GPS World, April 2017

Orbit Determination of LEO Satellites with Real Time Corrections By André Hauschild Javier Tegedor Oliver Montenbruck Hans Visser and Markus Markgraf 42 GPS WORLD WWW GPSWORLD COM APRIL 2017 WITH RICHARD B LANGLEY PRECISION ON BOARD P recise point positioning PPP with real time orbit and clock correction streams has become an established technique over the past decade Several free as well as commercial sources of precise correction streams are available through the internet or via a satellite link to geostationary satellites Many applications exist for land air and sea appications but use of real time corrections for precise positioning has not extended into orbit yet although a number of low Earth orbit LEO satellite missions have a demand for precise orbit determination POD Mission requirements often allow for a relatively high latency for the availability of the precise orbit products thus ground based near real time processing is sufficient However future satellites with altimeter and radiooccultation payloads may require real time POD to enable onboard processing of science data for short term forecasting or now casting of meteorology data open loop instrument operations of radar payloads or quick look onboard science data generation Also precise real time orbit information may be used for constellation maintenance of satellite formations Despite early technology readiness demonstrations by the Jet Propulsion Laboratory carried out one decade ago to transmit real time corrections via geostationary relay satellites to LEO spacecraft this technique has so far not been implemented and used in a space mission POD accuracy of a few decimeters or less with real time corrections has been demonstrated repeatedly by various groups For these studies it was assumed that the required realtime precise orbit and clock products are continuously available on board the LEO satellite Even though a network of several distributed geostationary Earth orbit GEO relay satellites may achieve seamless coverage in the equatorial region gaps at high latitude close to the North and South Poles may occur The extent of these gaps depends on the gain pattern of the transmitting antenna of the GEO relay satellite Likewise the availability of corrections depends on the LEO orbit characteristics the gain pattern and mounting of the receiving antenna and the attitude profile of the LEO satellite Most Earth observation and altimeter missions are launched into polar orbits to achieve global coverage Up to date real time corrections may therefore not be available for POD processing over the polar regions which are typically also affected by reduced GNSS satellite visibility As FIGURE 1 Close up view of the Swarm C satellite with Swarm A and B in the background artists impression The satellites booms point in the anti flight direction Two GPS antennas are located on the top side of each satellites structure credit ESA AOES Medialab a result the positioning performance will be degraded during this part of the orbit To study the effects of interrupted availability of precise correction data we simulated real time POD using real flight data of the Swarm C satellite a representative LEO satellite orbiting Earth at an altitude of about 440 kilometers in a polar orbit with approximately 87 inclination The satellite was launched into orbit in Nov 2013 and is part of a threesatellite constellation of identical spacecraft with the mission objective to study Earths magnetic field and the electric field in the atmosphere see FIGURE 1 The orbital period is 93 minutes The satellite is equipped with a dual frequency GPS receiver and two zenith pointing POD antennas The receiver provides dual frequency GPS observations of up to eight satellites simultaneously For the analysis we selected a test data period of Feb 1 15 2016 We processed the GPS observations using a highperformance navigation filter together with precise realtime orbit and clock corrections provided by Fugro a Dutch multi national company that provides a multi GNSS realtime PPP service tailored for maritime applications The complete processing emulates real time onboard POD and only uses information available up to the current epoch being