The existing backup is the Russian GLONASS system, which many consumer chips support since the start of this decade or so, and there are two more constellations being deployed, by ESA and China.

I'm not sure how you can test such things not in the production environment. The early development was done with the transmitters on airplanes, but i would guess that the jammability can't really be tested that way.

Commercial vendor Iridium as part of their "Iridium NEXT" constellation currently being deployed will have a secure alternative to GPS as well, their STL service. [1]

[1] http://gpsworld.com/iridium-launches-alternative-gps-pnt-ser...

And India's own: http://www.thehindu.com/sci-tech/science/irnss-launch-indias...

GPS uses a certain frequency. Use the high side band for prod, the low side band for staging. Simple :)

The nice thing about PRN codes is that you can just use a different set of gold codes in the same band, as long as they have low enough cross-correlation

Yup my GPS watch does GLONASS. Have never tried GLONASS only, but GPS + GLONASS is a fair bit faster to lock on to.

Most smartphones do both now as well. iPhones got it starting with the 4S, and various Android devices support it. I believe it's a requirement to be able to sell them in Russia, but it benefits us all.

I suspect this is because AWS hasn't yet added an API call for placing a fleet of satellites in semi-synchronous orbit

I can neither confirm nor deny that AWS is working on an API call for placing a fleet of satellites in orbit.

On a more serious note, I'm excited to see what happens to the NTP pools in the area, since GPS is often a timing source for them. It should be a non-event, but it could get interesting.

Oh, and what happens to CDMA cell towers that use GPS to sync? Do they go off-line?

Anywhere that GPS is used in a timing-critical application, they'll be using a timing receiver (like http://www.trimble.com/timing/thunderbolt-e.aspx) to handle their time reference; those use the GPS signal (when they have it) to discipline a (ovenized) quartz crystal, so that they have good performance during "holdover" (loss of GPS) especially for short-term outages. In its default configuration, the Thunderbolt-E linked above, for example, will be within +- 8us after being in holdover for 24 hours.

A Rubidium reference gets you down to the 1us/day range, cesium gets you down to a tenth of that. It all depends how much you're willing to pay.

±8µs can be rather bad, e.g. for financial applications.

There is certainly hardware available that can do better than this, but the parent asked about cell sites in specific (and cell sites are one of the top users of the Thunderbolt). You can get GPS hardware based on atomic references, for example, that will do better over these kinds of timespans.

(You really need GPS or some other precision time transfer mechanism for long-term accuracy, though ... there's no getting past that.)

Well...if you're in that position, you probably have money to spend on failover config. Pretty accurate hardware clock modules were available even before GPS. Not cheap, not simple, but reliable - if you have the need and the money.

Anything relying on GPS for timing is going to revert to an onboard crystal timer for the outage duration.

There already is a partial backups, and eventually there will be three: Galileo, Beidou, GLONASS. Many devices can access multiple systems.

I thought it was pretty obvious from the article that they're most likely testing GPS jammers. In the case of a war, the US probably doesn't want anyone else to be able to use their GPS network.