budden@manta.nosc.mil (Rex A. Buddenberg) (05/03/89)
From: budden@manta.nosc.mil (Rex A. Buddenberg) ------- Since several people commented, there seems to be enough interest for some history. During WWII, the Germans used US aids to Nav for their own purposes. Especially on the east coast in 1942. Further, both the Brits and the Jerries confused each others' nav signals mostly to get the bombers to dump over open space rather than cities. Meanwhile we Coasties were developing one of the Brits' inventions here -- Loran. Loran was pretty difficult to spoof, but we did generate some concerns after the war that it could be used by an enemy just as easily as by friendlies -- which is quite true. So, we had several editions of the Security of Aids to Navigation Plans. Aside from shutting down a few lighthouses, the plans mostly dealt with securing or reconfiguring the Loran system. Two things are required: 1) We had to know the reconfiguration setup so we could implement it. 2) the authorized users had to know what the auth setup was too. This plan had two major faults. First, it doesn't take long for a receiver who knows where he is to reverse engineer the reconfiguration, so the reconfig is rather perishable. Second, as the civil Loran user community grew, we couldn't manage the authorized users any more. Critical here were the aviation Loran users -- remember that a very sizable fraction of our mobilizatioin airlift comes from the Civil Reserve Air Fleet. The last edition of the SCATANA plan was unclassified and said don't change anything. We generally acknowledged that we'd hurt ourselves more than the enemy by trying to reorganize while under attack. As a result, only one Loran chain, the Northwest Pacific one (Master at Iwo Jima) has a reconfiguration plan. But the reason is so we can provide continuity of service when Iwo is off air for extended maintenance. Now, to GPS. Accuracy is closely related to the dynamic of the vehicle carrying the receiver. If you can stay in one place, like surveyors, then you integrate the data over a long enough period that all the dithering errors (including the selected availability noise) average themselves into oblivion. If Ivan is going to stick around for that long, he can have cm accuracy. The problem is to provide accuracy to high dynamic -- as in supersonic -- vehicles. For that, you need the p-code. There are other ways to handle the deliberate inaccuracies. The Soviet's method was to build their own GPS system -- theirs is called GLONASS. (Put yourself in a Soviet strategic planner's shoes -- would you be willing to bet the farm on your enemy's nav system?). Lower dynamic vehicles, like ships or land vehicles, can effectively average GPS data over time, even if moving. Use GPS as one input into an integrated positioning system along with Loran, inertial, dead reckoning, etc and you can get some pretty substantial performance even without p-code. But before we thrash that, what is your requirement? Are you measuring the settling rate of North Sea oil rigs (a real GPS application, incidentally), navigating open ocean or looking for mines on the continental shelf. Or, perhaps trying to negotiate a tricky harbor entrance in sloppy weather? Rex Buddenberg (usual disclaimer; I work for USCG, but speak for myself) -------
steve@uunet.UU.NET (Steve Nuchia) (05/05/89)
From: nuchat!steve@uunet.UU.NET (Steve Nuchia) In reference to the discussion of the relevance of the GPS system to ballistic missle operation... Note that the accuracy with which a mobile missle launcher can hit a known, immobile target depends on how well the launching platform knows its own position. This, rather than in-flight navigation, is the main importance of GPS to ballistic missles. Think about inertial navigation systems. Think about long-term drift. Think about nulling that drift with high accuracy and confidence every now and then. Cruise missles, on the other hand, may find it very convenient to know their exact position rather than perform a heuristic search through an 800Mb CD full of Microsoft Flight Simulator (tm) scenery when they loose track of the ground. -- Steve Nuchia South Coast Computing Services uunet!nuchat!steve POB 890952 Houston, Texas 77289 (713) 964 2462 Consultation & Systems, Support for PD Software.