Article: 1816 of rec.aviation.products Path: newshost.ncd.com!ncd.com!olivea!spool.mu.edu!howland.reston.ans.net!europa.eng.gtefsd.com!news.umbc.edu!eff!news.kei.com!news.byu.edu!news.mtholyoke.edu!news.umass.edu!noc.near.net!genrad.com!genrad.com!not-for-mail From: dls@genrad.com (Diana L. Carlson) Newsgroups: rec.aviation.products Subject: Report on a DGPS demonstration at Worcester, MA Date: 3 Jun 1994 13:35:13 -0400 Organization: GenRad, Inc. Lines: 99 Message-ID: <2snpkhINNl9s@sweetpea.genrad.com> NNTP-Posting-Host: sweetpea.genrad.com Keywords: GPS I attended a demonstration (along with about 50 other persons) of the Differential GPS that will be available in the future. I thought others may be interested hearing about the demonstration. First of all, DGPS is a system that allows airplanes to do PRECISION approaches using satellites (GPS stands for Global Positioning Satellites). There are currently 24 satellites in orbit that support the GPS, and there are numerous panel-installed and portable GPS units available for aviation and marine navigation with prices ranging from about $3-400 to about $10K. The FCC has just entered Phase III of implementation into using GPS's in NON-precision approaches. There is currently only one panel-installed GPS that is certified for IFR navigation for NON-precision approaches. A GPS unit, in and of itself, is not accurate enough to be used for precision approaches. There are numerous reasons for this, not the least of which is that the military intentionally introduces some error to prevent misuse of the satellites. The inaccuracies of the GPS can be gotten around if there is a ground station which has an accurately mapped position, which also receives the satellite data, then transmits error-correcting information. MIT Lincoln Lab has produced equipment to do this, and their equipment is simplified, because they use the already existing Mode S transponders that all commercial aircraft have. The ground station transmits the error-correcting information; the airplanes receive the satellite data PLUS the error-correcting information and use the data to produce a very accurate position. The accuracy is good enough for Category II approaches down to 200 feet AGL and 1/2 mile visibility. Some of the neat things about this new system: * It's cheap. The ground system for an ILS is about $1-$1 1/2 MILLION dollars. The ground system for DGPS is predicted to be around $100K (this assumes a 2-3 time markup from the cost of the pieces to manufacture). The equipment needed for a general aviation aircraft is on the order of $8K, whereas the cost for a commercial aircraft is on the order of $50K. * It uses already standard equipment. Mode S is already in use by commericial airlines and Mode S may (no promises either way) be required for general aviation aircraft sometime in the future. * In an ILS the accuracy between each dot gets increasingly larger, all the way to the threshold. With DGPS, on short final, the accuracy between each dot remains constant (the total needle swing is about 0.3 mile all the way to the threshold). * With DGPS, the actual course can be something other than a straight line. The vertical guidance still only kicks in on final approach. * A ground system can be received about 100nm radius (from the station), while it's accurate for precision approaches to about 20nm. That means a ground system located at Worcester airport (in MA) could be used for a PRECISION approach at Fitchburg airport 20 miles away (currently, Fitchburg only has a non- precision approach). * The ground system provides TWO capabilities. It provides data to allow for precision approaches. It ALSO provides aerial surveillance, similar to what is now provided by Mode C transponders. In many places, the radar coverage may only go down to about 3000 feet above the ground if using normal radar coverage. If there is a ground system installed in a place that normally only has 3000 feet AGL radar, the surveillance mode of the DGPS can change this to about 200 feet above ground (in other words, providing better coverage). * The actual location of the DGPS ground station is not as critical as for the ILS ground station. The DGPS ground station can be located virtually anywhere within the usable radius. The ILS ground station must be accurately placed. After the lecture, we got to see a demonstration of the DGPS system. They parked a truck (containing the equipment for the DGPS ground station) near the airport boundary. A receiving antenna was placed into the control tower, attached to a Sun SS10 sparcstation. We then watched as a Cessna 172 and a Cessna 411 did precision approaches to the various runways at Worcester. We could also see on the SS10 screen the precise location of the two aircraft at all times, as they moved toward (or away from) the airport. Well, that's the end of my report. Hope someone finds SOMETHING useful. Oh, and the guy said the DGPS ground stations should be available within about a year. They were using a Trimble GPS on the two aircrafts (obviously, since there are NO TSO'd GPSs available for precision approaches, they used a non-TSO'd GPS). -- ->Diana L. Carlson dls@genrad.com Ham: KC1SP (Sweet Pea) <- ->I'D RATHER BE FLYING! P-ASEL, INST CAP: CPT, NHWG, Profile 176 <- ->GenRad, 300 Baker Ave MS/1, Concord, MA 01742 (508)369-4400 x2459 <-