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2 years ago I wrote a small C program to synchronize a Windows PC to a Trimble GPS: easy. A RS232 library, a quick look at the Trimble communication protocol and it was done in under 4 hours.

Last week I've been synching my SGIs using ntp (Network Time Protocol) and I saw in the documentation that it can also synch to a GPS. Didn't look at the details though, but probably through the serial port. And ntp is free.

Trimble makes a product called Accutime 2000 http://www.trimble.com/acutime2000.html That can basically plug right into the ether.

Hokey name, but I'm told it works pretty good. The local telco (MTS) uses one for it's time source.

Cheers!
John

It's called Differential GPS (DGPS). (GPS is already triangulation of different signals sources)
AFAIK, the signals are delayed according to a certain algorithm (known to the DOD).
But there are several fixed GPS receivers which compare the measured position and the real position. The comparision yields a correction factor for the various signals from the different GPS-satellites.
Here is some short explanation

This is in contrast to WinCE's use of its power on a feeble attempt to replace a desktop computer.


Nah. I don't see why you think it will replace the desktop. it complements it.


This moves us onto the second question, which was about feature count. WinCE's trying to do much more than is appropriate for a handheld machine; things (like word processing) which are honestly better left to a desktop computer. The point I was making about "waaay too overpriced for too little functionality" was in comparison to the desktop machine that WinCE's apparently trying to overtake.


Pocket PCs come with Pocket Word not fully fledged word. It's hardly what I'd call something for word processing. It's not even trying to do that. It's just there so you can view your word documents. If you compare the functionality of a CE machine to a Palm, and then compare the prices, I think you'll find Palm is far too expensive.


That's just it. The pocketPC is simply overpowered for what's needed from a handheld.
Sure you could make a handheld computer that does everything a desktop does, but you'll pay for it in weight, size, battery life, cost, and complexity.


Like I said, Pocket PCs are very modular. Compaq have a very light grey scale Pocket PC that is on par (size wize) to the Palm V. Pocket PCs are as complex as you want them to be. Just don't buy the gadgets and install the right app launcher programs (like you would on a Palm).


The Palm handheld is simple. The core applications that it comes with it don't have complex user interfaces, don't require megabytes of ram (the original palm came with what, 128K?), and don't ask much of the user in order for them to find use for it.


Try out the latest Pocket PCs. The interface is much more simplified. That's the reason why since the update to the UI, Pocket PCs have been gaining in popularity faster than Palm.
Also I don't buy your memory argument. If small and simple is so good why are some of the latest palms equiped with 8MB of ram and colour?

Also remember that Pocket PCs aren't just for personal organizing. They have many other uses. The milatary use them. Trimble Navigation use them for their GPS MAP navigation systems. Stock taking is often done using Pocket PCs with bar code scanners...the list goes on...

BTW, the trimble systems are cool. One of the packages lets you hook a Pocket PC to a GPS receiver. You use a base site to map out plans for street lights and then the Pocket PC guides city planners as they patrol the streets.

Because I'm afraid we'd have to mod you down as well.

GPS signals contain two separate codes for position locating: C/A (Coarse Acquisition) and P (Precise). C/A was indeed degraded by Selective Availability, and SA was turned off by order of the President in May of last year. However, the way it works is that the C/A code only modulates the L1 carrier (1545.72 MHz) at a 1MHz rate, while the P code modulates both the L1 and L2 carriers at a 10MHz rate. Furthermore, the P code is encrypted (which is then referred to as the "Y" code.) The military GPS recievers typically acquire the easier L1 code first and from there scan for the P code.

The C/A code is still good only to a few meters, while the combination of the two carriers carrying the P code is able to detect and compensate for atmospheric disturbances yielding an accuracy of 10cm or less.

For a much better explanation, see Trimble's How GPS works article.

John

Because I'm afraid we'd have to mod you down as well.

GPS signals contain two separate codes for position locating: C/A (Coarse Acquisition) and P (Precise). C/A was indeed degraded by Selective Availability, and SA was turned off by order of the President in May of last year. However, the way it works is that the C/A code only modulates the L1 carrier (1545.72 MHz) at a 1MHz rate, while the P code modulates both the L1 and L2 carriers at a 10MHz rate. Furthermore, the P code is encrypted (which is then referred to as the "Y" code.) The military GPS recievers typically acquire the easier L1 code first and from there scan for the P code.

The C/A code is still good only to a few meters, while the combination of the two carriers carrying the P code is able to detect and compensate for atmospheric disturbances yielding an accuracy of 10cm or less.

For a much better explanation, see Trimble's How GPS works article.

John

For those that don't know, SA is a set of two different time signals broadcast by the satelites. The military time signal is pure, but the civilian one had some noise injected into it to degrade the accuracy. Now that the signal degradation is no longer being done, your commercial receiver is just as accurate as the military versions.

Khadaffi | Saddam | Osama Bin Laden | Joe Militia just needs a unit from Garmin | Magellan | Trimble, and they can pick off anything in range. This has been the case for quite a while, as even with SA in use, the accuracy was about 100 FT. A good large bomb/missile has a blast radius larger than that.

How vehicle location works. It requires a secondary communications network to actually send the GPS data back to be monitored. GPS alone does not do this.

Beeline, Integranautics, Modular Mining and Trimble Navigation already have self-navigating farm tractors and mining trucks.

These systems will to my knowledge measure their positions to within 10 cm horizontally and within 30 cm vertically. This is done using differential GPS (DGPS).

Selective availability has been turned off. Using only instantaneous measurements from a constellation the expected accuracy is on the order of 10 m today.

Differential GPS nulls out atmospheric errors by 'knowing' a fixed position and sending CMR (corrective measurement records) to other systems nearby. In dynamic situations (the tractor is moving) you use RTK (real-time kinematic) data to update the system.

Most of the self-navigating systems have inertial measurement as a patch method while the tractors are under trees or bridges.

This works and you can see it here .

The receivers use a combination of C/A code (the coarse code that inexpensive receivers use to get you within about 10M) and processing of the GPS carrier wave itself to perform measurements to that accuracy.

Five to Ten years ago you would set up two of these recievers recording GPS measurements and let each run for 15 miniutes or more, then process the data sets against each other to determine the relative positions of the two antennas. Then advances in computing the position for each epoch of satellite data recieved allowed one reciever to be mobile during data collection, only stopping to increase accuracy for each unknown point. Combine this with a radio transmitting the stationary receiver satellite data and a mobile processor powerful enough to do all the fun matrix math involved and you have a Real Time (within a second or so) Kinematic (moving) Survey Grade GPS system. Costs you about $40,000 or so.

One reason these aren't useful to Saddam is the fact that high dynamic situations (like an ICBM) break the entire system, from the C/A solution to the carrier wave processing.

If you want to know more, here is one article that goes beyond the basics of GPS positioning.

Oh, and to respond to some of the people on /., the military USED to introduce error (called Selective Availability) into the C/A code, reducing the accuracy of the measurements from a single GPS reciever from about 10m to 75m or more. That introduction of error has been turned off, though it can be turned back on in case of a national emergency. The military also has an encrypted transmission from GPS satellites called P code, it achieves a higher level of accuracy (with military recievers that can decrypt it) than the C/A code does without SA. Using two recievers (or a reciever and a differential correction signal such as that from the Coast Guard) narrows the error down to around 1m, the differrence is made up by errors introduced by the ionosphere, other atmospheric variables and the internal accuracy of the reciever clock itself. FWIW, GPS recievers are being used to measure atmospheric water vapor content, to aid in weather models and prediction.

While this WAS true for a long time, the U.S. government turned off Selective Availability in May of 2000, making it possible for civilian users to get what used to be strictly military-grade positioning.

Of course, they might turn it back on in case of a "strategic conflict", as they'd say.

The GPS system is actually much more accurate then 22 yards. However the only ones that can harness such accuracy are the military. Noise is intentionally added to the signal to decrease the accuracy for civilian receivers. Military receivers know of and account for the intentional noise and thus remain supprisingly accurate.

This site has an informative (albiet simplified) tutorial on the workings of the GPS system.

Farming is an extremely high-tech business these days. Check out Trimble, the first site in a Google search for "farming GPS."

Farmers are using sophisticated soil-sampling quality testing, with GPS, to determine fertilizer spreads. The GPS is used to mark the sample location and generate a "map" of the field... and the GPS is used to control the mix of fertilizer *as* it is being spread.

GPS is also used for yield monitoring, during harvest: volume and moisture content. Why is one area more productive than another? The soil/fertilizer/weather/etc data is reviewed and analysed, and plans made to improve yield the following year.

Some farms use GPS with insect infestation data to perform variable crop spraying. The most sophisticated systems mix the pesticide on-the-wing: concentration dependent on infestation level.

How about variable-rate planting? Overcrowding is ruinous in poor-yielding sections. Plant fewer seeds there, and save money. Variable-depth tilling: monitor the hardpan depth and till only deep enough to crack it.

Variable-rate irrigation will make a fortune for its inventor, particularly in water-poor states like California.

And so on. The farming business is as high-tech as one's imagination... satellite imagery mapping out stressed crops, so one doesn't need to sample all 4000 acres to locate the infestations? Why not!



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Trimble has a really good "Intro to GPS" Tutorial which covers how pseudo-random codes are used. The entire tutorial is here:
http://www.trimble.com/gps/fsectio ns/aa_f3.htm

while the section on pseudo-random codes is here:
http://www.trimble.com/gps/howworks /aa_hw3

It even discusses jamming (briefly).

Trimble has a really good "Intro to GPS" Tutorial which covers how pseudo-random codes are used. The entire tutorial is here:
http://www.trimble.com/gps/fsectio ns/aa_f3.htm

while the section on pseudo-random codes is here:
http://www.trimble.com/gps/howworks /aa_hw3

It even discusses jamming (briefly).

For a great introduction to the GPS system (and technologies such as differential GPS) check out the Trimble web site:

http://www.trimble.com/gps/index.htm