How Things Work: Global Positioning System Clay Graf October 12, 2017

Contents Background of GPS What does it do? How does it do it? GPS Structure and Operation Space module User module Control Module Setbacks – Ionosphere Conclusion

Background GPS is an American owned Global Navigation Satellite System Collection of over 30 satellites and 30 ground control stations At least 4 satellites in direct line of sight at any time Precise semi-synchronous orbit = reduced transmission power/cost Conceptually established in the ‘60s Military application, missile guidance Overcoming timing issues – test of General Relativity Handling ionospheric disturbances

What does it do? GPS is used primarily for location, navigation, tracking, etc. Atomic clocks onboard the satellites are used for precision timing and synchronization on the ground. Applications range from military to civilian, private to corporate: Air traffic control Oil and gas mining Agriculture Missile tracking Precision timing experiments

How does it do it? Each GPS satellite continuously transmits a unique signal of where it is, when it is (this is frequently corrected) A receiver obtains both position and time of transmission, along with a code from which time of arrival is deduced. Distance between the satellite to receiver is calculated “Trilateration” requires at least 3 satellites to obtain the height, latitude, and longitude of the receiver. Another satellite necessary to account for clock delay on ground

Structure of GPS System GPS is composed of 3 modules: Satellite – transmits data Receiver – processes data Ground Control – corrects and monitors the data/satellite

Satellite - Transmitter Continuously transmitting between 1.1 and 1.6 GHz (bands L1 – L5), each varying in application L1 and L2 (1575.42 MHz and 1227.60 MHz) are modernized for civilian use. Contain ranging code (C/A) and navigation message containing position and time of the satellites transmission

Transmitted signal Below, the carrier wave is loaded with the C/A code and the navigation message before being transmitted.

Transmitted signal The C/A code is used as PRN (pseudorandom noise) in order for receivers to distinguish between satellites (no correlation) Also used to calculate the time of arrival from autocorrelation function

User - Receiver The receiver contains the antenna (typically a tuned PIFA), processor(s), and a stable clock at the very least. Necessary to have at least 4 (typically 12-20 now) channels to deal with multiple satellite signals

Processing the signal Upon receival, both the carrier wave and PRN code are demodulated, and the spatial and temporal satellite coordinates extracted. Receiver location is ready to be established

Processing the signal Using at least 3 satellites allows for trilateration of the receiver by computing the distance to each of the satellites at the time of transmission. 4th satellite provides time delay correction

Ground Control Satellites need recalibration daily in order to stay precise Ground stations talk to the satellites, updating the navigation message data, as well as resynchronizing the clocks if necessary While the satellites are being corrected, a flag is set, notifying receivers not to use this satellite right now

Ionospheric Disturbances This is the major source of error in GPS calculations The signal undergoes dispersion (frequency dependent) as it passes through plasma layers How is this overcome?

Ionospheric Correction Modernized GPS is designed so the receiver listens to both L1C and L2C bands The processor can then account for scintillations and refraction Allows for correction and monitoring of the ionosphere in real time Although effective, will not resolve the issue completely

Conclusion GPS is composed of over 30 satellites, with at least 4 in direct LOS at any moment Constant corrections are made on the ground to keep the satellites up to date Unique signals are picked up by receivers which trilateration your position, and even derive your velocity Assuming the signal is not disrupted by turbulent ionospheric plasma, we can all enjoy the feeling of never being lost again

Thanks Garmin.com Wikipedia Google images

Questions?