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It is, actually, possible to measure such things.

Consider GPS, which relies on the accuracy of atomic clocks in orbit. Each GPS satellite has its own independent clock, which must be accurate to within about 40 billionths of a second, over the life of the satellite. http://gpsinformation.net/main... If the accuracy of one of the satellites' clocks is greater than that threshold, your GPS unit will incorrectly report your location. The accuracy of GPS coordinates is one way to calculate the accuracy of the atomic clocks in orbit. Multiply the error rate (in billionths of a second) times the life of the clock, and you can arrive at a number of years it will take for the clock to be 1 second off.

Similar types of calculations can be done with these new, faster clocks. No, it's not necessary to wait 300 million years to see if the clock is one second off. That number is simply an extrapolation.

GPS doesn't measure speed by looking at how your position changes; it uses the doppler effect, which is fairly accurate.
http://gpsinformation.net/main/gpsspeed.htm

Dude, your GPS receiver is 14 seconds off!

People who use GPS for accurate timing need to be aware that GPS doesn't account for leap-seconds. As such, GPS is ahead of UTC by about 14 seconds. You can read more about the problem here.

It gets more complicated, however, as some receivers correct for this. You can read more about the correction here.

Some have wondered how accurate the time display is on Garmin GPS receivers such as G-12XL, G-II+, and the G-III. Here is an answer provided by Garmin Engineering. This also explains why the GPS can be locked for awhile and still differ from UTC by 11 or 12 seconds. (This answer applies to other brands of GPS receivers as well.)

Start of Garmin quote>

Provided the unit has collected current leap second count from the navigation message, (current leap second difference from GPS time is only broadcast once in a 12.5 minute Nav. message), or current leap second has not changed since the last time the unit collected this variable, the time displayed on the front of the unit should be accurate to within 1 second of UTC.

>end of Garmin Quote

Joe Mehaffey comments:
This means that IF your GPS does not have (or does not save) the leap second offset from last time it was operated, your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS. Jack and I have observed that "typically" Garmin GPS receivers display time which is delayed from about 1/2 to 1 second behind UTC. Lowrance GPS receivers are usually between 1 and 2 seconds delayed behind UTC. In both cases, this is a result of the display driver subroutine having low priority as the "GPS internal clock" is within a few nanoseconds of correct.

Similarly, the NMEA time output on the serial link is typically delayed a second or two depending on various factors.

All GPS receivers (AFAIK) have a map of ephemerides

Sort of. It depends on whether you're cold or warm starting the GPS device.

http://gpsinformation.net/main/warmcold.htm

First, here is how Garmin defines their FOUR startup modes.

        Search the sky - Time, position, almanac, and ephemeris data all unknown.
        AutoLocate - Time, position, and ephemeris unknown, almanac known or partially known.
        Cold Start - Time and position known to within some limits, almanac known, ephemeris unknown
        Warm start - Time and position known to within some limits, almanac known, at least 3 SVs Ephemeris are known from previous operation.

The satellites (SVs) broadcast two types of data, Almanac and Ephemeris. Almanac data is course orbital parameters for all SVs. Each SV broadcasts Almanac data for ALL SVs. This Almanac data is not very precise and is considered valid for up to several months. Ephemeris data by comparison is very precise orbital and clock correction for each SV and is necessary for precise positioning. EACH SV broadcasts ONLY its own Ephemeris data. The validity of this data is dictated by the particular satellite and may be valid up to 4 to 6 hours. Each set of ephemeris data gives a "fit" indication which tells how long the particular Ephemeris data is valid. The Ephemeris data is broadcast by each SV every 30 seconds so GPS receivers have frequent opportunities to receive and log this essential information.

Correct me if I'm wrong . . .

You are wrong

GPS is receive-only, and many airlines allow it at cruising altitudes (like laptops, MP3 players, and anything else that doesn't try to transmit):

http://gpsinformation.net/airgps/airgps.htm

The grid-based system you're thinking of has already been implemented for years as UTM, MGRS, and new USNG.

MGRS is used in many technologies familiar to ground troops, such as the PLGR, DAGR, and BFT.

MGRS is used for land-navigation, as described in the Armys FM 3-25.26.

MGRS coordinates are regularly used in common radio traffic, such as the MEDEVAC request, UXO/IED spot report, and call for fire.

Many consumer GPS devices support MGRS.

I sincerely hope GPS isn't being used for primary altimetry:

New GPS buyers are frequently concerned about the accuracy (or lack of it) of the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL...Almost any calibrated altimeter will be more stable at reading altitude than a GPS....

http://gpsinformation.net/main/altitude.htm, emphasis mine

Air pressure altimeters are accurate, stable, and perfectly standardized by aviation processes and regulation. I have grave doubts that any aircraft primary avionics suite would ever be fielded that puts GPS altimetry above that.

On even the most advanced airliners GPS is only used for lateral guidance. The altitude is measured my good old outside air pressure. Same way it's been done for years.

I sincerely hope GPS isn't being used for primary altimetry:

New GPS buyers are frequently concerned about the accuracy (or lack of it) of the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL...Almost any calibrated altimeter will be more stable at reading altitude than a GPS....

http://gpsinformation.net/main/altitude.htm, emphasis mine

Air pressure altimeters are accurate, stable, and perfectly standardized by aviation processes and regulation. I have grave doubts that any aircraft primary avionics suite would ever be fielded that puts GPS altimetry above that.

GPS is not designed to provide an accurate measure of altitude. There are altimeters you could use to see if your animal got under a fence or up a tree, but GPS wouldn't be ideal for that purpose.

The use of GPS for the craft's altitude is indeed a problem. GPS units have the potential to give horrible altitude readings! Quite often, they do. 95% of the time their readings are within 15 meters of the actual altitude. That means two readings up to 30 meters apart would be considered normal. In actuality 95% of the time they are within 23 meters (source: http://gpsinformation.net/main/altitude.htm) And then for the other 5% of the time they can literally be any value whatsoever. It's mind-boggling why they chose GPS as an altimeter considering its (known) horrible accuracy.

http://www.gpsinformation.net/main/gpsspeed.htm

However, this is all a moot point. The defense department has the ability to selectively degrade the civilian signal in certain geographic regions, while leaving the military signal as well as the civilian signal outside of that area intact (and accurate).

Someone who is using an ICBM (or some other sort of long-range delivery system) is not going to be using GPS. They're going to be using a combination of radar, topographic map data/recognition systems, and inertial guidance (as to prevent navigation references to be screwed with during the cruise phase of the weapon in question).

All the answers are at gps information.

As for uploading waypoints, I think Google Earth, the paid version, can link to a gps unit. From there I believe it's easy to get it onto google maps.
J

GPS is only accurate to within a few metres for position. It's MUCH more accurate for speed.

Here's the handheld I own:

http://gpsinformation.net/etrexlegend.htm

- Speed Accuracy specification 0.1 knot RMS steady state.

Here's an interpretation.

http://gpsinformation.net/main/gpsspeed.htm

"Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on."

That's very probably more accurate than your car's speedo.

So please check your facts before spreading such nonsense.

GPS is only accurate to within a few metres for position. It's MUCH more accurate for speed.

Here's the handheld I own:

http://gpsinformation.net/etrexlegend.htm

- Speed Accuracy specification 0.1 knot RMS steady state.

Here's an interpretation.

http://gpsinformation.net/main/gpsspeed.htm

"Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on."

That's very probably more accurate than your car's speedo.

So please check your facts before spreading such nonsense.

Illegal meaning the country being visited might consider a GPS to be "military equipment". The US and most if not all Western countries don't care, but 3rd world countries may. Checking with the local consulate of the country being visited should clear the matter up.

Check here.

http://gpsinformation.net/

Hope this helps

I did, and I'm still right.

WAAS is essentialy just a variation of differential GPS.

It may not require an extra reveiver IF you have the RIGHT GPS ... but it does require wide open land or marine settings to obtain the high accuracies.

Not exactly what you find in a city. Just like differential GPS, you have to be away from structures like buildings due to the massive signal reflection and echo problem that buildings create and the WAAS satellites are in geostationary orbits, so you cant sit in one spot and hope the constellation geometry will improve, it aint going to move into a better spot.

Here's a happy link that claims 7m accuracy using WAAS. (not my 10m, but closer to my number than to yours)

http://gpsinformation.net/exe/waas.html

There's a link within that page that studies accuracy of hand-held units using WAAS ... note that to get 3m accuracy, readings were averaged for 30 minutes and done at night when atmospheric disturbances are much reduced. Recall me mentioning how you have to sit and average readings to get high accuracies?

What about all the other users? Ships and aircraft which may be relying on it?

Before turning off SA, both ships and aircraft used stationary beacons to augment their GPS accuracy (WAAS for airflight and Coast Guard Beacons for ships.) Both of these are fixed positions that greatly increase the accuracy of GPS systems becuase there is a KNOWN coordinate to triangulate from.

A lot of current handheld GPS devices utilize WAAS for accuracy but wouldn't function with the sats shut off for civilian usage as they only offer correction messages.

http://gpsinformation.net/exe/waas.html

The intentional degradation of the civillian signal was called Selective Availability, or SA. Clinton ordered it turned off in the late 1990's. I imagine since it was an intentional "fudging" of the signal that it really only took the flipping of a switch to turn it off. Here is a page showing tracks from the day it was turned off.

And on a side note, how well does GPS work at that alt?

To prevent people from cheaply building their own missile guidance systems, commercial GPS receivers are limited to a maximum altitude of 60,000 feet and a maximum speed of 1000mph. Link here.

These are legal restrictions though, not technical ones. I have no idea what the "real" limits are.

First of all, you should buy a GSPr with built-in mapping. Even for simple A->B uses (like geocaching), having a built-in map is invaluable. For example, a non-mapping GPSr will tell you that point B is 1.5 miles straight ahead. A mapping GPSr will show you that there is a stream and steep canyon straight ahead, and going a less direct route will be much easier. The only downside is cost - a mapping GPSr will cost about $100 more than a non-mapping model, and if you want detailed maps, you'll need additional software (~$100).

For a mapping GPSr, I highly recommend Magellen's SporTrak Pro. It is lightweight, durable, waterproof, and has been reported to be one of the most accurate GPS receivers available. It uses 2 AA batteries and has about a 16 hour battery life (using NiMH batteries). It also has 23MB of non-removable memory, which is enough for all of Southern California + the SF Bay area, for example. Some other receivers have more memory or removable memory cards (e.g. Magellen Meridian), but 23MB is plenty unless you will be travelling long distances with no access to a computer to upload new maps. The unit comes with some maps built in, but to get street level and topo maps, you'll need to buy the $100 MapSend Topo CD-ROM.

The SporTrak Pro also doesn't have a lot of the more gimmicky features, like a color screen, electronic compass, and barometers. All of those are more or less useless and just drain the batteries faster.

The less expensive alternative is a non-mapping GPSr. Look for a used Garmin GPS12 on eBay, or consider its new replacement, the GPS72 for around $150.

Finally, be sure to go to GPSInformation.net. It is an excellent source of information and reviews, and even an article which recommends various GPSr based on price and your needs.

First of all, you should buy a GSPr with built-in mapping. Even for simple A->B uses (like geocaching), having a built-in map is invaluable. For example, a non-mapping GPSr will tell you that point B is 1.5 miles straight ahead. A mapping GPSr will show you that there is a stream and steep canyon straight ahead, and going a less direct route will be much easier. The only downside is cost - a mapping GPSr will cost about $100 more than a non-mapping model, and if you want detailed maps, you'll need additional software (~$100).

For a mapping GPSr, I highly recommend Magellen's SporTrak Pro. It is lightweight, durable, waterproof, and has been reported to be one of the most accurate GPS receivers available. It uses 2 AA batteries and has about a 16 hour battery life (using NiMH batteries). It also has 23MB of non-removable memory, which is enough for all of Southern California + the SF Bay area, for example. Some other receivers have more memory or removable memory cards (e.g. Magellen Meridian), but 23MB is plenty unless you will be travelling long distances with no access to a computer to upload new maps. The unit comes with some maps built in, but to get street level and topo maps, you'll need to buy the $100 MapSend Topo CD-ROM.

The SporTrak Pro also doesn't have a lot of the more gimmicky features, like a color screen, electronic compass, and barometers. All of those are more or less useless and just drain the batteries faster.

The less expensive alternative is a non-mapping GPSr. Look for a used Garmin GPS12 on eBay, or consider its new replacement, the GPS72 for around $150.

Finally, be sure to go to GPSInformation.net. It is an excellent source of information and reviews, and even an article which recommends various GPSr based on price and your needs.

First of all, you should buy a GSPr with built-in mapping. Even for simple A->B uses (like geocaching), having a built-in map is invaluable. For example, a non-mapping GPSr will tell you that point B is 1.5 miles straight ahead. A mapping GPSr will show you that there is a stream and steep canyon straight ahead, and going a less direct route will be much easier. The only downside is cost - a mapping GPSr will cost about $100 more than a non-mapping model, and if you want detailed maps, you'll need additional software (~$100).

For a mapping GPSr, I highly recommend Magellen's SporTrak Pro. It is lightweight, durable, waterproof, and has been reported to be one of the most accurate GPS receivers available. It uses 2 AA batteries and has about a 16 hour battery life (using NiMH batteries). It also has 23MB of non-removable memory, which is enough for all of Southern California + the SF Bay area, for example. Some other receivers have more memory or removable memory cards (e.g. Magellen Meridian), but 23MB is plenty unless you will be travelling long distances with no access to a computer to upload new maps. The unit comes with some maps built in, but to get street level and topo maps, you'll need to buy the $100 MapSend Topo CD-ROM.

The SporTrak Pro also doesn't have a lot of the more gimmicky features, like a color screen, electronic compass, and barometers. All of those are more or less useless and just drain the batteries faster.

The less expensive alternative is a non-mapping GPSr. Look for a used Garmin GPS12 on eBay, or consider its new replacement, the GPS72 for around $150.

Finally, be sure to go to GPSInformation.net. It is an excellent source of information and reviews, and even an article which recommends various GPSr based on price and your needs.

For under $200, you can get some nice units. The market is being flooded with cheap units now. You don't say what you currently use.

For a bit over the $200 mark, you might want to look at the Lowrance GlobalMap 100. The only times I've seen problems with acquisition on one was in city environment where I was effectively in a valley, and the birds were blocked. Lowrance has a lower-priced line now called the iFinder. I haven't used one, so I don't know how it compares, but presumably they're still using the Rockwell design, in which case the only difference in reception would be the antenna design.

The important thing is that you're not going to be able to get the same kinds of street mapping and driving instructions on the lower priced units that the Street Pilot and the iQue provide.

If you, or anyone else is considering a new GPS, I'd highly recommend spending some time here. The guys who run the site are GPS geeks, and tend to give unbiased reviews of GPS products. Plus they've got lots of links to other sites, including software.

Did you know that there is only one product by one manufacturer if you're looking for an aftermarket car stereo that has NAV/DVD/Radio?

Check out this installation of the Pioneer NAV-SYS900DVD

I plan to put one into my Envoy.

GPS signals are not affected by rain and snow. At least the signal is NOT attenuated enough to effect accuracy. That's part of the reason the signals are in the frequency band they are. Check here and pages it links to if you're really interested in how it works.

I'd really recommend a legend or a vista instead. The mapping function really does make it much more fun and useable than the straight coords available on the Etrek.