NAV-RELPOSNED heading doesn't seem to be updating

[omuhr]

I have two ZED-F9P's set up in a moving base configuration, I am now trying to interface with the rover programatically in order to perform long term logging. Everything seems to work fine and I am able to parse the NAV-RELPOSNED messages and extract, among others, the relPosHeading field which I am after. But the problem is that the relPosHeading field doesn't seem to change whenever the antennae are rotated relative to each other. I know I am not reading the same message over and over, as the angle does change but only by a few decimal points when the platform is actually rotated by 15-30°. If I monitor the heading in u-center it does update in the expected way.

Here is a minimalist example of the code:

import serial
import serial.tools.list_ports as list_ports
from pyubx2 import UBXReader

try:
    stream = serial.Serial('COM7', 9600, timeout=3)

except serial.serialutil.SerialException:
    ports = list_ports.comports()
    for port, desc, hwid in sorted(ports):
        print("{}: {} [{}]".format(port, desc, hwid))

ubr = UBXReader(stream)

for angle in list_of_angles:
    data = []
    rotate_platform() # This line would rotate the antennae relative to eachother
    while True:
        (raw_data, parsed_data) = ubr.read()
        if parsed_data.identity == 'NAV-RELPOSNED':
            data.append(parsed_data.relPosHeading)
            break
    log_data(data)

Any ideas on where I might be going wrong? Does the receiver need to be given any messages in order to properly update the NAV-RELPOSNED message?

Regards,

Oscar

[semuadmin]

Hi @omuhr

Firstly, are you connecting to your F9P via UART or USB? If UART you might want to check the baud rate is correct - the default F9P UART baud rate is 38400, not 9600.

Your example code is fine as far as it goes, though one potential problem is your timeout value (which may be causing you to miss messages arriving more quickly than every 3 seconds) - see https://github.com/semuconsulting/pyubx2/blob/master/examples/ubxpoller.py for a more robust stream reader example.

Once you have the streaming mechanism sorted, you can simply read the parsed NAV-RELPOSNED message and reference any attributes you want - in your case relPosHeading. The raw heading value is scaled by 1e-5 degrees, but pyubx2 should apply this scaling automatically (assuming you haven't set the scaling keyword argument to False).

Could you provide the ACTUAL, COMPLETE Python script you are executing and an example of the output you're getting?

Could you also send me a screen shot of the comparative results you're getting in u-center, which you believe to be correct?

What values are you seeing for heading accuracy (e.g. accHeading)?

Thanks.

[semuadmin]

By way of illustration, I measured my own roving F9P against two different base locations in quick succession and compared the NAV-RELPOSNED relPosHeading and relPosLength readings with my own estimates of heading and distance (derived using the bearing() and haversine() helper functions provided by pyubx2).

"""
BASE1 - JoeSeelsGPS - 53.53, -1.33 60.9km

<UBX(NAV-RELPOSNED, version=1, reserved0=0, refStationID=0, iTOW=06:36:25, relPosN=-879166, relPosE=-6068417, relPosD=29273, relPosLength=6131841, relPosHeading=261.7566, reserved1=0, relPosHPN=0.0, relPosHPE=-3.9, relPosHPD=2.4, relPosHPLength=3.9, accN=270.1, accE=292.4, accD=493.1, accLength=291.9, accHeading=0.00025, reserved2=0, gnssFixOK=1, diffSoln=1, relPosValid=1, carrSoln=1, isMoving=0, refPosMiss=0, refObsMiss=0, relPosHeadingValid=1, relPosNormalized=0)>

reported relPosLength = 6131841.039
reported relPosHeading = 261.7566

BASE2 - ER_Pollybell_1 - 53.43, -0.91 88.2km

<UBX(NAV-RELPOSNED, version=1, reserved0=0, refStationID=2690, iTOW=06:37:30, relPosN=267987, relPosE=-8849794, relPosD=58193, relPosLength=8854041, relPosHeading=271.7345, reserved1=0, relPosHPN=5.1, relPosHPE=0.0, relPosHPD=9.0, relPosHPLength=7.8, accN=209.3, accE=217.4, accD=298.3, accLength=217.4, accHeading=0.00014, reserved2=0, gnssFixOK=1, diffSoln=1, relPosValid=1, carrSoln=1, isMoving=0, refPosMiss=0, refObsMiss=0, relPosHeadingValid=1, relPosNormalized=0)>

reported relPosLength = 8854041.078
reported relPosHeading = 271.7345
"""

from pyubx2 import haversine, bearing

BASE1 = {"name": "JoeSeelsGPS", "lat": 53.53, "lon": -1.33}
BASE2 = {"name": "ER_Pollybell_1", "lat": 53.43, "lon": -0.91}
ROVER = {"name": "pyubx2", "lat": 53.4506, "lon": -2.2403}

for base in (BASE1, BASE2):
    hdg = bearing(base["lat"], base["lon"], ROVER["lat"], ROVER["lon"])
    len = haversine(base["lat"], base["lon"], ROVER["lat"], ROVER["lon"]) * 100000
    print(
        f"name: {base['name']} ; estimated relPosLength: {len:.3f} ; estimated relPosHeading: {hdg:.4f}"
    )

name: JoeSeelsGPS ; estimated relPosLength: 6093366.427 ; estimated relPosHeading: 262.0256
name: ER_Pollybell_1 ; estimated relPosLength: 8823881.711 ; estimated relPosHeading: 272.0234

The estimates won't be exact because the base stations' positions are only given to 2 decimal places, but you can see that the NAV-RELPOSNED message is correctly reporting the relative heading and distance (+/- 0.6%) in each case.