Global Navigation Satellite System (GNSS)

A satellite navigation system is a system that uses satellites to determine its geographical position. When such a system has global coverage, it is a global navigation satellite system (GNSS). There are currently four fully operational GNSSs; GPS, GLONASS, BeiDou and Galileo, with GPS being the most mature technology consisting of 31 functional satellites.

Obtaining a fix

GNSS satellites orbit the earth on a known orbital path and transmit navigation messages to any GNSS receiver. These messages are then used to obtain the receiver’s position, also known as a fix.

Definition

GNSS fix (position fix): The result of the GNSS positioning results in a fix, indicating the receiver’s location with a given accuracy.

There are four unknowns when looking for a GNSS fix, latitude, longitude, altitude and clock bias, meaning a minimum of four satellites must be tracked to acquire a fix.

The GNSS receiver is able to calculate a PVT solution (Position, Velocity, Time) once it has at least 4 satellites from the same satellite system. However, the PVT solution isn’t necessarily a valid fix, which will depend on the accuracy of the solution.

To be able to calculate a fix, the GNSS receiver also needs information about the satellites, called the ephemeris and the almanac.

The ephemeris is unique to each satellite and contains information about the satellite such as the satellite accuracy and health, age of data, satellite clock correction coefficients and orbital parameters. It is only valid two hours before and two hours after the Time of Ephemeris (ToE).

The almanac is the same for all satellites and contains less accurate orbital information. It is valid for a period of up to 90 days, and is used to speed up the time to first fix (TTFF) by 15 seconds.

Time To First Fix

The Time To First Fix (TTFF) denotes the time it takes from the GNSS receiver is turned on until it has calculated a valid fix.

The TTFF is usually divided into three specific scenarios, depending on the start conditions.

ConditionUse-caseAlmanacEphemerisPositionTimeTTFF
Cold startFirst useUnknownUnknownUnknownUnknown2-4 minutes
Warm startDevice off for a dayKnownUnknown or outdatedWithin 100 km of last fixWithin 10 min precision45 seconds
Hot startDevice off for one or more hoursKnownKnownWithin 100 km of last fixWithin 10 min precision22 seconds

NMEA messages

NMEA is an ASCII representation of the GNSS data and is a standard data format supported by all GPS manufacturers. It has a standardized message structure that can be useful in certain scenarios.

For NMEA, you can select the frames you are interested in.

Below are all the NMEA messages supported by the nRF9160 and what they contain.

  • NMEA GGA – global positioning system fix data
  • NMEA GLL – geographic position
  • NMEA GSA – GNSS DOP and active satellites
  • NMEA GSV – satellites in view
  • NMEA RMC – recommended minimum specific GNS data

See the blog post Field verification of GNSS on the nRF91 Series for more detailed information.

GNSS TTFF improvements

To decrease the time it takes to acquire a fix, there are a few methods you can implement. We won’t go into much detail as that is beyond the scope of this course, but a few of the methods are presented briefly below.

Assisted GPS: Utilizes the cellular connection to retrieve the Almanac and Ephemeris data over the internet, which is several times quicker than using the data link to the satellites. Using A-GPS can improve the TTFF down to less than 5 seconds in a cold start, which also greatly reduces power consumption.

Predicted GPS: The device downloads up to two weeks of predicted satellite Ephemerides data to determine the exact orbital location of the satellite without having to download the information every two hours. This results in reduced accuracy over time. P-GPS is useful for devices that either cannot connect to the cellular network or don’t want to, in order to save power, but still want to reduce the TTFF.

Low accuracy mode: Lets the GNSS receiver accept a looser criterion for a fix with 4 or more satellites, or by using a reference altitude to allow for a fix with 3 satellites. This results in reduced accuracy of the fix. This is not a GNSS enhancement, but is considered an improvement in the TTFF.

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