GPS communication applications

[ak0b]

GPS has various functions which are applicable to communications projects. The primary cost item is the receiver and active antenna. Two things I have always looked for in a GPS module is NMEA data string and the 1 pulse per second output.

You can decode the NMEA string for location and time (field tracking, etc.) and/or you can use the 1 pps output to lock your local frequency counter and standard.

Many surplus 12 ch gps receivers have these outputs and can be found on the market. If you search the web you will find numerous DIY projects using the NMEA and the 1 pps outputs. I have also found the low cost handheld TREK GPS units in sporting stores for under $100 have the NMEA output, but the 1 pps is not available on them without doing internal surgery.

The full NMEA specification and protocol is available on the web. Two things one must remember when using these receivers. The location (+/-) is the location of the antenna and if you are forwarding the data to another location then you will have a timing and turn around error due to the transmission length and if other activity is on the same wireless channel. Tree cover will affect the accuracy and limestone cliffs are really bad on tracking. GPS does not work inside of a building you need a clear line of sight with the satellites.

I would be very interested in what others are doing with a GPS receiver and a PICAXE chip.

I am currently working on being able to track low cost wireless sensors and plan on using the 1 pps output to sync my transmissions.

Stan AK0B

 

[radcom]

Try looking at a delorme tripmate

I believe they (delorme tripmates) output a NMEA string @ 1 pulse per second with out any internal modification there is a string that needs to be sent to it to wake it up which if I remember is "astral" but it needs checking.
it should be relativity simple to do with a picaxe as I was looking in to interfacing my tripmate with a picaxe put I haven't got round to it yet as I have a lot of coursework

I just don't know how to interpret the strings so any help on that would be useful for me

 

[ak0b]

A typical GPS receiver data stream example

A standard GPS interface protocol NMEA (National Marine Electronics Association) 0183, is often used for real-time tracking and autopilot systems. Most Garmin products output their data in this format so they can easily talk to external devices. Garwin typically uses the GGA sentence format

The data is usually RS-232 compatible and it uses 4800 bps, 8 data bits, no parity and one stop bit ( 8N1 ). NMEA 0183 sentences are all ASCII. Each sentence begins with a dollarsign ($) and ends with a carriage return linefeed. Data is comma delimited. All commas must be included as they act as markers.

Some GPS units do not send some of the fields. A checksum is optionally added (in a few cases it is mandatory). Following the $ is the address field aaccc. aa is the device ID. GP is used to identify the GPS sentence data. Transmission of the device ID is usually optional. ccc is the sentence formatter, otherwise known as the sentence name (following example is GAA).

A sample NMAE 0183 sentence may look like this:
$GPGGA,121505,4807.038,N,01131.324,E,1,08,0,9,133.4,M,46.9,M,,*42

- $GPGGA is the NMEA 0183 sentence ID for the GPS fix data.
- 121505 is the fix taken at 12:15:05 UTC
- 4807.038, N is latitude 48d 07.038'N
- 01131.324,E is longitude 11d 31.324'E
- 1 is the fix quality. The fix quality can have a value between 0 and 3, defined as follows
- 0=no fix
- 1=GPS or standard positioning service (SPS) fix
- 2=DGPS fix
- 3=Precise positioning service (PPS) fix
- 08 is the number of satellites being tracked
- 0.9 is the horizontal dilution of position (HDOP)
- 133.4,M is the altitude, in meters, above mean sea level
- 46.9,M is the height of the geoid (mean sea level) above the WGS84 ellipsoid
- (empty field) is the DGPS station ID number
- *42 is the checksum field

As you can see nothing really hard about being able to decode the stream into a clock or location. You do need to verify which sentence structure is being used by your receiver in some cases you tell the receiver which format you desire check you instruction book.

Stan ak0b

For Reference:

The full NMEA-0183 standard is available from:
National Marine Electronics Association
NMEA Homepage: http://www.nmea.org

 

[MPep]

Hi Stan,

I agree that the sentences are easy reading, dependent on study of the NMEA protocol, of course.

I should warn you that some GPSs have variable field-lengths, i.e. some GPSs do not output a speed field eg ,, if there is no postion fix. Or they can add extra digitis as the speed goes up into tens, hundreds.

I have been caught out with this, and although you can program against it, it takes a fair bit of work to be seamless.

MP.