Global Positioning System

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Global Positioning System

• Douglas Daley
• FEG 437

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UTM Coordinate System

• The Universal Transverse Mercator Coordinate
  (UTM) system provides coordinates on a world wide
  flat grid for easy computation.
• 60 zones, each being 6 degrees longitude wide,
  extending from 80 degrees south latitude to 84
  degrees north latitude The polar regions are
  excluded.
• First zone starts at the International Date Line
  (longitude 180 degrees) proceeding eastward.
• Source URL http//www.ngs.noaa.gov/TOOLS/utm.html

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GPS

• The Global Positioning System (GPS) is a
  worldwide radio-navigation system formed from a
  constellation of 24 satellites and their ground
  stations.
• Source URL http//tycho.usno.navy.mil/gpscurr.html
  
• The current GPS constellation consists of 29
  Block II/IIA/IIR satellites.
• The design life of the Block II/IIA satellite is
  7.3 years each containfour atomic clocks  two
  Cesium (Cs) and two Rubidium (Rb)
• Selective Availability (SA) and Anti-Spoof (A-S)
  capabilities. 

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GPS constellation and individual satellite status

• updated every working day.
• Block I are referred to as the original concept
  validation satellites developed by Rockwell
  International and reflect various stages of
  system development.
• The first Block II satellite was launched in
  February 1989 the most recent Block IIR
  satellite was launched on March 20, 2004.

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Positioning

• Four GPS satellite signals are used to compute
  positions in three dimensions and the time offset
  in the receiver clock
• The Precise Positioning Service (PPS) is a highly
  accurate military positioning, velocity and
  timing service which will be available on a
  continuous, worldwide basis to users authorized
  by the U.S.
• P(Y) code capable military user equipment
  provides a predictable positioning accuracy of at
  least 22 meters (95 percent) horizontally and
  27.7 meters vertically and time transfer accuracy
  to UTC within 200 nanoseconds (95 percent).
• Civil users worldwide use the SPS without charge
  or restrictions. Most receivers are capable of
  receiving and using the SPS signal. The SPS
  accuracy is intentionally degraded by the DOD by
  the use of Selective Availability. SPS
  Predictable Accuracy 100 meter horizontal
  accuracy 156 meter vertical accuracy 340
  nanoseconds time accuracy

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The GPS

• consists of three major segments SPACE, CONTROL
  and USER.
• The SPACE segment consists of 24 operational
  satellites in six orbital planes (four satellites
  in each plane).
• The satellites operate in circular 20,200 km
  (10,900 nm) orbits at an inclination angle of 55
  degrees and with a 12-hour period.
• The position is therefore the same at the same
  sidereal time each day, i.e. the satellites
  appear 4 minutes earlier each day.

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The CONTROL segment

• consists of five Monitor Stations (Hawaii,
  Kwajalein, Ascension Island, Diego Garcia,
  Colorado Springs), three Ground Antennas,
  (Ascension Island, Diego Garcia, Kwajalein), and
  a Master Control Station (MCS) located at
  Schriever AFB in Colorado.
• The monitor stations passively track all
  satellites in view, accumulating ranging data.
  This information is processed at the MCS to
  determine satellite orbits and to update each
  satellite's navigation message.
• Updated information is transmitted to each
  satellite via the Ground Antennas.

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The USER segment

• consists of antennas and receiver-processors that
  provide positioning, velocity, and precise timing
  to the userGPS is at the present time the most
  competent system for time transfer , the
  distribution of Precise Time and Time Interval
  (PTTI).
• The system uses time of arrival (TOA)
  measurements for the determination of user
  position.
• A precisely timed clock is not essential for the
  user because time is obtained in addition to
  position by the measurement of TOA of FOUR
  satellites simultaneously in view.
• If altitude is known (i.e. for a surface user),
  then THREE satellites are sufficient.
• If time is being kept by a stable clock (say,
  since the last complete coverage), then TWO
  satellites in view are sufficient for a fix at
  known altitude.
• If the user is, in addition, stationary or has a
  known speed then, in principle, the position can
  be obtained by the observation of a complete pass
  of a SINGLE satellite.
• This could be called the "transit" mode, because
  the old TRANSIT system uses this method. In the
  case of GPS, however, the apparent motion of the
  satellite is much slower, requiring much more
  stability of the user clock.

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GPS time

• is given by its Composite Clock (CC). The CC or
  "paper" clock consists of all Monitor Station and
  satellite operational frequency standards.
• The GPS epoch is 0000 UT (midnight) on January 6,
  1980.
• GPS time is not adjusted and therefore is offset
  from UTC by an integer number of seconds, due to
  the insertion of leap seconds.
• The number remains constant until the next leap
  second occurs. This offset is also given in the
  navigation (NAV) message and your receiver should
  apply the correction automatically. As of January
  1, 1999, GPS time is ahead of UTC by thirteen
  (13) seconds. http//tycho.usno.navy.mil/gpstt.htm
  l
• Civil time is occasionally adjusted by one second
  increments to ensure that the difference between
  a uniform time scale defined by atomic clocks
  does not differ from the Earth's rotational time
  by more than 0.9 seconds.
• Coordinated Universal Time (UTC), an atomic time,
  is the basis for civil time. Corpsconhttp//crun
  ch.tec.army.mil/software/corpscon/corpscon.htmlCo
  rpscon, Version 5.11.08 (Year 2000 compliant), is
  a MS-Windows-based program which allows the user
  to convert coordinates between Geographic, State
  Plane and Universal Transverse Mercator (UTM)
  systems on the North American Datum of 1927 (NAD
  27), the North American Datum of 1983 (NAD 83)
  and High Accuracy Reference Networks (HARNs).
   http//aa.usno.navy.mil/faq/docs/UT.html

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Universal Time

• times of various events, particularly
  astronomical and weather phenomena, are often
  given in "Universal Time" (abbreviated UT)
• which is sometimes referred to, now
  colloquially, as "Greenwich Mean Time"
  (abbreviated GMT). The two terms are often used
  loosely to refer to time kept on the Greenwich
  meridian (longitude zero), five hours ahead of
  Eastern Standard Time.
• Times given in UT are almost always given in
  terms of a 24-hour clock. Thus, 1442 (often
  written simply 1442) is 242 p.m., and 2117
  (2117) is 917 p.m. Sometimes a Z is appended to
  a time to indicate UT, as in 0935Z.
• Scale 191302 (Centered at Lat 43.0348549 Lon
  -76.1363220 ESF Campus 

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GPS Satellite Prediction Results

• Navigation Datalink Section (Code
  471120D)Naval Air Warfare Center -- Weapons
  Division, China Lake, CAhttp//sirius.chinalake.n
  avy.mil/satpred/ Prediction forMarch 31, 2004
  154500 UTC (GPS week 240 GPS TOW 315900
  seconds)Altitude 250.0 MetersLatitude 43
  2'30.00"NLongitude 76 8'35.00"WUsing an
  elevation mask of 10 There are 7 satellites in
  view The best GDOP is obtained using satellites
  4, 9, 20 and 28 GDOP 2.4PDOP 2.2TDOP
  1.0HDOP 1.3VDOP 1.8
• Satellite Elevation Azimuth
• GPS BIIA-23 (PRN 04) 78.7 277.7
• GPS BIIA-22 (PRN 05) 25.6 311.6
• GPS BIIA-20 (PRN 07)65.6 91.4
• GPS BIIA-21 (PRN 09)13.0 271.0
• GPS BIIR-04 (PRN 20)26.6 47.5
• GPS BIIA-11 (PRN 24)39.2 245.5
• GPS BIIR-05 (PRN 28)21.5 163.7

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View

• Altitude 250.0 MetersLatitude 43
  2'30.00"NLongitude 76 8'35.00"W
• View from 20180 km above 4819'N 823'W
  Satellite GPS BIIA-23 (PRN 04) Mar 31 2004
  1602 UTC

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• http//www.ngs.noaa.gov/GPS/GPS.html