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Re: The n800: Ultimate Computer for Air Travel
GPS receivers download an almanac from the satellites, in order to know what satellites to look for. If you turn your GPS off and move it a few thousand miles, it has no idea you did that, and will look for the satellites it thinks will be overhead at that time. If it doesn't find them, then it has to start a new search, and find out what satellites are in view. The GPS satellites are not in geosynchronous orbits, but in polar orbits, moving from north to south, covering different areas as the earth spins below them. It can take up to half an hour for the GPS to find itself, although newer chipsets should find a position more quickly. On standalone models you can usually tell it to start a new search, which is quicker than continuing to search based on the last known location. If you move the GPS from the US to China, don't expect to get an immediate lock, it will take time even with a clear view of the sky. Other than that, there is nothing special about China. GPS is designed to work anywhere on the surface of the earth, thus the GLOBAL in the name.
Metal and water will block the GPS signals completely. The water in you hand is more than enough to block the signals, and the metal fuselage of an airliner will also block them. You can usually get a signal at a window, but it won't let you see all the satellites overhead, just the ones in the direction of the window. Whether the GPS will work at airliner speeds depends on the manufacturer. They can prevent it from working at whatever speeds or altitudes they want, or not. There is no technical reason they can't work at any altitude or speed, other than the US government not wanting them to. GPS receivers, bluetooth or not, won't affect aircraft navigation instruments. Neither will laptops or much of anything else. The reason for any prohibition is that the FAA, in its infinite wisdom, requires any airline (or any other commercial aircraft operator) to prohibit all electronic devices unless that operator has tested the specific devices, or types of device, and determined that they don't interfere with the avionics. Some airlines care about their customers and do the tests,which are minimal, and some don't. I've personally tested many devices, and have never found one which caused a problem. I used my own personal GPS mounted to the instrument panel for a long time before GPS was installed in our aircraft, and if it won't cause a problem there, it won't cause one from back in the passenger area. Many airliners still don't have GPS installed, because it really isn't necessary. Flying between major airports doesn't require GPS at all, but flying to other places can. |
Re: The n800: Ultimate Computer for Air Travel
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Re: The n800: Ultimate Computer for Air Travel
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Most/all newer units report speeds way above that now. |
Re: The n800: Ultimate Computer for Air Travel
ALL GPS receivers have a hard time getting a lock while in motion, even with a warm-start. This is because it is trying to get accurate timing references from the Satellites (which are moving), while it is moving, and not slaved to a satellite, which means it is using only the internal, unsynced quartz clock. Once it is able to get enough of a reading, it will usually train the internal clock to the satellite's atomic clock. Then it can look at the almanac data to figure out when the other sats are transmitting, and where they should be (ephemeris data). Once that happens, it can figure out where it should be (a second or so after it gets a lock on the first sat), provided it has an up-to-date almanac. If you watch the satellite status screen, you can see the almanac update itself when you see the satellite number jump around.
Now, if a GPSR is switched off and moved to another part of the world, it will need to do all of the above, and it will likely have an out of date almanac. Also, in most cases, the chipset makes a few assumptions about where it thinks it is, so it looks for satellites that the almanac says should be there. When they are not heard, it resets and goes into a cold-start mode. If it is way out, it may take 20-30 minutes for it to get fresh almanac data and gain a fix. The Wiki explains it better than I can... http://en.wikipedia.org/wiki/Global_...gation_signals The space shuttle and ISS use the GPS system to confirm their position, but I don't know if it is primary or not (likely not). There's no reason GPS wouldn't work above the satellites, except that the sats have directional antennas pointing at the Earth. Oh, and I would imagine that if a commercial airliner is using a GPS receiver, it will be confirmed that it has a position solution prior to pushback! |
Re: The n800: Ultimate Computer for Air Travel
Finally, thanks for these very good explanations! I never understood what all had to happen while my N810 says ''Establishing GPS Fix" but now it makes some sense at least. This explains why it took a while to get a fix when I went to Oregon last week, and is now trying to 'find itself' all over again. O well, at least I know my way around here:p
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Re: The n800: Ultimate Computer for Air Travel
a radio receiver, which includes a GPS, has to have a local oscillator as part of mixing down the high radio frequency to baseband; some have an intermediate frequency stage. The base band is then processed to extract the signal.
cheap receivers leak the local oscillator. computers can also leak the oscillator clocks used for the processor and logic - try operating an AM receiver near a computer and see what happens. an old fashioned glass-tube CRT leaks a bunch of signals and this is how the TV detector vans in the UK work - and it would probably be so expensive to shield the TV to make it "invisible" it's easier to pay the license! measurements of GPS *receivers* (not including any bluetooth stuff) show that they hardly leak anything and are practically undetectable it tends to be cheap FM radios and computer games that are unshielded and cause interference. CE marking and EMC has improved this situation quite a bit, but overall radio noise pollution is pretty dreadful round the world. HTH |
Re: The n800: Ultimate Computer for Air Travel
Largely correct, but I'm a hopeless pedant: strictly speaking, a local oscillator is not necessary. Simple AM receivers, back in the day, didn't have them. For anything else, and definitely for GPS, it's largely theoretically, but I couldn't let it go...
Incidentally (to add something of value to this comment :)) radar detectors (for detecting police speed-traps) are typically heterodyne, and simple ones are usually very noisy. So much so that (at least in states where they are illegal) police often use radar-detector-detectors to pick them up. Of course, those are also heterodyne receivers, so some motorists use radar-detector-detector-detectors to shut their radar detector down (and warn them) when they detect a radar-detector-detector... Electrical engineering (combined with government regulation) is almost as good a racket as being an impartial arms merchant. ;) |
Re: The n800: Ultimate Computer for Air Travel
I actually used my N800 as an in-flight laptop on the way to the Maemo Summit. It rocks, just as pronuke said it would. I finally got around to posting a picture:
http://farm4.static.flickr.com/3206/...12bf104b34.jpg |
Re: The n800: Ultimate Computer for Air Travel
heh, showing the app manager of all things ;)
btw, what battery pack is that? |
Re: The n800: Ultimate Computer for Air Travel
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Re: The n800: Ultimate Computer for Air Travel
Yes, it's a no-name clone of a battery pack talked about many times on the forums (you can buy the tekkeon? one in the US). I carry this and a bunch of charged 2800 mAh AAs around in my bag so I've always got power. Search my posts for "battery" for more information.
It may look a bit weird because I have some velcro on it; I can stick all my stuff together into little bundles (my N800 has velcro on the back, too ;) ). |
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