AP_Baro: Allow setting field elevation to 0

[stephendade]

This came up when using a vision-based vehicle (no GPS).

When using set_gps_global_origin_send to set the GPS origin of a vehicle, if the set altitude is exactly 0 then AP_Baro will continually keep resetting the field elevation.

This patch will warn the user if they try to do this.

This patch will allow setting the altitude to 0, but only once per boot.

Example pymavlink code to demonstrate issue:

#!/usr/bin/env python3
'''
Quick test script to send origin

'''
import argparse
import time
from pymavlink import mavutil


if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "--device", type=str, default="udpin:127.0.0.1:14550", help="MAVLink connection string")
    parser.add_argument("--baud", type=int, default=115200,
                        help="MAVLink baud rate, if using serial")
    parser.add_argument("--source-system", type=int,
                        default=1, help="MAVLink Source system")
    args = parser.parse_args()

    # Start mavlink connection
    try:
        conn = mavutil.mavlink_connection(args.device, autoreconnect=True, source_system=args.source_system,
                                          baud=args.baud, force_connected=False,
                                          source_component=mavutil.mavlink.MAV_COMP_ID_VISUAL_INERTIAL_ODOMETRY)
    except Exception as msg:
        print("Failed to start mavlink connection on %s: %s" %
              (args.device, msg,))
        raise

    # wait for the heartbeat msg to find the system ID. Need to exit from here too
    # We are sending a heartbeat signal too, to allow ardupilot to init the comms channel
    while True:
        conn.mav.heartbeat_send(mavutil.mavlink.MAV_TYPE_ONBOARD_CONTROLLER,
                                mavutil.mavlink.MAV_AUTOPILOT_GENERIC,
                                0,
                                0,
                                0)
        if conn.wait_heartbeat(timeout=0.5) is not None:
            # Got a hearbeart, go to next loop
            break

    print("Got Heartbeat from APM (system %u component %u)" %
          (conn.target_system, conn.target_component))

    time.sleep(0.05)
    print("Setting EKF origin")
    current_time_us = int(round(time.time() * 1000000))
    conn.mav.set_gps_global_origin_send(conn.target_system,
                                        int(-35.363261*1.0e7),
                                        int(149.165230*1.0e7),
                                        int(0*1.0e3),
                                        current_time_us)

[tpwrules]

Some slightly ignorant questions: Would it be a better experience to just convert it to 0.01? Could drifting back down to 0 cause the Baro reset to happen again?

[stephendade]

Would it be a better experience to just convert it to 0.01

That would work too. My only concern would be it's different to what the user specified, which may lead to confusion in later data analysis if they don't know about the 0 -> 0.01 change.

[menschel]

Wouldn't it make sense to name the parameter here?

[nexton-winjeel]

The issue is that new_field_elev can be set true even is the value hasn't changed. Probably should be something like:

            Location origin;
            if (!armed && AP::ahrs().get_origin(origin)) {
                new_field_elev = !is_equal(_field_elevation_active, origin.alt * 0.01f);
                _field_elevation_active = origin.alt * 0.01;
            }

[stephendade]

I've changed this to @nexton-winjeel's suggestion - we can now set the field elevation to 0.

However the field elevation can now only be set once. Not sure if this conflicts with any other assumptions in the codebase?