RTK Not Getting Fixed Solution? 9 Causes & Fixes (2026)

Symptom: Solution stays in Single and never reaches Float, even after waiting several minutes in open sky.

Cause: For CORS/NTRIP — the connection is not established. The server address, port, credentials, or APN may be wrong, or the SIM data connection is not active. For Base+Rover — the base station is not transmitting, or the radio channel and protocol on the base and rover do not match.

Fix: For CORS — open ApekSurv → Data Link → NTRIP Client and confirm status shows "Connected" and differential age is counting. If not connected, check SIM data signal, retype server address and credentials manually (never paste from email — hidden characters cause silent failures). For Base+Rover — confirm the base is powered on and transmitting, and that both base and rover are set to the same UHF channel and protocol. Single (not Float) almost always means zero corrections are arriving — fix the connection before anything else.

Symptom: Corrections are arriving (Float, not Single), but Float will not resolve to Fixed. The problem gets worse the further you move from the base or CORS station.

Cause: Beyond approximately 50 km from a CORS reference station, atmospheric decorrelation prevents reliable integer ambiguity resolution. For Base+Rover using a 2W UHF radio, the practical radio range limit is 8–15 km depending on terrain — beyond this, the correction link degrades or drops entirely.

Fix: For CORS — select a nearer mountpoint from the source table. Check source table coordinates to confirm which station is actually closest to your location. For Base+Rover — move the base station closer to the work area, or deploy an APEKS MAX5 base station with 5W LoRa radio for up to 25 km correction broadcast range on large remote project sites.

Symptom: Float when working near buildings, metal structures, vehicles, or large water surfaces. Fixed is achievable once you move to an open area away from these features.

Cause: GNSS signals bounce off reflective surfaces before reaching the antenna. The receiver tracks both the direct signal and the reflected copy, corrupting the carrier-phase measurement that RTK depends on for ambiguity resolution.

Fix: Move 3–5 metres away from the reflective surface and allow the receiver to reinitialise. Achieve Fixed in open sky first, then return to the work area. Full-constellation 1408-channel tracking (as on all APEKS receivers) helps maintain enough clean satellite signals even when some are contaminated by multipath — more usable satellites means multipath from a few satellites has less impact on the overall solution.

Symptom: Float at certain times of day even in open sky with no obstructions. Fixed may be achievable earlier or later in the day but not during a specific window.

Cause: When visible satellites are clustered in one part of the sky rather than distributed evenly, the geometric precision of the position calculation (expressed as PDOP — Position Dilution of Precision) is poor. High PDOP means weak geometry for carrier-phase ambiguity resolution.

Fix: Wait 15–30 minutes for satellite geometry to change as the constellation moves. Check PDOP in ApekSurv status — below 3 is good, above 6 makes Fixed resolution unreliable. Using a receiver that tracks all six constellations simultaneously (GPS, GLONASS, BeiDou, Galileo, QZSS, NavIC) maximises the number of visible satellites at any geometry, reducing PDOP and shortening the time-to-Fixed window.

Symptom: NTRIP shows Connected, differential age is below 3 seconds, but solution stays in Float for several minutes without resolving to Fixed.

Cause: An RTCM 2.x mountpoint delivers GPS-only corrections. If your receiver is tracking BeiDou, Galileo, and GLONASS (as all APEKS receivers do), the corrections for those constellations are absent — the receiver has satellite signals it cannot correct, creating ambiguity resolution instability. Alternatively, the selected mountpoint may be delivering a correction format (CMR, CMR+) that the receiver processes less efficiently than RTCM 3.x.

Fix: In the source table, select a mountpoint showing RTCM 3.2 or RTCM 3.3 format. These support multi-constellation corrections. Prefer VRS (Virtual Reference Station) or MAC (Master-Auxiliary Concept) mountpoints where available — these are computed corrections tailored to your specific rover location, rather than raw corrections from a single distant physical station.

Symptom: Float under tree canopy, in deep trenches, near tall buildings, or under scaffolding. Fixed is achievable in adjacent open areas.

Cause: RTK requires a minimum number of satellites tracked on multiple frequencies to resolve integer ambiguities. Physical obstruction of the sky reduces the number and quality of tracked signals below the threshold needed for Fixed resolution.

Fix: Move to a clearer sky view to initialise a Fixed solution first, then return to the obstructed area. Once Fixed is achieved, the receiver maintains it under moderate obstruction — the key is getting Fixed in the open before entering the obstructed zone. For points that cannot be reached with a vertical pole under dense obstruction, use a receiver with 120° calibration-free IMU (such as the APEKS AP40 Laser+) to measure the point with the pole tilted. For areas with permanent GNSS unavailability (indoors, deep shafts, tunnels), a total station is the correct instrument.

Symptom: The NTRIP connection was working on previous visits to the same site, but now returns an authentication failure (401 error) or the source table fails to load. Solution stays in Single.

Cause: CORS subscriptions are time-limited. An expired subscription causes silent authentication failure — the receiver appears to try to connect but receives no corrections. Credentials entered with a trailing space or incorrect capitalisation produce the same symptom.

Fix: Check subscription status in the CORS network portal before mobilising to site. Renew at least 5 working days before the project start date. Delete and retype credentials character by character — do not paste. As an immediate field contingency, deploy any APEKS receiver as a local base station to continue working without CORS while the subscription issue is resolved.

Symptom: NTRIP shows Connected and Fixed is achieved, but the solution is unstable — dropping in and out of Float. Differential age regularly spikes above 3 seconds.

Cause: Poor cellular signal at the survey location is delaying the NTRIP correction stream. The receiver is receiving corrections, but they arrive too late to be applied before the next observation epoch. The receiver extrapolates corrections beyond their valid age, degrading solution stability.

Fix: Check cellular signal strength in ApekSurv → Device Status. Switch to a different mountpoint on the same CORS network — oversubscribed mountpoints also produce high differential age. If cellular coverage at the site is genuinely marginal, switch to a local Base+Rover setup via UHF radio. UHF radio corrections have no internet dependency and maintain consistent sub-1-second differential age regardless of cellular signal.

Symptom: The receiver shows Fixed with good differential age and PDOP, but measured coordinates do not match known control points — offset by metres in a consistent direction across the entire site.

Cause: This is not a Fixed solution problem — the receiver IS in Fixed. The problem is that the wrong datum, projection, or geoid model is selected in ApekSurv. A Fixed solution confirms that carrier-phase ambiguities are resolved correctly. It does NOT confirm that the coordinate system is correct. Common errors: WGS84 selected instead of the national datum (GCS-80 in Saudi Arabia, Hartebeesthoek94 in South Africa, SIRGAS2000 in Brazil), wrong UTM zone, or missing geoid model causing systematic elevation offsets.

Fix: This cause is included here because it is frequently misdiagnosed as a signal or connection problem. Before recording any data on a new project, navigate to a known physical control monument and verify the displayed coordinate matches the registered value within 20 mm. If it does not, the problem is in the coordinate system settings — fix the datum, projection, and geoid model in ApekSurv before staking a single point.