Some of you may already know this, but to date the principle limiting factor in GPS accuracy has been an error intentionally introduced by the department of defense as a safety precaution. This error is called Selective Availability. President Clinton announced today that he has ordered that SA be turned off at midnight tonight. This will result in increased GPS accuracy for both standard GPS units and those with differential units. GPS works by measuring the time it takes for a signal to travel from a satellite in an 11000 mile orbit to the GPS receiver. The elapsed time establishes the distance the receiver is from the satellite. Assuming we know the position of the satellite, this narrows the possible receiver positions to a sphere. Add two more satellites to the equation and there are only two possible positions. One is your position on Earth. The other is either in the center of the Earth or out somewhere in space. The receiver is programmed to assume you are on the surface of Earth and disregards the bogus position. (In reality, the system uses a fourth satellite as an error checker and if possible up to twelve for redundancy, but three is enough to start with) The possible accuracy of the GPS system is less than one centimeter, though the systems aboard our boats are not sophisticated enought to do better than a meter or so. More on boat accuracy later. So, the government was concerned that accuracy of the GPS system would allow miscreants to target weapons against the United States. They decided to make the accuracy of the GPS system varry periodically to prevent this. This varriance is the selective availability that is about to be turned off (or may have been by the time you are reading this). Differential GPS was designed to overcome the limites of SA. It works like this: The DGPS sending station is attached to a GPS receiver whose position is known very precisely. The sending station then compares the position given by SA to the known position and extrapolates the offset. This information is then sent to the DGPS receiver on the boats of people with fewer student loans than I have, and the DGPS receiver fixes the position it had been receiving from the satellites. DGPS is no longer needed to overcome SA, but it is still a good thing to have aboard. In addition to SA, GPS accuracy is degraded by several factors including atmospheric transmission and local radio interference. For this reason, it is typically not more accurate than 15 yards even without SA. Under some conditions it is better, and some places experience those conditions more often than not (Casco Bay is one of them) but you still cannot trust the unit. Unless you have DGPS. DGPS doesn't care if the inaccuracy is coming from SA or from aliens. It just filters the inaccuraccy out and passes on good data. With it you can trust your GPS to a
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Bob England
Hurray!
SA was a silly idea that should have been turned off long ago. Navigation will be much more accurate and boating will be safer once SA is discontinued. You can read more about this at http://www.cnde.iastate.edu/staff/swormley/gps/check_sa.html
SA was a silly idea that should have been turned off long ago. Navigation will be much more accurate and boating will be safer once SA is discontinued. You can read more about this at http://www.cnde.iastate.edu/staff/swormley/gps/check_sa.html
Here's a clip on the deal.
President Clinton: Improving the Civilian Global Positioning System (GPS) THE WHITE HOUSE Office of the Press Secretary ___________________________________________________________________________ For Immediate Release May 1, 2000 President Clinton: Improving the Civilian Global Positioning System (GPS) May 1, 2000 "The decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide. --This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world." President Bill Clinton May 1, 2000 GPS IS A CRITICAL TECHNOLOGY FOR INDIVIDUALS AND BUSINESSES AROUND THE GLOBE. GPS is a dual-use system, providing highly accurate positioning and timing data for both military and civilian users. There are more than 4 million GPS users world wide, and the market for GPS applications is expected to double in the next three years, from $8 billion to over $16 billion. Some of these applications include: air, road, rail, and marine navigation, precision agriculture and mining, oil exploration, environmental research and management, telecommunications, electronic data transfer, construction, recreation and emergency response. GPS IS THE GLOBAL STANDARD. GPS has always been the dominant standard satellite navigation system thanks to the U.S. policy of making both the signal and the receiver design specification available to the public completely free of charge. NEW TECHNOLOGIES ENHANCE AMERICA'S NATIONAL SECURITY. The U.S. previously employed a technique called Selective Availability (SA) to globally degrade the civilian GPS signal. New technologies demonstrated by the military enable the U.S. to degrade the GPS signal on a regional basis. GPS users worldwide would not be affected by regional, security-motivated, GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency. GPS IMPROVED SIGNAL WILL BRING INSTANT BENEFITS TO MILLIONS OF GPS USERS. It?s rare that someone can press a button and make something you already own worth more, but that?s exactly what?s happening today. As of midnight tonight, all the people who?ve bought GPS receivers for boats, cars, or recreation will find that they are ten times more accurate. The technology that makes this extraordinary technology possible grows directly from our past research investments in basic physics, mathematics, and engineering supported from NSF, DARPA, NIST and other Federal agencies over a period of decades. GPS works because of super reliable atomic clocks -- no mechanical device could come close. These clocks resulted from Nobel-prize winning physics, and creative engineering that managed to package devices which once filled large physics laboratories into a compact, reliable, space-worthy device. The improved, non-degraded signal will increase civilian accuracy by an order of magnitude, and have immediate implications in areas such as: ? Car Navigation: Previously, a GPS-based car navigation could give the location of the vehicle to within a hundred meters. This was a problem, for example, in areas where multiple highways run in parallel, because the degraded signal made it difficult to determine which one the car was on. Terminating SA will eliminate such problems, leading to greater consumer confidence in the technology and higher adoption rates. It will also simplify the design of many systems (e.g., eliminate certain map matching software), thereby lowering their retail cost. ? Enhanced-911: The FCC will soon require that all new cellular phones be equipped with more accurate location determination technology to improve responses to emergency 911 calls. Removing SA will boost the accuracy of GPS to such a degree that it could become the method of choice for implementing the 911 requirement. A GPS-based solution might be simpler and more economical than alternative techniques such as radio tower triangulation, leading to lower consumer costs. ? Hiking, Camping, and Hunting: GPS is already popular among outdoor enthusiasts, but the degraded accuracy has not allowed them to precisely pin-point their location or the location of items (such as game) left behind for later recovery. With 20 meter accuracy or better, hikers, campers, and hunters should be able to navigate their way through unmarked wilderness terrain with increased confidence and safety. Moreover, users will find that the accuracy of GPS exceeds the resolution of U.S. Geological Survey (USGS) topographical quad maps. ? Boating and Fishing: Recreational boaters will enjoy safer, more accurate navigation around sandbars, rocks, and other obstacles. Anglers will be able to more precisely locate their favorite spot on a lake or river. Lobsterers will be able to find and recover their traps more quickly and efficiently. ? Increased Adoption of GPS Time: In addition to more accurate position information, the accuracy of the time data broadcast by GPS will improve to within 40 billionths of a second. Such precision may encourage adoption of GPS as a preferred means of acquiring Universal Coordinated Time (UTC) and for synchronizing everything from electrical power grids and cellular phone towers to telecommunications networks and the Internet. For example, with higher precision timing, a company can stream more data through a fiber optic cable by tightening the space between data packets. Using GPS to accomplish this is far less costly than maintaining private atomic clock equipment. Additional information about GPS and the Selective Availability decision is available online at the Interagency GPS Executive Board web site: http://www.igeb.gov
President Clinton: Improving the Civilian Global Positioning System (GPS) THE WHITE HOUSE Office of the Press Secretary ___________________________________________________________________________ For Immediate Release May 1, 2000 President Clinton: Improving the Civilian Global Positioning System (GPS) May 1, 2000 "The decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide. --This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world." President Bill Clinton May 1, 2000 GPS IS A CRITICAL TECHNOLOGY FOR INDIVIDUALS AND BUSINESSES AROUND THE GLOBE. GPS is a dual-use system, providing highly accurate positioning and timing data for both military and civilian users. There are more than 4 million GPS users world wide, and the market for GPS applications is expected to double in the next three years, from $8 billion to over $16 billion. Some of these applications include: air, road, rail, and marine navigation, precision agriculture and mining, oil exploration, environmental research and management, telecommunications, electronic data transfer, construction, recreation and emergency response. GPS IS THE GLOBAL STANDARD. GPS has always been the dominant standard satellite navigation system thanks to the U.S. policy of making both the signal and the receiver design specification available to the public completely free of charge. NEW TECHNOLOGIES ENHANCE AMERICA'S NATIONAL SECURITY. The U.S. previously employed a technique called Selective Availability (SA) to globally degrade the civilian GPS signal. New technologies demonstrated by the military enable the U.S. to degrade the GPS signal on a regional basis. GPS users worldwide would not be affected by regional, security-motivated, GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency. GPS IMPROVED SIGNAL WILL BRING INSTANT BENEFITS TO MILLIONS OF GPS USERS. It?s rare that someone can press a button and make something you already own worth more, but that?s exactly what?s happening today. As of midnight tonight, all the people who?ve bought GPS receivers for boats, cars, or recreation will find that they are ten times more accurate. The technology that makes this extraordinary technology possible grows directly from our past research investments in basic physics, mathematics, and engineering supported from NSF, DARPA, NIST and other Federal agencies over a period of decades. GPS works because of super reliable atomic clocks -- no mechanical device could come close. These clocks resulted from Nobel-prize winning physics, and creative engineering that managed to package devices which once filled large physics laboratories into a compact, reliable, space-worthy device. The improved, non-degraded signal will increase civilian accuracy by an order of magnitude, and have immediate implications in areas such as: ? Car Navigation: Previously, a GPS-based car navigation could give the location of the vehicle to within a hundred meters. This was a problem, for example, in areas where multiple highways run in parallel, because the degraded signal made it difficult to determine which one the car was on. Terminating SA will eliminate such problems, leading to greater consumer confidence in the technology and higher adoption rates. It will also simplify the design of many systems (e.g., eliminate certain map matching software), thereby lowering their retail cost. ? Enhanced-911: The FCC will soon require that all new cellular phones be equipped with more accurate location determination technology to improve responses to emergency 911 calls. Removing SA will boost the accuracy of GPS to such a degree that it could become the method of choice for implementing the 911 requirement. A GPS-based solution might be simpler and more economical than alternative techniques such as radio tower triangulation, leading to lower consumer costs. ? Hiking, Camping, and Hunting: GPS is already popular among outdoor enthusiasts, but the degraded accuracy has not allowed them to precisely pin-point their location or the location of items (such as game) left behind for later recovery. With 20 meter accuracy or better, hikers, campers, and hunters should be able to navigate their way through unmarked wilderness terrain with increased confidence and safety. Moreover, users will find that the accuracy of GPS exceeds the resolution of U.S. Geological Survey (USGS) topographical quad maps. ? Boating and Fishing: Recreational boaters will enjoy safer, more accurate navigation around sandbars, rocks, and other obstacles. Anglers will be able to more precisely locate their favorite spot on a lake or river. Lobsterers will be able to find and recover their traps more quickly and efficiently. ? Increased Adoption of GPS Time: In addition to more accurate position information, the accuracy of the time data broadcast by GPS will improve to within 40 billionths of a second. Such precision may encourage adoption of GPS as a preferred means of acquiring Universal Coordinated Time (UTC) and for synchronizing everything from electrical power grids and cellular phone towers to telecommunications networks and the Internet. For example, with higher precision timing, a company can stream more data through a fiber optic cable by tightening the space between data packets. Using GPS to accomplish this is far less costly than maintaining private atomic clock equipment. Additional information about GPS and the Selective Availability decision is available online at the Interagency GPS Executive Board web site: http://www.igeb.gov
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Gordon S.
Bravo for Sailors
For low velocity vehicles, like sailboats, GPS (without the differential add-on) hasn't been a very good instantaneous speed measure, or tracker. Now, with the bug not jumping around, we may be able to better guage our sail trim and get a track without jaggies!! I use the tracker on my portable GPS chartplotter to plan my next tack based on track history of the last several tacks.
For low velocity vehicles, like sailboats, GPS (without the differential add-on) hasn't been a very good instantaneous speed measure, or tracker. Now, with the bug not jumping around, we may be able to better guage our sail trim and get a track without jaggies!! I use the tracker on my portable GPS chartplotter to plan my next tack based on track history of the last several tacks.
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