Calculate GPS_DELAY_MS without Gps.Spd

Davide_Lentini · October 2, 2023, 11:15am

Hi all , I own a GPS module that provides NMEA messages with latitude and longitude , but does not provide information about GPS.Spd , speed.
Now , I would be interested in calculating the GPS_DELAY_MS of this module and I have referenced this script :

github.com

ArduPilot/pymavlink/blob/master/tools/mavgpslag.py

#!/usr/bin/env python

'''
calculate GPS lag from DF log

The DF log file needs to be generated with ATT, GPS and IMU bits:
-  On copter (bit0, bit2 and bit18) set on the LOG_BITMASK parameters
-  On rover  (bit0, bit2 and bit7) set on the LOG_BITMASK parameters
-  On plane  (bit0, bit2 and bit7) set on the LOG_BITMASK parameters
Make sure IMU_RAW (bit 19) is not set

For this to work, the vehicle must move at speeds above the --minspeed parameter (defaults to 6m/s)
The code really only works when there is significant acceleration as well.
You'll need to fly quite aggressively on a copter to get a result.

'''
from __future__ import print_function
from builtins import range

import os

This file has been truncated. show original

The problem that this script almost solely uses the velocity to calculate the delay , and my module does not provide this information .
So I would like to ask you what other methods exist to calculate the delay in MS for modules that are lacking GPS.Spd and provide only lat and lng. Thank you

And also ,if you can confirm me that ArduPilot use this value as default value for delay for NMEA modules :

AP_GPS_NMEA.cpp : 
`bool AP_GPS_NMEA::get_lag(float &lag_sec) const
{
    switch (get_type()) {
#if AP_GPS_NMEA_UNICORE_ENABLED
    case AP_GPS::GPS_TYPE_UNICORE_MOVINGBASE_NMEA:
    case AP_GPS::GPS_TYPE_UNICORE_NMEA:
        lag_sec = 0.14;
        break;
#endif // AP_GPS_NMEA_UNICORE_ENABLED

    default:
        lag_sec = 0.2; // <---- default value 200 ms ?
        break;
    }
    return true;
}`

Davide_Lentini · October 9, 2023, 8:05am

I know @tridge has partly developed this script , could he have some advice maybe ? Thanks
Maybe some ideas from other developers ?
@peterbarker
@rmackay9
@khancyr
Thanks !