Satellite gravimetry with small satellites: design and analysis of a pointing, acquisition and tracking unit for a ranging payload

Abstract Satellite gravimetry, the so-called satellite-to-satellite tracking in low-low mode (ll-SST), determines the Earth's gravitational field from changes in the distance between two satellites orbiting the Earth. Previous missions have used conventional satellites measuring several meters in size, which incur correspondingly high costs. Small satellites are being targeted as a cost-effective solution. Using multiple small satellite pairs can reduce temporal aliasing, a current main source of error in gravitational data. With the innovative Dynamic Optical Ranging & Timing (DORT) technology, a measurement technique is now available for the first time that has the necessary ranging resolution for ll-SST and is suitable for implementation in small satellites. This article provides a detailed description of the laser-based DORT ranging payload architecture. A one-way design is used, i.e., the transmitted and received laser beams use the same aperture. In addition to the actual DORT ranging system, the ranging payload includes an optical amplifier with > 20 dB to compensate for transmission losses between the satellites, a pointing, acquisition, and tracking unit, that establishes and maintains the optical link between the two satellites during operation, and the corresponding control and communication electronics. The focus of the work is on the design of the pointing, acquisition, and tracking unit. This includes a collimator, a 5 mm diameter fast steering mirror, a quadrant photodiode, and a beam expander with 10 × magnification. A tolerance analysis is performed for the optical components. Tolerances of 10 µm displacement on the image plane fiber per 1° tilt or 0.5 µm displacement on the image plane per 1 µm displacement are determined. In order to comply with these tolerances, the use of ultra-low-expansion glass or a 1 K temperature stabilization is necessary. Further steps like a system design of the small satellite to analyze the interactions with other components (accelerometer, GNSS receiver) are necessary.