Grzegorz Michalak

Recent Developments in CHAMP Orbit Determination at GFZ

The GeoForschungsZentrum Potsdam (GFZ) operationally provides CHAMP orbit products for various purposes. Here the rapid and ultra-rapid orbits are highlighted. Significant developments in Precise Orbit Determination (POD) for Low Earth Orbiters (LEOs), in particular SAC-C and GRACE besides CHAMP, are described. GFZ also started to generate CHAMP-like rapid orbits for SAC-C with good accuracy. Furtheron improved LEO orbit accuracies are demonstrated by simultaneous orbit solutions of the GPS satellites and one or more LEOs in an integrated approach.

CHAMP Rapid Science Orbit Determination - Status and Future Prospects

The CHAMP Rapid Science Orbit (RSO) is produced to support the atmospheric and ionospheric occultation processors as well as the magnetic/electric field analysis system of the CHAMP Science Data System. The RSO is computed using dynamical models in a two step approach. First the orbits and clocks of the GPS satellites are estimated, then the CHAMP orbit is determined by fixing the estimated GPS orbits and clocks. For quality control external GPS orbits and CHAMP laser observations are used. The GPS orbits are delivered with 14 h latency and with an accuracy of about 10 cm in 1-D position, while the CHAMP orbits are delivered with 16 h latency and an accuracy of 5 cm in 1-D position. For the future it is planned to decrease the RSO latency for use in a numerical weather prediction environment.

Monte Carlo simulations of the LARES space experiment to test General Relativity and fundamental physics

The LARES (LAser RElativity Satellite) satellite was successfully launched in February 2012. The LARES space experiment is based on the orbital determinations of the laser ranged satellites LARES, LAGEOS (LAser GEOdynamics Satellite) and LAGEOS 2 together with the determination of the Earth?s gravity field by the GRACE (Gravity Recovery And Climate Experiment) mission. It will test some fundamental physics predictions and provide accurate measurements of the frame-dragging effect predicted by Einstein?s theory of General Relativity. By 100 Monte Carlo simulations of the LARES experiment, with simulations of the orbits of LARES, LAGEOS and LAGEOS 2 according to the latest GRACE gravity field determinations, we found that the systematic errors in the measurement of frame-dragging amount to about 1.4% of the general relativistic effect, confirming previous error analyses.

Remarks on CHAMP Orbit Products

The GeoForschungsZentrum Potsdam (GFZ) runs an operational system for the CHAMP mission that provides precise orbits on a regular basis. Focus is put on recent analyses and achievements for the Rapid and Ultra-rapid Science Orbits.

GFZ RL05: An Improved Time-Series of Monthly GRACE Gravity Field Solutions

After publishing its release 04 (RL04) time-series of monthly GRACE gravity field solutions starting end of 2006, GFZ has reprocessed this time-series based on numerous changes covering reprocessed instrument data, observation and background models as well as updated processing environment and standards. The resulting GFZ RL05 time-series features significant improvements of about a factor of two compared to its precursor. By analyzing 72 monthly solutions for the time span 2005 till 2010, a remarkable noise reduction and a noticeably higher spatial resolution become obvious. The error level has significantly decreased and is now only about a factor of six above the pre-launch simulated baseline accuracy. GFZ RL05 solutions are publically available at ISDC and PO.DAAC archives.

The Tracking, Occultation and Ranging (TOR) instrument onboard TerraSAR-X and on TanDEM-X

TerraSAR-X, to be launched at the end of May, 2007, carries the tracking, occultation and ranging (TOR) Category A payload instrument package. The TOR consists of a high-precision dual-frequency GPS receiver, called integrated GPS occultation receiver (IGOR), for precise orbit determination and atmospheric sounding and a laser retro-reflector (LRR) serving as target for the global satellite laser ranging (SLR) ground station network. The TOR is supplied by the GeoForschungsZentrum Potsdam (GFZ) Germany, and the Center for Space Research (CSR), Austin, Texas. The objective of the German/US collaboration is twofold: provision of atmospheric profiles for use in numerical weather predictions and climate studies from the occultation data and precision SAR data processing based on precise orbits and atmospheric products. GFZ also supplies the TOR instrumentation for the follow-on mission TanDEM-X. For the scientific objectives of the TanDEM- X mission, i.e. bi-static SAR together with TerraSAR-X, the dual- frequency GPS receiver is of vital importance for the millimeter level determination of the baseline between the two spacecrafts. In the following, the TOR instrumentation is characterized by the features of and accuracies achievable with the GPS receiver and the LRR. The data flow is shown, as GFZ operates a high latitude data receiving station for fast data access, and runs a data system for preprocessing, archiving and retrieval. With this data system, higher level products are generated for TerraSAR- X, including near real-time orbits by GFZ and post-processed precise science orbits by GFZ and CSR. In the case of TanDEM- X, GFZ operationally determines the inter-satellite baseline on the millimeter level. CSR produces calibration orbits to support the GFZ operational activity. Orbit results in terms of latency and accuracy from current missions are given and achievable results for TerraSAR-X and TanDEM-X are discussed.

Atmospheric Profiling with CHAMP: Status of the Operational Data Analysis, Validation of the Recent Data Products and Future Prospects

Atmospheric sounding with the German CHAMP (CHAllenging Minisatellite Payload) satellite is successfully performed since February 2001. In total ∼145,000 precise globally distributed vertical profiles of refractivity, temperature and water vapor were provided as of April 2004. The operational occultation infrastructure from GFZ allows for the demonstration of Near-Real Time (NRT) data analysis since February 2003. An average delay of ∼5 hours between each measurement and provision of corresponding analysis results is continuously reached. A comparison with more than 10,000 radiosonde measurements shows nearly biasfree refractivity and temperature between ∼7 and ∼30 km. The standard deviation is ∼1 % and ∼2 K, respectively. Data of the SAC-C (Satelite de Aplicaciones Cientificas-C) occultation mission are used to prepare for multi-satellite capability of the operational data analysis system. Future prospects of the provision of occultation analysis results at GFZ are given.

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and GRACE

Precise and rapidly available orbits of GPS and Low-Earth-Orbiting (LEO) satellites are the prerequisite for processing of the radio occultation data from CHAMP, GRACE and other LEOs performing occultation measurements. For efficient occultation data assimilation by the weather prediction systems a 3 h timeline is required. In 2002 GFZ has started to generate orbits at fixed 3 h intervals with a mean latency of 2.2 h, so-called Ultra-rapid Science Orbits (USOs). The resulting delay of the occultation products was in the range 3–5 h, sufficient for building an operational radio occultation system and for performing data assimilation studies in weather centers. To better meet the 3 h timeline, a new Near-Real Time (NRT) orbit processing system was developed to generate GPS and LEO orbits every 1.5 h with low latency, typically less than 0.5 h. This reduces the average delay for occultation products to 1 h 45 min. The NRT system consists of three independent chains generating LEO orbits with latencies ranging from 13 to 30 min and accuracies in the range 6–10 cm validated by Satellite Laser Ranging (SLR).

Improvements for the CHAMP and GRACE Observation Model

In this chapter the theory and details of the implementation and validation of the GPS carrier phase wind-up correction and the GPS attitude model are presented. It is shown, that a correction of the carrier phase data due to the wind-up effect improves the GPS orbit fit together with the integer ambiguity fixing by 1–2 cm. It is also demonstrated, that changing the definition of the X-axis of the Block IIR satellites (towards Sun as for Block IIA) has no significant influence on orbits and clocks when integer ambiguity fixing is applied, since the difference due to the different axes definitions is completely absorbed by floating L3 ambiguities. The phase wind-up correction was also applied for one week of data for the Low Earth Orbiters CHAMP, GRACE-A and TerraSAR-X. The orbit improvement, measured by Satellite Laser Ranging (SLR) residuals is noticeable and amounts to 3 mm globally. The description of the GPS attitude model, including shadow crossing and post-shadow manoeuvre, noon and midnight turns is given in detail. It is demonstrated, that incorrect attitude modelling can cause yaw angle errors exceeding even the size of one full rotation of the satellite during the shadow crossing.

Precise Orbit and Baseline Determination for TerraSAR-X and TanDEM-X

TerraSAR-X (TSX), the recent German radar satellite launched on June 15, 2007, carries as primary instrument a synthetic aperture radar (SAR) sensor. The GFZ German Research Centre for Goesciences and the Center for Space Research (CSR) at the University of Texas in Austin, USA, provided the Tracking, Occultation and Ranging Category A instrument package (TOR) to the TSX mission to enhance the quality of the scientific SAR products and to collect occultation measurements for atmospheric/ionospheric sounding. The TOR consists of a precise dual frequency GPS flight receiver (Integrated GPS and Occultation Receiver - IGOR) and a Laser retro- reflector (LRR) that supports Satellite Laser Ranging (SLR) observations from the Earths surface. Also the CHAMP and GRACE satellites are equipped with the LRR originally developed for CHAMP by GFZ. TanDEM-X (TDX), a twin to TSX, to be launched in September 2009 will fly with TSX in a tandem orbit configuration. Its compatible SAR system enables bistatic application for the derivation of digital elevation models (DEMs). For TDX, GFZ again supplies the TOR payload. The IGORs on both satellites will become the key to the continuous determination of the baseline (the distance of the two individual SAR antennae) with millimeter accuracies. In this paper, we concentrate on results gained so far for TSX precise orbit determination (POD) and on the status of our preparations for precise baseline determination (PBD) for the tandem mission.