A GPS Question

[RogerI...@aol.com]

I recently purchased a GPS Receiver Dongle from GiSTEQ.com. I wrote a program that extracts and evaluates the information strings coming from the receiver so that I can see exactly what is being generated by it. In particular I am looking at the GPGGA message, which provides the lat/long/alt info. However, I'm getting some strange results and I don't know whether it's a faulty GPS receiver or what might be wrong.

 

Typically I'm getting a message like this:

 

$GPGGA,210126.583,2117.1955,N,15749.3757,W,2,07,4.6,23.1,M,2.2,M,,0000*49

 

The first thing I notice is that the 13th parameter is simply missing. It's just a comma followed immediately by a comma. According to the official documentation that parameter is the "Age in seconds since last update from diff. reference station". In addition, the very next parameter, which is the "Diff. reference station ID#", always shows up as 0000. Is this correct? Is it valid for parameter 13 to be missing and parameter 14 to be always 0000? I'm thinking that maybe there's no Reference Station available and that's why I'm always getting those values. Or is there something that I CAN do in order to force it to use a reference station?

 

The next thing I notice is that the lat and long seem to vary quite considerably. I'm using this GPS receiver in the center of Kapioloani Park that's almost all open space. The only buildings are many blocks away, so it should have excellent access to the satellites. In fact, it typically shows that of 13 actual satellites it's using 7 or 8 of them, so I'd think the values should be pretty good and steady. But still, the Latitude, for example, will vary from 2117.1955 to 2117.1946 (last t wo digits varying quite a bit). The Longitude has similar variations, even though I'm keeping the antenna solidly stationary. Is that to be expected?

 

And finally, the altitude value does not look correct at all. In the example above it shows 23.1 meters. Kapiolani Park is almost at sea level so at most I'd expect 2 or 3 meters above sea level, yet it's showing 23.1 meters (about 70 feet).

 

So, is my GPS Receiver not working correctly? Or am I not understanding how this is supposed to work?

 

Any advice you can share is welcome.

[Scott Seaburn]

Hi Roger,

I'm not familiar with the device you are using but I looked at the website and I'm guessing that you are really not using differential GPS.  For that you would need a reference base station that your device can communicate with.  If you have the GPS USB Dongle the GiSTEC FAQ shows the same format as you are seeing.

Regards,

Scott Seaburn

On Oct 10, 2012, at 11:42 AM, RogerI...@aol.com wrote:

GiSTEQ.com

[David Cornejo]

Differential GPS requires multiple receivers connected together to work, the output you get is proper in your case (presuming you have only one receiver).

Civilian GPS signals have some jitter thrown in to make it less useful for high precision targeting.  You would expect that the position you read will be within 10m of the actual position.  Sometimes it's better, and there are ways to smooth the readings out to get slightly better accuracy, maybe down to 3m.

It's been a while since I went through this stuff, so forgive any errors - I think I'm close to right...

Altitude on GPS is all but useless except as a differential - if I take a reading at one elevation, then go up 30 meters, then the reading should increase by 30m (subject to the noise thrown into civilian signals).  There are many models used to determine the apparent (NOAA says 28 different models) - GPS uses models based off of fixed points (like the distance from it's constant orbit to the point below it) where the most useful to us are orthometric models based on the mean sea level measured at a point.  Unfortunately MSL is not necessarily proportional to the GPS altitude for all points you may care about.

If you need accurate altitude, the best suggestion is to use a combo of GPS and an altimeter (some very good parts are available to do cheaply these days).  Adafruit has a $20 one good for 0.25m resolution.

There's a tool for converting between models here: http://vdatum.noaa.gov/ - which might help you relate the GPS reading to MSL.

dave c

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