The military uses "selective availability" within the GPS satellite broadcasts to make commercial GPS receivers less accurate than military ones. Well, thanks to Bill Clinton this accuracy-robbing feature is going to be turned OFF tonight!

My 2.7T doesn't have Nav, but I do have a GPS and I'm looking forward to seeing the accuracy improve by a big margin.

A lot of GPS receivers use components that are merely good enough for the accuracy provided by SA for commercial GPS. Many such GPS receivers will see minimal accuracy improvements because of these low-precision components.

Not to say that you won't see a difference, it all depends on how over-engineered your GPS receiver is.

Based on what I read at the link below, it should make a big difference indeed. Everything I've read about consumer GPS units is that they can easily handle the data once the SA is gone. The data is 'cleaner' now, they actually don't have to work as hard.

The link below leads to (among other things) information specifically about how it will affect auto navigation systems. Yesterday, yours could be off by 100m and tomorrow, it will be accurate enough to know what SIDE of the street you're on. It's supposed to take effect Midnight May 1.<ul><li><a href="http://www.igeb.gov/">Govt GPS site</a></li></ul>

Turning off selective availability will make a huge difference in the accuracy of civilian GPS. SA introduces artificial timing errors into the GPS signals that civilian GPS recievers use and decreases the accuracy from about 15 meters to something like ten times more than that. The military (I'm an ex USAF pilot) uses a different set of signals, that were never effected by SA, and is able to get about 1 meter accuracy. To get even better accuracy (better than 15 meter)with your civilian GPS you will need to use a differential receiver which can bring accuracy to about a meter or with some surveying systems down to a few centimeters.

The reason that the USAF (which built the GPS system) added the SA errors was to keep an enemy from using our own GPS system for aiming missles and smart bombs. GPS was designed to be very difficult to jam but I believe that the US military has found ways to reliably jam GPS in selected areas so that the need to degrade the accuracy is not so great. Besides, with differential GPS available in so many areas the SA wasn't really providing the intended accuracy degredation anyway it was just making consumers buy more expensive equipment.

I don't know enough about GPS to comment on if the increased accuracy is usable by our current in car navigation systems. However, I suspect even if the signal is usuable it will probably require a software update before it will use the additional accuracy as part of the route display (the Lat/Long display shouldn't require an update to make use of the more accurate data).

The good news is that it appears the operating system can be updated via flashing from the CD-ROM that contains the map data, the bad news is that the operating system appears to be Windows (there is a config.sys file on the CD-ROM and references to .DLL and .EXE files).

marcos

All consumer GPS systems got much more accurate (including the factory Audi system) as of May 1st. There is NO firmware update required.

Selective Availability (SA) was an encoding scheme that allowed the government to add a random amount of error to the civilian GPS signals. They have simply decided to stop adding this error. It doesn't matter what firmware your GPS has or "how overbuilt it is". GPS is a digital system, if your antenna receives a signal at the correct strength or greater, it will be accurate.

More expensive GPS's have better antenna's (to receive weaker signals), multiple receivers (to acquire the required satellites faster), and in extreme cases... multiple antennas (which allowed them to defeat the SA).

As it turns out most of the Military uses civilian GPS units (instead of the more expensive military ones), so during Desert Storm, we actually turned off SA over the region!

The government can still add SA back and or turn off the civilian frequencies all together if the need arises.

Jason

So I drove around today letting the Nav. System tell me how to get to places to see if the improved accuracy of the GPS data would allow it to give better instructions.

The short answer is no.

The two problems I've noticed with the nav system are that it doesn't realize where it is when starting out (i.e. gives the "Please proeeed to the nearest road" instruction even if you are on a road) and it takes a long time for it to realize that an instruction has not been followed (i.e. if it tells your to turn right and you proceed straight it doesn't immeadiately realize this and recalculate the route). If the higher resolution GPS data were being made use of it should be able to distinguish between being on the street or a parking lot when the system is switched on; it continues not to be able to do this. Siimilarly with the new GPS accuracy it should be able to realize that you didn't follow a turn instruction before you even leave the intersection; but that's still not the case, you have to drive down the road some distance (~100 yards?) before it figures that you aren't were you are supposed to be and recalculates the route.

It's possible that the Lat/Long display has become more accurate, but since it only displays data to the nearest second and that's about 100 feet (at the equator), it's hard to tell.

marcos

Even with SA off a civilian GPS without a differential receiver can only guarantee about 15 meter accuracy and since Audi had no way of knowing in advance that SA would be turned off they had to design their software to work with accuracy much worse than 15 meters. The software would presumably allow you quite wide lattitude before it would be sure you weren't following instructions since the position is suspect. Also remember that it takes a GPS receiver up to several minutes to get a fix when it is first turned on so you will get no position for a while when you first start the car. Also the GPS receiver needs a clear view of enough satelites (at least 4 for 3D position) so if they are trees or buildings around it could take even longer to get a fix.

With my cheapo $100 Garmin 12, if it can see six or more satellites, it is accurate to an amazing 13 feet, probably better if it catches 10 or 12 satellites. As for the comments of marcos, I think the issue here is driving away before it acquires position. When I have my GPS in the car, if I drive off before it acquires, it takes as long as 5 minutes to get a fix while on a straight highway - and likely a lot longer if I'm making a lot of turns. However giving the GPS about 30 seconds to get a fix when stationary makes a big difference.

And what I said earlier about now being able to discern what side of the road you are on, this is clearly evident on my GPS display which shows my path going to the grocery store and back and you can see both directions traced, side by side - slick!

Don't take this as Gospel but it's not far off!

Sensitivity/accuracy can be adjusted by the military in times of conflict. (Gulf War for example) Since both friend and foe can use GPS for weapon delivery there is a need to defeat the accuracy of potential threats.

That said, I recall reading that military versions could deliver a weapon to a target within a 50 ft diameter. Commercial applications somewhere around 100 ft or so.

I suspect our nav systems will not suddenly be more accurate. Besides there is a point when individual brain power, logic, and reasoning should be able to take over even when delivered to a point inside a 250 ft circle!