timing problem of two systems

hayee · Joined: 05 Sep 2007 Posts: 252

Hi,
I have two systems and both have GPS modules (GPS-310F).
The problem I am facing is that I start 1st system, it receives date and time from GPS and displays it on the lcd. Then after some minutes I start the 2nd system, it also gets the date and time from GPS module and displays it on the lcd. But the time of 2nd system is two seconds greater than the 1st system.
After several minutes both the systems synchronize and show the same time and then it works perfectly until I power off one of the systems and again power it on.
The time of the synchronization for both systems is varying. Sometimes it takes 4 minutes to synch, sometimes it takes 10 minutes to synch. Sometime it takes few seconds to synch.
I have to start the process at the same time, but due to this problem I am not able to do that.
Can anyone tell me how to solve this problem ?
Is there any signal generated by the GPS module that the time is now synch or any other indication?

Guest · Posted: Fri Feb 19, 2010 6:28 am

After a quick read of the GPS manual I'm not surprised at the results you're getting. The only easy way to get them 'in sync' is to have the GPS modules powered 24/7.

hayee · Joined: 05 Sep 2007 Posts: 252

Thanks.
Kindly tell me that on which page this is written. Also send me the link from where you download the datasheet.

GDetienne · Joined: 20 Sep 2003 Posts: 47 Location: Brussel - Belgium

Hi,
each GPS give the validity of the data. This flag is present in each NMEA sentence.

hayee · Joined: 05 Sep 2007 Posts: 252

Is there any way to solve this problem?

Ttelmah · Guest

I think you need to understand 'where' the time is coming from.
GPS was not really designed to give 'time'. It requires the local receiver to generate it's own local clock, which is then used to calculate the position, but this doesn't give UTC. Historically generating UTC from the local clock required the full ephemeris to be downloaded. Modern receivers 'cheat' in getting a position, by shortcutting the downloading of the ephemeris, but until this is done, accurate UTC time is not available.
When you first turn on, the 'time' you get is from the local module's own RTC, and can be very inaccurate. This is then updated when the ephemeris is downloaded. How long this takes to happen, depends on the number of satellites being seen, and the nature of the receiver. Worst case can be something like 12.5 minutes (the time for a full ephemeris from one satellite).
This is what you are seeing, with the two times being accurate, once both modules have the current ephemeris.
What error data is available, will depend on your GPS. You ought to be able to check if the almanac is within the last few hours, to know that the timing if likely to be right.

Best Wishes