Global Positioning Satellite System

(GPS)

- Prakash Kumar Sekar

Overview

The Navigation Problem

Earlier Approaches

GPS Description

How does GPS Work?

Acknowledgement

Many of the graphical images used in this

talk are courtesy of Peter H. Dana of the

Department of Geography, University of

Texas at Austin -

http://www.utexas.edu/depts/grg/gcraft/

notes/gps/gps.html

The best web site for GPS

The Navigation Problem

The ancient question:

Where am I?

Earth coordinates:

latitude and longitude

Lafayette: N40/W86

Latitude can be

determined by Sun

angle

What about longitude?

Latitude and Longitude

Latitude and Longitude

Longitude Problem

No easy way to determine longitude

On July 8, 1714 the Longitude Act was

established in England to solve the

“longitude problem”

Two solutions were proposed

-- use of stars and moons

-- the “time” solution

Longitude

Longitude : The True Story of a Lone

Genius Who Solved the Greatest Scientific

Problem of His Time

Dava Sobel

Longitude Problem: References

http://www.pbs.org/wgbh/nova/longitude/

The “Time” Solution

Where am I? <=> What time is it in

Greenwich, England?

The ability to find one’s position is based

on how well one can tell what time is it?

The development of the chronometer

To find longitude to within 0.5 degree

requires a clock that loses or gains no more

than 3 seconds/day

Longitude

How does this work?

The earth turns 360 degrees in 24 hours:

15 degrees = 1 hour

If you know the time in Greenwich when it

is local noon at your location one can find

your longitude relative to Greenwich

Must know “datum” reference to use maps

Datum Reference

Lone Pine Cemetery - N400 13.8’’ E260 17.24”

Satellite Navigation

US Department of Defense has need for

very precise navigation

In 1973, the US Air Force proposed a new

system for navigation using satellites

The system is known as: Navigation System

with Timing and Ranging: Global

Positioning System or NAVSTAR GPS

NAVSTAR GPS Goals

What time is it?

What is my position (including attitude)?

What is my velocity?

Other Goals:

- What is the local time?

- When is sunrise and sunset?

- What is the distance between two points?

- What is my estimated time arrival?

GSP System

Simply stated: The GPS satellites are nothing

more than a set of clocks in the sky

GPS Segments

Space Segment: the constellation of

satellites

Control Segment: control the satellites

User Segment: users with receivers

Space Segment

Space Segment

System consists of 24 satellites in the

operational mode: 21 in use and 3 spares

3 other satellites are used for testing

Altitude: 20,200 Km with periods of 12 hr.

Current Satellites: Block IIR- $25,000,000

2000 KG

Hydrogen Maser Atomic Clocks

Hydrogen Maser Clock

These clocks lose one second every

2,739,000 million years

GPS Orbits

GPS Orbits

Control Segment

Master Control Station is located at the

Consolidated Space Operations Center

(CSOC) at Flacon Air Force Station near

Colorado Springs

The Global Positioning System.

GPS antenna

GPS receiver

and batteries!

Windows CE handheld computer

Paper Map

with targets

Matt Evans of Abe591a and

Aaron Pierce of the Biology

Department; mapping the

Purdue University Ross

Reserve.

Sample Location

Flag/Pin

ASM 215 April

2009

User component

Individuals with GPS

receivers

Prakash Kumar Sekar

Basic GPS Surveying Techniques

Presented by:

Neil Gray, Teacher-in-Charge, Columboola EEC.

On behalf of ICT Innovators Centre, STiS Project.

Not sphere but spheroid

Newton and others in the 17th and 18th century

proposed that the Earth is flattened due to rotational

forces.

Complex, repeated, highly accurate measurements

established that the curvature of the Earth was greater

at the equator than the poles

Image from ESRI online coursePrakash Kumar Sekar

Push the “Page” button until you get the screen which

displays location.

They will be set to display degrees, minutes and decimal

seconds.

Combining range measurements

Range measurement from coded

signal

Topographic assessment at Davis Purdue

Ag Center

Control Segment

Differential

correction

• Post-processed

• Real-time

Differential Correction

Two receivers are used to

greatly improve the

accuracy of GPS

positional measurements

Base station at known

location

Prakash Kumar Sekar

CSOC

Track the satellites for orbit and clock

determination

Time synchronization

Upload the Navigation Message

Manage DOA

Operational Capabilities

Initial Operational Capability - December 8,

1993

Full Operational Capability declared by the

Secretary of Defense at 00:01 hours on

July 17, 1995

GPS Transmitted Signal

Two signals are transmitted on carriers:

L1 = 1575.42 MHz

L2 = 1227.60 MHz

These are derived from the system clock of

10.23 MHz (phase quadrature)

Modulation used is Direct Sequence Spread

Spectrum

(code division multiple access - CDMA)

GPS Signals

GPS Clock Signals

Two types of clock signals are transmitted

C/A Code - Coarse/Acquisition Code

available for civilian use on L1 provides

300 m resolution

P Code - Precise Code on L1 and L2 used

by the military provides 3m resolution

Spread Spectrum

Spread Spectrum is used because

- resistance to jamming

- masks the transmissions

- resist multipath effects

- multiple access

All 24 GPS satellites transmit on the same

two frequencies BUT use a different ID

sequence

GPS Signals

The satellites transmit as part of their

unique Spread Spectrum signal a clock or

timing signal

The range or distance to the satellite is

obtained by measuring how long it takes for

the transmitted signal to reach the receiver

This is not the “true” range due to clock

errors - what is obtained is know as the

“pseudo-range”

GPS Position

By knowing how far one is from three

satellites one can ideally find their 3D

coordinates

To correct for clock errors one needs to

receive four satellites

GPS: How does it work?

Typical receiver: one channel C/A code on

L1

During the “acquisition” time you are

receiving the navigation message also on L1

The receiver then reads the timing

information and computes the “pseudo-

ranges”

The pseudo-ranges are then corrected

GPS: How does it work?

Corrected ranges are used to compute the

position

This is a very complicated iterative

nonlinear equation

Navigation Message

To compute your position one must know

the position of the satellite

Navigation Message - transmitted on both

L1 and L2 at 50 bits/s for 30 s

Navigation message consists of two parts:

- satellite almanac

- clock bias

Why Do I Need

To See 4 Satellites?

The problem is that the clock signal from

the satellite is corrupted by atmospheric

refraction

Another major problem is that the receiver’s

clock is not very accurate

For a 2D fix <=> 3 satellites

Why Do I Need

To See 4 Satellites?

Denial of Accuracy (DOA)

The US military uses two approaches to

prohibit use of the full resolution of the

system

Selective Availability (SA) - noise is added

to the clock signal and the navigation

message has “lies” in it

Anti-Spoofing (AS) - P-code is encrypted

The military sometimes turns off both DOA

techniques

Differential GPS

Used to improve accuracy

Put a “satellite” on the ground at a precise

position

Differential signal is not “transmitted” on

standard satellite frequencies

Uses of GPS

Airplane and Boat Navigation

Continental Drift

Surveying

Precise Timing

Iceberg Tracking

Archaeological Expeditions

Mobile Multimedia

Applications of GPS

GPS Clock Rollover

GPS System Time rolled over at midnight

21-22 August 1999, 132 days before the

Year 2000

On 22 August 1999, unless repaired, many

GPS receivers claimed that it is 6 January

1980

http://www.navcen.uscg.mil/gps/geninfo/

y2k/gpsweek.htm

Conclusion

GPS will find more civilian uses

DOD has promised to eliminate SA

Russia has a system known as GLONASS

The EU is discussing deploying its own

system

References

B. Hofmann-Wellenhof, H. Lichtenegger,

and J. Collins, GPS: Theory and Practice,

Third Edition, Springer-Verlag, 1994.

T. Logsdon, The Navstar Global

Positioning System, Van Nostrand, 1992.

A. Leick, GPS Satellite Surveying, Second

edition, Wiley, 1995.

References

T. A. Herring, "The Global Positioning

System," Scientific American, pp. 44-50,

February 1996.

N. J. Hotchkiss, A Comprehensive Guide to

Land Navigation with GPS, Alexis, 1994.

Special Edition on the Global Positioning

System, Satellite Times, March/April 1996.

D. Sobel, Longitude, Walker, 1995.

Web Sites

GPS Program Office:

http://www.laafb.af.mil/SMC/CZ/homepage/

US Coast Guard Navaigation Center

http://www.navcen.uscg.mil/default.htm

GPS Precise Orbits

http://www.ngs.noaa.gov/GPS/GPS.html

GPS World Magazine

http://www.gpsworld.com/