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GPS Errors and Biases Flashcards

1
Q

Range (Distance)

A

D = d + e
D= (range)
d= (True Range)
e = (errors)

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2
Q

GPS satellite - related errors

A
  • Ephemeris (orbital) error
  • Selective availability
  • Clock Error
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3
Q

GPS Errors: GPS Receiver (Related Errors)

A
  1. Clock error
  2. Multipath error
  3. System noise
  4. Antenna phase center variations
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4
Q

GPS Errors: Signal Propagation (atmospheric refraction) errors

A

Ionospheric delay tropospheric delay

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5
Q

GPS Errors: Satellite Geometric Effects

A

Geometric Effects

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6
Q

GPS Satellite-related errors: Ephemeris (Orbital) error

A

 Caused by gravitational forces and solar radiation, affecting satellite orbits
 Errors range from 2m to 5m
 Ephemeris data:
 Show satellite’s position and velocity
 Predicts positions from past GPS observations at control stations

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7
Q

GPS Satellite-related errors - Ephemeris (Orbital) error – Continued:

A

 Mitigation:
a. Differential correction in DGPS positioning
b. Post-mission precise orbital service from global GPS
networks
 International GPS Service for Geodynamics (IGS)
 U.S. National Geodetic Survey (NGS)
 Geomatics Canada
 Accuracy: few centimeters to 10 centimeters

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8
Q

GPS Satellite Related Errors - Selective Availability (SA)

A

 Why?
 Implemented for national security
 Denying precise positioning to unauthorized users
 Began on March 25, 1990
 Ended on May 1, 2000

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9
Q

GPS Satellite-related Errors - Selective Availability (SA)

A

 Two errors:
1.Delta from satellite clock
2.Epsilon (orbit data manipulation)
 DGPS helps mitigate epsilon errors, particularly for close-proximity users

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10
Q

GPS Satellite-related Errors -Selective Availability

A

 Ended on May 1, 2000, greatly improving accuracy
 With SA: 100m horizontal error, 156m vertical error (95%
probability).
 Without SA: 22m horizontal error, 33m vertical error (95% probability)
 Its removal boosted GPS markets, including vehicle navigation
and enhanced-911

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11
Q

GPS Satellite-related Errors - Satellite clock error

A

 GPS satellite clocks have slight imperfections.
 Clock errors range from 8.64 to 17.28 ns/day.
 One nanosecond error equals about 30 cm range error, totaling 2.59 m to 5.18 m due to the clock.

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12
Q

GPS Satellite related Errors - Satellite clock error – Continued

A

 How to address it:
 Eliminate it through differencing between receivers (between-receiver single difference)
 Apply the satellite clock correction provided in the navigation message

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13
Q

GPS Receiver Related Errors - Receiver clock error

A

 GPS receivers use less accurate crystal clocks compared to
satellite atomic clocks
 Results in larger clock errors that that of GPS satellite clocks

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14
Q

GPS Receiver Related Errors - Receiver clock error – Continued

A

 How to fix:
* Remove through between-satellite single difference
* Treat as an additional parameter during estimation
* Use precise external clocks (costs vary from a few thousand to
about $20,000)

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15
Q

GPS Receiver Related Errors - Multipath Error

A

 Affects carrier-phase and pseudorange measurements, with pseudorange having a larger error.
 Occurs when GPS signals reach the
receiver antenna through multiple paths.
 Direct GPS signal
 Multipath signals

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16
Q

GPS Receiver Related Errors - Multipath error - Continued

A

 How to Fix:
 Utilize advanced receiver technology
 Select sites without nearby reflecting objects
 Employ a choke ring antenna

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17
Q

GPS Receiver Related Errors - Antenna Phase Center (APC) variation - continued

A

 How to fix:
 Align antennas in the same direction for short baselines
 Often overlooked in most practical GPS applications due to its
minor magnitude

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18
Q

GPS Receiver Related Errors - Antenna Phase Center (APC) variation

A

 A GPS antenna converts incoming satellite signals into electric
current.
 Antenna Phase Center (APC) is where the GPS signal is
received.
 Error
 APC isn’t always at the antenna’s physical center.
 Cause: Elevation, azimuth, and signal intensity
 Magnitude: A few centimeters

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19
Q

GPS Receiver Related Errors -Receiver measurement noise

A

 Arises from the receiver electronics limitations.
 High-quality systems have minimum noise.

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20
Q

GPS Receiver Related Errors - Receiver measurement noise - Continue

A

 GPS receivers conducts self-tests. High-cost systems need user evaluation, including
a. Zero baseline: Identifies biases, cycle slips
b. Short baseline: Detects noise, multipath, and antenna/preamplifier noise
 Error: 0.6m

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21
Q

Signal Propagation (Atmospheric Refraction) Errors

A

 The GPS signal experiences delays in the atmosphere as it
passes through:
 Ionospheric layer (from 50 km to 1,000 km)
 Tropospheric layer (up to 50 km)

22
Q

Signal Propagation (Atmospheric Refraction) Errors: Ionospheric delay error

A

 GPS signals slow down in Earth’s atmosphere, especially in the
ionosphere.
 Spans 50 km to 1,000 km, with varying electron density
 Varies with altitude, season, and time of day

23
Q

Signal Propagation (Atmospheric Refraction) Errors: Ionospheric delay error - Continued

A

 Ionosphere has D, E, F1, and F2 layers with varying electron
density.
 Impact on GPS:
* Bends and slows GPS signals as they pass through layers
* Introduces range error, frequency-dependent
22

24
Q

Signal Propagation (Atmospheric Refraction) Errors: Ionospheric delay error - Continued

A

 Delay increases with decreasing frequency.
* L2 (1227.6 MHz) has greater than that L1 (1575.42 MHz).
 Error: 5 m to 15 m
 Major GPS error source!

25
Signal Propagation (Atmospheric Refraction) Errors: Ionospheric delay error - Continued
How to fix: a. Differencing GPS observations between nearby users helps eliminate it. b. Use dual-frequency receivers (L1 and L2) to generate ionospheric-free linear combinations and mitigate the delay
26
Signal Propagation (Atmospheric Refraction) Errors: Tropospheric delay error
 Troposphere extends up to 50 km, delays GPS uniformly.  Unlike ionosphere, it's not frequency-dependent.  Affected by temperature, pressure, humidity  Longer path for low-angle satellites  Minimal at zenith (2.3 meters), maximum near horizon  Important for GPS mask angle settings (10-15 degrees)
27
Signal Propagation (Atmospheric Refraction) Errors: Tropospheric delay error - Continued
 Error:  2.3 meters at the zenith when the satellite is directly overhead  9.3 meters at a 15-degree elevation angle  Between 20 and 28 meters at a 5-degree elevation angle  How to fix:  Can’t be eliminated by combining L1 and L2  Frequency-independent, affecting both carriers and codes equally
28
Satellite Geometric Effects Satellite geometry and DOP (Dilution of Precision)
 DOP measures the influence of satellite positions on GPS accuracy, including;  Dilution of precision (DOP) or Geometric dilution of precision (GDOP)  Positional dilution of precision (PDOP)  Horizontal dilution of precision (HDOP)  Vertical dilution of precision (VDOP)  Time dilution of precision (TDOP)
29
Satellite Geometric Effects - Satellite geometry circle overlap
 Even satellite distribution results in lower GDOP, indicating a stronger geometric configuration and higher accuracy.  The margin of error decreases when satellites are widely spaced
30
Satellite Geometric Effects -  Characteristics of DOP
 Based on receiver-satellite geometry  Lower DOP means more precise positions.  Ideal DOP:  One overhead satellite and three evenly spaced ones around the horizon  Best accuracy: DOP < 4, Acceptable: DOP 4-8, Poor: DOP > 8  Changes with time and location but repeats daily due to the satellite constellation's consistency, making it predictable.
31
Satellite Geometric Effects - Positional dilution of precision (PDOP)
 Quantifies how satellite geometry affects 3-D positioning accuracy (latitude, longitude, and height)  Represents overall positioning uncertainty
32
Satellite Geometric Effects - PDOP
 Consists of: 1.HDOP (latitude and longitude) 2.VDOP (vertical)  To improve it, consider using pseudolites  Aim for a value of five or lower
33
GPS Mission Planning
 Under the full 24-satellite GPS constellation, certain satellites are visible at different times.  Goal: Assist users in finding optimal observation periods  Challenge: Satellite visibility varies  Ensures a minimum number of visible satellites and maintain a specific maximum DOP value
34
GPS Mission Planning
GPS manufacturers provide mission-planning software that predicts satellite visibility and geometry for precise planning of GPS surveys and missions.
35
GPS Mission Planning Plot
 Sky plot:  Displays the visible sky area  Input: User location, time period  Output: Path of each visible satellite
36
Satellite Availability Plot
Satellite availability plot:  A graph displaying PDOP, HDOP, and VDOP  Input: User-specified mask angle  Output: Total number of visible satellites.
37
User Equivalent Range Error (UERE)
UERE (User Equivalent Range Error) includes errors associated with satellite/receiver clocks, atmosphere, satellite orbits, and environmental conditions. It offers a more simplified means of examining GPS positioning
38
GPS Errors
1. GPS Satellite Related Errors 2. GPS Receiver Related Errors 3. Signal Propagation Errors 4. Satellite Geometric Effects
39
GPS Satellite Related Erros (Types)
1. Ephemeris (Orbital) error 2. Selective availability 3. Clock Error
40
GPS Receiver Related Errors (Types)
1. Clock Errors 2. Multipath Error 3. System Noise 4. Antenna Phase Center Variations
41
Satellite Geometric Effects (Types)
Geometric Effects
42
Signal Propagation Errors (Types)
1. Ionospheric Delay 2. Tropospheric Delay
43
GPS Satellite Related Erros: Ephemeris (Orbital) Errors
Caused by gravitational forces and solar radiation, affecting satellite orbits Mitigation: Differential correction in DGPS positioning
44
GPS Satellite Related Erros: Selective Availability (SA)
* Was implemented for national security * Denying precise positioning to unautorized users Two errors 1. Delta from satellite clock 2. Epsilon (orbit data manipulation) - DGPS helps mitigate epsilon errors, particularly for close proximity users.
45
GPS Satellite Related Erros: Satellite Clock Error
GPS satellite clocks have slight imperfections How to fix it:  Eliminate it through differencing between receivers (between-receiver single difference)  Apply the satellite clock correction provided in the navigation message.
46
GPS Receiver Related Errors: Receiver clock error
 GPS receivers use less accurate crystal clocks compared to satellite atomic clocks  Results in larger clock errors that of GPS satellite clocks How to fix: * Remove through between-satellite single difference * Treat as an additional parameter during estimation
47
GPS Receiver Related Errors: Multipath Error
Affects carrier-phase and pseudorange measurements, with pseudorange having a larger error. Occurs when GPS signals reach the receiver antenna through multiple paths  Direct GPS signal  Multipath signals How to Fix:  Utilize advanced receiver technology  Select sites without nearby reflecting objects  Employ a choke ring antenna
48
GPS Receiver Related Errors: Antenna Phase Center (APC) variation
A GPS antenna converts incoming satellite signals into electric current. Antenna Phase Center (APC) is where the GPS signal is received. Error  APC isn't always at the antenna's physical center.  Cause: Elevation, azimuth, and signal intensity  Magnitude: A few centimeters How to fix:  Align antennas in the same direction for short baselines.  Often overlooked in most practical GPS applications due to its minor magnitude
49
GPS Receiver Related Errors: Receiver measurement noise
 Arises from the receiver electronics limitations.  High-quality systems have minimum noise. GPS receivers conducts self-tests. High-cost systems need user evaluation, including: a. Zero baseline: Identifies biases, cycle slips b. Short baseline: Detects noise, multipath, and antenna/preamplifier noise
50
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