Posting this because it explains a failure I have seen reported from growers in Brazil and other equatorial regions, and it is easy to mistake for a base-station or radio problem.
The pattern: RTK is fine all afternoon, then around sunset the fix wanders, drops to float, and recovers by morning. Nothing is wrong with the base, the corrections or the tractor wiring.
The cause is the ionosphere. GNSS signals cross the 100-1000 km charged layer; when electron density becomes irregular, the signal is distorted in phase and amplitude - ionospheric scintillation. It is worst near the geomagnetic equator, follows the 11-year solar cycle, and has a daily rhythm: sunset sharply increases ionospheric activity for several hours.
Effect on a guidance system, in stages: metre-level accuracy loss → cycle slips (carrier phase jumps, so RTK has to re-resolve and you see float) → in extreme cases loss of lock. More satellites do not fix it, because the corrupted signal is the problem.
What helps is how the receiver handles it. Some Septentrio receivers run IONO+: they identify scintillation-affected signals and take them out of the position computation, and they estimate the ionospheric delay inside the receiver. Practically, that means a single base station is enough - 40 km baseline works, up to 80 km when the ionosphere is calm - rather than needing a dense reference network to interpolate the delay.
If you farm in the equatorial band, three cheap diagnostics: log C/N0 alongside the fix type (a dusk onset points at scintillation), test guidance in the evening rather than midday, and time how long the receiver needs to get back to fixed after a float excursion.
Anyone here farming at low latitude who has watched this pattern - does it line up with sunset for you, or does it persist all night around solar maximum?