Batboard

Anyone care to comment as to what would happen to our Simalcast and Trunked radio systems if the GPS System was shutdown. I guess Cellular would also have problems.

Look at the link for President's proposal to Shut down GPS during National Emergencys

http://tinyurl.com/5ode6

What other systems that we use daily depend on GPS for sync

MassFD wrote:Anyone care to comment as to what would happen to our Simalcast and Trunked radio systems if the GPS System was shutdown. I guess Cellular would also have problems.

Look at the link for President's proposal to Shut down GPS during National Emergencys

http://tinyurl.com/5ode6

What other systems that we use daily depend on GPS for sync

Nextel does - you can usually tell that it's a nextel site by the two GPS antenna's on the building that houses the site.

but I believe all their TDMA based timing is done by GPS...

Oh, simulcast systems will probably have a lot of issues...

-Alex

Actually, it would probably have little effect on those systems that use GPS for synchronization (relative timing), as opposed to time derivation (absolute timing).

The GPS system would not be "shut down," in the sense of signals turned off. Rather, what would be invoked is a routine, already built into the system, called Selective Availability (or "SA"). SA, which was running on the civilian GPS system (which is known as the "L1" or "CPS" system) from late 1991 until about two years ago, introduces timing offsets (errors) into the GPS ephemeris, so as to reduce the nominal positional accuracy of the L1 system from about 10 meters to about 150 meters. This was designed into the system because your pals the French will sell a GPS seeker head small enough to fit into a shoulder-fired missle to any Arab with a checkbook.

Since the SA shift would affect all of the transmitters (or other devices using GPS for synchronization) in a system the same way and by the same amount, the synchronization function would not be appreciably affected.

There are actually two GPS systems. The L1 system broadcasts its pseudocode in the clear, and is available to anyone with a commercial GPS receiver. The L2 system (also known as "PPS"), which uses a different down link frequency, broadcasts its pseudocode encrypted, and is only available to military GPS units. (Actually, in 1991, the Army did not have enough military GPS units and so employed commercial ones made by Garmin, which is why SA was turned off in 1991. The resulting increase in GPS accuracy via commercial units was astounding: under 10 yards across the board and under 10 feet for some high end stuff, far exceeding design specs.)

Well, the article make no mention of this policy simply being a re-enabling of SA during a crisis. It uses the word "OFF". Perhaps the SA thing is what they are actually intending, however at the moment there is nothing to suggest that.

It would be virtually impossible to turn the system "off" in the sense that the satellites ceased transmitting pseudocode, and if that were done, it would be impossible to get them going again. Nor is turning them "off" necessary to deny use of the system to hostile forces.

I just checked with NavCen (the USCG service that operates the DGPS services and coordinates marine GPS services with the DOD), and they had no information on this press release or its content.

Perhaps "rendered inoperable to the end-user" is a better term than "off"?

I talked to an engineer friend of mine that works for Telus Mobility up here in Canada (the same idea as your Nextel), he tells me that they got a bulliten about this from Nextel in the US. It says that the proposal that's being considered by the government would infact, stop transmitting accurate position information, however the time signal would still be 100% accurate, so the cell sites and trunking repeaters that rely on a time sync wouldn't be affected.

-M

Well, that's good news.

I'm trying to harass him to scan the thing they got from Nextel so I can post it on here...we'll see how that goes.

But regardless, I couldn't imagine them shutting down GPS. So many things (falsely) rely on it, even though they really shouldn't..GPS was never intended to be a primary means of navigation...

-M

Some sort of GPS satellite adjustment ocurred last month (coincident with Bush's visit) that caused cell site timing issues to some Nortel CDMA base stations all across Canada. People experienced this issue as dropped calls. Based on GPS input, somehow some (not all) base stations adjusted its chip delay to a point where the cell site became an island. The call would stay up, but would never hand off. Driving out of range would kill the call, but the cell phone RSSI could stay 100% since the caller was likely in perfectly good coverage area of another site. The solution was to figure out which base stations were affected by watching the OM logs and restart the site.

Don't know if this is some sort of Nortel "feature" affected by a GPS quirk, or if it affected other vendor equipment too.

Regarding GPS based timing, there is a double oven crystal oscillator in the cell site clock. It is GPS "trained" and will provide accurate timing up to 24 hours from loss of GPS before it drifts far enough to affect service. Timing is everything! If you turn off GPS timing, there will be telecommunications pandemonium. All synchronously timed communication would have difficulty. This includes T1/E1, etc. So in the interest of national security and commerce, this scenario is unlikely. RKG summs it up well.

RF Dude.

methinks i have a conspiracy theory.

it's entirely possible that the immediate vicinity of the president is jammed with broadband RF to prevent any kind of "fix" on the position of the motorcade, etc.

i put this together myself in that i remember reading something about bush's visit to canada also - i think when they absolutely cannot guarantee his safety they take some additional extreme measures like this.

i can see the reasoning i guess....overkill for sure.

GPS not a primary means of navigation>?

what?

it may not have been a primary CIVILIAN method of navigation... but it was damn sure put up there for MILITARY navigation and pinpointing.

and the rumor mill is that the GPS constellation was/is "Phase 1" of Reagan's Star Wars missile defense program....kick that around.

doug

Look at any system that requires clocking from a gps signal would be a problem .
Many trunking and simulcast systems-TAIT -Prodel etc use gps clocking for timeing requirements-they would have to update to a very stable timebase as a back-up for if and when such a shutdown occurs.These have to be netted periodically to increase longtime stability , usually by a tech which increases cost by itself.We currently have such a procedure on our GSM equipment and it is a long process.
Additional equipment may be required to allow "Hitless "switching as used in the long haul microwave systems.

A lot of the military equipment is strongly dependant on gps-perhaps too much.

Regards

Brad

http://www.ostp.gov/html/FactSheetSPACE ... TIMING.pdf

That is a PDF link to some of the information being discussed...kinda helpful

GPS IS Star Wars. It's that simple. During the beginning of Star Wars, everybody just thought and rumored that it was weapons mounted in outer space- and it wasn't. It was the beginning of GPS as it is today.

ExKa|iBuR wrote:It says that the proposal that's being considered by the government would infact, stop transmitting accurate position information, however the time signal would still be 100% accurate, so the cell sites and trunking repeaters that rely on a time sync wouldn't be affected.

-M

How would that work? The time signal and the position infomation that the recvicer comes up with are related. As far as I know the only way to degrade the accuracy is degrade the timing signal.

Cam

I always like the idea that one part of the government paying f-up the GPS system and another is paying to fix it (DGPS).

Oh well, I guess that why we pay so much taxes.

Cam

Cam22 wrote:How would that work? The time signal and the position infomation that the recvicer comes up with are related. As far as I know the only way to degrade the accuracy is degrade the timing signal.

Pretty Easy - Tell most of the constellation to stop transmitting C/A code. If you only have one bird in view, you can't derive a fix, but you still will have a time standard available. P-Code (the high precision signal) is unaffected by the command to do this.

The other option is to have the same number of satellites set the "bad signal" bit in the data stream. All of the commercial GPS systems I've worked with refuse to use the data. No valid data, no fix. P-Code (the high precision signal) is also unaffected by this.

I'm told by that the rewrite of SCATANA (CFR 87.395) will now include GPS denial details

Martin