Radio Control

Any transmitter that has an available receiver which outputs a CPPM / PPM sum signal, S-BUS or Spektrum Satellite is supported by the PX4 system, although these last two are only supported by the PIXHAWK module or a PX4FMU in combination with a PX4IO board.

After thorough testing by the developers and community, the following are recommended systems:

Transmitter Modes

The sticks can be mapped differently to the systems actuators and hence transmitters are sold with different “modes”. Some of the transmitters have some axes loaded with springs to auto-center and some not. On some transmitters those springs can be changed, on others not. If the shop mentions the mode somewhere in the product description, it is always a good idea to buy the right one, since it is an indication that swapping axes might not be easily done. More on this topic can be found here: A discussion of all four modes.

Function Assignments

The remote control has some common main functions. For full autonomous operation at least six channels are required.

  • Throttle
  • Yaw / Rudder
  • Roll / Ailerons
  • Pitch / Elevator
  • Mode switch (manual / auto)
  • Function switch (activation / deactivation of individual stabilization functions)

Calibration

In order to have correct minima / maxima on all RC channels, a calibration is needed. This can be done manually by entering the values via the parameter interface in MAVLink, or by using the automatic functionality of one of the ground control stations.

Radio Control Calibration with QGroundControl

Please follow the instructions on the screen.

Background Info: 2.4 GHz Spread Spectrum Modulation

DSSS - Direct Sequence Spread Spectrum

The DSSS modulation is a “wide-band” transmission in that the transmitter spreads the data across a wide range of frequencies and encrypts the data through the use of a mathematical key, the receiver then uses the same key to decode the data. In addition to the spreading of the data, extra redundant copies of the encoded data are transmitted to try to guarantee reception of the data. Any interference on a single channel appears to the receiver as a different transmission. When the complete received signal is then decoded, the DSSS transmission from all of the channels is decoded back to its original single channel format while the interference, as it is only received on a single channel, is a lot lower in power when compared with the original decoded signal and any potential effects on the signal are greatly diminished.

The opposite is true, however, when interference is present over a similar wide range of frequencies as the original signal. In this situation when the complete signal is decoded the interference can be of a similar power to the original multi-channel signal and thus “drowns” the signal out which can lead to connectivity loss and the inevitable consequences.

FHSS - Frequency Hopping Spread Spectrum

FHSS divides the frequency spectrum into different discrete channels, the signal data is then split up into packets and transmitted on these channels. The FHSS modulation transits the data on these channels in a random order which is known only by the transmitter and receiver. If any interference is experienced on a channel that results in data loss the FHSS modulation “hops” to the next channel in its random order and resends the data to try to ensure reception, whilst any interfering spread spectrum systems will use a different random order and can therefore co-exist without any interference.

It has been found that using FHSS can prevent interference from spurious RF signals up to ten times better than DSSS.

System Modulation Type

DSSS FHSS
Futaba T7C FASST * X X
FrKy ACCST X
Graupner HoTT X
Spektrum DSM-X * X X
Spektrum DSM-2 X

* FASST and DSM-X use a hybrid of both modulations.

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