Eberhard Gill

Ionospheric Correction for GPS Tracking of LEO Satellites

This paper describes an ionospheric correction technique for single frequency GPS measurements from satellites in low Earth orbit. The fractional total electron content (TEC) above the receiver altitude is obtained from global TEC maps of the International GPS Service network and an altitude dependent scale factor. By choosing a suitable eective height of the residual ionosphere, the resulting path delay for positive elevations is then computed from a thin layer approximation. The scale factor can be predicted from the assumption of a Chapman profile for the altitude variation of the electron density or adjusted as a free parameter in the processing of an extended set of single frequency measurements. The suitability of the proposed model is assessed by comparison with flight data from the Champ satellite that orbits the Earth at an altitude of 450 km. For the given test case, a 90% correction of the ionospheric error is achieved in a reduced dynamic orbit determination based on single frequency C}A-code measurements. .

State interpolation for on-board navigation systems

Abstract Common concepts for autonomous on-board navigation systems rely on the numerical integration of a spacecraft trajectory between subsequent measurements of a navigation sensor such as GPS. In combination with a Kalman filter, a predicted state vector becomes available at discrete, but not necessarily equidistant time steps. When used for real-time attitude control or geo-coding of image data, the on-board navigation system has to provide continuous dense output at equidistant time steps, which usually conflicts with the natural stepsize of the relevant integration methods and the non-equidistant measurement times. To cope with this problem, the integrator has to be supplemented by an interpolation scheme of compatible order and accuracy. After presenting a representative formulation of an on-board navigation system and deriving related timing and accuracy requirements, suitable Runge–Kutta methods and associated interpolants are selected and evaluated. Promising results are obtained for the classical RK4 method in combination with Richardson extrapolation and 5th-order Hermite interpolation. The 5th-order Fehlberg method with interpolation due to Enright and, for drag-free scenarios, the 5th-order Runge–Kutta–Nystrom method with 5th-order Hermite interpolation provide a good performance in terms of position interpolation. However, as both methods exhibit significant errors for the velocity interpolation, they are not recommended for use with the outlined navigation filter.

New approach for combined bundle block adjustment and orbit determination based on Mars-94 three-line scanner imagery and radio-tracking data

The high resolution stereo camera (HRSC) and the wide-angle optoelectronic stereo scanner (WAOSS) onboard the Russian Mars-94 spacecraft provide permanent stereo capability using the 3-line stereo concept. The photogrammetric bundle adjustment, which includes the reconstruction of the exterior orientation of the 3-line scanner imagery, is the prerequisite for all subsequently derived products (e.g., DTM, orthoimages). In order to properly utilize the image information contained in the conjugate point coordinates and the orbit information contained in the radio tracking data, both data types have to be evaluated in a combined adjustment process. To this end, the conventional bundle block adjustment algorithm is supplemented by a rigorous dynamic modeling of the spacecraft motion to account for long-arc orbital constraints. The camera position parameters, which have been estimated so far at certain time intervals, are now replaced by the components of the spacecraft epoch state vector and trajectory model parameters that may comprise the Mars rotation rate or solar radiation pressure coefficients. The focus of the paper is given to the combined approach, its mathematical model, and its main advantages.

AN INTERFEROMETRIC SAR SATELLITE MISSION

The paper provides a critical review of the achievements in SAR interferometry from the ERS mission as well as from the Shuttle Radar Topography Mission SRTM. It describes the development from the original idea of the Interferometric Cartwheel to the concept of a formation flight of identical and active SAR satellites. From the experience gained from ERS and SRTM interferometric data processing as well as from the analysis of the Cartwheel concept a list of mission requirements has been set up. The most demanding one is the autonomous configuration flight of a tight x-band constellation, where the satellites fly as close as up to 30 m with a dead-band of +/- 10 m. The guidance, navigation and control considerations come to the conclusion that such a mission is feasible.

Miniflex satellite concept for precursor and chemical missions

An upward trend in the development of microsatellites has been observed during the past few years mainly due to lower development and production costs and the wide range of applications. In order to assure technology know how for future missions Astrium GmbH has developed a MiniFlex Satellite Concept in the class from 80 kg to 250 kg derived from the Flexbus philosophy. The MiniFlex Satellite System is a modular concept that enables the design and manufacturing of tailor made solutions for specific missions, technically based on a reference architecture and a fixed set of recurring hardware elements (core elements). The classical subsystem or unit level design has been broken down to fuctional block levels (PCB´s). Potential fields of application are precursur missions in the field of constellation/formation flying, inter-satellite communication, verification of distributed system functions and on-board autonomy. This paper outlines the major features of the MiniFlex Satellite and describes an application example.