Remote Control RSSI

This page describes how to get RSSI feedback from PX4 for your remote control. Depending on your TX/RX system you might also get other readings, for example link quality.

RSSI for telemetry radios is a different topic, and is provided out of the box via MAVLink for radios with SiK firmware (e.g. 3DR Radio, RFD900, etc.).

There are different ways of getting RSSI input to PX4:

  • Automatic, for supported receivers
  • Analog input (from PWM or DAC, low-pass filtered)

Directly from the receiver

Surprisingly most vendors do not implement RSSI feedback via the digital protocols. Protocols not supporting it include:

  • Spektrum / DSM
  • S.BUS and S.BUS 2
  • Graupner SUMD

The only protocol directly providing RSSI is the Yuneec ST24. Others, like the ImmersionRC EzUHF receivers can be configured to provide RSSI through PWM (described below). All other setups require connecting the ADC/PWM RSSI output to the port labeled SBUS on Pixhawk for analog RSSI sampling (works with analog or PWM signals).

PWM input

If your RX outputs RSSI over one of its PWM channels, you can simply map this channel and specify the range from good to bad.

For example:

  • your RX outputs RSSI on channel 12
  • channel 12 has a value of 1500 for a good connection
  • channel 12 has a value of 1000 for a bad connection

In that case, set the parameters like this:

  • RC_RSSI_PWM_CHAN: 12
  • RC_RSSI_PWM_MIN: 1000
  • RC_RSSI_PWM_MAX: 1500

The following video shows a quick test with an EzUHF system:

Note for EzUFH Receivers

As explained above, the EzUHF receiver can be configured to output RSSI on one of its channels. That allows the use of one single connection to the PIXHAWK. For that to work however, you need to configure the proper channel mapping on the receiver itself using the ImmersionRC Config Tool (Windows only, which can be a pain if you don't have Windows). The following images show the receiver configured for 12 channels and the RSSI output assigned to channel 12.

And here is an example showing the above configuration. A single connection is used between the EzUHF receiver channel 1 and the PIXHAWK RCIN input. The EzUHF channel 1 is configured for PPM Muxed as shown in the screen shot above.

RSSI PWM Configuration

To configure the PIXHAWK for PWM RSSI, you need to tell it which PWM channel to use. In addition, you would also configure the minimum and maximum values as explained above. The configuration can be done using QGC in Setup/Parameters/Radio Calibration as shown below:

Measure values for good and bad

It is recommended that you measure the values for a good and bad connection with the actual setup you're going to fly. The values depend on your receiver in general, the antenna and antenna placement, how bad the signal can get before you loose control, etc..

Analog input

RSSI can be provided as analog input on the PIXHAWK SBUS port if SBUS out is not used. This is the default configuration of the SBUS / RSSI port. Connect the PWM / ADC RSSI output to this port.

As explained above, when using the ADC to read PWM RSSI, the PIXHAWK firmware uses a software low pass filter on the SBUS input, allowing a direct translation without the need of a hardware filter. See the example below using an FrSKY X8R, which has its RSSI output connected to the PIXHAWK's SBUS input.

FIXME: add example wiring/configuration

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