Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!tut.cis.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbnews!cbnews!military From: aardoom@donau.et.tudelft.nl (Eric Aardoom) Newsgroups: sci.military Subject: Re: Pinpoint navigation around Baghdad Message-ID: <1991Feb4.054457.16632@cbnews.att.com> Date: 4 Feb 91 05:44:57 GMT Sender: military@cbnews.att.com (William B. Thacker) Organization: Delft University of Technology, the Netherlands Lines: 58 Approved: military@att.att.com From: aardoom@donau.et.tudelft.nl (Eric Aardoom) A collegue of mine showed me a message (Hi, patrick!), which he picked up from sci.military, about the availability of GPS for military navigation in the Gulf. I would like to add some comments to this message. Also I included a shar file, which contains a bar chart of the number of visible satellites and predictions of GPS accuracy (DOP values) in the area around the Gulf during 24 hours, as well as an almanac, containing satellite orbital parameters. The DOP file is pretty large, so I compressed and uuencoded it. The predictions can be used for longer periods of time if a correction of approx. 4 minutes time shift per day forward is added. [mod.note: Due to the excessive size, I have deleted this shar file. I would presume it would be available upon request from the author. - Bill ] Currently there are 11 block II satellites available, but let's not ignore the fact that there are also 5 block I satellites, which can still be used. Consequently, the number of available satellites totals 16, not 10! I suspect the launching schedule of GPS has been given special priority. I have noticed that since my latest almanac update (about a month ago) SV 23 was added to the constellation of active satellites, although there weren't supposed to be any launches until the end of february '91 (please correct me if I'm wrong). Anyway, I have just downloaded a new almanac from SV 13 with our experimental GPS receiver, and I would like to share it with the readers. Included below are the results in my own almanac format (does anybody know of a data interchange standard for GPS data?). Though it is not (yet) a complete almanac, it should be enough to make an estimate of GPS coverage. Note that the block I satellites SV 3, 6, 9, 11, 12, and 13 are still alive and well. Also note that the anti-spoofing capability of the block II satellites is turned off! (or could this be a bug in my decoding program?) The almanac contains mainly the truncated Keplerian orbital parameters of the satellites and the time of almanac (TOA). It should be sufficient to predict satellite visibility and GDOP values anywhere on earth. I am still working on a new prediction program, which I am willing to make available when it is finished, if the war isn't over by then. Just let me know. As I said, also included are a bar graph of the number of visible satellites during 24 hours and a listing of Dilution Of Position (DOP) values. There is almost full 24 hour 4 satellite coverage at this moment. However, visibility does not garantee accuracy. Most important DOP values for airplanes are PDOP and GDOP. Large values result in low inaccuracy position fixes. GDOP values < 6 can be considered reasonably good values, GDOPs < 3 are excellent. For those who are not going to have a peek at the tables in the shar file I would still like to make one observation. Latest SV launched, no.23, is visible only during nighttime, presently from 23:40 until 7:00 (UTC). Adding this satellite to the constellation has significantly improved accuracy and continuous availability during nighttime (guess why). For those of you who do not have the tools to unpack this, I can also make this information available through anonymous ftp. I hope this information will be useful.