GPS World, April 2017

FIGURE 4 Coverage area of the GEO satellite network for orbit and clockcorrection dissemination colored circles and Swarm C satellite ground track black Dotted lines indicate the assumed coverage area limits at 66 N S and 75 N S APRIL 2017 WWW GPSWORLD COM GPS WORLD 45 errors of 60 centimeters This result demonstrates that the use of the real time orbits and clocks only leads to a small degradation in the orbit accuracy compared to the use of postprocessed GPS products EFFECTS OF CORRECTION DATA GAPS The analysis in the previous section has shown that the use of real time corrections enables high orbit accuracy when the corrections are continuously available However in an onorbit scenario the demodulator which keeps track of GEO satellites and delivers corrections to the navigation filter may not be able to track them continuously for various reasons Even though dedicated GEO satellite networks for spaceborne applications like NASAs Tracking and Data Relay Satellite System TDRSS or the European Data Relay Satellite EDRS system potentially offer a seamless service volume for LEO users anywhere on the globe this may not be feasible with a GEO network originally intended for ground based users These satellites typically have a more focused beam which potentially hinders reliable data transmission in polar regions This situation is depicted in FIGURE 4 which shows the approximate access areas of the GEO satellite network used to transmit Fugros corrections It also depicts the ground track of two orbital revolutions of the Swarm C satellite which leaves the access areas at latitudes beyond approximately 80 N S Even if the beamwidth of a GEO satellites antenna allows for a continuous link at high latitudes the receiving satellite demodulator on board the LEO spacecraft will have to switch signal reception to another GEO satellite when the tracked satellite drops out of the field of view These switches typically happen in polar regions The acquisition of the new GEO signal is not a trivial task as it is done under unfavorable conditions at the edge of the service area and requires for example correct prediction of the expected Doppler shift due to relative motion the GEO and LEO satellites Thus interruptions in the correction data streams are likely to occur and the extent of these interruptions depends on how the switching mechanism is implemented in the demodulator and how fast the acquisition of the new GEO satellites signal takes place It is worth mentioning in this context that GEO signal reception depends not only on the transmitting antenna gain pattern but also on the gain pattern of the receiving antenna on the LEO satellite the antenna placement on the satellite structure as well as its attitude profile Experience has shown that satellite design constraints may prevent the antenna from being placed in the most favorable position Operational constraints can force the satellite not to be oriented in the preferred way for GNSS and GEO signal reception Instead priority must often be given to the optimal orientation of body fixed solar panels for maximum power generation or the pointing of payload sensors such as optical instruments to certain target directions To study the impact of correction data outage on the LEO POD we defined reduced coverage areas The first scenario limits the reception of correction data beyond latitudes of 66 N S In the case of Swarm C at approximately 440 kilometers altitude the outage intervals over the North and South Poles extend to 13 minutes at maximum In the second case the corrections are received up to 75 N S which corresponds to a maximum outage of 8 minutes twice per orbit The smaller coverage area serves as a worst case scenario whereas the larger service area is more representative of the expected onorbit performance Prediction of Orbit and Clock Corrections When up to date corrections are no longer available due to an outage in the GEO satellite link the last received set of corrections must be extrapolated Up to a certain prediction interval this method still provides more precise orbit and clock information than the broadcast ephemerides and thus yields better positioning results The prediction of orbit and clock information is therefore crucial to bridge correction outages and still maintain a precise positioning solution The following analysis assesses the errors introduced by only extrapolating the orbit and clock corrections In addition to these errors the modeling of the observations is also affected by the absolute errors in the realtime orbit and clock product The satellite clock offsets are estimated based on predicted orbits Therefore the radial along track and cross track components of the orbit corrections can be computed so that prediction errors over a predefined time interval are minimized Taking advantage of this the prediction errors are typically less than 1 centimeter even for extrapolation times of 12 minutes and therefore have negligible effect on the POD In the case of the satellite clock offset corrections are only available up to the present epoch Thus the extrapolation is done based on a fit through the past hour of data