From: skunk-works-digest-owner@harbor.ecn.purdue.edu To: skunk-works-digest@harbor.ecn.purdue.edu Subject: Skunk Works Digest V2 #80 Reply-To: skunk-works-digest@harbor.ecn.purdue.edu Errors-To: skunk-works-digest-owner@harbor.ecn.purdue.edu Precedence: bulk Skunk Works Digest Sunday, 14 February 1993 Volume 02 : Number 080 In this issue: DGPS See the end of the digest for information on subscribing to the skunk-works or skunk-works-digest mailing lists and on how to retrieve back issues. ---------------------------------------------------------------------- From: THOMSONAL@CPVA.SAIC.COM Date: Sat, 13 Feb 1993 11:53:52 -0800 (PST) Subject: DGPS DGPS seems sufficiently removed from the theme of skunk-works that this should probably be continued by email (unless Ye Moderator wishes to invite further discussion here). However, let me say a couple of things about recent messages. Russ Schnapp said: >I had assumed that US DoD was permitting D-GPS because the range of >the correction signal is probably insufficient to provide correction to >ballistic warheads during reentry Not really. The thing about ballistic missiles is that they are, ah, _ballistic_. That is, the propulsion and guidance get done on the front end of the trajectory, after which it's Newton the rest of the way. This means that the guidance is done fairly near the launch site, and DGPS could be used, either by itself or as an adjunct to an inertial system. Accuracy MaRVs have been worked on for decades, but none that I know of have been deployed. With a nuclear warhead, you really don't need < 100 m accuracies unless the target is extremely hard. > But, they could avoid this problem by providing their own D-GPS > stations. Indeed. DGPS is easy to do, and full-feature base <----| stations costing in the $10,000 range are on the market today. | | | George (gwh@lurnix.com) said: | >What this kluge of a system (dithering, then D-GPS around | >airports) allows is the capability to deny precision targeting | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | >to the rest of the world (i.e. a regional conflict, like | >Iraq) while still letting US airliners land on GPS | >navigation systems. | Only if The Other Side is incredibly clueless. --->------------| (Not an impossibility by any means, but we shouldn't count on it.) Rick Emerson said: >DGPS is inherently a limited area feature. Beyond a certain range the >satellite constellation seen by a mobile station will not be the same >as that seen at the benchmark station. The USCG DGPS facilities, which >rely on RDF beacons, have ranges of approximately 75-100 nm. The studies that I've seen indicate that the inherent nature of the GPS system (the geometric factor that you mention, plus others) lets DGPS provide 10-meter accuracies out to around 1000 km from a base station. Inside 1000 km, you mostly have to worry about the communications link. Recommended Reading: The Effect of Selective Availability on Differential GPS Corrections Kremer, Kalafus, Loomis & Reynolds Navigation: Journal of The Institute of Navigation Vol.37, No.1 (Spring 1990) pp. 39-52 (This contains a wonderful quote: "Even though differential GPS also counters the effects of SA, the government determined that differential operation was acceptable. First, the major issues regarding security were found to be uncompromised by local-area differential corrections. Second, control of differential operations operated by foreign interests is virtually impossible. As a consequence, differential GPS was determined not to be a security issue.") Wide Area Differential GPS Kee, Parkinson & Axelrad Navigation: Journal of The Institute of Navigation Vol.38, No.2 (Summer 1991) pp. 123-143 Geostationary Augmentation of Global Satellite Navigation - 1991 Update Kinal & Nagle The Journal of Navigation Vol.45, No.2 (May 1992) pp. 166-174 Space News topical issue on Global Positioning Systems, Issue of May 11-17, 1992 Good general sources for GPS/DGPS stuff: Navigation: Journal of The Institute of Navigation The Journal of Navigation GPS World Aviation Week and Space Technology International Defense Review Space News Defense News ------------------------------ End of Skunk Works Digest V2 #80 ******************************** To subscribe to skunk-works-digest, send the command: subscribe skunk-works-digest in the body of a message to "listserv@harbor.ecn.purdue.edu". 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