DOD made changes, and some GPS receiver's have issues....

Jan, I found this info and it does mention the 12.5 minute interval you speak of, I wonder if this is what you are seeing? I wonder if you're seeing the break between each 12.5 minute segment?

The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.

Source for above: http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htm...

Wow! If this isn't an example of "Be careful what you ask for---You may get it" I don't know.....
!

So I did a search of "DOD changes GPS packets" in Google, and came up with more 'headache' material...

I have to call the MFG next week and report my findings to them (regarding the antenna change), and switch sync generators so as to swap out the other antenna..... and I'll see if I can get any more information from them....

Can anyone pin down when this happened? Date would be good, date and time even better.

good info thanks.

The link you provided is simply a description of the message format and specifies that it takes twelve and a half minutes to send. It doesn't say that it is 'new'.

Do you have a place you can point to that explains what the change in question is and when it was implemented?

My various navigators (the oldest being an SP3) do not exhibit the 12.5 minute unlock described by the OP.

Without seeing any supporting documentation to the contrary I would suspect that the OP's receivers have always been messed up (or that this is related to the same change in reference points that bricked a lot of Garmin devices last year) and that the manufacturer for his device was simply remiss in notifying it's customers and decided to (what else is new?) point a finger at the DOD.

After all, the changes were only published twenty years ago.

Garmin is no better.

I wondered what this 'change' was myself, and couldn't find any reference to it either. The original link also appears a bit old, as it talks about SA but not about it being off since 2000.

For much better info on the signal as well as any temporary or planned anomalies, why not go to the source? - http://www.navcen.uscg.gov/?pageName=GPS

This page, http://www.navcen.uscg.gov/?pageName=gpsModernization , explains gps modernization.. maybe this (modernization) is what you're finding references to?

How old are the models that are affected by this?

This jogged a memory that if the GPSr has been turned of for a number of days or it wakes up and finds itself a few hundred miles from its last fix, it must download a stream of data from the 1st satellite it sees to rebuild the almanac and ephemeris data it keeps in memory to reliably perform its functions and this takes 12+ minutes.

It was this way with my original StreetPilot (before they started numbering them) and it took that rascal a long time to figure out where it was if I didn't turn it on regularly, so the info described by TheProf is likely to have been part of the early specs for the system. I'm personally unaware of any changes the DOD may have implemented recently, nor have I seen any odd behavior with my nuvi or with the SP that I continue to use in my work vehicle.

Spoke with manufacturer today, and they forwarded a copy of the alert they got from GPS manufacturer, indicating that a firmware update had been developed...
And that the cause of the firmware update was a 12.5 minute unlock based upon "Recent changes by DoD"

Other than this, the manufacturer I spoke with knows nothing....

Date of this document that Mfg #2 sent Mfg #1(who I talked to) is January 26, 2010

That's all the info I got...

Sounds like CYA + Smoke + Mirrors to me. Although I did see a reference somewhere to the modernized L2 signal system being fully implemented by 2011 so maybe whatever system it is you are talking about uses it or L5 and is on the cutting edge of the system upgrades?

When what happened?
I stand corrected about the original link .. it does mention that SA was turned off on May 2, 2000 but the top of the page says "Revised: 05/01/2000 (first published in September, 1994)" so it's 10 years old.

I'd suggest you look at the Coast Guard links I posted above for GPS service alerts, and details of planned changes (modernization schedule),

And this one for current GPS operaing details- http://www.faa.gov/about/office_org/headquarters_offices/ato...

As near as I can tell, the L1 service that's been used forever (and likely the only one used by consumer GPSr's?) shouldn't have seen any glitches.

I'm not an expert whatsoever, but between the 3 links and those 2 sites all the verifiable info & details should be found, imo.

Exact wording of the document I received, with regard to the 'cause'........

"This firmware version was published to correct the signal tracking outage every
12.5 minutes due to the change in the GPS signal structure by the DoD."

I was referring to when the format of the message was changed. From what I'm reading it is not related to something that I had observed, but none the less this is interesting and I would be glad to hear more details about it. And as far as I can tell it doesn't have anything to do with the act of disabling SA.

Boy, I don't understand this one. the format for civilian L1 messages hasn't changed in a long time. Some manufacturers had a problem (more than a decade ago) with a GPS week rollover, but that was pretty isolated, and it's long gone.

Like to see some more information on this one.

I have not been given any further update on this subject, but I can inform that after a couple weeks I'm still seeing NO unlocks at 12.5Min (or any other) timeframe since the GPS receiver's were swapped out...

Jan

What's most likely is that there's been a latent bug in the supplier's code for a lot of years - they quietly fixed it and for those who asked, pointed the finger at the DoD. Better than admitting that they screwed up.

Does this explain the snowy TV picture I was getting in the 1980's, until I moved my rabbit ears?