Why are my GPS locations so damn accurate?

November 1, 2007 9:31 PM

Why is my Magellan Promark 3 GPS receiver picking up a correction signal from WAAS in northern Australia?

My lab has just bought a Magellan Promark 3 GPS system - when I'm using one of the units in the field, I notice that it is using WAAS corrections, and indeed the satellite map shows a WAAS satellite almost directly overhead.

I'm in Darwin, Australia - all the information I can find on the internet indicates that WAAS is only operational in North America, and I'm outside the footprint of the system - if I was able to pick up a WAAS satellite, surely it would be on the horizon somewhere, not directly overhead? And as far as I can tell, the European EGNOS system, which is WAAS-compatible, also doesn't have coverage over Northern Australia.

What am I missing? Has WAAS expanded it's coverage? I can't find any information online indicating that it has? GPS experts, show yourselves.

posted by Jimbob to technology (6 comments total)

Is it possible you're picking up the MTSAT-1R and MTSAT-2 satellites from MSAS? (Japan's version of WAAS).
posted by jaimev at 10:47 PM on November 1, 2007

MSAS provides GPS augmentation information to aircraft through MTSAT (Multi-functional Transport Satellite) located 36000km above the ground over the equator.

At 140 degrees east, in fact - that sounds pretty close to the satellite I'm picking up. Based on the field of view from the imager on it, it looks about right as well. If it's sending out a WAAS compatible signal, that would explain things.
posted by Jimbob at 11:55 PM on November 1, 2007

There's a good list of the various SBAS services here, which indicates that MTSAT is in the right spot, but might not be operational yet.

WAAS is the North American name for SBAS, but the other implementations are compatible so you could be picking up any of them. Does your receiver tell you the PRN of the signal you're getting? If it's 134, you're hearing the POR satellite.
posted by Myself at 12:54 AM on November 2, 2007

Also, there is another reason, which is peculiar, given that you're using a Magellan GPS. According to this old page, Magellan handhelds ignore the "do not use" flag associated with the EGNOS/MSAS/WAAS data (and Garmin, et al, heed them). There could also be some other geostationary satellite that isn't public, but transmitting the same signal. Be wary that while it is probably improving accuracy, it may not be. If your particular location and topography is allowing you to receive signals bouncing from very far away, the lag could hinder the accuracy. I'd be interested in the comparison of results..
posted by zachxman at 4:47 AM on November 2, 2007

How do you know your accuracy? Did you post-process your data in the office, then use that post-processed data with a different GPS unit to return to those same points? This is the only way to assess true accuracy, unless I am missing something...
posted by dontrockwobble at 7:54 AM on November 2, 2007

Well you're right, dontrockwobble - I was only kidding in the post topic. I don't know it's more accurate, I'm actually post-processing it against a base-station rather than relying on the WAAS signal. I was just interested to know why I was receiving it.
posted by Jimbob at 9:25 AM on November 2, 2007