Overview
This guide explains GNSS positioning functions available on compatible Siretta industrial modems. It covers GNSS activation, NMEA data, satellite positioning, antenna considerations, live NMEA streaming and troubleshooting GNSS performance.
| Part Number | Description | |
|---|---|---|
| ZETA-GEP-LTE4 (EU) | Low Power LTE Cat 4 European Modem with GPIO and GNSS | |
| ZETA-G-GPRS | GSM/GPRS Modem with GNSS |
This guide demonstrates how to:
Expected Result
At the end of this guide the modem should be capable of obtaining and reporting valid GNSS position information.
GNSS (Global Navigation Satellite System) is a satellite-based positioning technology that allows a modem to determine its geographic location, altitude, speed and time using signals transmitted by orbiting satellites.
Unlike cellular positioning, GNSS does not require network coverage and can operate anywhere with a sufficiently clear view of the sky.
Modern GNSS receivers typically support multiple satellite constellations simultaneously, improving positioning accuracy, satellite availability and acquisition speed.
| Constellation | Operator | Primary Coverage | Approximate Satellites | Orbital Altitude |
|---|---|---|---|---|
| GPS | United States | Global | 31+ | 20,200 km |
| GLONASS | Russia | Global | 24+ | 19,100 km |
| Galileo | European Union | Global | 24+ | 23,222 km |
| BeiDou | China | Global | 35+ | 21,500 km (typical) |
| QZSS | Japan | Japan & Asia-Pacific | 4+ | 32,000–40,000 km |
| NavIC | India | India & Surrounding Region | 7+ | 35,786 km |
Modern GNSS receivers often combine satellites from multiple constellations simultaneously.
Benefits include:
For example, a GPS-only receiver may see 8–12 satellites, whereas a multi-constellation GNSS receiver may simultaneously track 20–40 satellites.
Most GNSS satellites operate in Medium Earth Orbit (MEO), approximately 19,000–24,000 km above the Earth's surface.
At these altitudes, a relatively small number of satellites can provide continuous global coverage.
Signals transmitted from GNSS satellites typically take approximately 65–85 milliseconds to reach a receiver on Earth.
Typical civilian positioning accuracy is:
| Environment | Typical Accuracy |
|---|---|
| Open Sky | 1–5 metres |
| Urban Environment | 5–20 metres |
| Indoor Location | Poor or unavailable |
Position accuracy is influenced by:
Important Note
Supported GNSS constellations vary depending on modem hardware, firmware version and GNSS chipset capabilities. Some modem variants may support GPS only, while others support simultaneous multi-constellation operation.
Reliable GNSS operation requires:
| Recommendation | Reason |
|---|---|
| Use active GNSS antenna if supported | Improves sensitivity |
| Position antenna outdoors where possible | Improves satellite visibility |
| Avoid metal obstructions | Reduces signal attenuation |
| Keep antenna horizontal | Optimises reception pattern |
Important Note
GNSS performance indoors is often significantly reduced.
Before GNSS data can be read, the GNSS receiver must be enabled.
| COM3 - Tera Term VT |
¶ File Edit Setup Control Window Help |
|
AT$GPSP=1 |
| Command | Description |
|---|---|
AT$GPSP=1 |
Enables the GNSS receiver |
Expected Result
The modem should enable the GNSS receiver successfully.
| COM3 - Tera Term VT |
¶ File Edit Setup Control Window Help |
|
AT$GPSP=0 |
Once GNSS is enabled and satellites are visible, position information can be retrieved.
| COM3 - Tera Term VT |
¶ File Edit Setup Control Window Help |
|
AT$GPSACP $GPSACP: 12:30:15.000,5151.1234N,00107.5678W,84.0,0.13,309.62,3,08,1.2,0.9,120526,06 OK |
| Field | Description |
|---|---|
| UTC Time | GNSS UTC time |
| Latitude | Latitude coordinate |
| Longitude | Longitude coordinate |
| Altitude | Height above sea level |
| Speed | Movement speed |
| Heading | Direction of travel |
| Fix Type | GNSS fix validity |
| Satellites | Number of satellites used |
| HDOP | Horizontal accuracy |
| VDOP | Vertical accuracy |
| Date | Current GNSS date |
| GNSS Mode | Satellite system mode |
Important Note
Position data may be invalid until a successful satellite fix has been obtained.
DOP (Dilution of Precision) values provide an indication of the geometric quality of the satellite constellation being used to calculate the position fix.
Lower DOP values generally indicate better positioning accuracy.
| Parameter | Full Name | Description |
|---|---|---|
| PDOP | Position Dilution of Precision | Overall three-dimensional position accuracy estimate. |
| HDOP | Horizontal Dilution of Precision | Horizontal position accuracy estimate. |
| VDOP | Vertical Dilution of Precision | Vertical position accuracy estimate. |
| DOP Value | Accuracy Assessment |
|---|---|
| < 1 | Ideal |
| 1 - 2 | Excellent |
| 2 - 5 | Good |
| 5 - 10 | Moderate |
| 10 - 20 | Poor |
| > 20 | Unusable |
Important Note
DOP values describe the geometry of the visible satellites and not the strength of the received signals. Lower values indicate a more favourable satellite arrangement and generally result in more accurate position calculations.
Many GNSS-enabled modems support NMEA sentence output.
| Sentence | Full Name | Description |
|---|---|---|
| GGA | Global Positioning System Fix Data | Provides latitude, longitude, altitude, fix quality and satellite count. |
| GLL | Geographic Position – Latitude/Longitude | Provides current latitude and longitude coordinates. |
| GSA | GNSS DOP and Active Satellites | Provides active satellites used in the solution together with PDOP, HDOP and VDOP values. |
| GSV | GNSS Satellites in View | Provides information about all visible satellites including elevation, azimuth and signal strength. |
| RMC | Recommended Minimum Navigation Information | Provides time, date, position, speed and course information. |
| VTG | Course Over Ground and Ground Speed | Provides heading, track angle and movement speed. |
Compatible Siretta GNSS-enabled modems can stream live NMEA sentences directly from the GNSS receiver over the serial interface.
The AT$GPSNMUN command enables or disables individual NMEA sentence types.
| COM3 - Tera Term VT |
¶ File Edit Setup Control Window Help |
|
AT$GPSNMUN=2,1,1,1,1,1,1 |
| Parameter | Description |
|---|---|
2 |
Continuous NMEA streaming mode |
| Remaining parameters | Enable individual NMEA sentence types |
| COM4 - GNSS NMEA Output |
¶ File Edit Setup Control Window Help |
|
$GPGGA,123015.000,5151.1234,N,00107.5678,W,1,08,0.9,84.0,M,0.0,M,, |
Expected Result
The modem should continuously output live NMEA sentences while GNSS is active.
The modem also supports a dedicated GNSS streaming mode where the serial interface is converted into a continuous live GNSS data stream.
In this mode:
+++ escape sequence is required to return to command mode| COM4 - GNSS Streaming Mode |
¶ File Edit Setup Control Window Help |
|
AT$GPSNMUN=3,1,1,1,1,1,1 CONNECT |
| Parameter | Description |
|---|---|
3 |
Dedicated online streaming mode |
| Remaining parameters | Enable NMEA sentence types |
Important Note
Once enabled, the modem no longer accepts AT commands until online mode is exited.
| COM4 - GNSS Online Stream |
¶ File Edit Setup Control Window Help |
|
$GPGGA,123015.000,5151.1234,N,00107.5678,W,1,08,0.9,84.0,M,0.0,M,, |
To return to AT command mode, enter the standard modem escape sequence:
+++
| COM4 - GNSS Online Stream |
¶ File Edit Setup Control Window Help |
|
+++ |
Important Note
A guard time is normally required before and after the+++escape sequence.
| Problem | Possible Cause | Solution |
|---|---|---|
| No GNSS fix | Poor sky visibility | Move antenna outdoors |
| Invalid coordinates | GNSS not initialised | Wait for satellite lock |
| Slow TTFF | Cold start | Allow additional acquisition time |
| No NMEA output | GNSS disabled | Enable GNSS receiver |
| Streaming mode locked | Online mode active | Exit using +++ |
| Step | Action |
|---|---|
| 1 | Connect GNSS antenna |
| 2 | Enable GNSS receiver |
| 3 | Verify GNSS position |
| 4 | Configure NMEA output |
| 5 | Enable streaming mode |
| 6 | Verify NMEA data |
| 7 | Exit streaming mode if required |
Support
For further assistance, please contact the Siretta support team.