Trimble GCS900 No Correction Source: What It Means and How to Fix It

Applies to: Trimble GCS900 with CB460 display and MC-R3 GNSS receiver

What Does "No Correction Source" Mean?

The Trimble GCS900 requires RTK correction data from either a local base station (via UHF radio) or an NTRIP network (via cellular modem) to achieve the 10–20mm machine control accuracy needed for grade work. "No correction source" means this correction data stream has been interrupted — the receiver may still track satellites and provide a position, but without corrections it falls to autonomous or SBAS accuracy of 0.5–3m, which is completely inadequate for machine control.

Common Causes of No Correction Source

• Base station powered off or battery dead — the field base is not transmitting corrections
• UHF radio link blocked or out of range — machine has moved too far from base, or obstruction in radio path
• UHF antenna damaged — radio antenna on base or machine is broken or disconnected
• NTRIP connection lost — cellular signal dropped, NTRIP credentials expired, or VRS network outage
• Radio frequency mismatch — base and rover configured to different UHF channels
• Base station coordinates wrong — corrections technically received but geometrically incorrect

How to Fix GCS900 No Correction Source — Step by Step

1. Stop machine control operations immediately — do not grade with no correction source active
2. Check base station power — is it powered on, battery charged, and transmitting?
3. Check UHF radio link — verify base and machine are on the same UHF channel in CB460 settings
4. Check radio antenna connections — verify UHF antenna cable is connected at base and on MC-R3 receiver
5. If using NTRIP: check cellular signal on the CB460's modem, verify NTRIP credentials are current
6. Move the machine closer to the base station to test whether it's a range/obstruction issue
7. Power cycle the base station and allow 60 seconds for it to reacquire before rechecking CB460
8. If using a VRS network: call the NTRIP provider to check for service outages in your area

When to Send It In for Service

If the radio link is confirmed working (correct channel, close range, no obstruction) and the CB460 still shows no correction source, the UHF radio module in the MC-R3 receiver may have failed. This requires service center diagnosis. The GNSS receiver itself can be tested by substituting a different MC-R3 unit.

Preventing No Correction Source

Before mobilizing the machine: verify base station is operating and corrections are being received by the CB460 at the base location. Check base battery charge at the start of each shift. Monitor the CB460 correction age indicator during operation — corrections aging beyond 10 seconds indicate a weak or failing link.

Related Error Codes

Trimble GCS900 Full Guide | Trimble GPS ERR001