Opportunities for Orbit Improvement Using the Jason and Sentinel-6 Series of Satellites

Author:

Alexandre Couhert - (CNES)

Co-Investigator(s):

Pascal Bonnefond (Observatoire de Paris/SYRTE)
Sean Bruisnma (CNES)
Pierre Exertier (Observatoire de Paris/GET)
Sabine Houry (CNES)
Jean-Michel Lemoine (CNES)
Flavien Mercier (CNES)
John Moyard (CNES)
Franck Reinquin (CNES)
Hélène Roinard (CLS)

Abstract:

Based on experience gained with the past altimeter missions initiated by TOPEX/Poseidon and continued through Jason-1, Envisat, Jason-2 (OSTM), HY-2A, and with the currently flying satellites, CryoSat-2, SARAL, Jason-3, Sentinel-3A, Sentinel-3B, HY-2C (soon Sentinel-6, SWOT, …), for which we deliver precise and homogeneous orbit solutions, we propose to:

• assess quality by taking advantage of the three high precision tracking systems operating on the Jason/Sentinel6-serie missions (GNSS, DORIS, and SLR) and further explore the information provided when the same on-board oscillator is used by both the DORIS instrument and the GNSS receiver, making possible to estimate the clock using the GNSS measurements and use a GNSS-observed clock in the DORIS computation;
• compute precise orbits for all five satellites (TOPEX/Poseidon, Jason-1, OSTM/Jason-2, Jason-3, and Sentinel-6) with state of the art force and geometric models;
• examine force and measurement modeling improvements: improved surface force models (Solar Radiation Pressure (SRP) and thermosphere models) using accelerometer data, new GRACE/GRACE--FO-based mean geopotential models. For Galileo, new and promising results are expected on Sentinel-6 as the integer ambiguity fixing with Galileo and GPS receivers will be possible for the first time, enabling higher parameterization towards point positioning of LEO satellites;
• accurately determine the gravity changes at low spherical harmonic degrees making the most of all available space geodetic techniques (SLR, GNSS, DORIS): this requires improving the consistency of methods to solve for the variations of the three degree-1 coefficients (geocenter motion), i.e. look for strategies that mitigate sensitivity to miscentering effects on the orbit coming from the tracking measurements. The geocenter motion is expected to vary by as much as 50 mm over the course of the century (due to the melting of the Greenland and Antarctic ice sheets). Our involvements in two national (INSU-Programme National de Télédétection Spatiale research proposal) and international (IAG/IERS: “Toward reconciling Geocenter Motion estimates”) working groups on the topic of geocenter motion will be useful inputs towards this endeavor. From the spherical harmonic degree 2 (Earth’s figure axis orientation) further constraints on the rheology of the Earth from seasonal to decadal time scales (e.g., pole tide Love number, post-glacial rebound) could be inferred using long records of SLR observations: a specific PhD position will be requested on this topic for the period 2021-2024;
• alternative methods, e.g., using mascons based on historical multi-technique DORIS/SLR missions, will be analyzed to measure global mass changes prior to the GRACE era and reduce the orbit error budget of the fundamental T/P mission;
• characterize systematic errors in laser observations through precise orbit determination of multiple geodetic satellites (Lageos-1/2, Starlette, Stella, LARES, Ajisai), “zero-signature” targets (BLITS, Westpac-1, Larets), and altimeter satellites independently tracked by DORIS/GNSS. The observed measurement noise can be as good as 2 mm RMS, but the limitation is the value of the systematic biases (ranging from a few millimeters to several centimeters): station biases can no longer be ignored for the validation of altimeter satellite precise orbits. These robust estimates could later become official International Laser Ranging Service (ILRS) products;
• detect geographically correlated orbit errors using the laser-based short-arc technique;
• characterize orbit errors through the computation of altimeter sea level differences at crossovers, the Sea Surface Height (SSH) cross comparison between Jason/Sentinel-6 missions, other altimeter missions, and relative orbit analyses during the different tandem phases;
• take benefit of the colocation in space of high-quality SLR data and DORIS/GNSS-based orbits: analyze the forward and backward flying attitude regimes of the “flipping” satellites (Jason, Sentinel-6?) to disentangle time tagging from center of phase POD instrument offsets and observe separately any miscentering of the orbit around the Earth’s Center of Mass (CM), and the combined effects of miscalibrated SRP models and POD instrument locations. Our contribution to the IAG Study Group “integration and co-location of space geodetic observations and parameters“ could provide insights into these systematic errors;
• evaluate the benefits of a simultaneous orbit determination of LEO altimeter satellites and MEO (GPS, Galileo) constellations;
• periodically define and produce an updated set of orbits and geophysical standards to address short-term and long-term orbit errors impacting mean sea level change estimates. Radial orbit difference rates still display considerable regional signals, possibly due to systematic or unaccounted-for errors in the satellite tracking measurements and incomplete modeling of Time Variable Gravity (TVG);

Supported by CNES