The TD1204 is the GPS & Accelerometer version of the sigfox Telecom Design chip. After a previous post about the use of the accelerometers (here) I’m describing how to use the GPS module.

The embedded GPS is based on a UBX G7020 chip. This chip will give you the date, time, longitude, latitude, altitude, speed and direction of the device when activated.

Read more for programming details

Kickstart

To start using the GPS module, you will simply use this skeleton code

#include "config.h"
#include <efm32.h>
#include <em_cmu.h>
#include <em_adc.h>
#include <em_rmu.h>
#include <td_core.h>
#include <td_geoloc.h>
#include <nmea_parser.h>

#include <stdint.h>
#include <stdbool.h>

#include <td_config.h>

/* The GPSFix function is a callback function periodically called
 * by the GPS_Process
 */
static void GPSFix(TD_GEOLOC_Fix_t *fix, bool timeout)
{
    // Fix type is giving what level of information did we get
    // from the GPS system. Quality is indicating the precision of the 
    // given information. > 1000 is bad, < 1000 is acceptable, < 500 is good
    // <200 is excellent. 
    if (fix->type >= TD_GEOLOC_2D_FIX && fix->quality.hdop < 500 ) {
	lastLatitude = fix->position.latitude;
	lastLongitude = fix->position.longitude;
	TD_GEOLOC_StopFix(TD_GEOLOC_HW_BCKP);
    }
    if ( timeout ) {
	TD_GEOLOC_StopFix(TD_GEOLOC_HW_BCKP);
    }
}

void TD_USER_Setup(void)
{
   // Set the device as a Sensor transmitter
   // We are not using TD_SENSOR stuff but this call is needed
   // otherwize the GPS will not work
   TD_SENSOR_Init(SENSOR_TRANSMITTER, 0, 0);
   
   // Launch a GPS capture request with a timeout of 120s
   // The callback function is GPSFix.
   TD_GEOLOC_TryToFix(TD_GEOLOC_NAVIGATION, 120, GPSFix); 
}

void TD_USER_Loop(void)
{
   TD_GEOLOC_Process();
}

So the system has to be initialized then the TD_GEOLOC_TryToFix starts the GPS capture process. Once start, the callback function will be called in a regular basis. The GPS information will be updated asynchronously.

The fix->type variable will indicate the level of the captured information from the GPS system.

So callback after callback the information will be more and more precise. You will decide in the callback function when it is precise enough for you and decide to stop tracking or not.

After a given time, the  timeout flag will be set. There is no other action taken by the system when timeout is set. It means you have to manage it yourself and decide, as in the example, if you want to stop track after timeout.

Power mode

The GPS module have different PowerMode. To start tracking a position, you need to use TD_GEOLOC_NAVIGATION or TD_GEOLOC_POWERSAVE. Both are running GPS tracking.

• TD_GEOLOC_NAVIGATION will generate a load of 29-32mA on the battery during all the tracking activity. The duration of a first Track will be about 60 seconds dépendending on reception conditions. The next one will be about 15 seconds.
• TD_GEOLOC_POWERSAVE_MODE : this mode is like the previous one, the current consumption when fixing is between 27 and 32 mA but once fixed, the power consumption goes down to 10-14 mA. This option looks good for recording a path but seem to not provide a great saving for a single fixing.

When you have captured a location, you can stop or sleep the device until the next one. In this case, these mode will be used

• TD_GEOLOC_HW_BACKUP will sleep the device but it will keep the RTC and the RAM up. So the next synchronization will be done faster. The measured total consumption is around 15uA in this mode.
• TD_GEOLOC_SW_BACKUP should be equivalent but more efficient. But it seems to not yet implemented on the TD1204.
• TD_GEOLOC_OFF is stopping the GPS … Next start will be longer, at the end it will be efficient only if you have a long time between 2 measure. It not sure you are saving a lot of uA by using this mode.

To be more clear with GEOLOC_OFF : if you consider that consumption is 30mA for 60s to get a cold start fix you need about 0,5mAh.This is what you need each time if you switch OFF between capture. So per Hour with a capture every 15 minutes : 2mAh

To compare, if you go with HW_BACKUP, you will have an extra consumption of 5-7uA (?) and a fix time of only 15s after the first one so you will need 0,1mAh each. So for an hour : 0,5mAh + 3×0,1mAh   = 0,8mAh. The gain will be higher the next hour …

Callback function

The callback function is regularly called by TD_GEOLOC_Process(), it is about twice a second. Each time it is called you get more and more data. Localization becomes more accurate and you get a 2D, then 3D localization.

Basically you have to wait the expected status to get your data. fix->type help you to know what is the current status of the data. A first step is to get the time, a second step is to gate the date, then you get the 2D localization and after you are getting the 3D localization.

As the callback is just a flag, you have to decide what to do in case of timeout, so free to you to stop tracking or not.

If you stop tracking, the best is to switch to TD_GOLOC_HW_BACKUP to be ready for a next fix.

Precision of the localization

The accuracy of the localization is with fix->quality.hdop This is a good variable to get the quality of the localization. A value of 1000 is really poor, 500 is good, 200 is excellent.

According to some Internet search, an estimator of the precision for civil GPS use is 7m * Hdop / 100. So in this example with a hdop of 500 the result would be a precision of 35m.