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How GPS Works : GPS Machine Control : GPS Benefits : GPS Costs & Parts

GPS Machine Control Home

GPS Info:
What is GPS
How GPS Works
GPS Systems Accuracy
- US GPS
- Soviet GLONASS
- EU Galileo
- Why GPS, GLONASS & Galileo

Machine Control GPS:
How GPS Machine Control Works
- Base Stations intro
- Radio Base Stations
- GPS and RTK accuracy
- GPS + Laser = MM Accuracy
- GPS + Cell = 1200+ Sq Miles
- GPS on Machines
- GPS Control Boxes/Computers
- GPS Automate Vs. Indicate

GPS System Benefits:
GPS Machine Control Benefits
- Billing Controls
- Data and Management
- Job Management
- Move Dirt 1 Time!
- Many Machines

GPS Parts, Prices, and More:
GPS Machine Control Parts & Cost
- Pricing Automate vs Indicate
- GPS for Graders
- GPS for Dozers
- GPS for Blades
- GPS for Scrapers
- GPS for Excavators

Informative Grade Sites:
AccurateGrade.com
- Precision Grading Practices

LowCostMachineControl.com
- Laser Machine Systems

Sponsors:
Rocky Mtn Lasers
Construction Lasers

Construction Directory

GPS Accuracy

So how accurate is a GPS system? GPS without any calculation isn't very precise. Looking at the picture to the right, it is easy to see how the lines bend.

Refraction is the bending of radio and light waves done by the dense layers of the atmosphere which reduces the accuracy of GPS systems. Without having a system to compute this error and transmit the "Correction" GPS systems aren't very accurate. Military receivers can "Decode" a second error correction signal beamed from the GPS satellites, but the public is not able to receive this signal for security purposes. Without error correction, a single modern GPS receiver is accurate to roughly 8-10 feet. To get to tolerances necessary for construction a second receiver (Base Station) is used to create the "Correction" formulas for the roving receivers in it's general area. Once all system's data is compared, the accuracy is increased to reasonable tolerances.

GPS Systems can plot latitude and longitude very precisely (X and Y axis). But elevation, or vertical accuracy (Z axis), is more of a challenge to the system. Plotting x & y axis locations, or horizontal bearings, is easy because there are most likely satellites on all sides of you (Good DOP) reducing the error in the calculations (data available from one horizon to the other). Z axis calculations are difficult due to the GPS satellites location overhead, and a long way up. Since no GPS satellite signals are available from below the receiver, it’s impossible to be between two satellite points on the z axis and receive accurate data. The satellites location and distance reduces the accuracy range of vertical points.

So what is a typical vertical accuracy for GPS Machine Control systems without lasers? Anywhere from ½ to ¾ inch (12 to 18mm). Of course, the more GPS satellites you are receiving, the better your vertical accuracy will be. These tolerances can be decreased by a factor of 3 by adding lasers into the system....

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