Lecture notes

• Satellites of concern are positioning and mapping that produced the shape and size of the earth that has enabled us to produce accurate coordinates of points anywhere on of above the earth's surface

  
• USSR launches its first Sputnik satellite into space


  1957
• Determining Sputnik's orbit


  Measuring the Doppler frequency curve from one single satellite pass

  
• Shortly thereafter the US Navy Navigation Satellite System (NNSS) was conceived
• NNSS system


  Also known as the Transit system
• SECOR (Sequential Collation of Ranges) satellite launched by the US Army


  1960
• SECOR satellite


  • Used for geodesy, to map the whole universe
  • Allowed for improved global mapping and precise positioning of ground stations linking the Island States in the Pacific and continents of the universe

  
• SECOR satellite positioning


  1. Linked to four mobile ground stations: three in accurately determined known locations, and a fourth in an unknown location

  2. Measured satellite's distance from the three known stations to determine its position in space

  3. Ground stations transmitted phase modulated signal to satellite, satellite received and transmitted back

  4. Time taken for signal to travel multiplied by speed of light gave pseudorange distance

  5. Process repeated many times to enhance accuracy, minimum 3 satellite passes sufficient

  6. Unknown station's position determined, then it becomes a known coordinated station
• Precision of SECOR


  Repeatability brought about 5m precision with longer lines up to 1:100,000 i.e. 1m error for 100,000m distance
• 13 SECOR satellites launched and operational from 1964 to 1969
• Satellite Laser Ranging (SLR) became operational in 1970 and continues to today
• SLR

  • Distances to satellites equipped with retroreflectors can be measured to 1m accuracy
  • Use limited primarily to crustal or deformation studies due to complexity of instrumentations and data processing software
• Transit System operational


  1958-1996
• Transit System


  • Also known as NAVSAT or NNSS (Navy Navigation Satellite System)
  • First satellite navigation system to be used operationally
  • Primarily used by US Navy to provide accurate location information to Polaris ballistic missile submarines and for navigation of surface ships, hydrographic survey and geodetic surveying

  
• Transit System operation

  1. Satellites tracked by ground stations and command center that operated the satellites and generated navigation messages

  2. Operated on Doppler ranging principle - motion of satellite relative to user produced Doppler frequency shift, receiver compared received signal to reference signal to generate Doppler count as measure of change in slant range

  3. Position fix used several successive Doppler counts
• Strengths and characteristics of Transit

  • Requires only 4 satellites, was fully operational with 36 satellites by 1968
  • Position fix required only one satellite at a time
  • Satellites had 10 year lifespan
  • Used 150 and 400 MHz frequencies to cancel ionospheric refraction
  • Provided worldwide timekeeping service accurate to 50 microseconds
  • Circular, polar, low Earth orbits
  • Unlimited passive users, could operate in all weather
• Limitations of Transit


  • Position fix could take 30 minutes to compute, motion of receiver complicated calculations
  • Generally considered a 2D system (latitude and longitude), non-continuous coverage in many areas
• Transit system operated and maintained by Navy Astronautics Group from Point Mugu, California
• Strategic Systems Programs Office maintained a disaster backup system for Transit
• Last Transit-(Nova-3) satellite launched on 11th October, 1984
• TransNetwork Ground Monitoring Stations


  • Applied Physics Laboratory (APL)
  • Las Cruces, New Mexico
  • Lasham, England
  • Sao Paulo, Brazil
  • Wahiawa, Hawaii
  • San Miguel, Republic of the Philippines
  • Smithfield, Australia
  • Misawa, Japan
  • Anchorage, Alaska
  • Pretoria, Union of South Africa
  • American Samoa
  • Thule, Greenland
  • McMurdo Sound, Antarctica
• First GPS-1, NAVSTAR-1 satellite launched


  1978
• Transit system continued to be used till 1996 when it was made obsolete by GPS
• Initial Transit navigation results had errors up to 1km over 24-hr data spans and 2km from predicted orbits
• Dominant errors were related to Earth's gravity field modelling
• Timation (TIMe/navigATION) program conceived by Naval Research Laboratory's Naval Center for Space Technology


  1964
• First Timation I satellite launched


  1967
• Timation II satellite launched


  1969
• Timation program proved a passive ranging technique with accurate clocks could provide the basis for a new and revolutionary navigation system
• Timation program merged with Air Force's 621B program to form NAVSTAR GPS program


  1973
• Timation III satellite redesignated as Navigation Technology Satellite 1 (NTS-1) and launched


  1974
• First NAVSTAR GPS Phase I satellite launched

  1977
• NTS-2 satellite verified Einstein's relativistic clock shift
• NRL's pioneering contributions to GPS recognized with Collier trophy in 1993