Air Traffic Avoidance: ADSB/FLARM

Air Traffic Avoidance: ADS-B/FLARM

PX4 can use ADS-B or FLARM transponders to support simple air traffic avoidance in missions. If a potential collision is detected, PX4 can warn, immediately land, or return (depending on the value of NAV_TRAFF_AVOID).

Supported Hardware

PX4 traffic avoidance works with ADS-B or FLARM products that supply transponder data using the MAVLink ADSB_VEHICLE message.

It has been tested with the following devices:

• PingRX ADS-B Receiver (uAvionix)

• FLARM

Hardware Setup

Either device can be connected to any free/unused serial port on the flight controller. Most commonly it they are connected to TELEM2 (if this is not being use for some other purpose).

PingRX

The PingRX MAVLink port uses a JST ZHR-4 mating connector with pinout as shown below.

Pin	Signal	Volt
1 (red)	RX (IN)	+5V tolerant
2 (blk)	TX (OUT)
3 (blk)	Power	+4 to 6V
4 (blk)	GND	GND

The PingRX comes with connector cable that can be attached directly to the TELEM2 port (DF13-6P) on an mRo Pixhawk. For other ports or boards, you will need to obtain your own cable.

FLARM

FLARM has an on-board DF-13 6 Pin connector that has an identical pinout to the mRo Pixhawk.

Pin	Signal	Volt
1 (red)	VCC	+4V to +36V
2 (blk)	TX (OUT)	+3.3V
3 (blk)	RX (IN)	+3.3V
4 (blk)	-	+3.3V
5 (blk)	-	+3.3V
6 (blk)	GND	GND

:::note The TX and RX on the flight controller must be connected to the RX and TX on the FLARM, respectively. :::

Software Configuration

Port Configuration

Flarm/PingRX are configured in the same way as any other MAVLink Peripheral. The only specific setup is that the port baud rate must be set to 57600 and the a low-bandwidth profile (MAV_X_MODE).

Assuming you have connected the device to the TELEM2 port, set the parameters as shown:

• MAV_1_CONFIG = TELEM 2

• MAV_1_MODE = Normal

• MAV_1_RATE = 0 (default sending rate for port).

• MAV_1_FORWARD = Enabled

Then reboot the vehicle.

You will now find a new parameter called SER_TEL2_BAUD, which must be set to 57600.

Configure Traffic Avoidance

Configure the action when there is a potential collision using the parameter below:

Parameter	Description
NAV_TRAFF_AVOID	Enable traffic avoidance mode specify avoidance response. 0: Disable, 1: Warn only, 2: Return mode, 3: Land mode.
NAV_TRAFF_A_HOR	Horizonal radius of cylinder around the vehicle that defines its airspace (i.e. the airspace in the ground plane).
NAV_TRAFF_A_VER	Vertical height above and below vehicle of the cylinder that defines its airspace (also see NAV_TRAFF_A_HOR).
NAV_TRAFF_COLL_T	Collision time threshold. Avoidance will trigger if the estimated time until collision drops below this value (the estimated time is based on relative speed of traffic and UAV).

Implementation

PX4 listens for valid transponder reports during missions.

If a valid transponder report is received, PX4 first uses the traffic transponder information to estimate whether the traffic heading and height indicates there will be an intersection with the airspace of the UAV. The UAV airspace consists of a surrounding cylinder defined by the radius NAV_TRAFF_A_HOR and height NAV_TRAFF_A_VER, with the UAV at it's center. The traffic detector then checks if the time until intersection with the UAV airspace is below the NAV_TRAFF_COLL_T threshold based on the relative speed. If the both checks are true, the Traffic Avoidance Failsafe action is started, and the vehicle will either warn, land, or return.

The code can be found in Navigator::check_traffic (/src/modules/navigator/navigator_main.cpp).

PX4 will also forward the transponder data to a GCS if this has been configured for the MAVLink instance (this is recommended). The last 10 Digits of the GUID is displayed as Drone identification.

Testing/Simulated ADSB Traffic

You can simulate ADS-B traffic for testing. Note that this requires that you Build PX4.

:::note Simulated ADS-B traffic can trigger real failsafe actions. Use with care in real flight! :::

To enable this feature:

1. Uncomment the code in AdsbConflict::run_fake_traffic()(AdsbConflict.cpp).

2. Rebuild and run PX4.

3. Execute the navigator fake_traffic command in the QGroundControl MAVLink Shell (or some other PX4 Console or MAVLink shell, such as the PX4 simulator terminal).

The code in run_fake_traffic() is then executed. You should see ADS-B warnings in the Console/MAVLink shell, and QGC should also show an ADS-B traffic popup.

By default run_fake_traffic() publishes a number of traffic messages (it calls AdsbConflict::fake_traffic() to emit each report). These simulate ADS-B traffic where there may be a conflict, where there won't be a conflict, as well as spamming the traffic buffer.

::: details Information about the test methods

The relevent methods are defined in AdsbConflict.cpp.

run_fake_traffic() method

The run_fake_traffic() method is run when the navigator fake_traffic command is called.

The method calls the fake_traffic() method to generate simulated transponder messages around the current vehicle position. It passes in the current vehicle position, and information about the simulated traffic, such as callsign, distances, directions, altitude differences, velocities, and emitter types.

The (commented out) code in run_fake_traffic() simulates a number of different scenarios, including conflicts and non-conflicts, as well as spamming the traffic buffer.

fake_traffic() method

AdsbConflict::fake_traffic() is called by the run_fake_traffic() to create individual ADS-B transponder reports.

This takes several parameters, which specify the characteristics of the fake traffic:

• callsign: Callsign of the fake transponder.

• distance: Horizontal distance to the fake vehicle from the current vehicle.

• direction: Direction in NED from this vehicle to the fake in radians.

• traffic_heading: Travel direction of the traffic in NED in radians.

• altitude_diff: Altitude difference of the fake traffic. Positive is up.

• hor_velocity: Horizontal velocity of fake traffic, in m/s.

• ver_velocity: Vertical velocity of fake traffic, in m/s.

• emitter_type: Type of fake vehicle, as an enumerated value.

• icao_address: ICAO address.

• lat_uav: Lat of this vehicle (used to position fake traffic around vehicle)

• on_uav: Lon of this vehicle (used to position fake traffic around vehicle)

• alt_uav: Altitude of the vehicle (as reference - used to position fake traffic around vehicle)

The method creates a simulated transponder message near the vehicle, using following steps:

• Calculates the latitude and longitude of the traffic based on the UAV's position, distance, and direction.

• Computes the new altitude by adding the altitude difference to the UAV's altitude.

• Populates a TransponderReport topic with the simulated traffic data.

• If the board supports a Universally Unique Identifier (UUID), the method retrieves the UUID using board_get_px4_guid and copies it to the uas_id field of the structure. Otherwise, it generates a simulated GUID.

• Publishes the simulated traffic message using orb_publish.

:::

Further Information

• MAVLink Peripherals

• Serial Port Configuration