To interpret GPS frames, it is important to know and understand some concepts. Therefore we invite you to take note of this useful information.
NMEA Protocol
A GPS delivers its data through a protocol called NMEA (National Marine Electronics Association). Therefore most GPS receivers can communicate with each other, using this protocol.
The NMEA 0183 standard or format is a language in electronics. It is used globally in all satellite navigation devices.
A GPS returns a line of data, called a “frame”, with its respective regulatory separators and in some cases identification letters or symbols.
GPS Frames
The GPS return information through lines or data frames in BINARY or ASCII code depending on the device.
A data frame is a line of code that contains all the information evaluated by the tracking device.
It includes data such as: update time, position coordinates (latitude, longitude), orientation, percentage of internal battery, analog inputs, among others.
Additionally, GPS frames contain regulatory separators (usually commas) and identification letters or symbols.
In turn, the frames are made up of subframes that contain device-specific information.
In certain devices such as Queclinks, the data is transmitted through frames or sentences with ASCII characters, separated by commas. Each sentence is totally independent of others. It begins with the “$” sign and ends with a sequence of <CR> <LF> (carriage return, line feed).
The first two characters after the “$” sign, identify the team. For GPS receivers, the prefix is GP. This is followed by a sequence of three letters that define the type of information that is being sent. Example: GPGGA, GPGLL, GPGSV.
Examples:
$ GPGGA, 123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M, 46.9, M,,*47
$ GPGSV, 2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75
Using software provided by the manufacturer, you can configure which GPS frames you want to receive.
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How are GPS frames interpreted?
To send the information of a device protocols are used. These vary depending on the model of the same or the manufacturer. Several brands share the same manufacturer and the same protocol.
To be able to interpret GPS frames, the device’s protocol file must be taken into account. If the device is approved for a platform, the file with the protocol can be provided.
In this file you can find information about the construction of the plot; the information you can send, as well as all that is necessary to know the content of the line of code.
What does each symbol mean within a frame?
To better understand this topic, let’s look at an example.
Assuming that the GPS frames reach us in hexadecimal, like the following text.
HEX:
2b524553503a47544652492c3237303930332c3836373136323032363539303531332c2c2c31302c312c312c302e302c3237332c3635342e332c2d38392e3138353139392c31332e3731323930352c32303230303830333231333535302c303730362
c303030312c303139342c313333303743452c30302c31323830362e352c2c2c2c39372c3232303130312c2c2c2c32303230303830333231333630302c3846363424
The first thing to do is convert it to ASCII in order to evaluate the frame. For this we can use a translator or converter.
Once converted, in the case of Queclink devices, we will have a text similar to the one below. Each part of the plot (subplot) is separated by commas and has a meaning.
+RESP:GTFRI,270903,867162026590513,,,10,1,1,0.0,273,654.3, -89.185199,13.712905,20200803213550,0706,0001,0194,13307CE,00,12806.5,,,,97,220101,,,,20200803213600,8F64$
For example:
- + RESP: shows that it is a response frame. It is the first information sent by the GPS.
- GTFRI: is the acronym for “Fixed Report Information”. Describe the plot as an update. Depending on the information sent by the device, it can also be an on, on, off, command received plot, etc. It depends on the device and what it evaluates.
- 867162026590513: is the IMEI of the device. It consists of a series of 15 or 17 numbers that form a unique identification code for said device.
- -89.185199,13.712905: these are the coordinates of the device position (latitude and longitude).
- 20200803213550: is the date (year, month, day); followed by the time in format (hours, minutes and seconds)
You should know that depending on the protocol of each device, the frames are decoded and read. In addition, the order as well as all the information can arrive in a different way, depending on the protocol.
For example, in the case of COBAN devices, the frame is sent as follows (already translated into ASCII):
imei: 864893035517591,tracker,200820212337,,F, 212337.00,A,2516.15507,S,05735.12191,W,,;
At DeltaTracking we use software that analyzes GPS frames and translates the data into an understandable format.
For more information, contact us.