Importance of RSSI

The LUA scripts from yaapu and Craft and Theory (FlightDeck), running on a Taranis or similar TX, expect to receive a regular telemetry frame reporting adequate RSSI (rf signal strength). In the absence of such a frame, the Taranis reports "Telemetry Lost".

This translator receives the RSSI information from the Flight Computer via Mavlink telemetry, and creates a Frsky sensor frame type 0xF101 for the Taranis and LUA script to interpret. This reportedly must be done at a frequency less than 1Hz to be effective, however empirical evidence suggests the period should be lower to account for possible processing delays in the Taranis/Horus.

MavToPass supports three popular methods of determining RSSI

1. Receiver protocol - the RSSI is embedded in the FrSky sensor telemetry - default.
2. A designated RC PWM channel. ULRS, QLRS and some other long-range telemetry systems use a configuration tool to designate an unused RC channel that carries the RSSI value.
3. SiK telemetry modems, and long-range systems like RFD900x that use the SiK firmware, inject a special RSSI record into the Mavlink telemetry carried by the system.

From v2.60.2, MavToPass supports a compile time define option called RSSI_Pacemaker, with argument = [period], in mS. When this option is defined, the translator does not wait for each Mavlink RSSI message to forward, but forwards the most recent RSSI value received from Mavlink strictly every [period] mS. Default is 200 mS (5Hz).

From v2.43 RSSI is selected automatically if it is available in the telemetry. If no valid RSSI record is received you will get the "Telemetry Lost" announcement on your transmitter.

For debugging purposes, you may choose to un-comment the line below to force a regular RSSI frame to be sent

//#define RSSI_Override 70 // Dummy RSSI - fixed at [argument]

From v2.51 Hasi123 included a patch to force a default rssi of 69% if no rssi telemetry received. This is especially for SiK radios, like RFD900x because no RSSI telemetry message (#109) was being sent until Mission Planner connected.

Note that in Relay_Mode, two RSSI instances are reported:

1. The RSSI of the received signal at the aircraft - instance 28
2. The RSSI of the Taranis/Horus signal received at the relaying FrSky receiver (say XSR) - instance 25

Normally you would want 1. above to be reported on your transmitter screen, and not be particularly interested in 2. Delete all sensors on your Taranis/Horus, then discover new sensors. Be sure to un-tick "Ignore Instances". Two instances, as per 1. and 2. should be discovered for RSSI. Use OpenTX to delete instance 25, and change "Source" in the RSSI sub-section from "(default)" to "RSSI External"

Finally, be sure to define the RSSI parameters in Ardupilot (via Mission Planner) correctly.

1. Connect the Mission Planner
2. Open the CONFIG/TUNING tab
3. Click on Full Parameter Tree
4. Navigate to RSSI, and expand the tree
5. Examine RSSI_Type, and enter the type that applies to your system
6. Enter the RSSI-CHANNEL if appropriate
7. Click Write Params