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From: JF Mezei on 9 May 2010 17:37 Morten Reistad wrote: > GPS satellites have to use a lot of relativity to obtain their > time/space accuracy. To be pedantic: GPS satellites are programmed to compensate for relativistic effects so that receivers on earth will obtain the required time accuracy. In essence, they are programmed with a clock offset to compensate for the relativistic effects. This is because the time signals in the GPS system are critical to accurate location calculation.
From: Geoffrey S. Mendelson on 9 May 2010 18:14 JF Mezei wrote: > To be pedantic: GPS satellites are programmed to compensate for > relativistic effects so that receivers on earth will obtain the required > time accuracy. In essence, they are programmed with a clock offset to > compensate for the relativistic effects. > > This is because the time signals in the GPS system are critical to > accurate location calculation. Are they? I may be wrong, but I thought that the satellites where basically flying clocks, all syncrhonized. The receiver used the differences in the time received to plot the relative location, direction and speed (which would not work without relativity) and uses that to calculate it's position. From what I remember, the original GPS units need 12 satellites to be accurate down to 1 meter and a lot of time (with the high resolution data encryption turned off), then it became 9, then 6, now it can be done with 3. The satellites have not changed, but the receivers have gotten smarter. As for the data, orignaly it was good to 10 meters (30 feet) without the encryption keys or other data. There were companies in the US selling correction data which was sent of the subcarrier of FM broadcast stations, and GPS units that used LORAN* data to compensate. The Desert Storm occured and the US was caught without enough GPS units that could use the encryption. By order of the President, the encryption was turned off so that civilian units could be used. It has remained off. There have been proposed competing systems by the EU and the Soviet Union, now the Russian Federation, as the system could be at any time set to be encrypted again with the 10 meter limitation, or even worse, encrypted 100%, or just shut off. Geoff. * LORAN was a competing system, as it were, for locating ships near the coast. It used beacons just above the AM broadcast band, which would be good for several hundred miles most nights (and a lot less during the day). It was originally a manual system, based upon triangulation the source of three radio signals. -- Geoffrey S. Mendelson, Jerusalem, Israel gsm(a)mendelson.com N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia.
From: JF Mezei on 9 May 2010 18:59 Geoffrey S. Mendelson wrote: > Are they? I may be wrong, but I thought that the satellites where basically > flying clocks, all syncrhonized. The receiver used the differences in the > time received to plot the relative location, Yes. However, the satellites are flying fast enough that they "age" at a slightly different speed than people on earth. So they adjusted the clock rate of the GPS satellites to compensate for this so that the run at the same speed as on earth. Without this adjustement, 1 second on a satellite would be shorter than 1 second on earth. So receivers on earth would get a progressively more erroneous time. For other types of satellites, it doesn't really matter, but because the GPS system requires extremely precise clocks, they had to compensate for clocks running slower in orbit than on earth. > From what I remember, the original GPS units need 12 satellites to be accurate > down to 1 meter and a lot of time (with the high resolution data encryption > turned off), then it became 9, then 6, now it can be done with 3. The first generation GPSs could only lock in to 4 satellites at a time. Not enough CPU horsepower to process more signals. Second generation managed to get 12 satellites locked in at the same time. (mid/late 1990s).
From: Wes Groleau on 9 May 2010 21:29 On 05-09-2010 18:14, Geoffrey S. Mendelson wrote: > As for the data, orignaly it was good to 10 meters (30 feet) without > the encryption keys or other data. There were companies in the US selling > correction data which was sent of the subcarrier of FM broadcast stations, > and GPS units that used LORAN* data to compensate. Why LORAN? OMEGA superceded LORAN long ago. -- Wes Groleau Kids say … http://Ideas.Lang-Learn.us/barrett?itemid=1361
From: Morten Reistad on 10 May 2010 02:18
In article <4be73e5d$0$32093$c3e8da3(a)news.astraweb.com>, JF Mezei <jfmezei.spamnot(a)vaxination.ca> wrote: >Geoffrey S. Mendelson wrote: > >> Are they? I may be wrong, but I thought that the satellites where basically >> flying clocks, all syncrhonized. The receiver used the differences in the >> time received to plot the relative location, > >Yes. However, the satellites are flying fast enough that they "age" at a >slightly different speed than people on earth. So they adjusted the >clock rate of the GPS satellites to compensate for this so that the run >at the same speed as on earth. They are also at a different height in the gravity well we live in, and that affects aging as well. The signals reaching us also go through a gradient in that gravity well; and ISTR they compensate for that too. >Without this adjustement, 1 second on a satellite would be shorter than >1 second on earth. So receivers on earth would get a progressively more >erroneous time. > >For other types of satellites, it doesn't really matter, but because the >GPS system requires extremely precise clocks, they had to compensate for >clocks running slower in orbit than on earth. > > >> From what I remember, the original GPS units need 12 satellites to be accurate >> down to 1 meter and a lot of time (with the high resolution data encryption >> turned off), then it became 9, then 6, now it can be done with 3. > >The first generation GPSs could only lock in to 4 satellites at a time. >Not enough CPU horsepower to process more signals. Second generation >managed to get 12 satellites locked in at the same time. (mid/late 1990s). This is about custom hardware, not about cpu power. Even the relativistic calculations can be done in realtime with a few tens of kips. -- mrr |