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Question

What is the difference between an active and a passive GNSS antenna?

Solution

An active antenna has an low-noise amplifier built in. This provides gain to help overcome coax cable losses and at the same time provides the proper signal level to the GNSS receiver. The drawback is that an active antenna requires an external power source. Some module manufacturers makes their mPCIe modules provide the power. Others need a biased tee in between to avoid damaging the module.

A passive antenna doesn't have the low-noise amplifier. This means that the signal may be weaker when it's received by the module so a fix may take longer time, but there's no need for an external voltage source.

The general rule is that if the length of the cable is under one meter there's no need for an active antenna, but longer cable with an passive antenna works if the signal from the satellite is good enough.

Question

What does A-GPS mean?

Solution

Assisted GPS means that the module can use the cellular network to help the GPS get a fix if the connection to the GPS satelite is poor.

Question

I can see that NMEA data is sent from the module, but I can't get a fix from the GPS. What can I do?

Solution

When looking for a fix from GPS satelites, one has to keep in mind that it can take some time if the GPS is standalone. It can take up to 15 minutes before a fix is reached.

If there still is no fix you can try the following.
* Shorten the cable to the antenna
* Use an active antenna
* Set parameters for network assistance
* Make sure that there are no interference from other devices affecting the cable.

Question

The module states that it supports GNSS. What is the difference between GPS and GNSS?

Solution

GNSS is a collection of different techniques for global positioning. GNSS stands for Global Navigation Satelite System, and in GNSS different systems are included. It includes Global Positioning system (GPS), Global Navigation System (GLONASS) and Galileo.

If a module support GPS, it can only use the GPS satelites. If it's supporting GNSS there will be stated in the specifications which of the satelites it can use.

Question

I can see that my module supports GLONASS? What is that?

Solution

GLONASS is the Russian variant of the American GPS. There is no clear advantage in form of coverage or accuracy when using either of the technology, but when used together you will have a total of 55 satellites to help you pin-point your position.

Question

The Sierra Wireless EM74-series modules cannot get a fix on GPS/Satellite navigation system even if I have connected the antenna correctly?

Solution

The Sierra Wireless EM74-series modules can be adjusted to read the GPS signals from either the AUX/Diversity antenna connector or the dedicated GPS antenna connector on the module.

This can be configured through the AT!CUSTOM commands:
GPSSEL
0 = Use dedicated GPS antenna (Default)
1 = Use shared GPS/Rx diversity antenna

For example like this:
AT!ENTERCND="A710"
AT!CUSTOM="GPSSEL",0
AT!RESET

Now the dedicated GPS antenna port on module will be used for satellite location services.

Question

How can I enable DC voltage output for active GPS/GNSS antennas on modules that support this feature?

Solution

The 3 volt DC Low Noise Amplifier voltage supply from the cellular module can be enabled according to AT commands bellow:

For Sierra Wireless EM/MC74 series:
AT!ENTERCND="A710"
AT+WANT=1

For Sierra Wireless MC73 series:
AT!ENTERCND="A710"
AT+WANTGNSSPWR=1

For Simcom SIM7100, SIM7500, SIM7600 and SIM7600 -H series:
AT+CVAUXV=3050
AT+CVAUXS=1

Question

How do we use the GNSS location tracking (GPS) functionality of the Sierra Wireless EM/MC74** series cellular modules?

Solution

The Sierra Wireless EM/MC74 series modules include a GNSS tracker that can be enabled to acquire location data.

Pre-requirements:
EM/MC74 series module:
Connected to host system with sufficient power supply and correct drivers installed in the host system.

GNSS/GPS antenna:
The GNSS antenna should have clear sky view without obstacles in the way and be connected to the cellular modules antenna connector marked "GPS". If a passive antenna is used the antenna cable should be kept very short (preferably bellow 1 meter) as GNSS signals are very weak and not placed near interference sources.

Details of the available AT commands related to GNSS functionality are described in the "Sierra Wireless EM74XX and MC74XX AT Command Reference Manual" chapter 7: GNSS Commands.

The serial interface accepting AT commands is usually found under /dev/ttyUSB2 in Linux systems and under Modem tab in Windows Device manager (opening the device properties and Modem tab will show its given COM port number in the Windows system).

A basic GNSS tracking session outputting the location data on the NMEA serial interface can be started with the following AT command
AT!GPSTRACK=fixType,maxTime,maxDist,fixCount,fixRate

Bellow command starts a standalone tracking with 255 seconds timeout value, 250 meter accuracy, continuous tracking, fixed data output rate every second on the NMEA virtual serial interface:
AT!GPSTRACK=1,255,250,1000,1

Depending on if an GNSS antenna with or without low noise amplifier (active/passive antenna) are used, the AT+WANT command can adjust if the antenna power supply of 3 volt should be enabled or disabled.
Passive GPS antenna:
AT+WANT=0
Active GPS antenna:
AT+WANT=1

A GNSS tracking session can be interrupted and ended with the command:
AT!GPSEND

In Linux applications it is recommended to use the Sierra Wireless provided GobiNet and GobiSerial drivers for optimal performance. If the Linux in-kernel drivers are used, it might be needed also to send the bellow sentence to the ttyUSBx that represents the NMEA serial interface in order to activate the data stream on the interface:
$GPS_START

The antenna connector used by the GPS tracker can be either through the dedicated GNSS port or it can be combined with the Rx Diversity antenna located on the AUX marked connector. This is selected by changing a parameter with the AT!CUSTOM command:

To select usage of the dedicated GNSS antenna connector on cellular module, execute:
AT!ENTERCND="A710"
AT!CUSTOM="GPSSEL",0
This will select usage of the dedicated GPS antenna slot (Default value)

For further details on how to configure the GPS for different application and scenarios, such as using Assisted GPS, or changing output formats etc. please relate to the AT commands reference.

Question

How do we enable basic standalone GPS tracking with the SIMcom cellular modules that include GNSS tracking feature?

Solution

The SIM7100 series, SIM7500 series SIM7600 series and SIM7600 -H series cellular modules that include a GNSS tracker can be used to acquire current accurate location data with and active or passive GNSS antenna connected to the module.

Details of the available AT commands related to GNSS functionality are described in the AT command reference manuals under GPS section. The AT command reference manuals can be found under our product web pages under technical documentation tab.

Depending on if an GNSS antenna with or without low noise amplifier (active/passive antenna) are used, the AT+CVAUXV and AT+CVAUXS command can be used to adjust if the antenna power supply of 3 VDC should be enabled or disabled on the GNSS antenna connector.

Module configured for active GNSS antenna with 3VDC power supply enabled directly from module:
AT+CVAUXV=3050
AT+CVAUXS=1

Module configured for passive GNSS antenna and antenna voltage supply from cellular module is disabled:
AT+CVAUXS=0

A basic standalone GNSS tracking session outputting the location data on the dedicated NMEA virtual serial interface can be started with the following AT command:
AT+CGPS=1,1

Other useful commands:
(Please note that GNSS configuration parameters can only be changed when tracking is not active)

Stop ongoing GNSS tracking:
AT+CGPS=0

Clear GNSS fix related data stored in module:
AT+CGPSDEL

Set minimum accuracy threshold in meters for location fix (default value 50 meters):
AT+CGPSHOR=50

Enable autostart of GNSS tracking at module start up:
AT+CGPSAUTO=1

The latest GNSS fix can also be requested by AT command:
AT+CGPSINFO
And the output is in the form:
+CGPSINFO: [lat],[N/S],[log],[E/W],[date],[UTC time],[alt],[speed],[course]

For details on how to configure the GPS for different application and scenarios, such as using Assisted GPS, or changing output formats etc. please relate to the AT commands reference manuals for the specific SIMcom module.