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docs/ADSB.md

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+# ADS-B
+
+[Automatic Dependent Surveillance Broadcast](https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast)
+is an air traffic surveillance technology that enables aircraft to be accurately tracked by air traffic controllers and other pilots without the need for conventional radar.
+
+## Current state
+
+OSD can be configured to shows the closest aircraft.
+
+## OSD ADSB Info element
+* "-" no ADSB device detected
+* "H" IMU heading is not valid 
+* "G" no GPS fix or less than 4 stats
+* "[Number]" count of ADSB aircrafts
+
+## OSD ADSB Warning element
+OSD can be configured to simple view (one line) or to extended view (two lines) by \
+`set osd_adsb_warning_style=EXTENDED`
+
+### Simple view
+`{distance to vehicle} {direction to vehicle} {altitude diff}`
+
+### Extended view
+`{distance to vehicle} {direction to vehicle} {altitude diff}` \
+`{Emiter Type} {Vehicle direction} {Vehicle Speed}`
+
+![ADSB OSD](assets/images/adsb-info.png)
+
+
+## Hardware
+
+All ADSB receivers which can send Mavlink [ADSB_VEHICLE](https://mavlink.io/en/messages/common.html#ADSB_VEHICLE) message are supported 
+
+* [PINGRX](https://uavionix.com/product/pingrx-pro/) (not tested)
+* [TT-SC1](https://www.aerobits.pl/product/aero/) (tested)
+* [ADSBee1090](https://pantsforbirds.com/adsbee-1090/) (tested)
+* [SoftRF](https://github.com/lyusupov/SoftRF/wiki/Nano-Edition) (tested)
+
+## TT-SC1 settings
+* download software for ADSB TT-SC1 from https://www.aerobits.pl/product/aero/ , file Micro_ADSB_App-vX.XX.X_win_setup.zip and install it
+* connect your ADSB to FC, connect both RX and TX pins
+* in INAV configurator ports TAB set telemetry MAVLINK, and baudrate 115200
+* go to CLI in inav configurator and set serialpassthrough for port you connected ADSB ```serialpassthrough [PORT_YOU_SELECTED - 1] 115200 rxtx``` and close configurator
+* open ADSB program you installed, got to settings and set "telemetry" = MAVLINK,
+
+PCB board for TT-SC1-B module https://oshwlab.com/error414/adsb-power-board
+![TT-SC1 settings](Screenshots/ADSB_TTSC01_settings.png)
+
+## ADSBee 1090 settings
+* connect to ADSBee1090 via USB and set COMMS_UART to mavlink2 \
+``
+AT+PROTOCOL=COMMS_UART,MAVLINK2
+``\
+``
+AT+BAUDRATE=COMMS_UART,115200
+``\
+It's recommended to turn of wifi \
+``
+AT+ESP32_ENABLE=0
+``\
+``
+AT+SETTINGS=SAVE
+``
+* in INAV configurator ports TAB set telemetry MAVLINK, and baudrate 115200
+* https://pantsforbirds.com/adsbee-1090/quick-start/
+
+## SoftRF settings
+SoftRF supports only MAVLink version 1.
+```
+set mavlink_version = 1
+save
+```
+The baud rate for SoftRF is 57600. INAV provides minimal support for SoftRF and supports only 
+the mandatory MAVLink messages: `MAVLINK_MSG_ID_HEARTBEAT`, `MAVLINK_MSG_ID_SYSTEM_TIME`, and `MAVLINK_MSG_ID_GPS_RAW_INT`.
+
+The following messages are not supported: `MAVLINK_MSG_ID_SYS_STATUS`, `MAVLINK_MSG_ID_VFR_HUD`, and `MAVLINK_MSG_ID_ATTITUDE`.
+
+## Alert and Warning
+The ADS-B warning/alert system supports two operating modes, controlled by the parameter osd_adsb_calculation_use_cpa (ON or OFF).
+
+---
+
+### ADS-B Warning and Alert Messages (CPA Mode OFF)
+The ADS-B warning/alert system supports two operating modes, controlled by the parameter **osd_adsb_calculation_use_cpa** (ON or OFF).
+
+When **osd_adsb_calculation_use_cpa = OFF**, the system evaluates only the **current distance between the aircraft and the UAV**. The aircraft with the **shortest distance** is always selected for monitoring.
+
+- If the aircraft enters the **warning zone** (`adsb_distance_warning`), the corresponding **OSD element is displayed**.
+- If the aircraft enters the **alert zone** (`adsb_distance_alert`), the **OSD element starts blinking**, indicating a higher-priority alert.
+
+This mode therefore provides a simple proximity-based warning determined purely by real-time distance.
+
+---
+
+### ADS-B Warning and Alert Messages (CPA Mode ON)
+
+When **osd_adsb_calculation_use_cpa = ON**, the system evaluates aircraft using the **Closest Point of Approach (CPA)** and predicted trajectories, not only the current distance.
+
+1. **Aircraft already inside the alert zone**  
+   If one or more aircraft are currently inside the **alert zone** (`adsb_distance_alert`), the **closest aircraft** to the UAV is selected and the **OSD element blinks**.
+
+2. **Aircraft in the warning zone, none predicted to enter the alert zone**  
+   If aircraft are present in the **warning zone** (`adsb_distance_warning`), but none of them are predicted to enter the **alert zone** (their CPA distance is greater than `adsb_distance_alert`), the **closest aircraft to the UAV** is selected and the **OSD element remains steady** (no blinking).
+
+3. **Aircraft in the warning zone, one predicted to enter the alert zone**  
+   If at least one aircraft in the **warning zone** is predicted to enter the **alert zone**, that aircraft is selected and the **OSD element blinks**.
+
+4. **Aircraft in the warning zone, multiple predicted to enter the alert zone**  
+   If multiple aircraft are predicted to enter the **alert zone**, the system selects the aircraft that will **reach the alert zone first**, and the **OSD element blinks**.
+
+![ADSB CPA_ON](assets/images/adsb-CPA-on.png)