67 GPS 4 Flashcards
GPS Errors
GPS error is a combination of 2 types of errors:
The first error is the measurement of the distance from the satellite to the receiver
User range Error (URE)
Also reffered as user equivalent range error (UERE)
The second error is induced by having less than perfect fix geometry
Dilution of Precision (DoP)
User range error
Satellite errors
Clock drift accounts for errors up to 2 m 95% of the time
Control Segment Errors
Inaccurate ephemeris data gives errors up to 2.5 m 95% of the time
Over 8 hour period
Receiver errors make up the largest portion of User Radial Error
Atmospheric modeling is the largest cause of error
Ionosphere (refraction)
Troposphere (refraction)
Multi-path errors
associated with antenna location, and vehicle dynamics
total URE is expected to be:
P Code up to 6.6m 95% of the time
C/A Code up to 13.9m 95% of the time
Dilution of Precision
Comes from having a fix geometry of less than 90 degrees
Airborne we are concerned with 3 dimensional position error
DOP is considered to be 2.54, 95% of the time
Total System Accuracy
P code
C/A code
P Code
DOP times URE is (2.54 * 6.6) = 16.8m 95% of the time
C/A Code
DOP times URE is (2.54 * 13.9) = 35.3m 95% of the time
Selective Availability
Phased out in 2000
USA considered 35m accuracy to be a threat to national security
Selective availability gives an error of 100m, a threefold decrease in accuracy
Random clock errors and inaccurate ephemeris data were the methods of introducing error
GPS Receiver Types
Sequential receivers – 1 or more channels
Multiplex receivers – one or more channels plus software channels
Continuous receivers – four or more channels
Cost comparison
Sequential to multiplex to continuous
Cheap to expensive and simple to complex
Differential GPS/LAAS/WAAS
LAAS – Local Area Augmentation System
Ground based
WAAS – Wide area augmentation system
Space based
