Use this page to determine which link is active and where a degraded link is failing, by reading the unit LEDs together with what Horizon UI and the GCS application show. This complements the symptom index in Troubleshooting, which covers hard faults.

Read the LEDs first

The STATUS and LINK LEDs report unit power and LOS link state at a glance. Full table: LED Quick Reference.

What you see	What it means	Where to look next
STATUS solid green	Power good, unit healthy	Continue to LINK LED
LINK fast-blinking green (both units)	LOS link closed — both units locked on	LOS is healthy; verify GCS COM/UDP connection
STATUS blinking red	Bootloader mode — abnormal during operation	Power-cycle the unit; re-flash firmware if it persists
LINK blinking red	LOS link lost	Fall through to BLOS, then diagnose LOS below

Determine the active link

Horizon routes over LOS by default and falls back to BLOS automatically. If no LOS MAVLink packet is received for more than 3 seconds, Beam routes the supported command and telemetry set over the satellite link; when LOS returns, MAVLink traffic will re-route over it automatically. See System Description.

LOS LINK LED	Telemetry cadence in GCS app	Active link
Fast-blinking green	Real-time, low latency	LOS
Lost (red)	Slow / periodic updates	BLOS fallback
Lost (red)	No updates at all	Neither — troubleshoot below

Note

Under BLOS, telemetry updates at a slower cadence, and a command is acknowledged in the GCS application before it’s received by the aircraft. Confirm delivery by observing the Flight Log entry reach Delivered, not by the GCS acknowledgement alone.

Isolate a degraded LOS link

If LOS will not close or drops intermittently, the cause is usually physical-layer:

1. Channel separation — confirm the two LOS channels differ by at least 2 MHz and that both units share matching frequencies.
2. Antenna placement — confirm separation distances and clearance from conductive structure, including carbon fiber. See Horizon Aircraft Installation and Horizon GCS Setup.
3. RF cable routing — confirm runs are short, away from EMI sources, and not parallel to high-current or high-speed digital lines. See Horizon Aircraft Installation or Horizon GCS Setup.
4. EMI co-location — confirm the unit is not co-located with other 902–928 MHz or 1616–1626.5 MHz radios or major EMI sources. See Horizon Aircraft Installation.

Isolate a degraded BLOS link

If BLOS telemetry is slow, intermittent, or absent:

1. Sky view — confirm the BLOS antenna has a clear, unobstructed view of the sky; move away from buildings, canopy, and overhead structure. See Horizon Aircraft Installation or Horizon GCS Setup.
2. Coverage — confirm the operating latitude is within the modem’s coverage (Iridium is global; Inmarsat is approximately ±82° latitude).
3. Feedline loss — confirm BLOS feedline loss is within the 3 dB budget.
4. Subscription — confirm the satellite data plan is active.

If telemetry is still absent after these checks, treat it as a hard fault and follow No Telemetry Received Over BLOS.