Orbit Determination Error Analysis for a future Space Debris Tracking Radar

Kurzfassung

Independent of light and weather conditions radar systems provide observations of space debris in low and medium earth orbits. In general two operational principles can be distinguished - in survey mode all objects passing through the radar instrument’s field of view and above a certain size threshold are detected; subject to the tracking mode is the task to collect positional information on an already detected object. A recently proposed radar concept balances survey and tracking performance in a novel way by combining a mechanical steerable reflector antenna with digital beam-forming techniques. The utilization of multiple digital feed elements allows illumination of a larger survey zone, compared to the relative narrow pencil beam of conventional tracking radars. Signal processing from independent digital channels preserves a high antenna gain on the reception path. It is also a prerequisite for the implementation of an advanced Track While Scan operational mode. During design phase an optimal combination of surveillance and tracking functionality will have to be found. As the number of simultaneously observable space debris is directly linked to the angular extension of the survey zone plus the amount of transmitted/received power, it is straightforward to define system requirements to meet an envisaged survey performance. Moreover, it is less obvious to characterize tracking data that lead to reliable and accurate orbit determination results. Variables of influence are the type of involved measurement data, their temporal and geometrical distribution as well as systematic measurement errors, e.g. measurement bias and noise, or erroneous measurement correction and force modelling. The amount of error introduced by each variable can be quantized in a so called Consider Covariance Analysis. This paper presents the outcomes of this kind of orbit determination error analysis. Qualitative simulations for a small set of tracking scenarios serve as a starting point for the establishment of tracking requirements for a possible future space debris radar located at DLR ground station in Weilheim. The analyzed tracking scenarios are representative for three satellite missions currently operated at German Space Operation Centre (GSOC) and take into account the timeliness constrains inhered from a tracking request for refined orbit determination in case of a conjunction warning. Based on analysis findings the number of station passes, space debris can be tracked on, is investigated in more detail and an analytical formula is given for the optimization of minimum elevation angle.