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Mapping GPS coordinates

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If I have my GPS receiver mounted to the top of a train, and the train goes around the same track over and over (train is traveling a max of 20MPH)... I would think the GPS coordinates would be identically the same with every complete trip... but that doesn't happen.  Is there a way to get the same GPS coordinates each time?  Also the total number of GPS coordinates read for a complete trip varies... one time it may be a total of 1000, then the next it may be 1085. I've tried enabling and disabling Static Nav and WAAS, but didn't solve my problem.

Any help to point me in the right direction would be greatly appreciated!!

Hello zarkfoo,

Welcome to the forums! Just a couple of questions about this:

What is the model number of your device?
Are you using the device with a third-party software or with the program that came with it? Please provide details on this.
If the program is running on a computer, what is the operating system and version?

How about the testing environment? Does it always have access to a clear, open sky?
Since it's in a moving train, does it go through a tunnel or to an area where there are tall trees around?

I'm using the MR-350P Bulkhead connected to the serial port of a Windows XP SP2 Dell notebook. I'm a software developer and have written an app that reads the Comm port and records all $GPRMC message to a file.  I've also used the GpsInfo and SiRFDemo software with the same results. The environment is not always open sky... there are heavy wooded areas the train passes through... but even in the woods I continue getting $GPRMC messages.

Alright, thanks for the info.

The ideal environment for our GPS Receivers, or for any GPS devices out there, is that they should have clear access to an open sky, with no tall trees or buildings around. You can find more info about this in the following forum postings:

Whether the device is able to get a good GPS fix or not, you would still continually see the RMC sentences. In other words, if the device is not able to acquire a fix, then, you would get invalid RMC data. If it's able to acquire a fix, then, you would get a valid, accurate RMC data.

In my app I only accept and log valid RMC data (Status byte = A)... so whether in open sky or the woods I'm constantly receiving valid RMC messages.


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