View Full Version : ATPL Radio Nav exam 05 04 06 Geoid or ellipsoid?
GNSS references the world as a geoid or an ellipsoid?
Bristol site says ellipsoid, notes say geoid. I put geoid.
What is it?
High Wing Drifter
5th Apr 2006, 17:44
If I recall, GPS assumes a regular shape born from a mathematical model so I think that would be an ellipsoid. The geoid is an irregular definition. Not 100% certain but given the choice I think I would go for ellipsoid.
Alex Whittingham
5th Apr 2006, 19:56
I'm struggling with this one. The 'correct' answer is ellipsoid but, if you google WG84, you find the original GPS operated on an ellipsoid which seems to have been modified in several updates to what is currently a geoid. I'm still trying to get some definitive reference, I'd appreciate any authoritative information. We will probably appeal it anyway.
High Wing Drifter
6th Apr 2006, 07:34
Just found this, something to add to the mix. Trouble is with Google you can pretty much find something to confirm your point of view: http://www.esri.com/news/arcuser/0703/geoid1of3.html
Good find HWD
Seems the older systems use ellipsoid models and newer systems use geoid, therefore I think navstar/gnss will use an ellipsoid model and thus ellipsoid is the correct answer.
This reference - http://www.ngs.noaa.gov/CORS/Articles/Reference-Systems-Part-3.pdf - suggests that GPS is definitely based on WGS 84 and that WGS 84 has been modified since its initial iteration. Unfortunately, it is not specific as to what shape the latest WGS 84 is! However, all my references tell me that the original WGS 84 was an ellipsoid and I would certainly go with ellipsoid as the answer to the exam question.
BobC