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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
Q

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

A

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
Q

Signal Propagation (Atmospheric Refraction) Errors: Tropospheric delay error

A

 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
Q

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

A

 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
Q

Satellite Geometric Effects
Satellite geometry and DOP (Dilution of Precision)

A

 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
Q

Satellite Geometric Effects - Satellite geometry circle overlap

A

 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
Q

Satellite Geometric Effects -  Characteristics of DOP

A

 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
Q

Satellite Geometric Effects - Positional dilution of precision (PDOP)

A

 Quantifies how satellite geometry affects 3-D positioning
accuracy (latitude, longitude, and height)
 Represents overall positioning uncertainty

32
Q

Satellite Geometric Effects - PDOP

A

 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
Q

GPS Mission Planning

A

 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
Q

GPS Mission Planning

A

GPS manufacturers provide mission-planning software that
predicts satellite visibility and geometry for precise planning of
GPS surveys and missions.

35
Q

GPS Mission Planning Plot

A

 Sky plot:
 Displays the visible sky area
 Input: User location, time period
 Output: Path of each visible satellite

36
Q

Satellite Availability Plot

A

Satellite availability plot:
 A graph displaying PDOP,
HDOP, and VDOP
 Input: User-specified mask
angle
 Output: Total number of visible
satellites.

37
Q

User Equivalent Range Error (UERE)

A

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
Q

GPS Errors

A
  1. GPS Satellite Related Errors
  2. GPS Receiver Related Errors
  3. Signal Propagation Errors
  4. Satellite Geometric Effects
39
Q

GPS Satellite Related Erros (Types)

A
  1. Ephemeris (Orbital) error
  2. Selective availability
  3. Clock Error
40
Q

GPS Receiver Related Errors (Types)

A
  1. Clock Errors
  2. Multipath Error
  3. System Noise
  4. Antenna Phase Center Variations
41
Q

Satellite Geometric Effects (Types)

A

Geometric Effects

42
Q

Signal Propagation Errors (Types)

A
  1. Ionospheric Delay
  2. Tropospheric Delay
43
Q

GPS Satellite Related Erros: Ephemeris (Orbital) Errors

A

Caused by gravitational forces and solar radiation, affecting satellite orbits
Mitigation: Differential correction in DGPS positioning

44
Q

GPS Satellite Related Erros: Selective Availability (SA)

A
  • 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
Q

GPS Satellite Related Erros: Satellite Clock Error

A

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
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 of GPS satellite clocks
How to fix:
* Remove through between-satellite single difference
* Treat as an additional parameter during estimation

47
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

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

48
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

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
Q

GPS Receiver Related Errors: Receiver measurement noise

A

 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

GPS (9 decks)

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