GPS doesn't get a fix when combined with active LTE-M connection

[Sebastiaan Merckx]

We have a custom PCB with GPS circuit copied from the NR9160-dk board.

The default GPS example is working fine. However, we want to combine GPS tracking and LTE-M connection. The modem init configuration is OK, and the GPS receiver is able to track several (4-5) satellites, but it doesn't succeed in finding a fix. One specific error is printed repeatedly: "GNSS operation blocked by insufficient time windows".

When disabling the LTE connection, the GPS fix is found.

One explanation could be that the COEX0 (GPS_ENABLE) pin is going low regularly (interval of ~300msec) and maybe this results in the GPS receiver to miss GPS signals regularly. Is this a known limitation?

If not, what else could be the reason for this error message?

As a reference, modem initialization code is below.

  int err = nrf_modem_lib_init(NORMAL_MODE);
  if (err) {
    printk("Failed to initialize modem library!\n");
    return;
  }

  err = at_notif_init();
  if(err != 0) {
    printk("at_notif_init failed\n");
  }
  err = at_cmd_init();
  if(err != 0) {
    printk("at_cmd_init failed\n");
  }
  
  err = lte_lc_pdp_context_set(LTE_LC_PDP_TYPE_IP, "tsudp", false, false, false);
  if(err != 0) {
    printk("lte_lc_pdp_context_set err %d\n", err);
    return -1;
  }

  lte_lc_system_mode_set(LTE_LC_SYSTEM_MODE_LTEM_GPS);
  err = lte_lc_init();
  if(err != 0) {
    printk("lte_lc_init failed\n");
    return -1;
  }

  if (at_cmd_write(AT_COEX0, NULL, 0, NULL) != 0) {
    printk("AT_COEX0 failed\n");
    return -1;
  }
  
  //only called if lte_lc_connect below is not called.
  /*if (at_cmd_write(AT_ACTIVATE_GPS, NULL, 0, NULL) != 0) {
    printk("AT_ACTIVATE_GPS failed\n");
    return -1;
  }*/

  while(1) {
    printk("LTE Link Connecting ...\n");
    err = lte_lc_connect();
    if (err == 0) {
      printk("LTE Link Connected!\n");
      break;
    }
    printk("LTE link not established. err=%d\n", err);
    k_sleep(K_SECONDS(10));
  }

[Sebastiaan Merckx]

Modem firmware was mfw_nrf9160_1.1.0, but also seen on latest mfw_nrf9160_1.3.0

[Hakon]

Hello,

what NCS tag is this from?

[Sebastiaan Merckx]

Hello,

Sorry for the late reply, I was on a 2-weeks holiday.

I am using SDK 1.5.1-rc1 (I assume that's what you're asking for).

[Sebastiaan Merckx]

The same behavior is observed on the NRF9160DK reference board. The receiver is able to track 4-5 satellites but never able to "use" them, when LTE is also activated.

When LTE is de-activated, the GPS gets a fix in ~40 seconds (as expected).

Would the use of SUPL library help here, since I understand from documentation that it takes some time to download the GPS data from the satellite, and that "any loss of signal requires the whole process to be restarted" ?

[Sebastiaan Merckx]

This question is followed up privately.