UAS flight crew member reported GPS interference errors with several UAS during the flight and chose to land all of the UAS.

Narrative

The purpose of this report is to describe a GPS quality problem to other operators using sUA that rely on GPS with RTK (Real-time Kinetic) positioning. The issue described occurs approximately 350 ft. southwest of [the building] but could happen anywhere when operating in proximity to large reflective structures.On pre-flight; all XX drones checked ok: good heading; good position (within < 1 ft.); good data link; everything. Approximately 15 minutes prior to planned takeoff; the GPS position became erratic on 3 drones (varying anywhere from 2-10 ft.). Multiple attempts to resolve the issue were accomplished over the next hour. When the positions became reasonable; it was hoped that the RTK position would resolve when airborne. One flight was accomplished where XY of the XX drones had near perfect position; however; 1 drone was about 1 ft.-2 ft. off and another remained about 10 ft. off. The minimum drone separation for this show was 6 ft. The 2 drones out of position caused a bad looking show and increased the risk of collision. All XX drones landed without incident and there was never any risk of the drones hitting even the first safety zone. The second planned flight was canceled with concurrence by RPIC and Safety Pilot.On the following day; a large area was surveyed in the vicinity using s GPS Test unit for over 3 hours. The GPS Test unit was set to use only the GPS constellation (same as our sUA) without any augmentation (no SBAS; no RTK). Except for the following statement; the GPS accuracy was consistently reporting within 12 ft. However; when within 400 ft. south of [the building]; the GPS accuracy would randomly jump between 12 ft. and 35 ft. about 50% of the time. This is most probably a multipathing issue with reflections of the nearby building. Had we done this GPS prior to planning this Launch Recovery Zone; we would most likely have detected the issue and replanned an alternate location for the flight.In the future; our operation will include GPS observation time as part of the site survey. We have data link spectrum analysis as part of our site survey procedures but now will also include detailed procedures on GPS quality; especially when large reflective structures are present.