GPS World, June 2009

GPS Modernization INNOVATION longer code period provides better crosscorrelation properties but takes longer to search However one can speed up an acquisition to some extent with lower code cross correlation levels The L1 C A code period is 1023 chips or 1 millisecond The desire to maintain that epoch rate of 1 kHz with the 1023 Mcps chipping rate results in a code period of 10230 chips For both the I5 and Q5 ranging codes the 1 millisecond sequences are the modulo 2 sum of two subsequences referred to as XA and XB with lengths of 8190 and 8191 chips respectively The same XA sequence is used for both I5 and IQ whereas the XB sequence for I5 is different from that for Q5 The XB sequences are selectively advanced to produce different 1 millisecond long code sequences In this way a large number of unique codes can be generated Thirtyseven primary code pairs have been designated of which 32 are reserved for use by GPS satellites PRNs 1 32 An additional 173 pairs have been defined PRNs 38 210 PRN sequences 38 through 63 are reserved for satellites Demo Signal Verification The L5 signal transmitted by SVN49 contains only the dataless quadrature component modulated with the PRN63 Q5 sequence Furthermore the transmitted L5 signal power and the satellite antenna radiation pattern are different from those expected for the L5 signals to be transmitted by the Block IIF satellites as described in the L5 interface specification Over the past few weeks the German Aerospace Center Deutsches Zentrum für Luft und Raumfahrt or DLR has monitored SVN49 using its GNSS verification and analysis facility The core element of the facility is a 30 meter dish antenna at Weilheim near Munich Germany and is shown in FIGURE 1 The antenna which is based on a shaped Cassegrain system has a 30 meter diameter parabolic reflector and a hyperbolic sub reflector with a diameter of 4 meters The L Band gain of this high gain antenna is around 50 dB with a beam width of less than 05 The position accuracy in both azimuth and elevation directions is 0001 The antennas maximum slewing speeds are 15 per second in azimuth and 10 per second in elevation angle allowing it to easily track MEO satellites In September 2005 DLRs Institute of Communications and Navigation established an independent monitoring station for the analysis of GNSS signals using this powerful instrument For the new challenge the antenna was adapted to the requirements in the navigation field A newly developed broadband circularly polarized feed and a new receiving chain including an online calibration system were installed at the antenna during preparations for the GIOVE B in orbit test campaign in the spring of 2008 During this time intensive work on the system calibration was performed using well known signals from radio stars and EGNOS satellites for the antenna gain determination and sophisticated calibration O M N I S T A R H I G H P E R F O R M A N C E T M D G P S B R I N G I N G D E C I M E T E R A C C U R A C Y W I T H I N R E A C H Introducing OmniSTAR High Performance HP our revolutionary new DGPS system that redefines the standard for differential correction OmniSTAR has developed cost effective reliable real time corrections enabling high accuracy applications over entire continents without deploying local base stations Our innovative HP signal provides five times the accuracy of standard DGPS for horizontal accuracy of 10 15 cms HP means a wide area solution offering previously unobtainable levels of accuracy opening the door for high precision uses such as auto steering guidance and automation of agri vehicles increased hit probability for GIS utilities rough grading for construction applications survey projects oil exploration aerial photogrammetry and more We just raised the bar on accuracy and reliability but its way within your reach OmniSTAR HP Others promise we deliver Contact us for more information www omnistar com E x p e c t h i g h p e r f o r m a n c e 2005 OmniSTAR www gpsworld com June 2009 GPS World 51