New Orbit Determination and Clock synchronisation modules for EGNOS

The purpose of a Satellite Based Augmentation System (SBAS), such as EGNOS or WAAS, is to identify all range error sources and to distribute the corresponding corrections to the civil aviation user community with reliable navigation services for different flight phases. The effect of a satellite location error depends on the user location while a satellite clock error with respect to a reference time scale directly translates into a common pseudorange error to all the users. Therefore the SBAS shall broadcast a 3D vector that represents the satellite orbit error and a satellite clock correction. To achieve this objective the SBAS shall internally estimate the orbits and clocks for all the navigation satellites in view of the service area. The orbit determination function is in charge of computing the satellite ephemerides. The synchronization function computes the corresponding clock bias for each epoch and each satellite. Then the corrections are constructed from the differences between these orbits and clocks and the corresponding ones broadcasted inside the GNSS navigation messages. Starting from R&D activities, Thales Alenia Space has developed new orbit determination and synchronization modules that are part of the Thales Algorithm Navigation Chain. These modules have been designed in collaboration with the orbit determination team at CNES (the French Space Agency). The new proposed orbit determination module is based on real time processing using code carrier measurement only. This module provides a stable and metric GPS orbit performance using an SBAS set of receivers corresponding to the EGNOS service area. The new synchronization module solves clock errors directly steered to GPS reference time scale, for the stations and satellites. It uses both code carrier and phase carrier measurements as well as the orbits estimated by the orbit determination process. The clocks solution error Allan’s deviation is around 10-12 at 120s leading to 7cm of possible deviation for a prediction up to 120s. This performance is fully compatible with the needs of the SBAS mission. These modules are now fully integrated into the SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS Performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The performance evaluation shows a real improvement over the current EGNOS algorithms, particularly in terms of the distribution of the Satellite Residual Error for the Worst user location (SREW). This paper provides a high level architecture description of this new Thales solution. A set of performance figures showing the achieved improvements is also presented

GENESIS: Co-location of Geodetic Techniques in Space

Improving and homogenizing time and space reference systems on Earth and, more directly, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1mm and a long-term stability of 0.1mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation for predicting natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities which has enunciated geodesy requirements for Earth sciences. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, proposed as a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this white paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology.Comment: 31 pages, 9 figures, submitted to Earth, Planets and Space (EPS

Influence of the ionospheric model on DCB computation and added value of LEO satellites

In order to compute inter-frequency Differential Code Biases (DCBs), the Geometry-Free combination of a GNSS signal pair needs to be corrected from the ionospheric refraction effect. Such information is obtained using either Global Ionospheric Maps (GIMs) or local models. In this work we investigate the influence of GIMs on the final value and precision of DCB solution. The study covers different ionospheric conditions, ranging from very quiet ionospheric background up to a severe ionospheric storm. In a first step, the Slant Total Electron Content (STEC) between GIMs is assessed as a function of receiver latitude, elevation mask and ionospheric conditions. Then, daily DCBs are estimated using these different GIMs, receiver and satellite contributions being separated using a zero-mean constraint. At last, an independent estimation of DCBs is performed using Low Earth Orbit (LEO) observations (such as JASON's GPS data). This solution is compared with our ground network solution and with DCBs coming from Analysis Centers (ACs) of the International GNSS Service providing ionospheric and DCB solutions

Mapping and investigating phase anomalies in GPS data onboard Low Earth Orbiters

To face important societal challenges like sea level variations, climate change and natural hazards management (tsunami detection, earthquakes, crustal deformations…), modern science rely more and more on precise geodesy. Precise Orbit Determination (POD) is of major concern in the frame of altimetry or gravity recovery missions like GOCE or GRACE. Using the GPS receiver onboard, orbits at cm-level accuracy are generally achieved in both kinematic and reduced-dynamic approaches using dual frequency code and phase measurements. GPS data processing generally uses the Ionospheric-Free (IF) combination to get rid of the ionospheric delay, which is varying with the season, latitude, local time and solar activity. However, large discrepancies in the orbit determination are still observed over polar and equatorial regions, which turn into artefacts and errors in the derived scientific products (gravity field, sea surface height…). More precisely, large RMS values are strongly correlated to phase anomalies occurring on GPS receivers: cycle slips, data unavailability or enhanced measurement noise, especially on L2 signal. Phase anomalies are generally observed when the satellite orbit crosses regions where ionospheric scintillations occur, which are defined as rapid fluctuations in phase and amplitude of the GNSS signals. The occurrence of scintillations exhibits large day-to-day variations and depends mainly on geomagnetic latitude, season and local time. At low latitudes, maximum occurrence of scintillations is observed 15-20° on either side of the geomagnetic equator. Scintillations also occur at auroral and polar latitudes, where their intensity increases with increasing geomagnetic activity. This paper aims at detecting, mapping and understanding the phase anomalies experienced by LEO satellites and analyzing their correlation with geomagnetic activity, latitude, season and local time. Several LEO satellites at different altitudes are analyzed (e.g. SWARM, GRACE or JASON), which allows a multi-layer analysis of the underlying ionospheric phenomenon, including scintillation. The latter are generally measured with several indices, like the amplitude index S4 or the phase index SigmaPhi (σφ), which are usually derived from 100Hz measurements performed by dedicated scintillation monitors. In this study, we compute a similar index (called pseudo-σφ) using GPS phase data at 1Hz coming from POD GNSS antenna. A detailed study of the occurrence rate and the severity of pseudo-σφ, together with cycle slips and other spurious phase data, will be performed for different LEO satellites

Orbitography for next generation space clocks

18 pages, 7 figures, submitted to Journal Of GeodesyOver the last decade of the 20th century and the first few years of the 21st, the uncertainty of atomic clocks has decreased by about two orders of magnitude, passing from the low 10^-14 to below 10^-16, in relative frequency . Space applications in fundamental physics, geodesy, time/frequency metrology, navigation etc... are among the most promising for this new generation of clocks. Onboard terrestrial or solar system satellites, their exceptional frequency stability and accuracy makes them a prime tool to test the fundamental laws of nature, and to study gravitational potentials and their evolution. In this paper, we study in more detail the requirements on orbitography compatible with operation of next generation space clocks at the required uncertainty based on a completely relativistic model. Using the ACES (Atomic Clock Ensemble in Space) mission as an example, we show that the required accuracy goal can be reached with relatively modest constraints on the orbitography of the space clock, much less stringent than expected from "naive" estimates. Our results are generic to all space clocks and represent a significant step towards the generalised use of next generation space clocks in fundamental physics, geodesy, and time/frequency metrology

Orbit determination for next generation space clocks

Abstract. In this paper, we study the requirements on orbit determination compatible with operation of next generation space clocks at their expected uncertainty. Using the ACES (Atomic Clock Ensemble in Space) mission as an example, we develop a relativistic model for time and frequency transfer to investigate the effects of orbit determination errors. We show that, for the considered orbit error models, the required uncertainty goal can be reached with relatively modest constraints on the orbit determination of the space clock, significantly less stringent than expected from ”naive ” estimates. Our results are generic to all space clocks and represent a significant step towards the generalized use of next generation space clocks in fundamental physics, geodesy, and time/frequency metrology

Computation of GPS P1–P2 Differential Code Biases with JASON-2

GPS Differential Code Biases (DCBs) computation is usually based on ground networks of permanent stations. The drawback of the classical methods is the need for the ionospheric delay so that any error in this quantity will map into the solution. Nowadays, many low-orbiting satellites are equipped with GPS receivers which are initially used for precise orbitography. Considering spacecrafts at an altitude above the ionosphere, the ionized contribution comes from the plasmasphere, which is less variable in time and space. Based on GPS data collected onboard JASON-2 spacecraft, we present a methodology which computes in the same adjustment the satellite and receiver DCBs in addition to the plasmaspheric vertical total electron content (VTEC) above the satellite, the average satellite bias being set to zero. Results show that GPS satellite DCB solutions are very close to those of the IGS analysis centers using ground measurements. However, the receiver DCB and VTEC are closely correlated, and their value remains sensitive to the choice of the plasmaspheric parametrization