GPS signal loss on machine guidance systems is caused by: satellite obstruction (trees, buildings); high PDOP from poor satellite geometry; UHF radio interference or range exceeded (for base-and-rover systems); base station failure; or antenna damage. Diagnostic sequence: check the machine display for solution quality (Fixed vs Float vs none); check the base station transmit indicator; move the machine to an open area; check PDOP; verify base radio is transmitting on the correct channel.
Reading the System Diagnostics
Modern machine guidance systems display solution quality at all times:
| Display Status | Meaning | Action |
|---|---|---|
| Fixed RTK | Full accuracy (~0.02–0.05 ft) | Normal operations |
| Float | Degraded accuracy (~0.5–2.0 ft) | Do not grade to design surface; investigate |
| Autonomous / No Fix | No RTK corrections; positions are GPS only (~3–10 ft) | Stop precision work; check base station |
| No solution | Receiver not tracking satellites | Check antenna cable; move to open sky |
Diagnostic Sequence
- Check PDOP on the machine display. PDOP above 3.0–4.0 indicates poor satellite geometry — common in the early morning and late afternoon when satellite positions are low on the horizon. Wait for PDOP to improve.
- Check the base station display. Verify the base is Fixed (not Float), is transmitting corrections, and shows the expected number of satellites (typically 8+).
- Check the radio link. For UHF radio systems: verify the machine receiver and base transmitter are on the same channel. Check the radio display for signal strength. If signal is weak, the machine may be too far from the base or there may be interference from other equipment.
- Move the machine to open sky. If the machine is working near buildings or under a tree canopy, drive to an open area and check whether the fix returns.
- Restart the receiver. On many systems, a GPS signal outage that returns to Fixed after a restart was a momentary cycle-slip. Verify with a check shot after restart.
- Check antenna connections. A loose or damaged antenna cable is a common cause of persistent signal issues. Inspect the cable from the antenna on the blade mast to the receiver in the cab.
Multipath and Local Interference
Multipath occurs when the GPS signal reflects off a nearby surface (metal building, guardrail, stockpile) and creates a ghost signal that corrupts the position solution. Symptoms: Fixed solution but inconsistent cut/fill readings; machine appears to drift without moving. Solution: move the machine away from the reflective surface; change the antenna mounting height if the antenna is close to the blade or a metal surface.
When to Stop Work
If the machine guidance does not return to a Fixed solution after diagnostics, stop precision grading and switch to conventional grade staking with a laser and rod until the issue is resolved. Grading in Float mode to a ±0.10-ft toleranced design surface will fail verification. Do not try to “average out” Float mode errors by going slow — they are not random, they are systematic and will consistently point in the same wrong direction.