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| Science & Technology |
| Iridium as a GPS backup? |
| 2018-03-03 |
| [Wired] EARTH GOT A warning shot on January 25, 2016. On that day, Air Force engineers were scheduled to kill off a GPS satellite named SVN-23—the oldest in the navigation constellation. SVN-23 should have just gone to rest in peace. But when engineers took it offline, its disappearance triggered, according to the National Institute of Standards and Technology, a software bug that left the timing of some of the remaining GPS satellites—15 of them—off by 13.7 microseconds. That’s not a lot to you. If your watch is off by 13.7 microseconds, you’ll make it to your important meeting just fine. But it wasn't so nice for the first-responders in Arizona, Pennsylvania, Connecticut, and Louisiana, whose GPS devices wouldn’t lock with satellites. Nor for the FAA ground transceivers that got fault reports. Nor the Spanish digital TV networks that had receiver issues. Nor the BBC digital radio listeners, whose British broadcast got disrupted. It caused about 12 hours of problems—none too huge, all annoying. But it was a solid case study for what can happen when GPS messes up. With that kind of threat looming, GPS understudies are getting more attention. Like, for example, other satellites. A company called Satelles is looking to Iridium satellites—you know, the ones that link up to the satellite phone you take with you on Arctic expeditions. The Iridium satellites had a little-used channel that used to ping pagers, but people don't have those anymore. So Satelles made a deal with Iridium to reprogram the pager channel to beam down a GPS-esque signal. If it were outfitted to do so, your phone—or your ship or your plane or whatever—could catch that signal. And because it knows when Iridium sent the signal down, and it knows when the signal arrived, it can calculate how far away the satellite was. Get three Iridiums at once, and your phone calculate its position, similarly to how it does with GPS signals—except these signals were made with modern cryptographic techniques that make them, unlike GPS, basically unspoofable. The company calls it STL: Satellite Timing and Location, and it's selling it to customers like the US government, banks, data centers, and potentially wireless carriers. There are 66 Iridium satellites up there, and they’re about 25 times closer to the ground than GPS satellites are, and that proximity means their signals are much stronger when they arrive at Earth. Where a GPS signal might be too weak—like in an old brick apartment building amidst other brick apartment buildings—an Iridium signal does just fine. There’s gotta be a downside, though, or everyone would have used this for location and timing in the first place. And here it is: “The accuracy is not quite as good as GPS,” says O’Connor. Where GPS gives your spot with about 5 to 10 meters of accuracy, Satelles’ method is only about 20 to 30 meters (the timing accuracy is comparable). “When you’re in an environment where you can receive GPS, GPS is better,” says O’Connor. But if you’ve got no GPS, or GPS has got your spot wrong, or you need to know for sure for sure you’re not being spoofed, Iridium might be the way to go. |
| Posted by:3dc |
| #3 ..I believe that was 'dead' reckoning. |
| Posted by: Procopius2k 2018-03-03 09:22 |
| #2 Tech reckoning only works if you use it. See Navy collisions. |
| Posted by: Skidmark 2018-03-03 04:33 |
| #1 Still need manual nav, visual, geospatial for ground, and groundview nav. |
| Posted by: newc 2018-03-03 00:49 |