GPS World, August 2013
THE SYSTEM ephemeris data has not yet been evaluated nor compared to that of the L1 C A code navigation data Almanac The CNAV data can also provide a compact form of almanac data known as the reduced almanac It does not offer clock information that is not normally required for a signal search and assumes a circular orbit which reduces the overall accuracy Still it can be transmitted and repeated in a much shorter time interval than the legacy almanac which requires a total of 125 minutes Each reduced almanac message message type 12 provides orbit information for a total of seven satellites a set of five such messages conveys information for a complete constellation For the master frame layout described earlier the full constellation reduced almanac is repeated twice within 12 minutes on L2C and half this time on L5 Novel types of CNAV data not covered by the legacy navigation message include the differential code biases inter system corrections or ISCs which are required for pseudorange based positioning with signals other than the legacy P Y code in addition to the established Timing Group Delay parameter TGD An overview of TGD and ISC values broadcast by satellites participating in the CNAV test is given in TABLE 3 Earth Orientation Another important achievement is the provision of Earth orientation parameters EOP in message 32 which provides GPS users with access to the celestial reference frame EOPs were transmitted during the second test week and updated on a daily basis see TABLE 4 Knowledge of these parameters is of particular interest for GPS based orbit determination and navigation of spacecraft in low Earth orbit which is preferably referred to an inertial rather than an Earth fixed coordinate system Overall CNAV offers exciting prospects for improved GPS utilization and users may look forward to the next test campaigns which will tentatively be conducted once every six months OLIVER MONTENBRUCK is the head of the GNSS Technology and Navigation Group at DLRs German Space Operations Center in Oberpfaffenhofen Germany RICHARD B LANGLEY is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick Fredericton New Brunswick Canada PETER STEIGENBERGER is a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München TUM in Munich Germany ÞÜ iÀiÊ Ê Ì iÊÜ À ÌÊvÀ Ê Þ ÕÀÊLi V É Signal Simulator POWERFUL FUTURE PROOF EASY TO USE 1 À L iÊÌ Ê Ê V ÃÌi Ì ÃÊ Êà à GSG 5 Series GSG 6 Series Advanced Multi Channel GNSS Simulator UÊ Ê Ên Ê È V i UÊ Þ VÊÌÀ iVÌ ÀÞÊÃ Õ Ì UÊ Ê Ê i UÊ Û Vi Êvi ÌÕÀià AFFORDABLE Û Vi Ê Õ Ì ÀiμÕi VÞÊ GNSS Simulator UÊÎÓ Ê n ÊÈ V i UÊ1 ÊÌ Êv ÕÀÊvÀiμÕi VÞÊL à P Ê Ê ÓÊ Ê SAR P Ê ÓÊ Ó P Ê xÊ xÊ Ó P Ê ÈÊ Î North America 1 585 321 5800 France 33 0 1 6453 3980 UK 44 0 1 256 303630 Brazil 55 12 3882 4765 sales@ spectracomcorp com tinyurl com Spectracom GPS An Orolia Group Business www gpsworld com August 2013 GPS World 15
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