To set up a GPS/GNSS base and rover: set the base station over a known control point with the antenna height measured precisely; configure the base to broadcast corrections; initialize the rover over a known point to verify RTK fix quality; run a check shot on a second known control point before starting layout or as-built work. If the check shot is off by more than the accuracy specification, do not proceed — find the error first.
Base Station Setup
- Center and level the base over the control point. Use the tripod and tribrach. Measure the antenna height precisely — this is the distance from the control point monument to the antenna reference point (ARP), typically the bottom of the antenna or the phase center. Use the correct measurement method for your antenna (slant height or vertical height; check the manufacturer manual).
- Enter the control point coordinates and height. If using a known point, enter the exact GNSS coordinates (latitude/longitude/ellipsoid height or Northing/Easting/Elevation in project coordinates).
- Configure the correction broadcast. Set the base to broadcast RTK corrections via radio, cellular, or Bluetooth. Most modern bases use UHF radio for short ranges and cell modem (NTRIP) for longer ranges or VRS networks.
- Confirm the base is transmitting. The base display should show satellites acquired, solution quality (fixed or float), and broadcast active.
Rover Initialization
- Initialize over a known point. Hold the rover pole on a known control point and let the receiver acquire a fixed RTK solution. A fixed RTK solution is accurate to ~0.02 ft (6–12 mm); a float solution is not reliable for construction layout or as-built work. Wait for “Fixed” before proceeding.
- Run a check shot. After initialization, move the rover to a second known control point and measure its coordinates. Compare to the published values. Horizontal check should be within 0.05 ft (or the project accuracy spec); vertical within 0.05–0.10 ft depending on the spec.
- Verify PDOP is acceptable. Position Dilution of Precision (PDOP) should be below 3.0 for reliable results. Values above 4.0 indicate poor satellite geometry — wait for the constellation to improve before doing precision work.
Common Issues and Solutions
| Problem | Likely Cause | Fix |
|---|---|---|
| Rover will not achieve Fixed solution | Multipath from buildings/trees; poor satellite geometry; radio interference | Move to open area; wait for better PDOP; check radio channel |
| Check shot off by 0.10+ ft horizontal | Wrong base coordinates entered; wrong antenna height; base moved | Re-verify base setup; re-enter coordinates; re-initialize rover |
| Check shot off by 0.15+ ft vertical | Wrong antenna height; geoid model mismatch | Re-measure antenna height; verify geoid model matches project datum |
| Solution drops to Float during work | Satellite obstruction (trees, building edge) | Move to open sky; re-initialize after returning to open area |
When to Use GPS vs Total Station vs Level
| Method | Best For | Limitation |
|---|---|---|
| GPS/GNSS rover | Layout in open areas; as-built shots over large areas; real-time coordinate checking | Needs open sky; ±0.02–0.05 ft vertical accuracy |
| Total station | Layout in obstructed areas; high-precision work; stakeout with coordinate file | Needs line of sight to backsight; slower on large areas |
| Automatic level + rod | Differential leveling; grade checks; compaction area surveys | Relative elevations only; needs benchmark; manual record |
See also: GPS/GNSS Rover Layout: Setting Points from a Coordinate File and Machine Guidance Grade Control Overview.