GPS World - February 2008 - (Page 10)

EXPERT ADVICE can include higher-order or more general eﬀects, such as acceleration for tracking remote objects; verticality, drifts, and so on for navigation; rotations for coordinated relative motion between aircraft and aircraft carrier. Wide dynamic variations in some applications have prompted stipulation of more performance requirements such as settling time for reduction of velocity and higher-order errors within speciﬁed maxima. Some (not all) operations account for dynamics, and often not in suﬃcient depth. Let’s look at aircraft on a collision course while airborne (in traﬃc collision avoidance system, or TCAS) and on the ground (runway incursions). For this discussion, all encounters will be between an aircraft and an “intruder” — which may in fact be an intruder or anything from another aircraft to a truck or even a stationary object on a runway. For collision avoidance, an intruder’s position is of course needed, but the key issue is separation distance at time of closest approach. To make that determination in a timely manner — well in advance of that closest approach time — clearly requires knowledge of dynamics. Velocity plays a direct role in determining both the time to closest approach and the projected miss distance at that time. When miss distance is below a speciﬁed amount (that is, aircraft or intruder size plus a safety margin exceeding the eﬀects of estimation errors), guidance commands can be issued. In the airborne case (tomorrow’s horizontal TCAS modiﬁed to exploit accurate crossrange information), those commands could include evasive turns. To prevent incursions such as those involving aircraft on crossing runways, guidance commands could direct one vehicle to speed up, the other to slow down. In both two-dimensional (runway) and three-dimensional (in-air) operation, closest approach time is not the familiar ratio of range to closing-range rate (time-to-go, which characterizes collision courses only). Recognition of this is evident in TCAS design, with conservative adjustments (DMOD) introduced for unknown cross-range 10 GPS World | February 2008 motion at short distances. With more complete knowledge of velocity in cross-range as well as along-range, the conservatism can be eliminated without compromising safety. Current plans for collision avoidance preceded GPS and the commitment to use it for commercial aviation. I have since described a way to determine velocity at high accuracy in all three dimensions, using GPS to full eﬀectiveness, in “Send Measurements, not Coordinates,” ION Journal, Fall 1999. Although I have emphasized here one aspect of performance (accuracy), consideration of dynamics aﬀects other criteria. Integrity tests have successfully been applied to velocity information, and credible velocity histories can enable continued operation during brief data blackouts. In view of recent events, no serious proposal to use GNSS can ignore backup, especially where safety is involved. For occasional unavailability of GNSS, just use existing provisions. At most times in most places, enough GNSS information will be present to direct the function and vastly improve performance. Terry’s column and recent initiatives such as PNT architecture of the future highlight a signiﬁcant need: better and broader information for the departments of Transportation and Defense. What’s to be done? Again, the McGurn prescription: (1) ask the right questions and (2) provide answers. If industry won’t support that, we’ll repeat the past: performance deﬁciencies and, despite economical devices, uncontrolled system costs. at’s no exaggeration; there’s much at stake. My book GNSS-Aided Navigation and Tracking (2007, NavtechGPS) discusses ways to provide the right information, at both input and output, with special attention to robustness. Carrier phase, under siege from interference, scintillation, masking, obscuration, and so on, can be everywhere ambiguous, repeatedly interrupted — and still fully exploited. Worth repeating: better and broader information for DoT and DoD. Let’s really follow through this time. GPS WORLD ADVISORY BOARD Vidal Ashkenazi Sally Basker Nottingham Scientific Ltd., United Kingdom General Lighthouse Authorities, United Kingdom & Ireland Jean-François Bou Thales Group, France Alison K. Brown NAVSYS Corporation, United States Pascal Campagne France Developpement Conseil, France Olivier Colaïtis Alcatel Space Industries, France Ismael Colomina Institut de Geomàtica, Spain Jordi Corbera Spanish Institute of Navigation, Spain Paul A. Cross University College London, United Kingdom Nicolas de Chezelles Ministry of Defense, France Clem Driscoll C.J. Driscoll & Associates, United States Børje Forssell Norwegian University of Science and Technology, Norway Alain Geiger ETH Institute of Geodesy and Photogrammetry, Switzerland Art Gower Lockheed Martin Information Systems & Global Services, United States Sergio Greco Alcatel Alenia Spazio, Italy Jörg Hahn European Space Agency, The Netherlands Michael Healy Astrium Limited, United Kingdom Günter Hein Institute of Geodesy and Navigation, University of the Federal Armed Forces, Germany Larry D. Hothem U.S. Geological Survey, United States Len Jacobson Global Systems & Marketing, United States William J. Klepczynski Institute for Defense Analyses, United States Gérard Lachapelle The University of Calgary, Canada Wolfgang Lechner Telematica, Germany Jingnan Liu National Engineering Research Center for Satellite Systems, China Pietro Lo Galbo European Space Agency, The Netherlands Keith D. McDonald NavtechGPS, United States Terence J. McGurn Consultant, United States Jules G. McNeff Overlook Systems Technologies, United States James Miller National Aeronautics and Space Administration, United States Terry Moore Institute of Engineering, Surveying and Space Geodesy, University of Nottingham, United Kingdom Ruth Neilan Jet Propulsion Laboratory, United States Bradford W. Parkinson Stanford University, United States Ivan G. Petrovski NVS Technologies, Switzerland Mario Proietti TechnoCom Corporation, United States Martin U. Ripple European Aeronautics Defense and Space, Germany Michael E. Shaw National Space-based Positioning, Navigation and Timing Coordination Office, United States Giorgio Solari Galileo Supervisory Authority, Belgium Jac Spaans European Group of Institutes of Navigation, The Netherlands Thomas Stansell Jr. Stansell Consulting, United States F. Michael Swiek U.S. GPS Industry Council, United States David Turner The Aerospace Corporation, United States A.J. Van Dierendonck AJ Systems, United States Frantisek Vejrazka Czech Technical University, Czech Republic Akio Yasuda Tokyo University of Marine Science & Technology, Japan ADVISORY BOARD UPDATE Börje Forssell has a new book out. Or, an old book still going strong, and now stronger. With the growth of GPS, more and more electrical engineers are migrating towards work in the field and requiring knowledge of important GPS/GNSS Börje Forssell principles. Artech House will put his 1991 Prentice Hall title, Radionavigation Systems, back into print. Although technology has progressed at a remarkable pace since its first publication, the principles of navigation systems as described here remain immutable. The book’s reputation as an essential reference has led engineers to scour the Internet looking for used copies. www.gpsworld.com http://www.gpsworld.com

Table of Contents Feed for the Digital Edition of GPS World - February 2008

GPS World - February 2008 Contents Out in Front Expert Advice Galileo's Second Coming Soon UPS First to Use ADS-B European Industry Group Galileo Validation 2008 Antenna Survey Tsumani Detection by GPS Classifieds Advertisers Index Seen + Heard

GPS World - February 2008

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