GPS World, March 2010

Augmentation Assisstance GNSS DESIGN TEST s known to the authors Instead it is recommended that SBASs in equatorial areas wait for the forthcoming L5 signal to provide vertical guidance in their regions GPS L5 The next GPS satellite to be launched will contain a new civil signal L5 centered at 117645 MHz and in a protected aviation band As such it will be approved for use on aircraft When the L5 signal is used in combination with L1 the ionospheric delay for each line of sight can be directly estimated This will dramatically lower the uncertainty of the pseudorange measurement Thus if the SBAS is upgraded to provide corrections appropriate for an L1 L5 user and the user similarly upgrades his or her avionics SBAS service can be dramatically improved Another important advantage of the second civil frequency is its relative immunity to ionospheric storms Because the users are now directly eliminating the amount of delay they actually experience they are no longer affected by shortcomings in the MOPS ionospheric model The weaker effect of scintillation may have some impact however we do not expect to lose vertical guidance altogether Furthermore the availability of two civil frequencies offers protection against unintentional interference If either L1 or L5 is jammed the user still has access to guidance on the available frequency At the moment there is no MOPS for an L1 L5 user so any ground or user algorithms will have to be speculative We propose basing future L1 L5 algorithms on the existing L1 only algorithms Instead of using L1 only pseudorange measurements the user forms the ionosphere free combination For the confidence term representing the total pseudorange error on a line of sight the ionospheric correction terms and airborne multipath terms are replaced with a single value representing airborne noise and multipath for the ionospherefree combination For discussion of confidence terms for single frequency users related equations and VPL see the website version of this article at www gpsworld com dualsbas The improvement in performance for a dual frequency user can be seen in FIGURE 5 The coverage is significantly expanded Now each region is robustly covered with large margins surrounding their intended service regions However coverage is still limited to the areas around these first three SBASs GAGAN and SDCM Two additional SBASs are currently under development that will extend coverage to more regions India is developing GAGAN Currently it has eight Indian reference stations INRES all in India blue diamonds in FIGURE 6 There is one Indian master control center INMCC and plans to use the GSAT 4 as its initial GEO The GSAT 4 is planned for launch in 2010 and will be located near 82 E The geomagnetic equator passes through India and it therefore faces Availability as a function of user location 150 100 50 0 50 100 150 Longitude deg 50 50 75 85 90 95 99 995 999 Availability with VAL 35 HAL 40 Coverage 99 2803 Latitude deg 80 60 40 20 0 20 40 60 80 FIGURE 5 Potential dual frequency coverage of the first three SBASs including network improvements WAAS EGNOS MSAS GAGAN SDCM FIGURE 6 The networks of five SBAS systems In addition to the reference stations from Figure 3 the eight Indian stations are shown as blue diamonds and the 19 Russian stations are shown as red stars the full impact of equatorial ionosphere The advent of L5 will allow GAGAN to obtain high LPV 200 availability that is unlikely to be achievable for single frequency users Russia is developing SDCM It now has nine operational measuring points MPs and has plans for at least 10 more locations all in Russia red stars in Figure 6 There are also plans to use three GEOs Luch 5a planned for launch in 2010 and to be located near 16 W Luch 5b planned for launch in 2011 and to be located near 95 E and Luch 4 planned for launch in 2013 and to be located near 167 E FIGURE 7 shows the combined dual frequency coverage of all five systems WAAS EGNOS MSAS GAGAN and SDCM The vast majority of land masses in the northern hemisphere are now well covered by at least one of the SBASs Figures 6 and 7 clearly highlight that the majority of development has occurred www gpsworld com March 2010 GPS World 39