Navigation Tracking with Multiple Baselines Part I: High-Level Theory and System Concepts

Abstract

Delta Differential One-Way Ranging (ADOR) and Same Beam Interferometry (SBI) are deep space tracking techniques that use two widely separated ground antennas, known as a baseline, to simultaneously track a transmitting spacecraft to measure the time difference between signals arriving at the two stations. Errors are introduced into the delay measurements when the radio waves pass through the solar plasma and the Earth s atmosphere, and also due to clock bias and clock instability of the ground stations. These errors can be eliminated or calibrated by tracking a quasar in the angular vicinity of the spacecraft (ADOR), or by tracking another close-by spacecraft whose trajectory/orbit is accurately known (SBI). Both ADOR and SBI uses this double-differencing of signal arrival time to eliminate the aforementioned error sources, and to generates highly accurate angular measurements with respect to the baseline. The three Deep Space Network (DSN) sites cover three approximately equally-spaced longitudes to provide near-continuous coverage of deep space. Spacecraft occasionally see two DSN sites simultaneously, but never three. Current DSN s ΔDOR and SBI techniques are based on one baseline of two sites. But recent additions of non-DSN deep space antennas and increased cross-support collaborations between space agencies allow spacecraft seeing more than one baselines simultaneously. In this paper, we consider simultaneous SBI of two or more baselines that share one common ground station. We show that under certain condition, precise pointing vector between the common ground station and the spacecraft can be computed using simultaneous ADOR measurements from the two baselines. When there is another spacecraft in the vicinity of the first spacecraft, precise pointing vector of the second spacecraft can be derived from the simultaneous SBI measurements. Also, precise angular distance between the two spacecraft can be computed in real-time. We expect these new data types could enhance ground antenna pointing, and deep space spacecraft traj ectory estimation and orbit determination. This technique can have near-Earth applications. We describe a system concept that detects and locates dead and noncooperative spacecraft in the Geostationary Orbit (GEO). This can be done by making use of an existing GEO satellite with accurately known location as a reference, and/or by placing a dedicated “reference” spacecraft into an eccentric geosynchronous orbit over a region of interest (e.g. above North America). By adjusting the orbit, the “reference” spacecraft can sweep through the sky back-and-forth in the vicinity of the GEO over the region. In this way, the reference spacecraft can be close to any “static” GEO targets along its path. Using the multi-static radar approach, the ground transmitting radar illuminates both the reference and target spacecraft, and the ground receiving radars measure the different time-delays of signal arrival. Applying a variant of the aforementioned simultaneous SBI scheme, the time-delay measurements can be used to compute the precise relative position of the target spacecraft with respect to the reference spacecraft, whose position can be accurately estimated using the weak Global Positioning Satellite (GPS) signals.