8. GNSS Positioning and Modern Survey Technology
GNSS Positioning and Modern Survey Technology
This domain tests satellite positioning principles, error sources, observation modes and the integration of GNSS with total stations and digital workflows. Expect accuracy statements, coordinate frames and field procedures.
Core rules
- GNSS has space, control and user segments; pseudorange and carrier-phase observations have different precision.
- Errors include ionosphere, troposphere, multipath, satellite clock, orbit and receiver noise; differencing reduces common errors.
- RTK provides centimetre-level positioning over short baselines; static and PPP suit longer occupations or remote control.
- GNSS heights are ellipsoidal; convert to orthometric heights with a geoid model.
- A modern survey workflow ties GNSS control to total-station detail and checks redundant observations.
Worked example
An RTK specification is 10 mm + 1 ppm horizontally. At a 5 km baseline, the ppm term is 1 mm/km x 5 km = 5 mm. Total expected accuracy is 10 + 5 = 15 mm. If the manufacturer states 10 mm + 1 ppm and the baseline is 12 km, the term is 12 mm, giving 22 mm. Do not quote the 10 mm alone for a long baseline.
Traps
Using RTK ellipsoidal heights as levelled heights; ignoring multipath under canopy; treating a fixed solution as error-free; mixing datum epochs; failing to check a known point.