Transcript GPS Satellites - UNL School of Natural Resources

GPS Satellites
• Satellite-based navigation system
originally developed for military
purposes (NAVSTAR1 -1978).
NAVSTAR Global Positioning
System (GPS)
• Globally available since 1994
• Maintained and controlled by the
United States Department of
Defense ( 50th Space Wing (50 SW))
Landsat
90 minute orbit
• There are two GPS systems:
NAVSTAR - United State's system,
and GLONASS - the Russian version
GLONASS
• GPS permits users to determine
their three-dimensional position,
velocity, and time.
NAVSTAR
Constellation Arrangement
• GPS satellites fly in Medium Earth orbit (MEO) at an
altitude of approximately 20,200 km (12,550 miles).
• Each satellite circles the Earth twice a day. 12 hour
return interval for each satellite
• GPS uses radio transmissions.
• The satellites transmit timing information and satellite
location information.
• The United States is committed to maintaining the
availability of at least 24 operational GPS satellites,
95% of the time.
• To ensure this commitment, the Air Force has been
flying 31 operational GPS satellites for the past few
years.
• Satellites are distributed among six offset orbital
planes
How GPS works in five logical steps:
1.
2.
3.
4.
5.
The basis of GPS is triangulation from satellites
GPS receiver measures distance from satellite using
the travel time of radio signals
To measure travel time, GPS needs very accurate
timing
Along with distance, you need to know exactly
where the satellites are in space. Satellite location.
High orbits and careful monitoring are the secret
You must correct for any delays the signal
experiences as it travels through the atmosphere
Distance from satellite
• Radio waves = speed of light
• Receivers have nanosecond
accuracy (0.000000001
second)
• All satellites transmit same
signal “string” at same time
• Difference in time from
satellite to time received
gives distance from satellite
Speed of light - -approximate values
kilometres per second
300,000
kilometres per hour
1,080 million
miles per second
186,000
miles per hour
671 million
Triangulation
Triangulation
Differential GPS
• Differential GPS uses the time
sequence of observed errors at
fixed locations to adjust
simultaneous measurements at
mobile receivers
• A location measurement
accurate to 1 cm horizontally
and 2cm vertically is now
possible in 3 minutes with a
mobile receiver
• More accurate measurements
if the instrument is left in place
longer