Intro to GPS

Intro to GPS
- Overview
  - Global Navigation Satellite System (GNSS)
    - NAVSTAR GPS
      - US military
      - 24 satellites (= SV)
    - GLONASS - Russia
    - [Galileo - Europe & BeiDou - China]
  - DNews (4 min)
- How It Works
  - surveying: triangulation/ intersection; navigating: resection & trilateration
  - Link: Trimble (GREAT resource)
  - analogy: ripples & time
    - 1 ripple = circle of possibilities
    - 2 ripples = 2 locations (intersection)
    - 3 ripples = 1 location
  - Trilateration video (4 min)
  - satellites (SV)
    - 1 SV = sphere of possibilities
    - 2 SV's = circle of possibilities
    - 3 SV's = 2 locations ... 1 usu. ridiculous
    - but we need 4 SV's ...
      - consider triangulation ...
      - ... only really need 2 angles (bearings)
      - ... purpose of the 3rd bearing?
      - same for GPS
      - ... we need a 4th because ...
  - analogy: sound
    - stereo with 2 speakers far apart
    - stand by one speaker, delay with sound from 2nd speaker
    - delay due to distance for sound to travel
  - pseudo-random code & time
    - pseudo-random code
      - long digital code emitted from SV
      - each SV has it's own code ("song", e.g. Shoot to Thrill)
      - receiver has all codes (all "songs = greatest hits album)
      - SV & receiver emit code ("play song") at exact same time
    - Time  Distance
      - receiver compares incoming code (song) with it's own
      - calculates time diff.
      - time diff * speed of light = distance
      - distance = radius of sphere
  - Exact time measures are critical
    - time for signal to reach earth is ~0.06seconds
    - speed of light
      - 300,000 km/ sec
      - 300,000,000 m/s
      - ... 0.001 sec = 300,000 m
      - ... 0.000001 sec = 300m
    - ... atomic clocks
- 3 Segments
  - User
    - GPS units we use (iPad to Trimble)
    - you get what you pay for
  - Satellites
    - 24 SV's - 6 orbits with 4 SV's
    - high orbit (20,000 km)
    - each SV has 2 orbits / day
    - solar powered - low powered transmissions ...
      - ... pseudo-random code
      - each SV has a unique code
      - GPS unit receives these codes
  - Ground Control
    - many monitoring stations
    - monitor SV's location & health (clocks)
    - send ephemeris data to SV's
    - thus SV's transmit
      - pseudo-random code
      - updated info on their true location (ephemeris)
- Sources of Error
  - Clock error (1-2m)
  - Ephemeris (2-3m) ... vs almanac
  - Receiver (0.5m)
  - Ionosphere (5m)
  - Troposphere (0.5 m)
  - Multi-path (1m)
  - PDOP (position dilution of of precision) (factor of <5 = good)
  - S/A (selective availability) - now turned off
- Differential Correction
  - eliminates errors!
  - stationary unit & roving unit
  - error correction for stn. unit gets applied to roving unit - use same SV's and time of day
  - post survey correction OR real time correction
- Trimble GPS --> GIS --> iPad (geo-pdf)
- GPS & elevation
  - profile image
  - Geoid ht calculator
  - Real Elev. = GPS Elev - correction
    - if correction is -, then it's added