Ionospheric Interference With GPS Signals 127
Roland Piquepaille writes "In recent years, we have become increasingly dependent on applications using the Global Positioning System, such as railway control, highway traffic management, emergency response, and commercial aviation. But the American Geophysical Union warns us that we can't always trust our GPS gadgets because 'electrical activity in the... ionosphere can tamper with signals from GPS satellites.' However, new research studies are under way and 'may lead to regional predictions of reduced GPS reliability and accuracy.'" Roland's blog has useful links and a summary of a free introduction, up at the AGU site, to a special edition of the journal Space Weather with seven articles (not free) regarding ionospheric effects on GPS.
Dual Frequency (Score:5, Interesting)
Good Grief! (Score:4, Interesting)
Re:Good Grief! (Score:5, Interesting)
Re:Good Grief! (Score:5, Interesting)
Re:GPS is digital! (Score:5, Interesting)
So you might hear the tune fine, but if the ionosphere delays the tune every so slightly, your reading will be off and your position will be inaccurate.
What about timekeeping? (Score:1, Interesting)
My understanding is that you need to see a constellation of 4 sattelites to get accurate time. Use 3 to pinpoint your exact position, and then use that knowledge, and your knowledge of the 4th sattelite's position, to compensate for the delay in receiving the time signal.
If the precision of your position lock is degrated or unreliable, would the decreased precision of the reference time be enough to cause problems?
Re:Dual Frequency (Score:3, Interesting)
Also, it looks like military personnel ended up buying there own civilian units a large percentage of the time with obvious problems.
Looks like it was officially disabled around 2000 or so.
However, if you're a Ham.... (Score:5, Interesting)
I check the "Space dials [rice.edu]" regularly, and can't wait for them to be in the red! 73s.
Re:Dual Frequency (Score:5, Interesting)
That's not to say that it was a total piece of shit. It was water-proof and pretty durable. It was really extensible; it could be plugged into a variety of other things, which made them really useful *if* you had the proper hardware. The problem was that all the needed gear to take full advantage of it required a vehicle to transport and provide power. The PLGR was a fantastic piece of gear for anyone but the infantry. Problem is, there's a hell of a lot more infantry that needs coordination on the ground than there is anyone else. So, many of us bought our own.
Re:Good Grief! (Score:5, Interesting)
Roland has an extremely high ratio of postings and a *much* higher ratio of accepted postings. So much higher that for the longest time I figured he was a sockpuppet for one of the
The discrepancy is too large to be ignored or brushed under the carpet.
After all, the
The standards that most postings are held to would mean that *none* of Rolands postings would have been accepted, they are the very definition of blog spam.
Something is smelly here, even if I can't quite put my finger on it. Maybe it's time to do some scripting to get some real hard stats on this whole thing.
Re:Dual Frequency - Not just for the military (Score:3, Interesting)
Re:Dual Frequency (Score:3, Interesting)
Less positioning, more navigation. GPS is rapidly becoming (if it hasn't already) a level 1 navigational device (trustable on its own). Right now, it's level 2, which means it's good for general use, but must be compared against another source of navigational information (VORs, etc). The reason is, GPS is cheap compared to maintaining the entire network of VORs and NDBs (and sometimes LORAN) equipment.
The other issue is that if they degrade the GPS accuracy, there is a huge loss - a number of airports have instrument approaches that rely on GPS, and good GPS units can often be used at lower minimums than regular ILS approaches (not landings, though, for obvious reasons). Airports are happy with GPS approaches because they're cheap, they avoid having to maintain expensive ILS equipment. Thus there are a number of GPS-approach-only airports (the requirements are quite strict - WAAS must be available, and enough GPS satellites must be available to compensate for satellite irregularity. Aviation GPSes that are certifiable for instrument approaches have calculators that can tell you if an approach is possible at the destination based on current GPS almanac data).
If they started degrading GPS again, the impact on aviation would be quite significant.
The other thing is WAAS. FAA wanted a way to compensate for GPS signal degradation, so they had WAAS put in, which broadcasts correction data... from the GPS satellite! (That's why most modern GPS receivers can pick up WAAS easily - the satellite is already transmitting the information, so picking up the WAAS information is trivial). Of course, if you degrate the main GPS signal and don't degrade WAAS, the whole exercise is pointless.
(DGPS requires an external receiver - higher end units have a bidirectional serial port so they can transmit NMEA data to a host, but also receive a DGPS correction data from a DGPS receiver, which is why almost no GPS come with built in DGPS - it doesn't come "for free" like WAAS does).
Re:Roland the Plogger, again (Score:3, Interesting)