EdDekker
Silver Member
- Joined
- Apr 11, 2000
- Messages
- 192
- Location
- New Ipswich, New Hampshire
- Tractor
- Kubota B2400, Bobcat 331 Mini-Excavator
When the GPS system was new there was a random noise, called GPS Selective Availability (SA), added to the civil signals which added some error to the locations plotted by a GPS receiver. The system allowed the GPS controllers to control the amount of extra error added to the signal. The error was worldwide.
The SA error could be averaged out by the surveyor's GPS systems with long periods stationary (15 minutes and up).
During the Gulf war 12 years ago SA was turned off allowing American military units to use civilian pocket GPS units, there were not enough military units to go around. The solution was for the military to buy civil units by the truckload. (The military GPS receivers do other good things using signals not available to civil receivers which prevents them from being fooled by jamming signals etc.)
In the current war I have read that many of the soldiers have had commercial pocket GPS units sent to them by family members as a backup. You can occasionally see a Garmin Streetpilot in the FOXNEWS Humvee in their reports from Iraq. I have not read of any plan to restrict the civil GPS signals in the Middle East during this war.
As other uses were found for the GPS system other Government departments and commercial subscription services added (ground based) transmitters sending out a signal which could be integrated with the GPS signal to eliminate the error and in fact considerably improve the GPS accuracy. This is called Differential GPS or DGPS. A Differential GPS ground station receives the GPS signal and computes a correction signal (it is installed at a fixed location) and broadcasts the correction signal. The user will have a Differential GPS receiver connected to his/her GPS. The GPS unit will use the correction information received by the Differential receiver and use it in computing the location displayed to the user. Since the transmitter is ground based it can only be received in the immediate vicinity. Differential GPS ground stations have been installed by the Coast Guard to enable the LORAN network to be dismantled, and by the FAA to allow GPS approaches to airports too small to warrant an ILS (Instrument Landing System).
There is another system being implemented to improve accuracy Wide Area Augmentation Service uses a ground station to receive the GPS signal compute a correction signal and send it to a satellite for broadcast to WAAS enabled GPS receivers. The WAAS signal is sent in the same frequency band as the GPS signals so it is not necessary to have a second receiver, it is possible that a software change in the GPS can handle it (given enough memory and unused processing capacity).
There are currently two geosynchronous satellites broadcasting the WAAS signal. At ground level the signal from these satellites can not be received in most of the US. The WAAS signal will eventually be broadcast by more satellites and therefore be more useful.
The Russians have launched a system equivalent to GPS GLOSNASS. The EU is in the development stage of their own navigation system (When the new European system is fully implemented sale of GPS units in Europe will be banned and users of the EU system in Europe will pay a stiff annual tax for each receiver they own.)
The situation was not sustainable.
<ul type="square">
[*]The Civil users depended on GPS so the signal could not be turned off in a crisis (e.g. the airplane on approach needs the GPS signal to land AND to execute the missed approach procedure).
[*]While the military was introducing error in the signal, the FAA and Coast Guard were providing a correction signal which removed the introduced error and provided additional accuracy.
[*]There was another system GLONASS operating that any hostile military/terrorist could use.
[/list]
On May 2 2000 SA was turned off permanently. At that time, the United States Government stated that it has no intent to ever use SA again. The Government is not adding error worldwide but may 'prevent hostile use' ... 'in a theater of operations' (see below for longer quote from the Interagency GPS Executive Board web site).
<font color="blue">
U.S. Policy Statement Regarding Civil GPS Availability
March 21, 2003
The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace.
The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations.
We believe we can ensure that GPS continues to be available as an invaluable global utility at all time, while at the same time, protecting U.S. and coalition security requirements.
</font>
The SA error could be averaged out by the surveyor's GPS systems with long periods stationary (15 minutes and up).
During the Gulf war 12 years ago SA was turned off allowing American military units to use civilian pocket GPS units, there were not enough military units to go around. The solution was for the military to buy civil units by the truckload. (The military GPS receivers do other good things using signals not available to civil receivers which prevents them from being fooled by jamming signals etc.)
In the current war I have read that many of the soldiers have had commercial pocket GPS units sent to them by family members as a backup. You can occasionally see a Garmin Streetpilot in the FOXNEWS Humvee in their reports from Iraq. I have not read of any plan to restrict the civil GPS signals in the Middle East during this war.
As other uses were found for the GPS system other Government departments and commercial subscription services added (ground based) transmitters sending out a signal which could be integrated with the GPS signal to eliminate the error and in fact considerably improve the GPS accuracy. This is called Differential GPS or DGPS. A Differential GPS ground station receives the GPS signal and computes a correction signal (it is installed at a fixed location) and broadcasts the correction signal. The user will have a Differential GPS receiver connected to his/her GPS. The GPS unit will use the correction information received by the Differential receiver and use it in computing the location displayed to the user. Since the transmitter is ground based it can only be received in the immediate vicinity. Differential GPS ground stations have been installed by the Coast Guard to enable the LORAN network to be dismantled, and by the FAA to allow GPS approaches to airports too small to warrant an ILS (Instrument Landing System).
There is another system being implemented to improve accuracy Wide Area Augmentation Service uses a ground station to receive the GPS signal compute a correction signal and send it to a satellite for broadcast to WAAS enabled GPS receivers. The WAAS signal is sent in the same frequency band as the GPS signals so it is not necessary to have a second receiver, it is possible that a software change in the GPS can handle it (given enough memory and unused processing capacity).
There are currently two geosynchronous satellites broadcasting the WAAS signal. At ground level the signal from these satellites can not be received in most of the US. The WAAS signal will eventually be broadcast by more satellites and therefore be more useful.
The Russians have launched a system equivalent to GPS GLOSNASS. The EU is in the development stage of their own navigation system (When the new European system is fully implemented sale of GPS units in Europe will be banned and users of the EU system in Europe will pay a stiff annual tax for each receiver they own.)
The situation was not sustainable.
<ul type="square">
[*]The Civil users depended on GPS so the signal could not be turned off in a crisis (e.g. the airplane on approach needs the GPS signal to land AND to execute the missed approach procedure).
[*]While the military was introducing error in the signal, the FAA and Coast Guard were providing a correction signal which removed the introduced error and provided additional accuracy.
[*]There was another system GLONASS operating that any hostile military/terrorist could use.
[/list]
On May 2 2000 SA was turned off permanently. At that time, the United States Government stated that it has no intent to ever use SA again. The Government is not adding error worldwide but may 'prevent hostile use' ... 'in a theater of operations' (see below for longer quote from the Interagency GPS Executive Board web site).
<font color="blue">
U.S. Policy Statement Regarding Civil GPS Availability
March 21, 2003
The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace.
The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations.
We believe we can ensure that GPS continues to be available as an invaluable global utility at all time, while at the same time, protecting U.S. and coalition security requirements.
</font>
