Date: Sat, 06 May 2000 11:10:21 -0700 From: Kent Stewart <kstewart@3-cities.com> To: Nate Williams <nate@yogotech.com> Cc: Wes Peters <wes@softweyr.com>, Poul-Henning Kamp <phk@critter.freebsd.dk>, Warner Losh <imp@village.org>, hackers@FreeBSD.ORG Subject: Re: GPS heads up Message-ID: <3914600D.D7657143@3-cities.com> References: <6827.957594577@critter.freebsd.dk> <3913BD91.1803945E@softweyr.com> <200005061607.KAA17627@nomad.yogotech.com>
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Nate Williams wrote: > > > > >: With 12-channel chipsets becoming common, new devices are getting quite > > > >: good at this. > > > > > > > >Yes. Most of the data I have is for 6 channel models. > > > > > > 12-chanel chipsets are overkill if you don't live more or les exactly > > > on the equator or one of the poles. Here where I live (56 north) about > > > 30% of the sky is never covered by a satelite because of the inclination > > > of the satelites being non-zero. > > > > Get used to them, both Garmin and Trimble are using 12-channel engines > > now. Even the cheapo Rockwell chipset is 6 channels, going to 12 by > > the time the start putting them into mobile phones next year. There's > > a mandate in the USA that all mobile phones have GPS in them by 2003, > > Actually, the phones don't have to have GPS, but they must be able to > get a fix on them good to 100m. I talked to one of the engineers at > Qualcomm last year who is working on this, and GPS isn't going to be > used because it doesn't work in buildings. And, most of the world's > cell-phones are used in buildings. They are doing triangulation and > putting GPS units in the cell towers instead, which is far cheaper. > > Plus, they can get a fix on the phone in 300ms (good to about 25m), > which is far faster than a GPS unit can do it. Basically, the phone is > 'locked on' as soon as you turn it on and it finds a cell tower. And, > apparently they've figured out a way to get a coarse fix on it even > where there is only one tower, although when I pressured him, he just > smiled and claimed it was a trade secret. > > Or so I've been told, but I trust the source since he's one of the > smartest guys I ever met. :) The direction and distance doesn't seem to be too much of a challenge. Most of you are HAM's and some of you may even remember the old Bunny hunts. This game at Hamfests disappeared when the directional finders were developed that detected the phase difference between 4 antennas. With the phase differences the unit would calculate a relative heading and light up that LED. All you had to do then was just drive to the hidden transmitter using the bearing present by the lighted LED. The cell towers have ~10 antennas, which should provide more accuracy than 4 did. The cell phone probably responds similarly to an IFF on an airplane and any delay would pin point the cell phone within a reasonable distance down that heading they could calculate. You are going to have to calibrate the delay but that would be a function of the logic chips. It could also be close to a constant. Multipath would cause problems in the larger cities. Anyone that has tried to use a 2m handheld in Dallas is familiar with that problem :). But you have more cell towers and the cities and you could triangulate that way. -- Kent Stewart Richland, WA mailto:kstewart@3-cities.com http://www.3-cities.com/~kstewart/index.html FreeBSD News http://daily.daemonnews.org/ SETI(Search for Extraterrestrial Intelligence) @ HOME http://setiathome.ssl.berkeley.edu/ Hunting Archibald Stewart, b 1802 in Ballymena, Antrim Co., NIR http://www.3-cities.com/~kstewart/genealogy/archibald_stewart.html To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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