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

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

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)

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??

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

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

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)

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)

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?

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

GPS Navigation

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