GPS World, April 2014

INNOVATION System Design Test point at which the corrections may be used Dmax as well as parameters to calculate the remaining risk of incorrect GNSS ephemeris data Kmd e Within ADB3 additional information required for GAST D is transmitted ADB4 implements the VDB authentication feature If this ADB is broadcast by a ground facility MT2 messages must be WUDQVPLWWHG UVW DQG FRQWDLQ DGGLWLRQDO indications about which VDB slots are allocated to the ground facility MT2 messages must be transmitted at least each 20th frame but may be repeated up to once per frame Message Type 3 MT3 The VDB 0HVVDJH 7 SH LV D OO PHVVDJH ZKLFK is only used in conjunction with the GBAS authentication feature MT2 ADB4 Among other things this feature requires a minimum slot occupancy of at least 95 percent Thus MT3 messages are broadcast only by ground facilities that support the authentication feature and are completely ignored by airborne GBAS receivers Message Type 4 MT4 With VDB Message Type 4 approach information can be broadcast to approaching aircraft SLORW FDQ VHOHFW D VSHFL F DSSURDFK E simply tuning to a given channel number Currently GBAS only uses Instrument Landing System look alike straight in approaches called Final Approach Segments FAS Each FAS represents one approach This way a single GBAS ground facility can provide multiple approaches for all runways of an airport All approaches must be broadcast at least once per 20 consecutive frames Message Type 11 MT11 The VDB Message Type 11 provides differential corrections in a way very similar to MT1 messages The main difference is that MT11 corrections are based on 30 second smoothing which is required for GAST D service As for MT1 all MT11 measurement blocks must be transmitted at least once per frame Enhancements for GBAS with Galileo At the moment the GBAS standardization documents include information on GPS GLONASS and SBAS ranging sources No information on Galileo or other constellations has been added yet Thus to include Galileo for GBAS some Galileo VSHFL F H SHULPHQWDO DGGLWLRQV WR WKH standards are necessary These proposed PRGL FDWLRQV KDYH EHHQ PDGH LQ VXFK a way as to keep as close to the other system standards as possible to preserve consistency This way hardly any new functionality is added but additional satellites can be used The additional Galileo signals E5a E5b E6 are not used at the moment however they might EH KLJKO EHQH FLDO IRU PXOWL IUHTXHQF applications in the future All modifications presented here DUH SXUHO H SHULPHQWDO DQG ZLOO PRVW SUREDEO QRW EH H DFWO WKH same as those in future standards documents Nevertheless they provide a way to test Galileo together with GPS and GLONASS for GBAS on an H SHULPHQWDO EDVLV Ranging Source ID The Ranging Source ID uniquely addresses a single satellite It is used in MT1 and MT11 to transmit the differential corrections and other information for each ranging source In ICAO Annex 10 Standards and Recommended Practices the Ranging Source ID is defined for GPS GLONASS and SBAS only To provide Galileo corrections as well DQ H SHULPHQWDO PDSSLQJ IRU DOLOHR satellites was added see TABLE 1 In this way up to 36 Galileo satellites can be addressed Navigation Data Galileo provides two different sets of navigation data The I NAV data corresponds to the Safety of Life SoL service and is broadcast on E1 and E5b The F NAV data corresponds to the Open Service OS and is broadcast on E5a In order to remain as close as possible to the legacy navigation systems we selected the I NAV navigation data for use as it is broadcast on the E1 frequency and can thus be received with an L1 only GNSS receiver The navigation data is primarily used LQ 9 07 RU WKH UVW WUDQVPLWWHG correction in this message the ephemeris set that shall be used in the aircraft is LGHQWL HG YLD WKH VVXH RI DWD 2 HOG 7R EH FRQVLVWHQW ZLWK WKH 36 ephemeris we used Galileos IODnav parameter 7RJHWKHU ZLWK WKH LGHQWL FDWLRQ RI the navigation data a CRC parameter is WUDQVPLWWHG LQ 07 IRU WKH UVW VDWHOOLWH within the differential corrections This parameter ensures that the receiver as well as the ground facility use identical navigation data for all calculations The CRC algorithm uses the raw navigation data to generate a distinct CRC value For GPS and GLONASS two ephemeris masks are defined These masks ensure that only information relevant for GBAS processing are covered by the CRC For Galileo a similar mask had to be designed Additional Data Blocks in MT2 Within VDB MT2 station parameters and integrity information are transmitted Some parameters for the over bounding RI SRVVLEOH HSKHPHULV HUURUV DUH VSHFL F Ranging source ID Satellite ID 1 36 GPS PRN 38 61 GLONASS slot number 37 63 99 Galileo PRN 62 120 138 SBAS PRN TABLE 1 GBAS Ranging Source IDs Data content Bits used ADB Length 5 8 ADB Number 5 8 Kmd e POS GAL 8 Kmd e C GAL 8 Kmd e D GAL 8 TABLE 2 Additional Data Block 5 in Message Type 2 for Galileo parameters Message type Size in bytes MT1 17 N Â MT2 basis 28 MT2 ADB 1 6 MT2 ADB 3 6 MT2 ADB 4 3 MT2 ADB 5 5 MT2 max 48 MT4 FAS 51 MT11 14 N Â TABLE 3 Size of different VDB message types including message block header and CRC Variable length message types are dependent on the number of corrections N GPS World April 2014 www gpsworld com 46