Speedmeters!!

[hoytedow]

http://ocsdata.ncd.noaa.gov/BookletC...kletCharts.htm

[Poida]

Michael, one probable reason why the USA supplies satellite navigation and charts to the rest of the world is so, the rest of the world doesn't have to do it.

We use USA satellite navigation (thanks guys) but in the case of a war, the US can code the signals or throw them off a desired distance and only the US and allied forces would be able to decode them.

If America didn't share this technology other countries would have to have their own.

In fact the US was very edgy when the European Union was considering launching thier own navigation system.

I have been told that the system is slightly offset to avoid being hit by satellite guided missiles.

Some GPSs have a correction function in them so you can plot your actual position against what the GPS reads, allowing your GPS to display your correct location by making the adjustment automatically.

Poida

[rwatson]

Originally Posted by Poida

.....
I have been told that the system is slightly offset to avoid being hit by satellite guided missiles.

Some GPSs have a correction function in them so you can plot your actual position against what the GPS reads, allowing your GPS to display your correct location by making the adjustment automatically.

Poida

This doesnt make a lot of sense to me.

What is 'offset to avoid guided missiles' - the satellites ? the GPs Reading ?

If I wanted to shoot down a satellite, I could home in on the signal, even if the signal was 'saying' it was in a different place.

Gps with correction ? - how would they 'correct' an error - in what direction by how much ?

[michael pierzga]

Ya...there is an error programed into the GPS signal that renders is of little use for foreign power military ballistic adventures......and the rest of the world may be shut off in times of crisis or for political reasons. . This is why the new Galileo system is being developed in Europe.

[rwatson]

Not any more

"Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded (Selective Availability). This changed with President Bill Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 meters (about 300 feet) to 20 meters (about 65 feet)."

http://en.wikipedia.org/wiki/Gps

Modern cartographers and surveyors equipment can get to within a few feet with optimum satellite orientation.

[michael pierzga]

Im not upto date on GPS signals. Horizontal and verticle accuracy. But you can be assured that the US government will degrade the signal if it sees a need. This degradation would present no problems for vessel navigation...even when degraded the gps position exceeds the accuracy of the chart and a navigator abilty to plot.

[Poida]

Rwatson, the enemy would not shoot down the satellite because they use it too.

How much it is out and how do they correct it?

I've had a bit to do with satellite communications, not a lot, but signals from a satellite, microwave signals, have to be decrypted by the receiver.

Not only can a satellite send an encrypted signal, it can also send a code to decrypt it, which can be changed at regular intervals.

I am sure that if the US wanted too, they could shut down the original signal that we all use and send encrypted signals that only their military can decrypt.

Further to this, each receiver receives a different signal and a different encryption code, so if a receiver is captured by the enemy it will not be sent the decryption code and will stop working.

Poida

[CatBuilder]

And... they us GPS jammers on the battle field.

[michael pierzga]

Has anyone seen a Galileo gps receiver yet. They say its the future of sat navigation

[Stumble]

While at some point degrading the US GOS signal was a reasonable possibility, it just isn't anymore. The Russians have a correlate system, Europe is creating Galileo, and radio triangulation (loran) is now accurate to within a couple of feet as well. Practically shutting down the GPS system would shut down commercial air fair, marine shipping, and pretty much all civilian navigation. Absent a direct assault on the US I can't imagine we would ever turn it off, or even degrade the signal very much.

As for carrying paper charts... The navy doesn't even have them on board any more. The reality is that it is cheaper, easier, and more reliable to carry multiple independent gps receivers on board thn to rely on paper charts.

That being said, I still pilot with paper charts because I find it easier to have a chart on the bridge than deal with the computer up there, but that is just personal preference.