Global Positioning System Technical Overview

What is GPS? Global Positioning System is: –Satellite based time distribution system 32 Satellites in all numbered Orbiting satellites in 6 inclined orbits About 12K nautical miles up Each satellite orbits the earth twice every 24 hours When & Why? –1st satellite launched Feb 28, 1978 by DOD –Intended for exclusive military ops: Navigation, positioning, time distribution Consolidate overlapping existing systems Enhance precision weapons delivery –Approved for civilian use in 1980 by President Carter –Each Satellite transmits L1 and L2 signals L1 at MHz is for both civilian and military use L2 at MHz is exclusive to DOD and U.S. Gov (until recently)

3 Segments of GPS Space Segment –Consists of Solar powered satellites No more than 50 watts of transmit power lbs each and 17 feet across & travels at 7000 MPH Each satellite transmits a unique CDMA code Can operate autonomously in the absence of the control segment Control Segment –11 control stations throughout the world Master - Schriever AFB Colorado (Top) Passive- Hawaii (Bottom), Ascension Islands, Diego Garcia, Kawajalein, DC, England, Argentina, Ecuador Bahrain, Australia Track and controls satellites and provides correct clock (timing) information User Segment –Consists of a user and a GPS receiver –Commercial Applications Outdoor activities such as hiking, biking and boating Navigation and directions Fleet Management & Telematics –Military Applications Weapons systems Field operations Force management

How It All Works The GPS receiver must know two things: –Where the satellites are. (Location of each Satellite) –Time at which the GPS signal was broadcast in order to calculate how far away they are. (Distance to each Satellite) How Does It Know Where They Are? –Each satellite broadcasts its ephemeris model (astronomical position) along with time information and information on the propagation delay through the ionosphere. –The ephemeris model describes satellite position, and velocity at time of transmission. –An un-aided receiver may take 30 seconds to collect the data that repeats twice per minute How Does The Receiver Calculate The Distance To Each Satellite? –Radio waves travel at the speed of light (~ one billion feet/sec) –The distance to a satellite is the time elapsed from transmission to reception times the speed of light. [d = (t 1 – t 0 ) x c] –The receiver uses the time information encoded in the signal to calculate the actual time that elapsed since transmission and then calculates distance. –This process produces fixes with errors on the order of 30 feet –Similarly, the receiver then also knows the correct time to about 30 nanoseconds

Calculating Latitude & Longitude (3 Satellites) Latitude Longitude Sat 1 = (lat 1,lon 1 ) Sat 2 = (lat 2, lon 2 )` Sat 3 = (lat 3, lon 3 ) Rcv = (lat R, lon R ) d3d3 d1d1 d2d2 Only unknowns are lat R & lon R With 3 independent equations and only 2 unknowns, the Lat & Lon of the Receiver can be easily calculated (lat R – lat 1 ) 2 + (lon R – lon 1 ) 2 = (d 1 ) 2 (lat R – lat 2 ) 2 + (lon R – lon 2 ) 2 = (d 2 ) 2 (lat R – lat 3 ) 2 + (lon R – lon 3 ) 2 = (d 3 ) 2 Knowing the location and distance of the satellites, Latitude and Longitude can be calculated using Geometry and Linear Algebra techniques The Receiver’s Lat & Lon coordinates will be the intersection of circles drawn around the satellites with radius = distance from satellites

GPS Fix & Errors How Many Satellites Are Needed For A Fix? –A minimum of 3 satellites = 2D fix Latitude Longitude Includes Timestamp –4 satellites = 3D fix Latitude Longitude Altitude Includes Timestamp –By continuously updating your position, a GPS receiver can also accurately provide speed and direction of travel Sources of Error in GPS –Obstructions - Buildings & Tunnels, Metallic Structures, Dense Foliage, Terrain –Environment - Atmosphere & Weather (minor effects) –Selective Availability - Intentional errors by DoD (disabled May 1, 2000) –Multipath - GPS signals reflect off of structures causing timing errors

GPS Types Autonomous GPS: –Using the GPS receiver only to calculate current position –TTFF (Time to First Fix) ~ 50 seconds for a Bluetooth GPS Receiver. Assisted GPS (AGPS): –Data Aiding - Wireless network “Assists” the GPS receiver by sending it ephemeris data from the Base Station Almanac that tells the receiver which satellites are visible in sky. Saves secs. –Temporal Aiding – Wireless Network shares its knowledge of GPS time with GPS Receiver so it does not have to solve ambiguities. Saves 4-6 seconds. –Main benefit of AGPS is that it provides TTFF in ~10-20 seconds. AGPS - iDEN –MLS (Mobile Location server) provides AGPS information one way from MLS to receiver inside the handset. –Both Handset Application and Network initiated are available AGPS – CDMA –Uses Qualcomm GPSOne Technology –PDE (Position Determining Equipment) provides AGPS to the handset –MS-Assist (User Plane) – Handset Application initiated; Uses PDE on every fix –MS-Based (User Plane) – Handset Application initiated; Uses PDE on first fix and only when needed (every min) –Control Plane (BMF) – Network initiated via web services calls to BMF Gateway

AGPS Fix... open sky (5m - 50m) GPS Fix requires a minimum of four(4) GPS measurements. If there three(3) or fewer GPS measurements, proceed to Hybrid Fix below. Hybrid FIX... partial view of sky (20m - 100m) AFLT FIX... indoors (50m – 200m) Cell Sector Center FIX... anywhere (0.5mi – 3mi) Most Precise Least Precise Hybrid Fix is activated when GPS Fix fails. CDMA pilot phase measurements can be used as virtual GPS measurements in fixing position. AFLT Fix can be done when Hybrid Fix fails due to no GPS measurements. AFLT Fix requires a minimum of three(3) CDMA pilot phase measurements to fix position in accordance with the TDOA(Time Difference Of Arrival) theory. AGPS If AFLT Fix fails in case when there are less than three (3) CDMA pilot phase measurements, location server reports cell sector center as the handset position. Accuracy CDMA Location Accuracy

FAQs & More Information FAQs Q: What happens to GPS when users are on a voice call? A: On most current CDMA handsets, voice calls prevent GPS activity. More Information –Wiki Page