The potential of technical developments in future GNSSs such as Kepler on important geodetic parameters is being analyzed by the GFZ German Research Center for Geosciences. These include satellite orbits, geodetic reference frames, Earth’s rotation, Earth’s gravity field, monitoring of sea surface height and troposphere. To predict the effects of climate changes, it is essential to determine these parameters with the highest accuracy and reliability, since climate indicators such as ice sheet and glacier changes as well as the global sea level rise need highly accurate long-term monitoring. The accuracy provided by current GNSSs does not yet meet the requirements needed for drawing reliable conclusions: the developments in optical technologies applied to future GNSSs such as Kepler will allow overcoming current limitations.
Precise orbit determination
The dynamics of the satellites in the Kepler MEO constellation is similar to the one of the current GALILEO system, which assures long term orbit stability and limits the number of constellation-keeping maneuvers. The baseline LEO segment considered here consists of four satellites distributed over two circular near-polar orbits on two perpendicular orbital planes.
The relative phasing between LEO satellites and between LEO and MEO satellites is designed to guarantee 100% availability of connections between the three MEO planes via the LEO segment at all times. This infrastructure provides two types of measurements: GNSS-type L-band ranging data (true ranges and carrier phase) broadcast by MEO satellites, and precise inter-satellite ranges on optical carriers between neighboring MEO satellites (continuously) and between MEO and LEO satellites (sporadic connections to all MEO satellites).
The navigation signals broadcast by MEO satellites are logged by LEO satellites and by a single ground station (or a very limited number of ground stations).
Using GFZ’s Earth Parameters and Orbit System – Orbit Computation (EPOS-OC) software package, aforementioned data types were simulated and a number of precis orbit determination (POD) tests (orbit recoveries) were performed.
In the data simulation step standards and models from daily processing of real GNSS data are replicated. These include GPS-like attitude, different solar radiation pressure models, MEO satellite antenna thrust, EIGEN-6C gravity field model, Earth and ocean tide models, ocean loading etc. The GNSS range data are simulated on 2 GALILEO frequencies E1 and E5 and, for the purpose of orbit determination, converted into ionosphere-free combinations, with Gaussian noise, characterized by a standard deviation of 50 cm (pseudo-ranges) and 3 or 5 mm (carrier phases) on LEO satellites or ground station(s), respectively. Cycle slips are simulated as well. The Kepler system time is assumed to be ultra-stable: the system clock offsets are simulated as constant null values. Optical ranging between MEO satellites and between MEO and LEO satellites are simulated with a conservative noise threshold of 1mm.
Initial results show that with just one ground station and appropriate compensation of mismodeling of non-conservative forces (e.g. solar radiation pressure, LEO air drag, MEO antenna thrust) the orbits of the Kepler satellites may be recovered offline to sub-centimeter precision for the radial coordinate and better than 15 micrometers per second for velocity estimation, fulfilling preliminary requirements for navigation and clock synchronization.
Precise orbit determination for geodetic applications
For the system operation and navigation purposes a very limited regional ground infrastructure is considered (see above). For satellite geodesy applications (e.g. realization of a terrestrial reference frame), a global network with a large number of stations is used (see figure).
This global network is the basis for simulation of geodetic parameters estimation.
Realization of terrestrial reference frames
Global terrestrial reference frames (TRFs) provide the metrological basis for measuring and monitoring the Earth system. Nowadays, the accuracy of the most recent TRFs is still up to 8 times worse than required by the Global Geodetic Observing System: 1 mm accuracy, 1 mm/decade stability.
Gravity field
Current gravity field models based on the GRACE (Gravity Recovery and Climate Explorer) and GRACE-FO (GRACE-Follow On) missions do not meet all user requirements for spatial and temporal resolution and accuracy. The benefits of improved orbits of the Kepler constellation on the gravity field coefficients to investigate potential improvements of Next Generation Gravity Missions are being assessed.
Water vapor
The troposphere is a major contributor to the systematic and random error budget in microwave-based space geodetic techniques. A more accurate quantification of the related signal propagation delay leads to the estimation of more accurate geodetic parameters in the subsequent geodetic analysis. The improvement of tropospheric products attributed to the characteristics of the Kepler constellation, and especially its LEO segment and inter-satellite links, is being examined.
Reflectometry
The ocean altimetric perspectives of the new GNSS constellation are investigated in view of the space-based application of the innovative GNSS reflectometry (GNSS-R). Assuming GNSS-R transmitters and receivers aboard the constellation’s MEO satellites (MEO to MEO links), a novel altimetric concept is explored. Preliminary results indicate that the MEO to MEO links can enhance the estimation of crucial altimetric parameters compared to current scenarios.