Satellite Network

1
Satellite Network
2
Satellite Network

• 24 satellites in orbit, plus 3 backups
• 3000-4000 lbs, solar powered
• Orbit height is roughly 20,000 km (Earth radius
  is 6400 km)
• Complete orbit twice per day
• At any point on Earth, at least four should be
  visible at any time

3
Finding Location

• Based on principle of triangulation or
  trilateration
• E.g. in two dimensions, three overlapping
  circles is enough to find a precise point
• In 3 dimensions, three spheres overlap in a
  single point
• Minimum you need two satellites and the Earths
  surface

4
Distance calculation

• Satellites send out low-power, high frequency
  radio signals with time stamps
• Time stamps come from long sequences of
  pseudo-random numbers that restart each day at
  midnight
• Satellites have extremely accurate atomic clocks

5
Distance calculation

• Receivers have normal quartz clocks not
  accurate enough for this
• Quartz clocks are calibrated by using four
  satellites for measurements
• Travel time can be calculated from delay, because
  speed is known (300,000 km per second)

6
Possible Problems

• Waves travel through atmosphere this changes
  speed of light
• Orbits can change due to moon/sun gravity (DoD
  monitors and includes corrections in signal)
• Reflections off of ground stuff
• Bad position data from satellite foils almanac
  plan

7
Differential GPS

• Use a base station to improve accuracy
• Fixes some systematic inaccuracies, because the
  position of base station is known, and offsets
  can be calculated

8
Accuracy

• Hand-held horizontal accuracy of 10-20 meters,
  worse for elevations
• Differential submeter accuracy, but calculation
  is difficult and not real-time
• In May 2000, military ended the practice of
  including errors in the signals, which used to
  limit accuracy to 100m for non-military GPS units.

9
Horizontal ErrorDavid Wilsons GPS Accuracy Page
(URL next slide)
10
Vertical Errorhttp//users.erols.com/dlwilson/gps
vert.htm