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74
Troubleshooting
GPS Receiver Accuracy
Accuracy of the GPS receiver is dependent on the positioning service available for the machine location. Listed
below are the positioning services that the GPS receivers recognize.
• GPS Standard Positioning Service (SPS) Available any where in the world.
- Position Accuracy
Garmin: < 15 meters, 95% typical (with Selective Availability OFF).
• DGPS Wide Area Augmentation System (WAAS) Available in North America with limited availability Internationally.
- Position Accuracy
Garmin: < 5 meters, 95% typical.
GPS Signal Errors
Listed below are several factors that can degrade the GPS signal and affect accuracy.
• Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The
GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type
of error.
• Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large
rock surfaces before it reaches the receiver. This increases the travel time of the signal, causing errors.
• Receiver clock errors — A very slight timing error between the receiver’s built-in clock and the very accurate
atomic clocks onboard the GPS satellites.
• Orbital errors — Are inaccuracies of the satellite’s reported location.
• Number of satellites visible — The more satellites a GPS receiver can see, the better the accuracy. Build-
ings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing
position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or
underground.
• Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal
satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry
results when the satellites are located in a line or in a tight grouping.
• Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the
signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries
from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
Wire Color Function
White / Red Pulse Per Second Output ( Not Used)
White Port 1 Data In
Gray Port 1 Data Out
Red Power
Orange Remote On/Off (Grounded)
Black Power And Signal Ground
White / Orange Port 2 Data In (Grounded)
Purple
Port 2 Data Out (Not Used)
Figure 74-1
Garmin 19x GPS Power Data Cable Wire Color/Function