Is it possible to simulate HDOP error(degradation) in SITL?

ArduRover ArduBoat
1

Hello.

I tried the parameters below, but the SITL’s HDOP value always stayed the same.(1.21) :

SIM_GPS1_ACC
SIM_GPS1_BYTELOS
SIM_GPS1_GLTCH_X
SIM_GPS_JAM
SIM_GPS1_NOISE
SIM_GPS1_NUMSATS

Is there any way to simulate HDOP errors?(e.g. values higher than 2.0 or even 100.0)

Simply changing the value in the MAVlink message would be sufficient for my testing.

Alternatively, is it possible to request that feature be added in a future SITL firmware updates?

2

I changed the code : libraries/SITL/SIM_GPS_NMEA.cpp as belows(with nano) :

/*
  send a new GPS NMEA packet
 */
void GPS_NMEA::publish(const GPS_Data *d)
{
    struct timeval tv;
    struct tm *tm;
    char tstring[20];
    char dstring[20];
    char lat_string[20];
    char lng_string[20];

    double yj_seconds = tv.tv_sec + tv.tv_usec * 1e-6; // seconds
    double yj_t = fmod(yj_seconds, 20.0);
    float yj_hdop;
    if(yj_t < 10.0f)
    {
        yj_hdop = 1.2f;
    }
    else
    {
        yj_hdop = 2.1f;
    }

    simulation_timeval(&tv);

    struct tm tvd {};
    tm = gmtime_r(&tv.tv_sec, &tvd);

    // format time string
    hal.util->snprintf(tstring, sizeof(tstring), "%02u%02u%06.3f", tm->tm_hour, tm->tm_>

    // format date string
    hal.util->snprintf(dstring, sizeof(dstring), "%02u%02u%02u", tm->tm_mday, tm->tm_mo>

    // format latitude
    double deg = fabs(d->latitude);
    hal.util->snprintf(lat_string, sizeof(lat_string), "%02u%08.5f,%c",
             (unsigned)deg,
             (deg - int(deg))*60,
             d->latitude<0?'S':'N');

    // format longitude
    deg = fabs(d->longitude);
    hal.util->snprintf(lng_string, sizeof(lng_string), "%03u%08.5f,%c",
             (unsigned)deg,
             (deg - int(deg))*60,
 d->longitude<0?'W':'E');

    nmea_printf("$GPGGA,%s,%s,%s,%01d,%02d,%04.1f,%07.2f,M,0.0,M,,",
                     tstring,
                     lat_string,
                     lng_string,
                     d->have_lock?1:0,
                     d->have_lock?d->num_sats:3,
                     yj_hdop,
                     d->altitude);

I built the code with waf and it detected my code changes.

but above didn’t work.

I changed GPS_UBlox::publish() in SIM_GPS_UBLOX.cpp like below and it worked :

dop.time = gps_tow.ms;
dop.gDOP = 65535;
dop.pDOP = 65535;
dop.tDOP = 65535;
dop.vDOP = 200;
dop.hDOP = 221; // modified from 121
dop.nDOP = 65535;
dop.eDOP = 65535;

I added code like below and it worked great :

static uint32_t yj_next_switch_ms = 0;
static uint16_t yj_current_hdop = 121;
uint32_t yj_now_ms = AP_HAL::millis();

if(yj_now_ms >= yj_next_switch_ms)
{
    if(yj_current_hdop == 121)
    {
        yj_current_hdop = 221;
    }
    else
    {
        yj_current_hdop = 121;
    }
    yj_next_switch_ms = yj_now_ms + 10000; // 10 seconds
}
dop.hDOP = yj_current_hdop;

I could change value of hdop with above code.

3

HDOP is purely related to spatial configuration of satellites used to calculate position. You can have inaccurate or imprecise information about your location even with good HDOP.

1 Like
4

Thank you.

Since I operate the rover in areas where the sky can be obstructed, I needed a mission planner plugin that can detect HDOP degradation, so I’ve been developing and testing one.