do an air swing? I'd have thought that a ground swing would be more straight forward and, undoubtedly, a lot more cost-effective.

About the only thing I can think of is the possibility of having more equipment working in flight than on the ground. I'm not convinced that this is a valid point though, in view of the many years that compass swings have been conducted on the ground.

Am I missing something here?

[captain cumulonimbus]

I can't get my head around this...howcome is it Great circle? If you punch in eg FACT as a dest from FAJS,it gives you ONE heading to steer all the way.That would make it Rhumb line,not GC surely? :confused:

[Old Smokey]

captain cumulonimbus,

It's not what it seems. If you do operate the flight from Johannesberg to Capetown, the Initial Magnetic Track is 241.7°(M), and the Final Magnetic Track is 253.6°(M), with a mean of 247.7°(M).

What your GPS is indicating is the Great Circle Magnetic Track at that particular point in time as you proceed along track, that is, initially it will say 242°, but steadily change to 248° by the time of arrival at FACT.

You've given a 686 nm flight as an example, that's a change of 1° for every 114.3 miles, the slow and subtle change is not very noticeable. Even at B737 type speeds, that's only about 1° every 15 minutes.

It would be much more noticeable on a High Latitude East/West flight, particularly in a region of rapid change of isogonals.

Regards, and Happy New Year,

Old Smokey

[compressor stall]

I fly in the higher latitudes (approximately 66deg).
From the GPS:
Yesterday's flight A-B: 273degTrue, 343nm
Return flight B-A: 107degTrue, 343nm.

That's a fourteen degree change in True heading in 343nm - and the GPS in "direct to" mode does change as you fly along. :8

If you are interested in this and want to study further, download the excellent Ed Williams' Aviation Formulary (http://williams.best.vwh.net/avform.htm)

BTW: I work in true as the magnetic pole is just down the road. Magnetic variation at A is 78degE and B is 66degE :eek:

[None]

1) What type of map is the plotting chart and all enroute charts?

a) Minnesota Multi-phasic Personality Inventory (MMPI)
b) Lambert Conformal Conic Projection
c) Azimuthal Equidistant Projection
d) Mercator

2) A straight line on a Lambert Conformal Conic Projection is a:

a) Great oval
b) Rhumb line
c) Great circle route
d) Tangential line

3) A straight line is a great circle on:

a) The Plotting Chart
b) Jeppesen Enroute Charts
c) HSI
d) All of the above.

4) A Rhumb line:

a) Crosses all meridians at the same angle
b) Is a straight line on a Mercator map
c) Is a curved line on a conformal map
d) All of the above

[neilfraser]

Air swing: how can you fly a precise *heading* with a GPS, presuming an unknown wind vector? A GPS measures track over the ground, not the direction an aircraft is pointing in ...:confused: