GPS World, April 2015

COVER STORY Galileo E1 and E5a Performance For Multi Frequency Multi Constellation GBAS Analysis of new Galileo signals at an experimental ground based augmentation system GBAS compares noise and multipath in their performance to GPS L1 and L5 Raw noise and multipath level of the Galileo signals is shown to be smaller than those of GPS Even after smoothing Galileo signals perform somewhat better than GPS and are less sensitive to the smoothing time constant Mihaela Simona Circiu Michael Felux German Aerospace Center DLR and Sam Pullen Stanford University S everal ground based augmentation system GBAS stations have become operational in recent years and are used on a regular basis for approach guidance These include airports at Sydney Malaga Frankfurt and Zurich These stations are so called GBAS Approach Service Type C GAST C stations and support approaches only under CAT I weather conditions that is with a certain minimum visibility Standards for stations supporting CAT II III operations low visibility or automatic landing called GAST D are expected to be agreed upon by the International Civil Aviation Organization ICAO later this year Stations could be commercially available as soon as 2018 However for both GAST C and D the availability of the GBAS approach service can be significantly reduced under active ionospheric conditions One potential solution is the use of two frequencies and multiple constellations in order to be able to correct for ionospheric impacts detect and remove any compromised satellites and improve the overall satellite geometry and thus the availability of the system A new multi frequency and multi constellation MFMC GBAS will have different potential error sources and failure modes that have to be considered and bounded Thus all performance and integrity assumptions of the existing single frequency GBAS must be carefully reviewed before they can be applied to an MFMC system A central element for ensuring the integrity of the estimated position solution is the calculation of protection levels This is done by modeling all disturbances to the navigation signals in a conservative way and then estimating a bound on the resulting positioning errors that is valid at an allocated integrity risk probability One of the parameters that is different for the new signals and must be recharacterized is the residual uncertainty GPS World April 2015 www gpsworld com 30