1. 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)) There are two GPS systems: NAVSTAR - United State's system, and GLONASS - the Russian version GPS permits users to determine their three-dimensional position, velocity, and time. NAVSTAR GLONASS Landsat 90 minute orbit

2. 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

3. How GPS works in five logical steps: 1. The basis of GPS is triangulation from satellites 2. GPS receiver measures distance from satellite using the travel time of radio signals 3. To measure travel time, GPS needs very accurate timing 4. Along with distance, you need to know exactly where the satellites are in space. Satellite location. High orbits and careful monitoring are the secret 5. You must correct for any delays the signal experiences as it travels through the atmosphere

4. 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 second300,000 kilometres per hour1,080 million miles per second186,000 miles per hour671 million

5. Triangulation

7. 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