The GNSS receiver should be able to record GCP coordinates with cm-level accuracy.
To obtain this level of accuracy, you need to use a dual-frequency receiver.
Furthermore, GNSS data should be differentially corrected either in real-time or in post-processing, i.e. the GNSS receiver should be capable of RTK correction using correction data received either from a base station nearby or a virtual reference station, over radio or from a NTRIP provider, or the GNSS receiver should log raw GNSS observables for use in PPK correction (done by the surveyor before sending the GCP coordinates to VITO).
A measurement pole should always be used to ensure good placement of the central position of the GNSS:
(Robotic) total station
To determine an absolute location of ground control points, a total station requires line of sight observations with a reflector pole held at the GCP locations while the total station itself is set up over a known point, or by making use of 2 line of sight observations with known points before proceeding to measure the GCPs. A typical total station can measure distances up to 1500 m with an accuracy of about 1.5 mm ± 2 ppm, and therefore enables approximately 10x higher relative accuracy (coherence between the GCP measurements) than can be expected from multiple GNSS measurements only. While the absolute geo-referencing accuracy of the whole block is the same as that of the reference point measurement made by GNSS, the much higher relative accuracy of total station measurements is ideally suited to process sub-cm GSD imagery to obtain pixel-level alignment between multi-temporal layers.
Measurements data format
Ensure the measurements are done according to the format requested for MapEO.