F. J. Perosanz

Earth Gravity Field and Seasonal Variability from CHAMP

GPS-CHAMP satellite-to-satellite and accelerometry data covering 2.5 years of the CHAMP mission period were exploited to generate the global gravity field model EIGEN-3p revealing considerable improvements in both accuracy and resolution with respect to the previous model EIGEN-2. For the year 2001, CHAMP and satellite laser ranging data of four satellites were combined to recover largest scale monthly gravity field variations that are subsequently analyzed for the annually varying constituents. The temporal gravity field variations observed by CHAMP and the SLR satellites are compared in the spectral and spatial domain with geophysically (atmosphere, ocean, hydrology) predicted gravity variations that do not reflect the large observed scattering in the monthly solutions but are of comparable size and distribution on the annual time scale.

1 × 10 −16 frequency transfer by GPS PPP with integer ambiguity resolution

For many years, the time community has been using the precise point positioning (PPP) technique which uses GPS phase and code observations to compute time and frequency links. However, progress in atomic clocks implies that the performance of PPP frequency comparisons is a limiting factor in comparing the best frequency standards. We show that a PPP technique where the integer nature of phase ambiguities is preserved consitutes significant improvement of the classical use of floating ambiguities. We demonstrate that this integer-PPP technique allows frequency comparisons with 1 × 10−16 accuracy in a few days and can be readily operated with existing products.

Global Gravity Field Recovery Using Solely GPS Tracking and Accelerometer Data from Champ

A new long-wavelength global gravity field model, called EIGEN-1. has been derived in a joint German-French effort from orbit perturbations of the CHAMP satellite, exploiting CHAMPGPS satellite-to-satellite tracking and on-board accelerometer data over a three months time span. For the first time it becomes possible to recover the gravity field from one satellite only. Thanks to CHAMP’s tailored orbit characteristics and dedicated instrumentation, providing continuous tracking and on-orbit measurements of non-gravitational satellite accelerations. the three months CHAMP-only solution provides the geoid and gravity with an accuracy of 20 cm and 1 mgal, respectively, at a half wavelength resolution of 550 km, which is already an improvement by a factor of two compared to any pre-CHAMP satellite-only gravity field model.

On Board Evaluation of the STAR Accelerometer

The main results of the on-board evaluation of the STAR accelerometer are presented after 18 months of mission. The instrument demonstrates high performances in terms of resolution and reliability and its contribution to dynamic orbit determination is clear. However, unexplained signal jumps have been detected and analysed. In addition, an anomalous behaviour of the X3 electrode of the accelerometer, affects the Radial, Roll and Pitch accelerations. Nevertheless, corrected observations can be recovered from a new combination of the electrode voltages. The in-orbit calibration will also benefit from the new EIGEN gravity field model that includes CHAMP data.

The time stability of PPP links for TAI

In the last few years the BIPM has started using GNSS phase and code observations and the Precise Point Positioning technique to compute time links for the generation of TAI. The estimated instability of such links for averaging time up to 1 month has been taken as 0.3 ns. In this paper, we investigate methods to estimate this instability, following several approaches.

The 2013 Ibiza Calibration Campaign of Jason-2 and SARAL Altimeters

This study presents the results of the 2013 Ibiza (Western Mediterranean) calibration campaign of Jason-2 and SARAL altimeters. It took place from 14 to 16 September 2013 and comprised two phases: the calibration of the GNSS (Global Navigation Satellite System) buoys to estimate the antenna height of each of them and the absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was performed at ∼ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and SARAL nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and SARAL, respectively. The GNSS data were processed using precise point positioning technique. The biases found are of (−0.1 ± 0.9) and (−3.1 ± 1.5) cm for Jason-2 and SARAL, respectively.

Accelerometry Aboard CHAMP

One of the key instruments aboard the CHAMP geoscientific small satellite is the three-axes STAR accelerometer, which shall measure the resultant acceleration of the spacecraft due to non-gravitational forces. Then, the purely gravitational signal in CHAMP’s orbit, measured with GPS satellite-to-satellite tracking, can be extracted for use in Earth gravity field modelling. Besides the noise in the accelerometer measurements, environmental systematic error sources may affect the accelerometer data: e.g. accommodation at CoG, orientation knowledge and spurious, linear and rotational accelerations from the attitude control system and due to boom oscillations. A crucial point in the exploitation of the measured accelerations is the determination of the instrument’s calibration parameters, i.e. bias, and scale factor which shall be done with an in-flight data adjustment. Results of numerical simulations of CHAMP’s tracking and accelerometer scenario, taking into account the GPS and accelerometer error sources and the estimation of calibration and nuisance parameters, are presented with regard to impacts on orbit and gravity field recovery accuracy. The objective is to develop an optimal data processing approach which can be applied to real mission data after the launch of CHAMP, end of 1999.

Evaluation of the CHAMP Accelerometer on Two Years of Mission

This paper presents a synthesis of the CNES/GRGS routine activities regarding the analysis and calibration of the CHAMP accelerometer data. The impact of instrument operations and out of specification temperature variation on bias and scale factor estimation is demonstrated. Precise dynamic solutions of orbits and calibration parameters have been obtained from the processing of two years of data. The results presented here could be obtained thanks to the high performances of the accelerometer and GPS tracking.

STAR Accelerometer Contribution to Dynamic Orbit and Gravity Field Model Adjustment

We investigate the contribution of the STAR accelerometer on board the CHAMP satellite to the dynamic orbit restitution and gravity field recovery. The quality of the dynamic orbits is evaluated in term of tracking data residuals (GPS and Laser data) and by comparisons to precise reduced-dynamic orbits. The use of STAR data for dynamic orbits allow a gain of 20 centimeters in term of Laser residuals and around 50 cm in terms of 3D comparisons to reduced-dynamic orbits. The gravity fields obtained with only 1.5 months of CHAMP observations are significantly improved by the use of the STAR accelerometer data.

New Global Gravity Field Models from Selected CHAMP Data Sets

Several sets of data were carefully selected out of the first 12 months of CHAMP GPS and STAR micro-accelerometer observations to derive a new model of the Earth’s static gravity field. These data have been dynamically processed as 63 orbital arcs, each of one to one and a half day duration. The approach consists of two steps: the GPS satellite orbits and clocks are first determined, then the CHAMP orbital arcs are adjusted using undifferenced phase and pseudo-range SST observables, and the residuals of those are used for retrieving the gravity harmonic coefficients. Micro-accelerometer, attitude and manoeuver information are taken into account to determine the surface accelerations as well as the residual thrusts acting on the spacecraft. Temporal variations of the gravity field are introduced in the observation equations for further usage in subsequent models, but are not here solved for. Several solutions have been derived, with or without additional information coming from the previous GRIM 5 satellite model, or from other laser satellite observations over a time period encompassing the CHAMP first year mission, with regularization (based on Kaula’s rule) and with different weighting factors. The solution which is presented is complete to degree and order 120, although a lack of power in the coefficients power spectrum is obvious above degree 40 approximately, as may be expected from a satellite solution, due to the orbit decreasing sensitivity to gravity at the present altitude. The quality of the field is assessed mainly through orbit determinations of several geodetic satellites, of CHAMP itself (especially with the residuals of the laser range measurements — not included in the gravity modelling process), and through comparisons with the geoidal surface derived from a mean altimetric sea surface — corrected for the ocean circulation, at a comparable resolution. A significant improvement in the long to medium wavelength harmonics of the model can already be seen as compared to previous satellite solutions.