Parallax GPS Receiver Off by 168 km

meiradam

Hello,

We are using the Parallax GPS receiver with arduino.
And in setting it up and just reading raw data we found that
the lat and long is about 168 km off from our actual location.
The readings where taken outside and are consistent over
a 15 minute period of time, i.e., there is little fluctuation
in the readings. Also, the sensor was stationary.

Has anyone ever experienced this? Or have a possible
explanation of the reason?

Thanks in advance,
Adam

Post Edited (meiradam) : 12/22/2009 9:02:30 AM GMT

Phil Pilgrim (PhiPi)

I would check the code that you used to parse the NMEA data. An error like that is very unlikely to be the fault of the GPS unit itself.

-Phil

meiradam

There is no code. We are reading raw data.

Adam

Bean

I think the parallax GPS outputs Degrees, Minutes. (with minutes having a decimal value).

If you are comparing that to a GPS reading that is Degrees, Minutes, SECONDS then you will get a large (apparent) error.

I suspect that is what is happening.

If you post the actual raw data, and your location we could tell for sure.

http://en.wikipedia.org/wiki/Geographic_coordinate_conversion

Bean.

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Post Edited (Bean (Hitt Consulting)) : 12/23/2009 5:54:49 PM GMT

meiradam

Here is a sample of the raw data:

$GPRMC,062842,A,3656.9344,N,12754.4580,E,000.9,000.0,221209,,,A*79
$GPGGA,062843,3656.9305,N,12754.4566,E,1,03,12.0,00000.2,M,020.5,M,,*41
$GPGSA,A,2,05,10,17,,,,,,,,,,12.0,12.0,01.0*00
$GPGSV,2,1,05,05,27,240,35,10,42,242,45,12,21,282,00,17,40,155,49*7B
$GPGSV,2,2,05,30,09,311,00,,,,,,,,,,,,*45
$GPRMC,062843,A,3656.9305,N,12754.4566,E,001.7,000.0,221209,,,A*7A
$GPGGA,062844,3656.9314,N,12754.4568,E,1,03,12.0,00000.2,M,020.5,M,,*48
$GPGSA,A,2,05,10,17,,,,,,,,,,12.0,12.0,01.0*00
$GPGSV,2,1,05,05,27,240,34,10,42,242,44,12,21,282,00,17,40,155,48*7A
$GPGSV,2,2,05,30,09,311,00,,,,,,,,,,,,*45
$GPRMC,062844,A,3656.9314,N,12754.4568,E,001.7,000.0,221209,,,A*73
$GPGGA,062845,3656.9387,N,12754.4603,E,1,03,12.0,00000.2,M,020.5,M,,*4D
$GPGSA,A,2,05,10,17,,,,,,,,,,12.0,12.0,00.9*08
$GPGSV,2,1,05,05,27,240,33,10,42,242,45,12,21,282,00,17,40,155,48*7C
$GPGSV,2,2,05,30,09,311,00,,,,,,,,,,,,*45
$GPRMC,062845,A,3656.9387,N,12754.4603,E,001.7,000.0,221209,,,A*76

Our location should be in Chungju, South Korea: 36.949945N, 127.907371K

I will look into the minutes/seconds issue and see if that resolves the problem.

Thanks,
Adam

Adam

Duffer

Same place, different notations.

GPS: DDMM.mmmm and DDDMM.mmmm (Degrees, Minutes & decimal Minutes)

Your comparison: DD.dddddd· and· DDD.dddddd (Degrees & decimal Degrees)

Converting the GPS's notation to your data notation·gives:

56.9305 / 60 = .948841· and 54.4566 /60 = .907610

or

36.948841 N· 127.907610 E

That's a lot less than 168 km difference (~0.12 km of LAT).

Duffer

SRLM

So, from the first RMC string your location is 3656.9344 N, or 36 degrees and 56.9344 minutes. From the wikipedia article (en.wikipedia.org/wiki/Geographic_coordinate_conversion) we find that we need to divide the minutes by 60 in order to produce degrees dot decimal degrees (as your given location is shown in). So, 56.9344 / 60 = .9489. The GPS therefore has a position of 36.9489 N, which is close to your given position (36.9499 N). The same applies for longitude.

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meiradam

Thank you. I understand now. Appreciate the help!

Adam

Bean

Your welcome.
I ran into the same thing trying to use the Parallax GPS coordinates in Google Earth (I think).

Bean.

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