Precise Orbit Information Research Articles

Improved kHz-SLR tracking techniques and orbit quality analysis for LEO missions

Satellite gravity field missions such as CHAMP, GRACE and GOCE are designed as low Earth orbiting spacecraft (LEO) with orbit heights of about 250–500 km. The challenging mission objectives require a very precise knowledge of the satellite orbit position in space. For these missions precise orbit information is typically provided by GPS satellite-to-satellite tracking (SST) observations supported by satellite laser ranging (SLR). The role of SLR is primarily devoted to serve as an independent tracking instrument used to calibrate and validate the on-board GPS flight receiver. However, the very limited visibility of LEOs from SLR ground stations together with the accordingly high angular rates necessary for the laser mounting to follow the satellite make it more difficult to track LEO missions. At the Observatory Graz–Lustbühel, the Space Research Institute of the Austrian Academy of Sciences operates a very novel SLR facility which was continuously upgraded during the recent years, and is today the only station worldwide capable to operate at kHz-firing rates. The activities presented here focus on a number of hardware upgrades and methodical improvements at the SLR station Graz aiming for a faster and more reliable target acquisition. These include upgrades of laser tracking algorithms as well as a redesign of the laser detection package in particular for LEO spacecraft. These improvements allow an extension of the measurement durations and thus increase the number of observations per pass. As a result we are able to raise the normal point accuracy as well as the overall system performance for LEO tracking. Improvements of the pointing accuracy and the range gate control system lead to a data quantity raise of about 5%. Another task addresses both a geometric and dynamic arc comparison of SLR derived orbits with GPS SST orbit solutions. In the case of CHAMP, the resulting one-way SLR range residuals are in the order of a few centimetres. This allows to draw conclusions on the accuracy of orbit solutions. In order to evaluate the performance of this technique for the upcoming GOCE mission, investigations are carried out in an analogous manner based on simulated GOCE SLR observations. In this study, it is demonstrated that satellite laser ranging, in particular with high-rate tracking capabilities and low orbit optimizations, offers a valuable tool for orbit validation purposes.

GPS clock correction estimation for near real-time orbit determination applications

Precise orbit determination for low Earth orbiting (LEO) satellites using the global positioning system (GPS) is usually done in a post-processing mode introducing GPS satellite orbits and clock corrections. Depending on the mission objectives, the latency and required accuracy of precise LEO orbit information can vary significantly. Near real-time (NRT) orbits for certain geodetic missions (e.g. atmospheric sounding or altimetry) may require accuracy down to 10 cm (radial) or 0.1 mm s − 1 (along-track) and with a latency of less than 2–3 hours after the GPS measurements have been taken on the satellite. In order to fulfill such a demand in NRT, it is crucial that GPS orbit and clock products are available within the desired time frame and with adequate accuracy. In response to the demanding NRT processing for future geodetic satellite missions (i.e. ESA's Sentinel series), the Astronomical Institute of the University of Bern (AIUB) and Deutsches Zentrum für Luft- und Raumfahrt (DLR) have made an effort in finding an alternative approach to obtain high-rate NRT GPS orbit and clock solutions by way of GPS clock corrections estimation. A realistic NRT orbit determination (OD) scenario has been simulated in this joint-study assuming a radial orbit accuracy of 10 cm and of 3 hours latency for the orbit product. A summary of all currently available NRT GPS orbit and clock products and a description of the generation of NRT GPS clock corrections are given in detail in this paper. The estimated GPS clock corrections are validated with a kinematic precise point positioning (PPP) for static ground stations. For direct evaluation of this innovative approach on the LEO orbits, the resultant merged high-rate NRT GPS product was tested using GPS data from the GRACE (Gravity Recover And Climate Experiment) mission. The results from the NRT simulation show promising aspect of such a concept to achieve unprecedented LEO orbit accuracy in NRT.

Remote Synchronization System of Quasi-Zenith Satellites Using Multiple Positioning Signals for Feedback Control

We have developed a new control method for the QuasiZenith Satellite (QZS) remote synchronization system for an onboard crystal oscillator (RESSOX). The new method utilizes both L1 and L2 positioning signals of the QZS. We also found that precise orbit information and the estimation of delays, such as those caused by the ionosphere and troposphere, are not necessary to realize the RESSOX technology. The estimation of the delays of the L1-, L2- and Ku-band signals caused by the ionosphere and other sources can be calculated separately during feedback control, and the results of Ku-band delay are used to generate the RESSOX control signal. The simulation results showed that synchronization of within 3 ns between the ground site clock and the QZS site clock is achievable. Experimental results on the ground also confirmed that synchronization within 3 ns is feasible.

Derivation of the CHAMP-only global gravity field model TUG-CHAMP04 applying the energy integral approach

A global gravity field model TUG-CHAMP04, derived from CHAMP (CHAllenging Minisatellite Payload) satellite-to-satellite GPS tracking observations in the high-low mode (SST-hl) in combination with CHAMP accelerometry, is presented and described in detail in this paper. For this purpose the energy integral approach was applied to precise kinematic orbits and accelerometer data. The advantage of these kinds of orbits is that they are derived from purely geometrical information, hence no external gravity field information is used for the determination of the positions. The disadvantage of precise kinematic orbit information is, that no velocities are delivered and hence a procedure has to be elaborated to deduce the velocities from kinematic positions. This work is done in preparation for ESA’s GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite mission (scheduled launch November 2006), aiming at a high precision and high-resolution gravity field model on a global scale. This paper concentrates on the CHAMP data processing, where, in contrast to the usual standard method (processing in the Earth fixed frame), an approach in the inertial frame is chosen. Focus is taken on the data preprocessing of both accelerometer and orbit data, emphasising on the correct treatment of data-gaps and outlier detection. Furthermore an arc-wise weighting strategy is introduced and the advantages/disadvantages of this approach are discussed. Finally, the TUG-CHAMP04 model, calculated from one year of CHAMP data is compared with the official CHAMP gravity field model EIGEN-3p and terrestrial data (GPS levelling data).

The bering small vehicle asteroid mission concept.

The study of asteroids is traditionally performed by means of large Earth based telescopes, by means of which orbital elements and spectral properties are acquired. Space borne research, has so far been limited to a few occasional flybys and a couple of dedicated flights to a single selected target. Although the telescope based research offers precise orbital information, it is limited to the brighter, larger objects, and taxonomy as well as morphology resolution is limited. Conversely, dedicated missions offer detailed surface mapping in radar, visual, and prompt gamma, but only for a few selected targets. The dilemma obviously being the resolution versus distance and the statistics versus DeltaV requirements. Using advanced instrumentation and onboard autonomy, we have developed a space mission concept whose goal is to map the flux, size, and taxonomy distributions of asteroids. The main focus is on main belt objects, but the mission profile will enable mapping of objects inside the Earth orbit as well.

Assessment of pseudolite layout under urban environments using a simulation system for seamless positioning

Since the launch of the first Global Positioning System satellite, the demand for higher accuracy and wider applicability has been continuously increasing. In particular, personal navigation stresses requirements on “seamless” or continuous positioning with higher reliability are needed in urban areas. Unfortunately, urban canyons causing shadowing and multipath represent an adverse condition for Global Navigation Satellite System (GNSS) signal reception, thus leading to poor satellite visibility and low positioning accuracy. In order to overcome such problems, pseudolites can be set up to provide additional ranging signals and aid the carrier phase ambiguity resolution which is crucial for utilizing the full accuracy and wide range capability of GNSS. Pseudolites, which are ground-based instruments that transmit GPS-like signals, can improve the satellite-receiver layout and be used as additional range observations to improve the performance of a GPS-based deformation monitoring system. However, due to economic and environment constraints, the number of pseudolites that can be installed will be limited. More importantly, in order to reduce the areas affected by multi-path of pseudolite signals, locations and antenna pattern of pseudolites have to be carefully examined. This paper shows the development of a simulation system for evaluating the appropriateness and economics of installing pseudolites using precise orbital information of the satellite and a three-dimensional digital map.

A new algorithm for retrieving GPS radio occultation total electron content

A new algorithm for retrieving the relative ionospheric satellite‐to‐satellite total electron content during Global Positioning System (GPS) radio occultations is proposed. The algorithm consists of a linear combination of the L1 and L2 excess phase data using precise orbit information of the transmitter and receiver. The combination eliminates the effects of refractive bending and dispersion to first order. Simulations, representative of solar maximum conditions, show that using the “traditional” combination of the L1 and L2 phases, bending and dispersion may cause a systematic residual error of up to 20 TECU. Using only the L1 excess phase, the corresponding maximum residual error is about 7 TECU. The proposed combination results in a residual error due to bending and dispersion less than 0.1 TECU. Uncertainty in satellite orbit determination, as well as an unmodeled residual related to the geomagnetic field, result in a total systematic error of 1–2 TECU.

Filtered Geosat altimetry as a tool for global bathymetric chart development

Now that the Geosat altimetry and precise orbit information collected during the 1985–1986 Geodetic mission is declassified, the Naval Oceanographic Office is releasing its chart of Geosat altimetry of the world's oceans (Figure 1). This data set has been used at the Naval Oceanographic Office since 1987.

Radio tracking of artificial earth satellites

The accuracy of position determination required for the various purpo for which earth satellites may be used is discussed briefly. The capabilities of optical tracking methods are indicated.The three principal radio methods are those using radar, Doppler fre quency shift and interferometer techniques. Although radar has already been used to some extent in satellite tracking, its main use is likely to be in connection with manned satellites and the arrangement for the U.S. project Mercury is described. The use of the Doppler frequency shift of the signals received from a moving satellite may be utilized in the determination of its orbit and, provided full use is made of the observed data and allowances made for ionospheric refraction, very precise orbital information can be obtained, as illustrated by the development of the Transit navigation system.The tracking system in most general use at the present time is the inter ferometer in which directional information is obtained from measure ments of the phase difference between the satellite signals received on a spaced aerial system. The U.S. Minitrack system is of this type and the construction, operation and performance of the station installed at the Radio Research Station Winkfield Field Station are described in detail.The paper concludes with a brief review of future trends in satellite tracking by radio.