Global Positioning System (GPS)

1
Global Positioning System (GPS)
GPS Presentation

• Mohammed Ibrahim Assafany
• mdyemeni_at_hotmail.com
• GPS Presentation
• 2004

2
Contents

• Basic GPS Design
• Basic GPS Receiver
• Basic GPS Concept
• Operating Modes
• Doppler Frequency Shift
• Basic GPS Signal Structure (Interface)
• GPS Data Errors

3
Basic GPS Design

• GPS (DoD with 28 satellites, 1993), Glonass 1995
  (Russian Federation with only nine active
  satellites), European Galileo (planning stage
  2008)
• GPS Three segments - Space, Control, and User
• Space Segment 28 satellites in 12 hour, 20,000
  km, 55º orbits, four satellites in each of the
  six orbital planes
• Control Segment Ground stations adjust satellite
  clocks, almanac and ephemeris information for
  each satellite
• User Segment GPS receivers provide navigational
  and time information to users

4
Basic GPS Receiver
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Basic GPS ConceptFor Finding User Position

• One-dimensional user position
• Two-dimensional user position

6
Basic GPS ConceptFor Finding User Position
(cont)

• Three known positions to find one unknown
  position
• Four Satellites

(?c?T) bu is the user clock bias error
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Basic GPS ConceptFor Finding User Position
(cont)

• User Position in Spherical Coordinate System
• The distance from the center of the earth to the
  user is
• The latitude Lc is
• The longitude l is
• The altitude h is
• where re is the radius of an ideal spherical
  earth or the average radius of the earth, r0 re
  6368 km

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Operating Modes

• Precise Positioning System (PPS) - US and allied
  military, authorized government agencies - 22 m
  horizontal, 27.7 m vertical, 100 ns accuracy
  (95)
• Standard Positioning System (SPS) - civilian use
  - 100 m horizontal, 156 m vertical, 340 ns
  accuracy with S/A, improved to 22 m horizontal,
  30 m vertical, 60 ns without S/A as of March 2001
• Differential Carrier Phase for Surveying - at
  least two receivers - sub cm accuracy

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Operating Modes (cont)

• Differential GPS (DGPS) - ground signal required
• 1 to 10 m horizontal accuracy
• Wide Area Augmentation System (WAAS) - as of
  February 2002 implemented only in US with 25
  ground stations and 2 geosynchronous satellites.
  Accuracy 7 m vertical/horizontal.

10
Doppler Frequency Shift

• Constellation
• Angular velocity d? /dt and the Speed vs of the
  satellite
• where rs is the average radius of the satellite
  orbit

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Doppler Frequency Shift (cont)

• The Doppler frequency is caused by the satellite
  velocity component vd toward the user where
• Using the law of cosine in
• triangle OAS, the result is
• because of a ? ?

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Doppler Frequency Shift (cont)

• Maximum Doppler velocity vdm
• For the L1 frequency (f1575.42 MHz), which is
  modulated by the C /A signal, the maximum Doppler
  frequency shift is
• Therefore, in designing a GPS receiver, if the
  receiver is used for a low-speed vehicle, the
  Doppler shift can be considered as 5 KHz. If the
  receiver is used in a high speed vehicle, it is
  reasonable to assume that the maximum Doppler
  shift is 10 KHz.

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Basic GPS Signal Structure (Interface)

• The GPS signal contains two frequency components
  L1 and L2.
• These frequencies are coherent with a 10.23 MHz
  clock. These two frequencies can be related to
  the clock frequency as
• The signal structure of the satellite may be
  modified as L1 frequency contains the C /A and
  P(Y) signals, while the L2 frequency contains
  only the P(Y) signal. The C /A and P(Y) signals
  in the L1 frequency are in quadrant phase of each
  other and they can be written as

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(Interface) (cont)

• Satellite Signal Modulation

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(Interface) (cont)

• Simplified Model of GPS L1 Signal Generation

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(Interface) (cont)

• GPS Signal Frequency Spectrum
• The spectrum width is proportional
• to the chip rate.
• (f10.23M?BW20.46M)
• Therefore, this type of signal is also
• referred to as a spread-spectrum signal.
• If the modulation code is a digital
• sequence with a frequency higher than the
• data rate, the system can be called
• a direct-sequence modulated system.

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(Interface) (cont)

• GPS Receiver Tracking System

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(Interface) (cont)

• Code Division-Multiple Access (CDMA) Signals
• A signal S can be written in the following form
• The GPS signal is a phase-modulated signal with f
  0, ? this type of phase modulation is referred
  to as bi-phase shift keying (BPSK). The phase
  change rate is often referred to as the chip
  rate.
• A code division multiple access (CDMA) signal in
  general is a spread-spectrum system. All the
  signals in the system use the same center
  frequency.
• The signals are modulated by a set of
  near-orthogonal codes. In order to acquire an
  individual signal, the code of that signal must
  be used to correlate with the received signal.
• The GPS signal is CDMA using direct sequence to
  bi-phase modulate the carrier frequency

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(Interface) (cont)

• CDMA (cont)
• Since the CDMA signals all use the same carrier
  frequency, there is a possibility that the
  signals will interfere with one another.
• This effect will be more prominent when strong
  and weak signals are mixed together.
• In order to avoid the interference, all the
  signals should have approximately the same power
  levels at the receiver.

20
(Interface) (cont)

• P CODE
• The P code is bi-phase modulated at 10.23 MHz.
• The code is generated from two pseudorandom noise
  (PRN) codes with the same chip rate. One PRN
  sequence has 15,345,000 chips, which has a period
  of 1.5 seconds, the other one has 15,345,037
  chips, and the difference is 37 chips.
• Therefore, the code length generated by these two
  codes is 23,017,555.5 (1.5 15,345,037) seconds,
  which is slightly longer than 38 weeks. However,
  the actual length of the P code is 1 week as the
  code is reset every week. This 38-week-long code
  can be divided into 37 different P codes and each
  satellite can use a different portion of the
  code.

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(Interface) (cont)

• P CODE (cont)
• There are a total of 32 satellite identification
  numbers although only 24 of them are in the
  orbit. Five of the P code signals (3337) are
  reserved for other uses such as ground
  transmission.
• The P-code is normally encrypted into the Y-code
  to protect the user.
• The P(Y)-code is transmitted by each satellite on
  both L1 and L2. On L1, the P(Y)-code is 90
  degrees out of carrier phase with the C/A-code.
  The navigation data rate carried by the P code
  through phase modulation is at a 50 Hz rate.

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(Interface) (cont)

• C /A Code Generation
• The C /A code is a bi-phase modulated signal with
  a chip rate of 1.023 MHz.
• The total code period contains
• 1,023 chips.
• The C /A code belongs to
• the family of Gold codes.

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(Interface) (cont)

• C /A Code Correlation
• In order to detect a weak signal in the presence
  of strong signals, the autocorrelation peak of
  the weak signal must be stronger than the
• cross-correlation peaks from the
• strong signals.
• However, the Gold codes are near
• orthogonal, implying that the cross
• correlations are not zero but have
• small values.
• For the C /A code n even 10,
• thus, Peak 1023.

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(Interface) (cont)

• GPS DATA FORMAT
• The user location can
• be found
• Theoretically, 18 sec.
• Actually, take 30 sec.

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GPS Data Error
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GPS Data Error (cont)

• Multi-Path Errors
• With multi-path reception, the receiver collects
  both the direct signal from the satellite and a
  delayed, reflective signal.
• Minimizing the problem
• Use Rake Receiver.
• Use Semi-directional, ground-plane
• antennas.

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Conclusion and Future

• GPS is one of International Navigation Systems,
  was made by USA for Ranging and Time.
• Now GPS offers a large precision in user position
  at three-dimension, Reliability, Availability,
  and Repeatability.
• GPS is a DS-SS CDMA System. BPSK Modulation.
• While the Future Satellite Systems
• 1. Galileo E.C. 2008
• 2. MSAS Japan.