File formats

NMEA File Format

Simulation PSY can read files that contain NMEA sentences with NMEA 0183, version 2.1.

The NMEA file is in 7-bit ASCII format (see NMEA 0183 version 2.1). All lines end with the character ’\n’. The NMEA sentences are case sensitive.

Simulation PSY parses position information from the NMEA sentences GGA, RMC, GSA and GSV. Data from other sentences are not parsed. The NMEA sentences GLL, GGA, GSA, and RMC are required to make a location estimate.

Simulation PSY has to understand which sentences constitute the location estimate. When a required NMEA sentence is read for the second time, the parsing of NMEA data stops, and Simulation PSY checks if all required sentences have been read. The location estimation continues if all the required sentences have not been read.

When the end of a file is reached, Simulation PSY starts re-reading again from the beginning of the file. The parser is reset, meaning that all the required sentences need to be read again starting from the beginning of the file before a location estimate is returned.

Satellite time is read from the RMC sentence and stored in the incoming position class if it is an instance of TPositionSatelliteInfo or HPositionGenericInfo. In the latter case, satellite time is stored only if it is a requested field in the HPositionGenericInfo instance. System time is set in the base class TPositionInfo at each position reading.

The following table shows which position data, parsed from an NMEA data file, each position class supports. Basic info in the table below indicates that TPositionInfo supports position data. All read NMEA sentences are included in HPositionGenericInfo if EPositionFieldNMEASentences is requested.

An example of correct NMEA file content:

$GPGSV,3,2,12,24,32,051,,22,21,292,,01,20,315,,14,20,321,*71
$GPGSV,3,3,12,05,19,134,,13,18,348,,17,14,180,,10,10,110,*7F
$GPGLL,6459.8757,N,01433.8547,E,091931.375,A*3E
$GPGGA,091931.38,6459.8757,N,01433.8547,E,1,04,2.0,-0034,M,,,,*3D
$GPRMB,A,0.21,L,SIM003,SIM001,6500.0900,N,01433.8589,E,000.2,00..,000.0,V*18
$GPRMC,091931.38,A,6459.8757,N,01433.8547,E,21.7,177.3,200302,02.,E*6F
$GPAPB,A,A,0.2,L,N,,,87.6,M,SIM001,358.1,M,,,*16
$GPGSA,A,3,01,02,03,04,,,,,,,,,1.0,2.0,3.0*34

Position information is obtained after parsing the GLL, GGA, RMC and GSA sentences. Latitude is parsed from “6459.8757,N”, longitude from “01433.8547,E”, altitude from “-0034”, and the value “2.0” is used for calculating horizontal and vertical accuracy. All retrieved from GGA sentence.

$GPGLL,6459.8757,N,01433.8547,E,091931.375,A*3E
$GPGSV,3,1,12,06,53,095,,30,52,150,,15,51,090,,25,37,230,*7F
$GPGSV,3,2,12,24,32,051,,22,21,292,,01,20,315,,14,20,321,*71
$GPGGA,091931.38,6459.8757,N,01433.8547,E,1,04,2.0,-0034,M,,,,*3D
$GPRMB,A,0.21,L,SIM003,SIM001,6500.0900,N,01433.8589,E,000.2,00..,000.0,V*18
$GPRMC,091931.38,A,6459.8757,N,01433.8547,E,21.7,177.3,200302,02.,E*6F
$GPAPB,A,A,0.2,L,N,,,87.6,M,SIM001,358.1,M,,,*16
$GPGSA,A,3,01,02,03,04,,,,,,,,,1.0,2.0,3.0*34

The first line in the following textbox starts with a GLL sentence which has already been parsed. The other three sentences required for calculating position information, i.e. GGA, RMC and GSA, have also been parsed. Hence a second position fix is obtained from the parsed sentences.

$GPGLL,6459.8757,N,01433.8547,E,091931.375,A*3E
$GPGSV,3,1,12,06,53,095,,30,52,150,,15,51,090,,25,37,230,*7F
$GPGSV,3,2,12,24,32,051,,22,21,292,,01,20,315,,14,20,321,*71

Rest of NMEA file.

An example of incorrect NMEA file content..

$GPGSV,3,2,12,24,32,051,,22,21,292,,01,20,315,,14,20,321,*71
$GPGSV,3,3,12,05,19,134,,13,18,348,,17,14,180,,10,10,110,*7F
$GPGLL,6459.8757,N,01433.8547,E,091931.375,A*3E
$GPGGA,091931.38,6459.8757,N,01433.8547,E,1,04,2.0,-0034,M,,,,*3D
$GPRMB,A,0.21,L,SIM003,SIM001,6500.0900,N,01433.8589,E,000.2,00..,000.0,V*18
$GPRMC,091931.38,A,6459.8757,N,01433.8547,E,21.7,177.3,200302,02.,E*6F
$GPGGA,091931.38,6434.8744,N,01343.8637,E,1,04,2.0,-0032,M,,,,*3D

GGA sentence exists for the second time before a GSA sentence is read. Location estimation for read NMEA sentences is not used. Simulation PSY will continue reading NMEA sentences trying to read a correct fix.

The Simulation PSY supports the NMEA sentences from the following GPS devices:

• Garmin:

  • eTrex – basic

  • eTrex – Legend

  • III plus

  • 12 cx

• Magellan:

  • GPS 315

  • Meridian Gold

  • Tracker

Simulated Movement Format

Simulation PSY can generate location data from simulated information.

The Simulated Movement file has the following format:

Horizontal accuracy={float};
Vertical accuracy={float};
TimeToFix min={integer};
TimeToFix max={integer}; 
Powerup time={integer};
Longitude={float};
Latitude={float};
Speed={float};
Course={float};
[Deterministic|Random]={integer};

The Simulated Movement file is in 7-bit ASCII format. All lines end with the character ‘\n‘. The Simulated Movement file words are case insensitive. The Simulated Movement file is read when a sub-session to Simulation PSY is opened.

The Simulation Movement file words before the ’=’ mark must have spaces as shown above. However, it is legal to have spaces before and after the ’=’ mark and before the semicolon (;).

All Simulated Movement sentences end with a semicolon (;).

No comments are allowed in the Simulated Movement file.

The order of different lines does not matter, but it is required that all simulated movement sentences exist in the file.

The float value decimal separator is a period (.).

If Speed is not 0, new locations are calculated for each location estimate.

The Course value is the direction of the simulated movement (0 and 360 = North, 90 = East, 180 = South and 270 = West).

The last row specifies how to simulate errors. If Deterministic is used, positioning fails at the specified frequency. Deterministic=3 means that every third request fails. Random means that positioning fails sporadically, but at the specified mean frequency. Random=3 means that after a large amount of requests, one third of the requests have failed. Error code KPositionQualityLoss will be returned.

It is also possible not to simulate errors. This is specified by setting Deterministic=0.

Horizontal and vertical accuracy, are same for all location estimates. For each new location estimate, horizontal accuracy is multiplied with a random generated value. The multiplied value is added as “error” for longitude and latitude to the location.

Altitude always starts at 0 meters over the WGS-84 ellipsoid model. For each new location estimate, vertical accuracy is multiplied with a random generated value. The multiplied value is added to the current altitude.

If the syntax of the file is incorrect or all settings are not present, the system wide error code KErrCorrupt is returned. This error code is returned when opening the positioner, RPositioner::Open().

For example, if you want to simulate the locations of a car that drives 20 m/s (72 km/h) straight to the North and every seventh location request fails:

Horizontal accuracy=20;
Vertical accuracy=30;
TimeToFix min=2;
TimeToFix max=7; 
Powerup time=3;
Longitude=11.34;
Latitude=57.11;
Speed=20;
Course=0;
Deterministic=7;

Using this example would give locations between 2 and 7 seconds at each location update that is requested, except for the first request when a powerup time of three seconds is added.