1. Lesson VIII: Global Positioning System (GPS)
Nautical Navigation
Lesson VIII: Global Positioning System (GPS)
Preserving America’s Sailing Legacy * Engaging Sailing’s Next Generation

2. First became operational in 1995
History of GPS
GPS is a part of our everyday lives and it’s hard to imagine life without it. How long has it been around?
Owned and operated by US Department of Defense
First became operational in 1995

3. History of GPS
Civilian use of GPS was not reliable until President Clinton directed DOD to turn off “Selective Availability” in 2000
Prior to that, civilian GPS could be over 100 yards off
10x more accurate (~30 ft off)

4. GPS Overview Satellite-based navigation system
NASA launches satellites into space
Made up of a network of 24 operational satellites orbiting the earth
31 satellites orbiting today
Why 31??

5. How GPS Works
GPS Satellites orbit the earth twice a day and constantly transmit a radio signal
Satellites travel at speeds of up to 7,000 mph
Radio signal travels at speed of light
At least 4 Satellites are visible
anywhere on the earth at all times

6. How GPS Works
GPS receivers (e.g. your cell phone) take this information and use trilateration to determine your exact location

7. Distance = Speed * (Satellite Time – Receiver Time)
How GPS Works
Step 1: GPS Satellite transmits radio signal providing their exact location and time (t1)
Step 2: Radio SignalTravels through space at the speed of light (s)
Step 3: GPS Receiver takes the time the signal is received (t2) and calculates the distance (D)
D = S * (t2 – t1)
Distance = Speed * (Satellite Time – Receiver Time)

8. Distance = Speed * (Satellite Time – Receiver Time)
How GPS Works
Once a GPS device knows the distance between you and those 4 Satellites, it can use geometry to pinpoint exactly where you are
D = S * (t2 – t1)
Distance = Speed * (Satellite Time – Receiver Time)

9. How GPS Works What are those large panels on the satellites used for??
Powered by solar energy
How do they work during a solar eclipse?

10. Makes a HUGE difference!
GPS Uses
What happens if the GPS satellite is 1 minute off?
Makes a HUGE difference!
Each satellite has an atomic clock that is accurate to a nanosecond (1/1,000,000,000)
So accurate, that most clocks are now synchronized off GPS satellites

11. GPS Navigation

12. GPS is NOT a primary form of Navigation
GPS Navigation
Could a terrorist with a GPS jammer cause airplanes to crash?
YES! Commercial aircraft using GPS are required to maintain alternative means of navigation.
Same goes for US Navy and Merchant ships
GPS is NOT a primary form of Navigation