how to separate two GPS signal in one COM Port

[jtc]

Dear Mr Chow,

Thank you for this message. That is true, I have experienced "overwrite received data" before. In my present application, I am getting about 4000++ of data and so I set the receive buffer to 5000.

In the GPS Utility Software I can set the baud rate from 9600 to as big as 921,600. If I will put the baud rate to 9600 only, I am getting strange data, on the otherhand if I will set to 57600, I can read the actual data. Similar to Mr Beal suggestion, I must flushed the buffer. Kindly show me how can I flushed the buffer memory. What VI function should I use?

"Make sure  your recieve buffer is large enough !! Else you will overwrite  recieved data !
If I remember correctly the NMEA protocol is set at 9600baud rate. Which means that even though you set your
com port at 57600 baud your GPS reciever only recieves data at 9600 !!. But you transfer data between the reciever and
Com port at the higher rate. So if the GPS reciever buffer has not been flushed before then you could actually be transferring 'old' data."

[Brian_Beal]

You can receive NMEA messages at whatever baud rate that your GPS unit and terminal support.  You will want to use the VISA Clear function to clear out the serial buffer of your computer whenever you run your VI.  I don't believe that messages get stacked up in the GPS unit.  What messages are you receiving that you get that much data at once?  Do you need all of that?  Please explain you application a little more.  I haven't had time to hook up my GPS, maybe in the next day or so I will be able to do it.

[xseadog]

Can we just recap !
NMEA 083 (standard protocolfor last 20 + yrs) tranmits at 4800 baud. Remember that the signal level is less than -100dBm.
Your BER would be extremely high if you could use a higher baud rate.
IF you trawl the net : use google -NMEA  protocol  as key word  further specific information avaliable.

You can communicate between your GPS reciever and PC com port/interface at far higher baud rates but  the bottle neck/weak link
is the GPS satellite transmitter at 4800baud.



It would be helpful if you identified which GPS engine you are using plus all other salient info.

xseadog.

[Brian_Beal]

I'm not following you, xseadog.  Are you saying that the GPS receiver and the satellite communicate using the NMEA protocol at 4800 baud?  I used the Thales (Ashtech) DG16 for a project, and it could output 20 position messages per second.  I could poll it at about 8Hz for position updates, and that was essentially real time. 

[Brian_Beal]

Here is a working program that will parse the GGA and RMC messages.  You may only need to change the comm port settings and the number of bytes at the port, which triggers that parsing.  I tested this on a Garmin GPS18 that was outputting only the GGA and RMC messages at 1Hz.  Everything worked as it should.

[xseadog]

Hi Brian

see attached web link:

http://www.nmea.org/pub/0183/

as stated before the higher baud rate is the interface between the GPS engine and the pc port. The GPS engine recieves data at 4800 baud. You cannot adjust the GPS satellite  transmitter parameters !!!

 Image what would happen if everyone had that option.

You would also have to have a powerful transmitter !!!

chow

 xseadog

[Brian_Beal]

xseadog-

I believe that the NMEA standard only applies to instruments that strictly adhere to it.  I also don't think that the GPS engine and the satellite use this standard for communications.  A GPS receiver can output data in whatever format the manufacturer wants, and most of them do.  They have the NMEA option, but also have their own sentence structure.  Also, the NMEA sentence can be communicated at whatever baud rates are supported, although this doesn't adhere to the NMEA communications standard.

The NMEA standard is for devices that want to exist on the same communications bus, many of them which could be somewhat "dumb".  In other words, the standard allows you to wire all of your marine navigation equipment up and have it working, without having to worry about different comms protocols and settings.

Most low cost GPS receivers only allow update rates of 1 Hz, except for one of the Garmin units that offers 5 Hz, but those are predicted positions, not actual.  Higher end units offer much higher rates of real time position updates.

Brian

[jtc]

Dear Mr Beal,

Thank you again for this VI. But I would like to use first the one you have sent me before (GET GGA VI and GET RMC VI). So far I dont have problem with the VI you have sent me before.

Thanks a lot.

[jtc]

Dear Mr Chow,

Thank you for the advises.

I do agree with Brians comments. In addition, please find the following;

Baud rate is 50 bps for navigation data such as almanac and ephemeris, but position, speed and other calculated data in receiver are not shown by baud rate. Rather they are shown by the performance of update, maximum 20 Hz. The baud rate depends on the setting between PC and GPS Receiver. Baud rate is not a good factor for the faster the data comes. Baud rate is the total number of data in one second. The faster the data comes is update speed, such as 5 Hz,10 Hz and 20 Hz.

[jtc]

Dear Mr Beal,

Kindly teach me where will I insert the VISA Clear Function. Is it after the read function or at the end of the VI beside a VISA Close Function?

Attached please find the sample GPS data I gathered while my application moves. My present setting for baud rate is 57600, but If i will reset to 9600, I cant get all the data. It will come as strange data. I need all those data for my positioning, calculation, indicators, etc. 

"You can receive NMEA messages at whatever baud rate that your GPS unit and terminal support.  You will want to use the VISA Clear function to clear out the serial buffer of your computer whenever you run your VI.  I don't believe that messages get stacked up in the GPS unit.  What messages are you receiving that you get that much data at once?  Do you need all of that?  Please explain you application a little more.  I haven't had time to hook up my GPS, maybe in the next day or so I will be able to do it."