Xref: utzoo rec.ham-radio:30342 sci.electronics:17370 sci.physics:16557 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!usc!julius.cs.uiuc.edu!rpi!uupsi!njin!princeton!puppsr!marty From: marty@puppsr.Princeton.EDU (Marty Ryba) Newsgroups: rec.ham-radio,sci.electronics,sci.physics Subject: Re: GPS accuracy in wake of Desert Storm? Message-ID: <5806@idunno.Princeton.EDU> Date: 30 Jan 91 16:17:47 GMT References: <1154.27A2ECFD@w8grt.fidonet.org> <1991Jan29.044137.20914@zoo.toronto.edu> Sender: news@idunno.Princeton.EDU Reply-To: marty@pulsar.princeton.edu Followup-To: rec.ham-radio Organization: Dept. of Physics, Princeton U. Lines: 33 In article <1991Jan29.044137.20914@zoo.toronto.edu>, henry@zoo.toronto.edu (Henry Spencer) writes: |> The last word I heard was that accuracy had actually improved, because the |> US military forces in the Gulf are making extensive use of commercial |> Navstar receivers, while the Iraqis have little or no Navstar equipment, |> so the public-access code is being kept as undegraded as possible. A little deeper information: all the new GPS/Navstar satellites (so-called "Block II") have what is called SA, Selective Availability. SA allows the DoD to dither the effective phase and frequency in the pseudocode that contains the range information. I forget, but I think they are also allowed to lie a little about their orbits. Anyway, they can then add uncertainty in several places. Starting in April or so of last year, they turned on SA on the Block II satellites, but only part way: they just added about 100 ns pure white phase noise. So, if you had time to track for a while (say, if you are NIST and want to transfer atomic time accurately) you could get around this. The SA limits triangulation accuracy for positioning to about 100 meters, compared to 10 m or so with SA off. Note the few older Block I satellites still in operation have no SA capability; if you can control which satellite you talk to, you can get around it (until they launch enough new ones to retire the old ones). In August, when Desert Shield started, the DoD realized they needed Navstar stuff, but their military versions of the receivers are not yet ready, so they switched SA off. As Henry stated, this is little strategic loss, with much to gain (the commercial Navstar receivers are portable enough for infantry). If anything, I'd imagine the Iraqis may have some GLONASS equipment from the Soviets, if they trusted them enough. GLONASS is just as good as GPS (but I think the receivers are a little less portable). -- Marty Ryba | slave physics grad student Princeton University | They don't care if I exist, Pulsars Unlimited | let alone what my opinions are! marty@pulsar.princeton.edu | Asbestos gloves always on when reading mail