GPS World, January 2013

GNSS DESIGN Integration with Other Technologies models Communication system service providers typically use propagation models that are conservative in their estimates of received power levels in the sense that they overestimate propagation losses This conservatism is necessary so that the service can be provided to end users with high availability From the standpoint of potential victims of interference however it is seen as far more desirable to underestimate propagation losses so that interference can be kept below an acceptable level a very high percentage of time As shown in Figure 3 some received power measurements from the Las Vegas live sky test indicate values even greater than would be predicted using free space propagation model Statistical models that allow for this possible were used in the FAA Status Report The general topic of propagation models is worthy of future additional study if GPS interference standards are to be developed Future Considerations GPS is being modernized Additionally satellite navigation users now enjoy the fact that the Russian GLONASS system has recently returned to full strength with the repopulation of its constellation In the next decade satellite navigation users also eagerly anticipate the completion of two other global GNSS constellations Europes Galileo and Chinas Compass Notably between the GPS modernization program and the deployment of these other systems satellite navigation users are expected to soon be relying upon equipment that is multifrequency and that needs to process many more signals with varied characteristics New equipment offers an opportunity to insert new technologies such as improved ltering but of course the need to process additional signals and carrier frequencies may make GNSS equipment more susceptible to interference as well Clearly these developments will need to be carefully assessed to support the establishment of GPS spectrum interference standards Summary This article has identified a number of considerations for the development of GPS interference standards which have been proposed by the PNT EXCOM If the United States proceeds with the development of such standards it is hoped that the information within this article will prove useful to those involved Appendix AGL Heights of CORS Network Sites The National Geodetic Survey Continuously Operating Reference Station CORS website provides lists of CORS site locations in a number of different reference frames To determine the height above ground level h agl for each site within this study two of these les igs08_ xyz_ comp txt and igs08_ xyz_ htdp txt were used These two les provide the x y z coordinates of the antenna reference point ARP for each site in the International GNSS Service 2008 IGS08 reference frame which is consistent with the International Terrestrial Reference Frame ITRF of 2008 These coordinates are divided into two les by NGS since the site listings also provide site velocities and velocities are either computed for sites that have produced data for at least 25 years or estimated for newer sites The comp le includes sites with computed velocities and the htdp le includes sites with estimated velocities using a NGS program known as HTDP The data files can be used to readily produce height above the ellipsoid h ellipsoid for each site This height can be found using well known equations to convert from x y z to latitude longitude height Obtaining estimates of h agl requires information on the geoid height and terrain data per the relationship h agl hellipsoid N h terrain A 1 For the results presented in this article terrain data was obtained from http earthexplorer usgs gov in the Shuttle Radar Topography Mission SRTM Digital Terrain Elevation Data DTED Level 2 format For this terrain data the horizontal datum is the World Geodetic System WGS 84 The vertical datum is Mean Sea Level MSL as determined by the Earth Gravitational Model EGM 1996 Each data file covers a 1 º by 1 º degree cell in latitude longitude and individual points are spaced 1 arcsec in both latitude and longitude The SRTM DTED Level 2 has a system design 16 meter absolute vertical height accuracy 10 meters relative vertical height accuracy and 20 meter absolute horizontal circular accuracy All accuracies are at the 90 percent level Considering the accuracies of the DTED data the differences between WGS 84 and IGS08 as well as between the ARP and antenna phase center were considered negligible Geoid heights were interpolated from 15 arcmin data available in the MATLAB Mapping Toolbox using the egm96geoid function Lower AGL heights are preferred for CORS sites to minimize motion between the antenna and the Earths crust However many sites are at significant heights above the ground by necessity particularly in urban areas due to the competing desire for good sky visibility Christopher J hegarty is the director for communications navigation and surveillance engineering and spectrum with The MITRE Corporation He received a D Sc degree in electrical engineering from George Washington University He is currently the chair of the Program Management Committee of the RTCA Inc and co chairs RTCA Special Committee 159 GNSS He is the co editor co author of the textbook Understanding GPS Principles and Applications 2nd Edition GPS World January 2013 www gpsworld com 66