GPS World, December 2009

DIRECTIONS 2010 exploited with receivers described next FFT Based Processing Correlators and track loops in GNSS receivers can be replaced The theory is age old multiplication in the frequency domain corresponds to convolution in time and vice versa Thus a term by term product of a digitized receiver inputs fast Fourier transform FFT with the reference patterns FFT can after an inverse FFT provide outputs equivalent to full sets of The key lies not in new inventions nor provisions but in use of newer methods while ditching some old habits correlator responses Todays processing and analog to digital converter capabilities offer feasibility In addition to reduced vulnerability to jamming not covered here advantages include access to all cells not only a track loops subset guaranteed access stability is not conditional linear phase versus frequency no phase distortion Features from the preceding section combined with these traits off er extreme robustness Extension to Surveillance The practice of transmitting responses to RF interrogations has for many decades been quite vulnerable to overload garble one users information is everyone elses interference One report described the unsurprisingly poor performance during the first Gulf War and identified a remedy squitters with separate assigned time slots spontaneously firing the transponder transmitter without interrogation Immediately a sea change in capability offers every participant an opportunity to track every other participant With no interrogations garble would disappear This dramatic increase in capacity has been successfully demonstrated with the use of an existing communication link and existing airborne equipment GPS receivers and Mode S squitters Subsequently I enthusiastically advocated adoption of the technique with one fundamental modification replace the data bits of the transmitted messages with measurements instead of coordinates Additional improvements include small shifts in time reducing bits needed for time tags and recomputation of measurements that would have occurred at the center of gravity to mitigate rotation effects Collectively the full set of procedures offers a vast and compelling list of benefits Conclusions Capability and dependability of navigation and surveillance can be enormously increased The key lies not in new inventions nor provisions but in use of newer methods among them FFTbased receivers segmented estimation and 1 second carrier phase changes while abandoning habits such as dismissal of partial fix data preoccupation with full fixes for instantaneous position irrespective of dynamics preference for location pseudomeasurements rather than the measurements themselves reliance on proprietary software in equipment boxes RF interrogation response sequences instead of squitters The industry can either adopt changes or continue to settle for performance levels at a minor fraction of the intrinsic capabilities available from our present and future systems EDITORIAL ADVISORY BOARD Vidal Ashkenazi Nottingham Scientific Ltd United Kingdom Sally Basker General Lighthouse Authorities United Kingdom Ireland Alison K Brown NAVSYS Corporation United States Pascal Campagne France Developpement Conseil 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 Institute of Geodesy and Photogrammetry Switzerland Art Gower Lockheed Martin United States Sergio Greco Alcatel Alenia Spazio Italy Jörg Hahn European Space Agency The Netherlands Michael Healy Astrium Limited United Kingdom Günter Hein 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 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 NASA United States Terry Moore University of Nottingham United Kingdom Ruth Neilan Jet Propulsion Laboratory United States Bradford W Parkinson Stanford University United States Ivan G Petrovski iP Solutions Japan Mario Proietti TechnoCom Corporation United States Jayanta Ray Accord Software and Systems India Martin U Ripple European Aeronautics Defense and Space Germany Michael E Shaw Lockheed Martin Space Systems United States Giorgio Solari Galileo Supervisory Authority Belgium Jac Spaans European Group of Institutes of Navigation Netherlands Thomas Stansell Jr Stansell Consulting United States F Michael Swiek U S GPS Industry Council United States David Turner Department of State 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 ALAN CAMERON l and DOROTA BRZEZINSKA r moderator of GPS Worlds online Tech Talk EDITORIAL UPDATE Forum co chaired a technical session at the World Congress of the International Association of Institutes of Navigation in which Hideo Usui c of Kobe University Japan presented Effectiveness of Estimated Environmental Stress Value Monitor GPS World December 2009 www gpsworld com 12