Hi all, I just got back from four days away and found a few questions from you. I also notice that the GPS/DGPS/Loran-C thread is winding down, but I promised to answer questions if I could, so here goes one last time. (1) From Jack Haring: >Some very knowledgeable people have given us detailed information about >DGPS. The USCG Web site showing DGPS coverages is very handy too. >What can 'you' tell us about how the FAA is implementing DGPS? I know that >pilots can take advantage of the accurate altitude information too, where >it's a pretty useless feature for boaters (except at high tide <g>). I'm not familiar with the use of DGPS by pilots, but I can infer a couple of things from the technical characteristics. First, DGPS is only available on the coasts and Great Lakes (plus HI), so it can't be used universally, at least for now. I have heard rumors of the FAA adding interior stations, but as far as I'm concerned, they are just that--rumors. They can, of course always use ordinary GPS for general navigation, but it is not accurate enough for precision approaches. Any pilots out there who know more? As far as the altitude information is concerned, it is not really very useful, except perhaps as a backup. A simple pressure altimeter is much more accurate, as long as the appropriate surface pressure is set in as required by standard procedures, and (most important) any errors are shared by all airplanes in the vicinity. Why is this? Because GPS receivers simply calculate the range from the satellite to the receiver and then triangulate to get position. Most of the time, the satellites being received are NOT overhead, but near the horizon (simple geometry), so there is no direct measurement of the VERTICAL distance to a satellite--it is calculated from a three-dimensional fix. In such a fix, the error from any one measurement is magnified by 1/cos(angle between satellite and direction desired). If one happens to be nearly overhead, the altitude reported is significantly improved. >Are these two separate systems? Are the receivers compatible... etc? I'm not sure which two systems you refer to. In the US, there is only GPS (which is global), DGPS--run by the coast guard for coastal and inland waterway navigation but usable by anyone with the appropriate equipment (and dependent on GPS to work), and Loran-C--again run by the coast guard, but usable by anyone. There is not separate DGPS for pilots, but they have yet a fourth system, called VOR (VHF Omni-Range) for cross-country navigation. I have never heard of boaters using VOR--probably because it is VHF and therefore a line-of-sight transmission--it is designed for things up in the air, not on the ground or water. >I forgot. Do you know the reliable range of the Differential stations?? The reliable range differs from station to station, depending on soil moisture conditions (or proximity to large bodies of water), and the terrain (and maybe also transmitter power--to avoid mutual interference). I ran a quick AVERAGE range from the officially-reported ranges of each station in the network, and got 131 NM, with a standard deviation of 49 NM. The values range from a high of 300 NM for Kokole Point, HI (mostly flat water) to 35 NM for Wisconsin Point, WI (don't know why so small--perhaps low power?). I don't know how these individual numbers were determined. I know that the USCG has been surveying all of the sites, but many of the entries in the table are suspiciously "round" and the same for many stations. I suspect they are a mix of measured and calculated or "nominal" ranges. They also don't say if they are averages for 360 degrees, the minimum for 360 degrees, or something else. In any case, it looks like 130 NM is a good nominal number, with a conservative estimate (mean minus one standard deviation) of 80 NM. For whatever its worth! BTW, anyone wanting to look up their nearby sites, go to http://www.navcen.uscg.mil/dgps/dgeninfo/default.htm and download "USCG Differential GPS Navigation Service" and look in table 2 (page 6-7)-- you will need an Adobe reader, which you can get free from almost any FTP site. I neglected to cut the names for the following clips, but had a couple of comments: >Now of course the troops were equipped with civilian GPS units and of course >these units cannot correct for SA. So what does DOD do? They turn off SA! >So in a war when you should really be worried about the other guy using your >GPS you have to make it more accurate for him so your own troops can use it! >Makes this whole SA thing kinda silly, doesn't it? Actually, it wasn't silly. At the time, commercial GPS receivers were not as ubiquitous as they are today and the Iraqis would have had very few of them. They also didn't have the weapons that could take advantage of GPS, nor the time to integrate GPS in their weapons (SCUDS are, after all 1950's technology). I think it was a very smart move, something you normally see from the military ONLY in wartime (I can say that, since I am retired from a 27-year career in the USAF). >Not to beat this thing to death, but in reading the recent postings, I >wondered if some enterprising individual or company had thought of >combining DGPS and a simple Loran receiver in one? They both work at low >frequency, both require an external whip, maybe there is some synergy to >be had here. I haven't heard of such a thing, but I wouldn't be surprised. Actually, though, for maximum performance, you would need two antennas and two receivers (probably packaged in the same box), since the DGPS corrections are transmitted at around 300 kHz (285-325), and the Loran-C at around 100 kHz. Clever engineering might find a compromise on the antenna (I would favor the DGPS, since the Loran-C signal is stronger), and one could design a receiver with two bands with some parts savings (power supply, "audio" stage, etc.) If the decision is made to extend Loran-C, that might be profitable (i.e. enough customers to pay for the development). But of course, then they would not be truly redundant systems, would they? >I need to share this with the TW listees. Today I had a chance to try >out my new Garmin 48 on land. I drove from Nanaimo BC back home to Sooke >a distance of 91 km as the birdie flies according to my GPS. Home was a >waypoint and the Garmin tracked us a map all the way home. When we got >into the driveway we were out 6 (six) meters from our destination as far >as the Garmin was concerned. Several posts (sorry to pick on this one) have implied that the postee believes that a long round trip is a better test of accuracy for GPS/DGPS. This is not true, since each position determination is independent of the others (though the receiver might do some short-term averaging). Someone else had the right idea. Find a place with a very-accurately known position and let the "tracking" feature of your system run for as long as you like. The resulting "random walk" pattern on the screen will give you a visual picture of the precision (the variation) and accuracy (how close the middle matches the known coordinates) for that location and time. Cheers! John Mill, Fairfax, VA 27' "go-fast" M/V Paradox, but looking for a trawler. JDMill@ix.netcom.com