Wrong current time on GPS receiver while generating GPS signal with GNSS Toolkit

[kostyuk.a]

Hello everyone!

I have a problem with generating GPS signal. As it shown in attachment receiver's current time differs from current GPS time of week on niGPS Direct Streaming Generation.vi

Initial GPS time set to 0, so I expect the current while generating should start from 0:00 of 6 December, 2015. But receiver shows 23:59:40 12.06.2015 on start

Could anyone explain what can be wrong?

[Sastry]

Hi,

1. "Initial GPS Time of a Week" is a control that is, it is a user input.

2. Regarding the mismatch, please note that on the "Current GPS Time of Week" on NI GPS Direct Streaming Generation.vi, the value is GPS time. But the value on the uBLOX receiver is UTC time. And the GPS time differs from UTC time by 16 seconds based on the file. The additional difference (of around 6 seconds?) could be because of the amount of buffer. By buffer I mean, the amount of data that has been pre-loaded onto the generator onboard memory. So, lets say if you pre-loaded with around 6 seconds worth of data, then what shows up on the NI GPS Direct Streaming VI will be ahead from actual generation by that time (6 seconds). So, the total difference between GPS receiver and LabVIEW VI will be around 6 seconds if you compare GPS Time to GPS Time but could be 22 seconds if you compare GPS time on VI to UTC time on Receiver. You can reduce the 6 seconds difference by redudcing the amount of pre-loaded data. You can do this on the block diagram of the NI GPS Direct Streaming Generation VI.

Hope the above helps.

[kostyuk.a]

Hi, Sastry

Thaks for your reply!

Let me explain the problem better. The ephemeris file is for 340th day of the 2015 year. So this day is December, 6, Sunday. The initial GPS Time of Week control is set to 0, so I expect that corresponded UTC initial time should be 0:00 AM December, 6. But as we can see, uBlox shows time 23:59:54, December, 6. It is not 16 seconds difference, it is difference of about 24 hours, isn't it? 

[Sastry]

Can you run the example with "Initial GPS Time of Week" set to say 7200 and report the time and date? I am curious to see what is shown on receiver screen.

[kostyuk.a]

I'll try to set initial time to 7200 and post screenshots tomorrow (it's too late now in my timezone). But for now two more screenshots made today on morning. One - gps simulating with other ephemiris file(there is the problem too ) and other for glonass simulation(no problems, everything seems to be right)

[kostyuk.a]

Hello Sastry,

I've tried run vi with initial gps time set to 7200. Results are in attachments. Yesterdays results are not repeatable and I think there was a mistake doing screenshots. BUT STILL I ask for help in explanation.

Look at pics in attachment:

1. Initial GPS Time - 7200, so initial UTC time is 02:00:00 AM December, 13, 2015. Current GPS Time on generator is 7219 = 02:00:19 AM. There are about 6M samples in buffer, IQ Rate is 1.5MS/s, so buffer give us a 4 seconds delay. I expect the receiver current time to be about 02:00:15 AM, but it is 01:59:58 AM. It is even early then the inital time! Why is there ~15-17 seconds difference.

2. On the second picture, later screenshot, we can see the same difference between expected time on receiver and real current time on receiver

Why does this delay appear? 

I have to generate GPS and GLONASS signal simultaneously and I suppose that my GPS/GLONASS receiver would go crazy because of signals' current time difference. How can I synchronize GPS and GLONASS current times?

[Sastry · Accepted solution]

This was what I was talking about yesterday.

Receiver Time is showing UTC Time

The time on GPS Direct Streaming VI is GPS Time.

GPS Time - UTC Time = 16 seconds   (Leap Seconds).

[kostyuk.a]

Thanks a lot! I missed that yesterday

[MaxEngee]

Is it posible to change Leap Seconds in GNSS toolkit?

Because the current value is 17

I compare the results of the generation R&S SMBV100A vs GNSS toolkit (and usrp). I see that there is a problem in the wrong value of Leap Seconds

[Sastry]

Hi,

The Leap Seconds value is taken directly from the ephemeris file you are using. So, if you are using an older ephemeris file which had the leap second value of 16, you will be getting 16. In order to get the newer value of 17, try using an ephemeris file that was published after the leap second of 17 seconds went effective.

You can download the ephemeris files from here :

http://cddis.nasa.gov/Data_and_Derived_Products/GNSS/broadcast_ephemeris_data.html#GPSdaily

Make sure you have the almanac file that corresponds to ephemeris file. For example, if your ephemeris file is brdc3190, then the almanac file should be 319.al3.

You can get almanac files here :

http://www.navcen.uscg.gov/?pageName=gpsAlmanacs

Look for the almanac files of SEM format with extension  .al3.

Hope the above helps.