# Air Traffic Avoidance: ADS-B/FLARM
PX4 can use ADS-B (opens new window) or FLARM (opens new window) 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 (opens new window) message.
It has been tested with the following devices:
# 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 (opens new window)).
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.