How Global Positioning Devices (GPS) work

1. How Global Positioning Devices (GPS) work Presented by William Thompson Jamaes Payne

2. GPS is a Satellite Navigation System • GPS is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military. • GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. • Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

3. Space Segment • The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space. • The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites

4. Control Segment • The Control Segment consists of a system of tracking stations located around the world. • The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites.

5. User Segment Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies

6. GPS Positioning Services Specified In the Federal Radio navigation PlanPrecise Positioning Service(PPS) • Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military. • PPS Predictable Accuracy • 22 meter Horizontal accuracy • 27.7 meter vertical accuracy • 200 nanosecond time accuracy

7. Standard Positioning Service (SPS) • Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. • SPS Predictable Accuracy • 100 meter horizontal accuracy • 156 meter vertical accuracy • 340 nanoseconds time accuracy

8. GPS Satellite Signals • The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers

9. GPS Data • The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds.

10. GPS interference and jamming • Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to overpower the receiver, making acquiring and tracking the satellite signals difficult or impossible. • Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun

11. Differential GPS (DGPS) Techniques • The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal

12. Differential Carrier GPS (Survey) • In order to correctly estimate the number of carrier wavelengths at the reference and remote receivers, they must be close enough to insure that the ionospheric delay difference is less than a carrier wavelength. This usually means that carrier-phase GPS measurements must be taken with a remote and reference station within about 30 kilometers of each other

13. Common Mode Time Transfer • When time information is transferred from one site to another, differential techniques can result in time transfers of around 10 ns over baselines as long as 2000 km.

14. GPS Techniques and Project Costs • Receiver costs vary depending on capabilities. Small civil SPS receivers can be purchased for under $200, some can accept differential corrections. Receivers that can store files for post-processing with base station files cost more ($2000-5000). Receivers that can act as DGPS reference receivers (computing and providing correction data) and carrier phase tracking receivers (and two are often required) can cost many thousands of dollars ($5,000 to $40,000). Military PPS receivers may cost more or be difficult to obtain.

15. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS. • GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.