Navigation is one of the key enablers for autonomous air vehicle operations for urban air mobility vehicles and unmanned aircraft systems (UAS). The most critical phases of vertical take-off and landing (VTOL) vehicle autonomous flight, especially in terms of navigation accuracy and integrity requirements, are the approach and landing phases in complex/urban terrain surrounding highly constrained sites.
The navigation solution for these phases assumes hybridization of various sensors and systems to create solutions that meet stringent navigation accuracy and integrity requirements in GNSS available and denied environments. One potential sensor set for the navigation system includes a radar altimeter with extended capability to provide not only 1-dimensional range measurements to the ground but full 3-dimensional position measurements. This can be achieved with the help of dedicated ground-based beacons at known positions, using the multilateration method and advanced detection algorithms. The beacons can be either passive or active.
This paper introduces a novel concept utilizing onboard radar altimeter(s) and active Radio Frequency (RF) repeaters on the ground. In contrast to passive reflectors that just reflect the radar signal, active repeaters can each modify and retransmit the incoming signal to get unique echoes. Signal modification, with possible amplification, delay, and transmission of multiple copies of the received signal, overcomes the issues with reflector’s installation and identification. As a result, the radar altimeter will receive not only the reflected signal from the ground but additional unique signal from each repeater. Thus, the onboard radar altimeter detects the surface and the artificial echoes. Unique sets of artificial echoes can be used to enable detection and unique identification of individual repeaters and landing zones.
This contribution describes the novel navigation concept and UAS flight test experiments with the repeaters. Flight tests included three repeater prototypes with known positions relative to the landing zone.
