RSSI

Purpose

• determine signal strength from different sources / GPIO pins

Considerations

• Need to keep functions generic for GPIOs

• What is providing the RSSI signal? What is the form of the data (Need to ask Aidan Bowers)

• Related hardware: An RC Link

  • RFD900.

    • SiK Manual: https://files.rfdesign.com.au/Files/documents/Software manual.pdf

    • Datasheet: http://files.rfdesign.com.au/Files/documents/RFD900 DataSheet.pdf

    • Need to see how RFD900 sends RSSI signals and how it interfaces with the actual flight controller. Then can determine what signal needs to be "decoded"

• Will the PPM driver be what calls the RSSI module?

  • (2021 year was PWM, this year 2022 will need to look into RFD900)

• We are no longer using singleton, need to get rid of that

• Functions:

  • Normalized and unnormalized function returning signal strength

  • Threshold function returning a bool representing ‘good signal' and bad signal, or a function returning one out of a scale of judgements, e.g. ‘no signal', ‘bad signal’, ’ok signal' , ‘good signal', 'strong signal’ represented as an integer

• For pull request:

  • Make LOS/RSSI branch (after weekend)

Research

Notes about RSSI:

• https://www.youtube.com/watch?v=nZxMvTCN_pI&t=340s&ab_channel=AndyRC

• Some RSSI Types

  • Basic analog signal (eg. voltage range between 0.0 and 5.0, or 0.0 and 3.3)

  • PWM signal

  • PPM signal

PPM vs. PWM:

• https://www.youtube.com/watch?v=FLl837l8o_Q&ab_channel=TeamPanic

Notes about RFD900:

• According to SiK Manual, RFD900 has:

  • TX pin that is UART

    • which protocol?

  • GPIOs

    • Digital

    • ADC

    • PWM capable

• “Mavlink radio status reporting”, including local RSSI and remote RSSI

  • https://mavlink.io/en/messages/common.html

  • https://mavlink.io/en/

  • So the RFD900 can use Mavlink serial protocol for RSSI reporting

• Can be configured by AT commands (use RT commands for remote modem):

  • you can send AT commands to get RSSI report

• Accoring to https://robokits.co.in/multirotor-spare-parts/fpv-video-telemetry-camera/rfd900-txmod-and-rfd-900x-modem-original

  • RFD900 uses PPM, but do we ever directly deal w/ the PPM, or does it get converted to a serial protocol first?

  • RFD900 carries RC PPM and Mavlink telemetry at the same time, so either or can be chosen

Ardupilot implementation reference

• AP_RSSI.cpp calls on AP_HAL_SITL/AnalopIn.cpp class for its ‘channel’ function

  • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_RSSI/AP_RSSI.cpp

  • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_SITL/AnalogIn.h

• AnalogIn.cpp has ‘channel’ function which returns a pointer to an AnalogSource object by calling the ADCSource object constructor. Pointer is stored in rssi_analog_source variable

  • (ADCSource inherits from AnalogSource, is declared in AP_HAL_SITL/AnalogIn)

  • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_SITL/AnalogIn.cpp

  • AnalogSource is an abstract class, defined in a different AnalogIn file, AP_HAL/AnalogIn

    • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL/AnalogIn.h

  • AnalogSource constructor requires inputting a pointer to a SITL_State object

    • SITL_State class files:

      • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_SITL/SITL_State.h

      • https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_SITL/SITL_State.cpp

• In AP_RSSI.cpp, AnalogIn object is actually a member variable of AP_HAL::HAL.h class

  • A hal.h object called hal is accessed using ‘extern’ keyword

• In AP_RSSI.cpp read_pin_rssi function

  • ADCSource voltage_average() function is called on rssi_analog_source variable

  • voltage_average() returns a manipulated (normalized to between 0.0f and 5.0f) output of ADCSource read_average() function. read_average() returns return of ADCSource read_latest() function.

    • read_latest(), depending on the pin, returns the appropriate pin value from the _sitlState member variable of ADCSource, which holds a pointer to a sitlState object

      • (the desired pin is an argument of the ‘channel’ function mentioned earlier)

      • the pin values are from 0.0f to 1023.0f

• AP_RSSI.cpp also has functions which get PWM values