dodgeman
Well-Known Member
- Region
- USA
- City
- Macomb, Illinois
I see this discussed all the time. I’m a retired land surveyor and started using it in the early 1990’s. A couple of points.
Originally this was designed for the United States military. Of course smart people found other uses. Until the year 2000 there was something called Selective Availability (SA). This degraded the signal to an accuracy of 100 meters. In 2000 SA was turned off and the accuracy is now about 10 meters in open sky’s.
This 10 meter accuracy is for an uncorrected signal. Surveyors have generally always us a corrected signal through the use of a second unit set on a known point broadcasting a radio signal. This is now often available through a cell phone link so a second unit isn’t needed. Surveyors get an accuracy of around 1 cm.
There are also other corrections available, often called mapping grade. This gives an accuracy of about 1 meter and is typically what something like a Garmin will give you.
GPS needs open sky’s to perform its best. GPS signals are weak and are prone to bouncing around, called multi path. This degrades the accuracy. There has been very little improvement made to the GPS satellites themselves. The biggest improvements have been the receivers and the software.
There are now multiple constellations. Europe=Galileo, the Russians= Glonass, the Chinese=Bediou, Japanese=QZSS, USA= GPS. Depending on the equipment it might use some or all of these.
When I first started using GPS we had to use mission planning software to see when we could work. There weren’t always enough satellites available. Most of the systems the satellites are rising and setting over a period of hours. There might have been several hours there weren’t enough visible. It takes multiple satellites to get a position, 5 is the normal number needed. That is no longer a problem, there are often more than 20 visible.
The clocks are the weak point in the satellites. Most are up there still functioning except for the clocks. Time is a critical component of how it works.
Originally this was designed for the United States military. Of course smart people found other uses. Until the year 2000 there was something called Selective Availability (SA). This degraded the signal to an accuracy of 100 meters. In 2000 SA was turned off and the accuracy is now about 10 meters in open sky’s.
This 10 meter accuracy is for an uncorrected signal. Surveyors have generally always us a corrected signal through the use of a second unit set on a known point broadcasting a radio signal. This is now often available through a cell phone link so a second unit isn’t needed. Surveyors get an accuracy of around 1 cm.
There are also other corrections available, often called mapping grade. This gives an accuracy of about 1 meter and is typically what something like a Garmin will give you.
GPS needs open sky’s to perform its best. GPS signals are weak and are prone to bouncing around, called multi path. This degrades the accuracy. There has been very little improvement made to the GPS satellites themselves. The biggest improvements have been the receivers and the software.
There are now multiple constellations. Europe=Galileo, the Russians= Glonass, the Chinese=Bediou, Japanese=QZSS, USA= GPS. Depending on the equipment it might use some or all of these.
When I first started using GPS we had to use mission planning software to see when we could work. There weren’t always enough satellites available. Most of the systems the satellites are rising and setting over a period of hours. There might have been several hours there weren’t enough visible. It takes multiple satellites to get a position, 5 is the normal number needed. That is no longer a problem, there are often more than 20 visible.
The clocks are the weak point in the satellites. Most are up there still functioning except for the clocks. Time is a critical component of how it works.
