Well, I wrote a long article in response to yesterday's anonymous comment claiming that there were no extra GPS satellites involved in the SBAS system. It linked to plans, pictures and launch timetables for SBAS GEO satellites in Europe, India and Austraila. It quoted from the AIP, the FAA and the ESA. And it concluded triumphantly with a picture of a GEO satellite. It completely soothed the initial sense of guilt I felt when it was implied that I had given out incorrect information. And then Blogger ate it, saying on its status page, "We will also be down for about 10 minutes right now to reboot some servers."
Here are a couple of links salvaged out of my history. EGNOS, the European effort and an AvWeb article showing the network of stations.
I'm not rewriting it, and I'm ticked at Blogger now, for wasting my time so I'm not posting anything more until all my mammals are up to date.
Perhaps it's just a confusion about terminology rather than about the nature of the system, or perhaps I'm being unreasonably picky. SBAS certainly uses geosynchronous satellites to pass along the correction data. Those satellites, however, are completely separate from the GPS (or GLONASS) satellite "fleet".
From the point of view of a user, of course, it doesn't matter what things are called as long as the system works as designed.
The AIP tells us that "the SBAS GEOs also serve as additional sources of GPS ranging signals."
Oops. There are many phrases I could use here, but "I was wrong, sorry" seems most appropriate.
I've been following GPS technology closely since its inception, but I apparently never got the memo that redefined "GPS satellite" as "satellite broadcasting signals to a GPS receiver". While many official descriptions of WAAS use the phrase "GPS-like signals", some go so far as to say that "GPS and WAAS signals are sent using the same frequency band, so the WAAS geostationary satellites themselves can be considered a part of the GPS constellation."
(The quoted phrase from the AIP doesn't quite prove the point by itself, though. Certain US Coast Guard beacons and LAAS ground stations serve as sources of GPS ranging signals too, but calling them GPS satellites is overstating the case.)
Hmm ... this link may help:
"... Precisely surveyed wide-area ground reference stations (WRS) are linked to form the U.S. WAAS network. Signals from the GPS satellites are monitored by these WRSs to determine satellite clock and ephemeris corrections and to model the propagation effects of the ionosphere. Each station in the network relays the data to a wide-area master station (WMS) where the correction information is computed. A correction message is prepared and uplinked to a geostationary satellite (GEO) via a ground uplink station (GUS). The message is then broadcast on the same frequency as GPS (L1, 1575.42 MHz) to WAAS receivers within the broadcast coverage area of the WAAS GEO."
Apology not required, and thanks for the links and additional information. Anon is right, it's a terminology difference. I didn't get any memos on this either. If something orbits the earth, I call it a satellite, and if it's up there for the purpose of enhancing GPS, as opposed to facilitating television or targetting death rays, I call it a GPS satellite. Ground stations would be GPS satellites, except that they are not satellites. Raccoon actually has one of those. I refer to it as "a GPS satellite, except that it's on the ground, not a satellite"
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