Use Payload Control to switch payloads on and off from a mission plan (e.g. on arrival at a waypoint) or by hand. This feature can control the LOS radio, BLOS satellite link, or also an external payload connected to Horizon’s GPIO. Payload Control lets a mission plan or operator switch the aircraft’s You set it up in three stages: map each piece of equipment to an output index in Horizon UI, configure a matching relay on the autopilot, then actuate it from Mission Planner.

Note

This page documents the ArduPilot relay flow. PX4 is supported but uses different parameter names (ACTUATOR_OUTPUT_STATUS where ArduPilot emits RELAY_STATUS)

What you’ll need

• Horizon UI 1.15.0 or later, with the Horizon unit connected over USB (see Configure Horizon).
• Autopilot firmware — ArduPilot v4.5.0+ or PX4 v1.14.0+. Equipment-channel control does not work on older versions.
• Mission Planner (or an equivalent ground control application, or “GCS”) connected to the flight controller.
• The flight controller’s serial port already configured for the Horizon link (see Flight Controller Configuration).

Indexing

Each equipment channel is identified by a number in three places, and the number is not the same in all three. Get this wrong and the relay UI will appear to work while the payload never switches.

Where	What you enter	Value to use
Horizon UI → Output Index	The index you assign to a channel	your choice, 1–64
Autopilot → RELAYx parameter	The relay you configure	Output Index + 1
Mission plan → DO_SET_RELAY	The relay a plan actuates	the Output Index
Direct actuation → Servo/Relay tab	The relay you toggle by hand	RELAYx = Output Index + 1

Caution

The autopilot relay is always one greater than the Horizon UI Output Index. If you assign a channel to Output Index 3, configure RELAY4 on the autopilot. A mission plan will then actuate it with DO_SET_RELAY Relay No 3 (the Output Index), but the manual Servo/Relay tab shows it as Relay 4 (the RELAYx value).

Map equipment to output indices in Horizon UI

The Advanced Settings modal in Connected Device holds both the serial-link settings and the equipment map. (This is separate from Preferences, which sets the GCS connection — see the Horizon UI Tour.)

1. In Connected Device with the unit attached, select Advanced Settings.
2. Turn on MAVLink Equipment Control. While it is off, no equipment mapping is written to the Horizon firmware during a settings sync.
3. For each channel you want to control — BLOS (Iridium), LOS Radio (LoRa mesh), and IO Payload (GPIO) — set an Output Index and enable its toggle.
4. Select Configure Aircraft to write the mapping to the unit.

Note the index you assign to each channel — you will use it, and its + 1 relay number, on the autopilot.

The Advanced Settings button sits at the top right of Connected Device.

Each channel — BLOS, LOS Radio, and IO Payload — takes an Output Index and an Enabled toggle.

Set the serial link (baud rate and hardware flow control)

The Serial Link section of the same Advanced Settings modal sets how the Horizon unit talks to the flight controller.

• Baudrate — must match the autopilot telemetry port; see Flight Controller Configuration (SERIAL1_BAUD) for the value the standard Horizon link requires.
• Hardware Flow Control — RTS/CTS on the flight-controller serial connection. This is the setting that matters for payload control — see below.

The Baudrate dropdown lists the standard rates; select the one that matches the autopilot telemetry port.

Caution

Hardware flow control and the IO Payload (GPIO) channel use the same GPIO pins — you can enable one or the other, not both. With hardware flow control on, the IO Payload channel is unavailable and cannot be indexed; enabling IO Payload makes Hardware Flow Control unavailable in turn. To control an external payload over GPIO, turn hardware flow control off. The standard Horizon wiring uses hardware flow control (RTS/CTS on I/O connector pins 2 and 3 — see Hardware Description).

With hardware flow control on, the IO Payload row is greyed out and its toggle is forced off.

Configure the autopilot relay (ArduPilot)

For each equipment channel you mapped in Horizon UI, configure the matching relay on the autopilot. Remember the relay number is the Output Index + 1.

1. Identify a physical output pin on the flight controller that is available for assignment (see the pin tables in C.5).

2. In Mission Planner, open Config → Full Parameter List (or Full Parameter Tree).

3. Set the chosen output’s SERVOn_FUNCTION parameter to -1 to put the channel in GPIO mode. Leave any TRIM, MIN, or MAX parameters for that channel unchanged.

4. Configure the logical relay (x = Horizon UI Output Index + 1):

   Parameter	Value
   RELAYx_FUNCTION	1
   RELAYx_PIN	(pin value from C.5)
   RELAYx_DEFAULT	0
   RELAYx_INVERTED	0

5. Write the parameters to the flight controller and reboot the autopilot.

6. Verify from Mission Planner before relying on it in a plan.

Caution

Two distinct ways to get this wrong:

1. Assigning a relay to a pin that isn’t a real output. The UI reports success, but the autopilot never emits the status message the relay UI needs — the toggle appears to work while the payload never switches.
2. Assigning a relay to a pin already wired to a servo, ESC, or other actuator. That hardware fires whenever the payload state changes, cross-contaminating a real actuator function with payload control.

Note

Once one aircraft is configured, save its parameters to a .param file and load it onto the rest of the fleet instead of repeating this by hand.

In the Full Parameter Tree, each RELAYx shows its _FUNCTION, _PIN, and _DEFAULT. The _PIN dropdown lists every selectable output — the numeric values in the next section correspond to these entries.

Relay pin values by autopilot board

Use the numeric RELAYx_PIN value for the output pin you identified in C.4. The configuration is conceptually identical across boards, but the available pins differ — confirm the values for any board not listed here against its documentation.

Cube Orange / Pixhawk family

Prefer an AUX output when one is available; otherwise use an unused MAIN output.

Output	ArduPilot channel	Required setting	RELAYx_PIN
AUX OUT 1	SERVO9	SERVO9_FUNCTION = -1	50
AUX OUT 2	SERVO10	SERVO10_FUNCTION = -1	51
AUX OUT 3	SERVO11	SERVO11_FUNCTION = -1	52
AUX OUT 4	SERVO12	SERVO12_FUNCTION = -1	53
AUX OUT 5	SERVO13	SERVO13_FUNCTION = -1	54
AUX OUT 6	SERVO14	SERVO14_FUNCTION = -1	55
MAIN OUT 1	SERVO1	SERVO1_FUNCTION = -1	101
MAIN OUT 2	SERVO2	SERVO2_FUNCTION = -1	102
MAIN OUT 3	SERVO3	SERVO3_FUNCTION = -1	103
MAIN OUT 4	SERVO4	SERVO4_FUNCTION = -1	104
MAIN OUT 5	SERVO5	SERVO5_FUNCTION = -1	105
MAIN OUT 6	SERVO6	SERVO6_FUNCTION = -1	106
MAIN OUT 7	SERVO7	SERVO7_FUNCTION = -1	107
MAIN OUT 8	SERVO8	SERVO8_FUNCTION = -1	108

ARKV6X (no-IOMCU firmware — built without the separate I/O co-processor)

Mission Planner’s labels for 50–57 may be generic or incomplete — use the numeric values below.

Caution

Do not use 101–108 with the standard no-IOMCU ARKV6X firmware.

Output	ArduPilot channel	Required setting	RELAYx_PIN
PWM 1	SERVO1	SERVO1_FUNCTION = -1	50
PWM 2	SERVO2	SERVO2_FUNCTION = -1	51
PWM 3	SERVO3	SERVO3_FUNCTION = -1	52
PWM 4	SERVO4	SERVO4_FUNCTION = -1	53
PWM 5	SERVO5	SERVO5_FUNCTION = -1	54
PWM 6	SERVO6	SERVO6_FUNCTION = -1	55
PWM 7	SERVO7	SERVO7_FUNCTION = -1	56
PWM 8	SERVO8	SERVO8_FUNCTION = -1	57

Verify the relay from Mission Planner

Before trusting a relay to a mission plan, confirm it actually switches the payload — from the Servo/Relay tab on Mission Planner’s Data screen, while connected to the flight controller.

Each configured relay appears here as Relay x — using the RELAYx value (Output Index + 1), not the Output Index itself.

• Low commands the relay low (the box turns red) — the payload’s On state.
• High commands the relay high (the box turns green) — the payload’s Off state.
• Toggle sends Low, High, Low in sequence; we do not recommend using this setting.

Caution

If you are controlling payloads over an active LOS link and use this tab to disable the LOS radio, you will need to reconnect to the autopilot over USB and command that relay Low to bring the link back.

Actuate a payload from a mission plan (proximity trigger)

After confirming the configured relay switches the payload correctly, you can embed the actuation in a mission plan. This enables operations like disabling the LOS radio on entry to a defined area to reduce RF signature.

Relay state: 1 = Off, 0 = On. (Each equipment channel’s default output is treated as On.)

1. In Mission Planner, plan the mission up to the waypoint N where the change should take effect. Set that waypoint’s lat / long / alt to the trigger location.
2. Immediately after waypoint N, insert a new mission item and set its Command to DO_SET_RELAY.
3. Set Relay No to the Horizon UI Output Index for the equipment — not the RELAYx value. Set the relay value to 1 to switch the payload off, or 0 to switch it on.

Note

To switch several payloads at the same point, be sure to add one DO_SET_RELAY per channel. Each command only affects a single relay.

DO_SET_RELAY will fire only after the preceding waypoint is reached; this enables automatic change based on location or altitude.

Troubleshooting

The relay appears configured but the payload doesn’t respond:

1. Confirm the output pin is a real, unoccupied output — not one already wired to a servo, ESC, or other actuator.
2. Confirm the Output Index is unique per channel in Horizon UI — BLOS, LOS Radio, and IO Payload each need their own.
3. Confirm Hardware Flow Control isn’t silently disabling the IO Payload channel.
4. Confirm the autopilot parameters were written and the autopilot rebooted.
5. Re-verify from the Servo/Relay tab before suspecting the mission plan itself.

Related

• Configure Horizon — naming, radio settings, and firmware for the unit you configure here
• Flight Controller Configuration — the SERIAL1 baud and flow-control settings the serial link must match
• Hardware Description — the I/O connector pinout, including the GPIO pins shared with hardware flow control
• Horizon UI Tour — where Advanced Settings and Preferences live in the interface