Global Positioning System

Technical Overview

What is GPS?

•

Global Positioning System is:

– Satellite based time distribution system • 32 Satellites in all numbered 1-32 • • • 24 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 1575.42 MHz is for both civilian and military use L2 at 1227.60 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 • • • 2100-3700 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

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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)

The Receiver’s Lat & Lon coordinates will be the intersection of circles drawn around the satellites with radius = distance from satellites Sat 1 = (lat 1 ,lon 1 ) d 1 Rcv = (lat R , lon R ) d 2 Sat 2 = (lat 2 , lon 2 )`

• •

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 R (lat R – lat 1 ) 2 + (lon R – lat 2 ) 2 + (lon R – lat 3 ) 2 + (lon R – lon 1 ) 2 – lon 2 ) 2 – lon 3 ) 2 = (d 1 ) 2 = (d 2 ) 2 = (d 3 ) 2 d 3 Sat 3 = (lat 3 , lon 3 ) Latitude Knowing the location and distance of the satellites, Latitude and Longitude can be calculated using Geometry and Linear Algebra techniques

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GPS Fix & Errors How Many Satellites Are Needed For A Fix?

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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 Environment -

Buildings & Tunnels, Metallic Structures, Dense Foliage, Terrain 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 18-30 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 60-90 min) – Control Plane (BMF) – Network initiated via web services calls to BMF Gateway

CDMA Location Accuracy

Accuracy

AGPS Fix

... open sky

(5m - 50m)

AGPS

Most Precise

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) Hybrid Fix is activated when GPS Fix fails. CDMA pilot phase measurements can be used as virtual GPS measurements in fixing position.

AFLT FIX

... indoors

(50m – 200m) 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.

Least Precise

Cell Sector Center FIX

... anywhere

(0.5mi – 3mi) 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.

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

• http://en.wikipedia.org/wiki/Global_Positioning_System#