GPS: I keep stumbling over this in books, films and on The TV and not only is it jarring, it's gonna get some tenderfoot killed dead if they don't knock it off.
A GPS device knows where it is (if it can receive signals from enough of the satellites). It cannot tell others where you are. Can not.
There is no transmitter in your handheld or dash-mounted GPS. It's not talking to the Global Positioning System Satellites; all it does is listen to them and then do some math. (As far as I know, the only people talking to GPS birds would be DoD and last I knew, that was in the form of admin-type stuff not in-the-field position info).
That said, there are ways to hang additional hardware on the GPS device and use that to send the GPS position to others; hams have been doing it for years. But it takes a transmitter and most of 'em are power-hungry. It is limited to the range the combination of frequency, power, antenna system and propagation conditions permit.
Your cellular telephone can be tracked back to the particular cell it is in and if your phone includes a GPS receiver, it can send that info, too. This is smaller and more power-efficient than the APRS alluded to in the previous paragraph but if you're in an area where there is no cellular coverage, oopsie! She no work.
(Update: Joe Huffman on phone tracking, privacy and how caution and even paranoia can't stop stupidity).
And again, the first thing is the GPS receiver has to have a lock on enough of the GPS satellites to be able to figure out where it is; in tunnels or between tall buildings, it may not work so well. Or at all.
So, writers, will you please, pleeeeeeeze stop breezily tossing "GPS tracking" into your work in the form of Character holding up GPS Receiver and remarking, "They can track us with this," 'cos they can't. Now, that iPhone you've been taking for granted, on the other hand....
(Side note: the slick way to write this for most adventure-type apps is to use a transponder, something that is polled and spits out the current or last-known-good GPS position, suitably flagged. This is economical of batteries, plausible, and darned hard for anyone who doesn't know how to poll it to locate using normal bug-hunting tools. For extra credit, use some form of decent data encryption and/or spread-spectrum; it can easily be about celphone-sized).
(P.S. GPS receivers can be spoofed. It's a neat trick. Uncle Sam knows how and he's not talkin').
As far as I know, all cell phones sold today are required to transmit their GPS coordinates when 911 is dialed. That would seem to indicate that all cells sold now have some sort of GPS receiver built in to them, even if the GPS info is not available to the user.
ReplyDeleteNo, not all cell phones are GPS enabled. The FCC issued a requirement for mobile phone providers to provide location data for 911 calls. Some providers met this requirement through signal triangulation, other decided to use GPS.
ReplyDeleteI admit, most now have some GPS capability installed because of mapping and other apps running on the phones. It is not, however, a requirement by the feds.
Frankly, OnStar irritates me more. When I owned a Yukon, I didn't subscribe to OnStar even though is was standard equipment, and I disabled it since it could be used to remotely track and monitor me without my knowledge. (Hint: google, OnStar, FBI, Law Enforcement, etc.)
Roberta:
ReplyDeleteDid you know that all GPS satellites transmit on the same frequency?
Hank
There's a combo FRS/GPS called the Garmin Rino that actually DOES allow you to send your GPS coordinates to another Rino. Cool idea.
ReplyDeleteHank: can you say "Gold codes?" Same freq but the birds are rather niftily sorted out!
ReplyDeleteMany cell phones transmit on the same frequency - it was ahrd trick 10 years ago; but now it's common-place. CDMA cellular radio is fascinating, both technically and from a business development standpoint.
ReplyDeletenot a direct GPS transceiver but might as well be...
ReplyDeletehttp://www.rei.com/product/791972
Ok, most of the handheld GPS probably don't transceve (?)
ReplyDeleteI'm hearing radio commercials for either Garman or TomTom that tell in realtime where the traffic is bad and able to rerout around back-ups.
That tells me either they're talking to the unit, or the units signature is known somehow and relayed to the mothership.
They require a data connection - typically provided by attaching a cell phone; or by embedding a cellular data module.
ReplyDeleteThe software that runs on cell phones/handhelds uses that unit's data connection to provide.
GPS live traffic can also work via low powered FM transmitters, so you only get traffic info from statins nearest your current location. (That data stream CAN, however, include data from as far away as the system designers wish and have bandwidth for.)
ReplyDeleteISTR the traffic info can be embedded in regular FM transmissions as well; but I may have mis-heard a description of the low-power FM transmitter. Or you could use something like that now-defunct Microsoft system that had the nifty watches
ReplyDeleteIf I'm thinking right, the traffic info is digital and encoded - you pay a fee for the decrypt key.
So thats how she found me in that bar!
ReplyDeleteWell Pshaa on cel phone thingys.
There's a nifty device called the "Spot", which is an emergency locator device. You can set it up to leave a breadcrumb trail of where you are and have been on a Google map. It also uses satellite phone to enable you to call for rescue from designated friends if you break down, or to dial 911 if you're in a life or death situation. It's been out for over a year. www.findmespot.com
ReplyDeleteYeah, but you pay WAY extra for that "Come find me!" feature, over and above the basic GPS receiver found in everyday gadgets.
ReplyDeleteWhich just sort of indicates how unrealistic the usual TV "We'll track him by his Garmin!" meme is.
Selective availability, disabled in 2000. GPS Selective availability adds a pseudo random error of up to 100M to the signals. This was done to keep enemies from using Civilian GPS receivers for precision attacks. For more geeky information than you could ever want on GPS and how it works see This Wiki entry
ReplyDeleteAssisted GPS (A-GPS) found commonly on cellphones may transmit GPS signal data to the cellphone towers and have a server on the provider's network process it. This form of A-GPS works just like the writers think all GPS does in terms of revealing location, but does not work where there is no signal. Some phones have both A-GPS and regular GPS, because A-GPS is much faster initially in areas with bad GPS reception and passable celluar reception such as cities.
ReplyDeleteI'd forgotten about that. A-GPS is also used by cellphones whose processing power is insufficient to run the calculations in a sufficient time. These phones provide the raw data to the netwrok, which crunches and returns.
ReplyDeleteThe increase in accuracy by having precalculated part of the answer makes up for the minor additional lag in communications
The garmin rino is a two-way walkie-talkie as well; it piggy-backs the location signal on the walkie-talkie frequency. As such, while the GPS portion is good anywhere, the communicating where friends are is restricted to the same Line Of Sight communication as the talkie part. As I have had to patiently explain to several hunters, it's not a magic cure if "their wife" "just happens" to split off from the group and gets on the other side of a ridge.
ReplyDeleteThe SPOT uses a satphone network coupled to a GPS and PLB - it's essentially a one-way satellite phone. If you are down in the lower 48 down to the equator, it works most of the time, as long as it can see both the GPS constellation and the satphone constellation. Deep canyons and large angled boulders that bounce signals need not apply. North of 60, the satphone constellation the use is low on the horizon, and you start to have problems when on the north side of a mountain, or under a heavy low cloud layer, etc.
The Spidertracks unit has no external power, so it must be attached to a power source, but it uses the Iridium satellite network, and therefore will report reliably even north of 60. It also comes with bluetooth hardware and pushes data packets from compatible cell phones along with the location pings - essentially, sattelite texting. On the other hand, it's not made for handheld use and the hardware is expensive.
And all of this knowledge amounts to no good point except feeling the urge to massage the bridge of my nose and sigh when writers insert "the plot needs the device to do this" instead of plotting with what the actual devices do.