Overview of GPS

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Overview of GPS
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GPS Signal Frequency

• Fundamental Frequency 10.23MHz (f0)
• 2 Carrier Frequencies
• L1 (1575.42 MHz) (154 f0)
• L2 (1227.60 MHz) (120 f0)
• 2 Codes
• Coarse Acquisition (C/A) 1.023 MHz
• Precise (P) 10.23 MHz
• Spread Spectrum
• Harder to jam

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Frequency and Wavelength
c vl Where c speed of light in vacuum,
2.9979246x108 m/s v frequency l
wavelength E.G. Thermal IR, l 12x10-6 m What
is frequency, v? 2.9979246x108 m/s v (12x10-6
m) v 2.49827x1013 s-1 25x1012 s-1 25
terahertz
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Amplitude Modulation
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Frequency Modulation
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Phase Modulation
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Codes

• Stream of binary digits known as bits or chips
• Sometimes called pseudorandom noise (PRN) codes
• Code state 1 and 1
• C/A code on L1
• P code on L1 and L2
• Phase modulated

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C/A Code

• 1023 binary digits
• Repeats every millisecond
• Each satellite assigned a unique C/A-code
• Enables identification of satellite
• Available to all users
• Sometimes referred to as Standard Positioning
  Service (SPS)
• Used to be degraded by Selective Availability (SA)

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P Code

• 10 times faster than C/A code
• Split into 38 segments
• 32 are assigned to GPS satellites
• Satellites often identified by which part of the
  message they are broadcasting
• PRN number
• Sometimes referred to as Precise Positioning
  Service (PPS)
• When encrypted, called Y code
• Known as antispoofing (AS)

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Future Signal

• C/A code on L2
• 2 additional military codes on L1 and L2
• 3rd civil signal on L5 (1176.45 MHz)
• Better accuracy under noisy and multipath
  conditions
• Should improve real-time kinematic (RTK) surveys

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Time Systems

• Each satellite has multiple atomic clocks
• Used for time and frequency on satellite
• GPS uses GPS Time
• Atomic time started 6 January 1980
• Not adjusted for leap seconds
• Used for time tagging GPS signals
• Coordinated Universal Time (UTC)
• Atomic time adjusted for leap seconds to be
  within 0.9 s of UT1 (Earth rotation time)

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Pseudorange Measurements

• Can use either C/A- or P-code
• Determine time from transmission of signal to
  when the signal is received
• Distance timespeed of light
• Since the position of the satellite is assumed to
  be known, a new position on the ground can be
  determined from multiple measurements

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Carrier-phase Measurements

• The range is the sum of the number of full cycles
  (measured in wavelengths) plus a fractional cycle
• ? N? n ?
• The fraction of a cycle can be measured very
  accurately
• Determining the total number of full cycles (N)
  is not trivial
• Initial cycle ambiguity
• Once determined, can be tracked unless

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Cycle Slips

• Discontinuity or jump in phase measurements
• Changes by an integer number
• Caused by signal loss
• Obstructions
• Radio interference
• Ionospheric disturbance
• Receiver dynamics
• Receiver malfunction

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How to Fix Cycle Slips?

• Slips need to be detected and fixed
• Triple differences can aid in cycle slips
• Will only affect one of the series
• Should stand out
• Once detected, it can be fixed

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GPS Errors and Biases

• Satellite Errors
• Potentially different for each satellite
• Transmission Errors
• Depends on path of signal
• Receiver Errors
• Potentially different for each receiver

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Linear Combination

• Errors and biases, which cannot be modeled,
  degrade the data
• Receivers that are close enough have very
  similar errors and biases
• Data can be combined in ways to mitigate the
  effects of errors and biases

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Linear Combination

• Combine data from two receivers to one satellite
• Should have same satellite and atmospheric errors
• Differences should cancel these effects out

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Linear Combination

• Combine data from one receiver to two satellites
• Should have same receiver and atmospheric errors
• Differences should cancel these effects out

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Linear Combination

• Combine data from two receivers to two satellites
• Should have same receiver, satellite and
  atmospheric errors
• Differences should cancel out

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Linear Combination

• Can also combine the L1 and L2 data to eliminate
  the effects of the ionosphere
• Ionosphere-free combination
• L1 and L2 phases can also be combined to form the
  wide-lane observable
• Long wavelength
• Useful in resolving integer ambiguity