Evaluating differential GNSS techniques for landing the 1st stage of an RLV – with a skydiver experiment

Abstract

Reusable launch vehicles (RLV) in a vertical take-off vertical landing configuration require accurate measurements of the height above the ground during the final descent just before landing. It is proposed to use real-time kinematic positioning (RTK), which is a differential GNSS navigation method requiring a nearby reference GNSS antenna and receiver, to measure the rover’s position with centimetre accuracy. The effects of the engine plume, vibration and multipath are challenging when using this carrier-phase based positioning method on an RLV. Especially the residual differential tropospheric error due to the rover and reference GNSS antennas being at different heights, primarily caused by humidity in the air, may hinder the successful fixing of the carrier phase ambiguities. To gain a better understanding of the RTK performance to be expected on a landing RLV, a skydiver experiment was conducted in April 2022. The skydiver was equipped with a rover GNSS receiver and a helmet with a GNSS antenna on top and an independent integrated navigation system serving as reference. The skydiver reached free fall velocities of up to 385 km/h, which is seen representative of the vertical velocity of an RLV. The paper describes the experiment procedure and the results of the experiment.