how to separate two GPS signal in one COM Port

[Brian_Beal]

If you look at the block diagram of the last program I posted, you will see where the VISA Clear should go.  You shouldn't have any problem getting just GGA and RMC at 9600.  What problems do you have when you run the last program I posted?  That one should work fine.  Could you post the program that you are now using.  Have you investigated if you can poll your GPS for a standard or custom message?  That is a good way to go if you want to get position updates at the rate your program runs.  In other words your progam isn't limited to 1 Hz, and you don't have to worry about not getting the latest data because messages will build up in the buffer if they aren't read or cleared.

[jtc]

Dear Mr Beal,

Greetings!

"If you look at the block diagram of the last program I posted, you will see where the VISA Clear should go.  You shouldn't have any problem getting just GGA and RMC at 9600.  What problems do you have when you run the last program I posted?  That one should work fine."

Today I went to the testing area but I forgot to copy all the ctl files of the Garmin GPS VI. I will reply to you on this shortly.

I have found another issue on the GPS data. The GPS Latitude and Longtitude data are not stable. It is always fluctuating (sometimes value goes up, sometimes value goes down). Even if the antenna is not moving the data is fluctuating. The Latitude and Longtitude data are very important for my positioning. Kindly teach me the way to make this data fix if antenna is not moving and if the antenna starts to move either left or right or down or up, then thats is the time the data updates. Thank you very much.

[Brian_Beal]

I'm not familiar with the Novatel unit, but there may be some settings that you can change.  My Ashtech had different modes that you could set(static, walking, boat, car, plane) that helped with that.  How big are the distances that you are seeing?  I did a study with my Garmin GPS 18 that tracked the position data over several hours.  Even though the unit was fixed, I saw excursions as big as 200 feet.  Garmin said that this was probably due to multipath problems where I had the unit.  It was next to my house.  What environment is your antenna in?

You could write a filter in Labview.  If the movement that the unit sees doesn't exceed a preset value, then you can assume that it is not truly moving.  Even with the best conditions and the best hardware, the values will always change somewhat.

[Jason Stallard]

Another thing to consider is the number of satellites in view.  The more the receiver can see, the more accurate your reading will be.  Also you might consider upgrading to a WAAS enabled unit.  I work with the Garmin 17 series and we often are able to get <5meters of resolution.  There are parameters in the NEMA spec for # of sats, horizontal/vertical/positional error estimates which you can use to estimate the precision you're getting.

Here's my library, that looks for the sentences $GPRMC, $GPGGA, $PGRMV, and $PGRME....and it can be easily modified to look for others.

-Jason

[jtc]

I tried to ask the maker and hopefully i can get answer nextweek.

The excursions are not that big but the data is fluctuating rapidly. For instance, i placed the end of the boom near the limit line i created. Its not yet touching the limit line but because of the fluctuations, sometimes it will trigger an alarm and sometimes it will not even the antenna is not moving. My environment is fine, on a clear sky area. I tried to reduce the digits of precision to 2 (from 4) but I am still getting the same problem.

"You could write a filter in Labview."- Can you please teach me which filter function I can try to use. Thank you very much.

[jtc]

Thank you very much for the VI Library. But most of them I cant open,  they are looking for sub-vi.

The GPS receiver I am using I can get up to 12 satellites in view. I agree with you, the more the receiver can see, the more accurate the reading will be. I might try the WAAS unit in my next project if i am successful in my present project .

Also, the more sensitive it is the better, but I wish I can make the data fix at one position (if the antenna is not moving)

Thanks a lot.

[xseadog]

Mr Bong
you are seeing the max number of satellites possible (12); even though there are in fact 24 in orbit !!
You are correct regarding the sensitivity this can be helped by having a stable antenne platform and an antenna that has a high signal gain.
What you should also investigate is the carrier to noise ratio (CNR),on a clear day ( in northern Europe) the value can be as high as 45dB. The terminal systems that I previosly worked on could ' see'  satellites down to 32dB but not all the transmission data was readable. Our test specification required that the CNR was above 38dB. Usual figures of around 40 to 42 dB were norm.
Hope that helps.

chow
xseadog

[jtc]

"If you look at the block diagram of the last program I posted, you will see where the VISA Clear should go.  You shouldn't have any problem getting just GGA and RMC at 9600.  What problems do you have when you run the last program I posted?  That one should work fine."

Dear Brian,

I ran the program you last posted but at 9600 baud rate it will give me strange data too. I confirmed the PC and GPS Receiver baud rates and the
PC hardware settings at 9600 (device manager).

I changed it to 57600 and it ran fine. I can get both GGA and RMC data/s. 

"Have you investigated if you can poll your GPS for a standard or custom message?  That is a good way to go if you want to get position updates at the rate your program runs.  In other words your progam isn't limited to 1 Hz, and you don't have to worry about not getting the latest data because messages will build up in the buffer if they aren't read or cleared."

With my GPS receiver i can poll for custom message and timing. For instance "log com1 gpgga ontime 1", meaning i want to get gpgga data every one second, but i can changed the timing for 0.2, 0.5 second and so on. 

[Brian_Beal]

Bong-

I'm not sure why you are having problems at 9600, but if it works at the other speed, that is OK.

By polling your unit, I mean you serially write "get custom message x", then you read what the result is.

[jtc]

"By polling your unit, I mean you serially write "get custom message x", then you read what the result is."

No. I write custom log commands in the GPS receiver and SAVE IT. So everytime I turn ON the GPS receiver and run my application, via serial link, it will read the log data that i requested. As long as I made the "saveconfig" command in the GPS receiver, it will always output the requested log data that i need.

Do you have any idea on what kind of filter function I can use? I understand that the fluctuations on GPS data is caused by a lot of reasons especially NOISE (electrical noise, environmental noise, multipath, etc.)