GPS World, April 2014

System Design Test INNOVATION to each satellite navigation system To extend MT2 to Galileo parameters for the DCPS GAST C and GAST D must be added for Galileo For downward compatibility these parameters cannot be included in the existing Additional Data Blocks beside the existing parameters Thus a new Additional Data Block ADB5 was GH QHG RQ DQ H SHULPHQWDO EDVLV 7KLV GGLWLRQDO DWD ORFN is dedicated to Galileo and is structured as shown in TABLE 2 The coding of all values corresponds to the coding of the parameters for the existing systems Optimized VDB Transmission Scheme Having available a large number of ranging sources for differential corrections the VHF VDB is a bottleneck for WKH WUDQVPLVVLRQ RI WKLV GDWD 7R GHPRQVWUDWH WKLV ZH UVW consider the number of visible satellites that there will be in the future This leads to construction rules for an optimal VDB transmission scheme which allows transmitting the maximum number of differential corrections Number of Satellites Available To demonstrate the number of differential corrections enabled by the different systems in the future we computed the number of visible satellites over a day for a stationary GNSS receiver in Braunschweig Germany Even though only four Galileo satellites were in orbit at that time up to 26 different satellites GPS GLONASS and Galileo were in view simultaneously Keeping in mind the preliminary Galileo constellation it is obvious that more than 30 satellites will be available simultaneously in the future considering only GPS GLONASS and Galileo Adding BeiDou satellites for GBAS would further boost these numbers The broadcast of such a large number of differential corrections is limited by the capacity of the VDB and thus by the number of slots assigned to a GBAS ground facility The number of assigned slots for a facility should be limited as far as possible to be able to use the same frequency for other GBAS ground facilities Thus the available capacity must be used as effectively as possible Number of Bytes Required Each VDB message is framed by a message block header 6 bytes and the message block CRC 4 bytes The length of each message depends on the message type and the amount of information to be transmitted The resulting length for a message of each type is given in TABLE 3 VDB Constraints A GBAS ground facility must transmit the VDB data following some constraints These are MT2 messages including all Additional Data Blocks required must be transmitted at least each 20th frame that is every 10 seconds If authentication is required each MT2 message must be WUDQVPLWWHG LQ WKH UVW VORW DVVLJQHG WR WKH 6 JURXQG facility All differential corrections both MT1 and MT11 must be transmitted at least once in each frame However it is possible to split the differential corrections into two adjacent slots using the Additional Message Flags in MT1 and MT11 messages Slot A 986 MT 2 or MT 4 48 or 51 B MT 11 N 18 140 B MT 1 N 1 28 B MT 3 18 B MT 1 N 17 204 B Slot B 100 FIGURE 3 VDB messages for two slots and 18 satellites MT1 and MT11 Within each MT1 message the ephemeris decorrelation parameter P eph the Issue of Data IOD and the HSKHPHULV 5 LV WUDQVPLWWHG IRU WKH UVW VDWHOOLWH LQ WKH PHVVDJH 7KXV WKH UVW VDWHOOLWH PXVW EH DOWHUQDWHG in order to broadcast the ephemeris information for all satellites SSURDFK GH QLWLRQV DUH WUDQVPLWWHG LQ 07 PHVVDJHV All MT4 messages must be transmitted within at least each 20th slot Based on these constraints a VDB encoding scheme has EHHQ GHYHORSHG ZKLFK DOORZV XV WR IXO OO DOO WKH UHTXLUHPHQWV listed above while optimizing the number of differential corrections that can be transmitted Even though it is optimized for GAST D like services including authentication parameters MT11 messages and experimental Galileo extensions it can be used for legacy GAST C systems too Rules for Optimal VDB Transmission 7R IXO OO WKH UHTXLUHPHQW IRU WKH 07 PHVVDJH WR EH WUDQVPLWWHG UVW D FRPSOHWH MT2 message must be transmitted each 20th frame at the EHJLQQLQJ RI WKH UVW VORW DVVLJQHG I QR 07 PHVVDJH KDV WR be transmitted an MT4 message is transmitted instead Thus all messages are arranged in proper order by three simple rules 1 MT2 each 20th frame or MT4 otherwise 2 MT11 all corrections can be split into two messages 3 MT1 all corrections can be split into two messages GGLWLRQDOO WZR PRUH UXOHV PXVW EH IXO OOHG 2Q WKH RQH hand if supporting the authentication feature each slot in ZKLFK WKH JURXQG IDFLOLW PD WUDQVPLW 9 GDWD PXVW EH OOHG to at least 95 percent For this MT3 null messages may be used WR HQVXUH WKDW HDFK VORW LV OOHG VXI FLHQWO 2Q WKH RWKHU KDQG an additional rule for MT1 messages is necessary if more than three slots are assigned to the GBAS ground facility In this case to maximize the number of differential corrections the MT1 messages may be transmitted in the last two assigned slots only This rule is necessary because the Additional Message Flag is limited to two slots for differential corrections Using this transmission scheme the number of differential corrections is maximized while fulfilling the minimum requirements on the VDB data Even in case of the maximum QXPEHU RI GLIIHUHQWLDO FRUUHFWLRQV 07 DSSURDFK GH QLWLRQV can still be broadcast However in this case the number of transmittable FAS segments is limited to 19 If more approaches or different approach types such as Terminal Area Paths TAPs have to be transmitted the VDB generation scheme must be adapted Number of Transmittable Corrections Using the optimized transmission scheme explained earlier the number of www gpsworld com April 2014 GPS World 47