CS 128/ES 228 - Lecture 10b1 GPS. CS 128/ES 228 - Lecture 10b2 A guide to GPS theory … .

CS 128/ES 228 - Lecture 10b 1

GPS


A guide to GPS theory …

www.usace.army.mil


… and practice.

Recreational

GIS inputs

Surveying

Transportation

and of course, the military


A brief history of GPS…

Forerunners- LORAN-C: marine navigation. Used radio beacons along shoreline. Localized coverage and low accuracy (CEP ~ 200 m) - TRANSIT: developed by U. S. Navy. Used 6 satellites, low orbits. Global coverage & high accuracy (sub-meter) but “fix” took hours to days

Department of Defense: 1970s study showed ~120 navigation systems in use. Proposed a single system, called NAVSTAR


The NAVSTAR system

GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996

Military wanted portable, fast, passive positioningsystem

Navstar I launched Feb ’78. Now 24 satellites + “spares”

Global, 24/7 coverage by at least 4 satellites

Satellites carry atomic clocks


GPS basics

Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993


1, 2 …

Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993


3!

Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

Technically 4 satellites are necessary, but normally only 1 of the 2 points is on the geoid

But, as we’ll see later, a 4th satellite is still useful


How to time the signal?

Standard EDMs bounce a signal off a reflector and measure time of return

But, GPS requires a low-power transmission and passive receivers

Imagine 2 people with synchronized stop watches, standing 1 mile apart. A gun fires near 1, and each records the time when they hear the shot. What do the 2 times tell you?


Pseudocode

GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993

A string of pseudo-random bits

Predetermined sequence – can be generated by the satellite and the receivers


Code correlation



But wait: for $19.95 you get all these extra codes …

C/A (coarse acquisition) code:- short (1 ms) & slow (1.023 Mbps)- meant to enable receivers to get a crude “fix” quickly

P (precision) code:- long (267 d) & fast (10.23 Mbps) - permits sub-meter accuracy


Why use the P code?


C/A code bits are ~ 1 µs wide

Even a good receiver can be out of phase with the code by 1-5%

1% phase error ~ 3 m position error


Still not sure? There’s more …

Each code is broadcast on 2 frequencies, the L1 and L2 bands

“Dual frequency” receivers utilize the frequency difference between L1 & L2 to compensate for atmospheric distortions – more on that later

Mucho expensive


Sources of error

1. [Selective availability]

2. Clock errors

3. Ephemeris errors

4. Atmospheric delays

5. Multipath effects

6. Receiver errors Differential GPS Explained. J. Hurn. Trimble Navigation, 1993


A small clock error matters!

Microwaves travel at approximately the speed of light:

300,000 km per second

A clock error of only 1 µs could produce a positional error of ~ 300 m!

How can a $100 GPS receiver have a clock that stays accurate to the µs??


Voila – the 4th satellite!

Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993

A 2-D example:

Clocks synchronized Clocks not synchronized


If the 4 ranges don’t overlap:


Receiver adjusts its clocks until they do

[Actually: done by algebra, not trial & error]

The time on a $100 GPS unit is really accurate!


Ephemeris errors


Trimble Navigation

Ephemeris: mathematical description of an orbit


Atmospheric delays


Signal slowed by:- charged particles in the ionosphere- water vapor in the troposphere

Dual-frequency receivers can correct for ionosphere delays


Multi-path errors

Top: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993Bottom: www.gpsw.co.uk

Worse:- near buildings, other obstructions- satellites near horizon: use “elevation mask”

Better:- more sophisticated antenna- ground plane to block

low-angle reflections


Typical error “budget” (m)*

Source:Standard GPS

Differential GPS

Satellite clocks 1.5 ~ 0

Ephemeris errors 2.5 ~ 0

Ionosphere delays 5.0 0.4

Troposphere delays 0.5 0.2

Multi-path (reflections) 0.6 0.6

Receiver errors 0.3 0.3

Typical totals 10 – 15 1-2Differential GPS Explained. J. Hurn. Trimble Navigation

* Horizontal position; vertical errors typically 2x or greater


Improving GPS accuracy

DOP and mission planning

Differential GPS

Surveying-grade GPS


DOP & “mission” planning


Dilution of precision: a small number of satellites or poor positions degrades accuracy

Advance software lets you plan when to use GPS for maximum accuracy


“mission” planning software

Pathfinder Office. Trimble Navigation


Differential GPS


Fixed base station:- knows its location- records any shifts in its readings in correction file

Differential corrections:- real time- post-processing


Real-time DGPS

GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1989

Radio link with base station

U. S. Coast Guard beacons

WAAS


Coast Guard beacons

www.navcen.uscg.gov/dgps/coverage/NYork.htm

Trimble’s Beacon on a Belt

www.trimble.com


WAAS Wide Area Augmentation System

Designed by FAA &DOT for generalaviation

25 ground basestations collect DGPS data & uplinkto 2 geostationary satellites, 1 over east coast, the other over the west coast

www.garmin.com/aboutGPS/waas.html


Effectiveness of WAAS

Corrects for satellite orbit & clock errors, plus ionosphere & troposphere distortions

Capable of improving accuracy to < 3 m for WAAS enabled receivers

Vertical accuracy is not yet sufficient for landing airplanes at uninstrumented airports, the original program objective

The eastern satellite (#35) is low on the SE horizon and signal can be hard to receive


Post-processing GPS receiver logs all signals received

during data collection phase

Data log compared to similar record from a base station receiver

U. S. NGS operates a series of public base stations and an internet file processing system (OPUS) for free postprocessing

www.ngs.noaa.gov/OPUS/What_is_OPUS.html


Survey-grade GPS Uses high-quality, dual-frequency receivers

and DGPS

Carrier phase processing- utilizes the L1 or L2 carrier signal, 1.2 / 1.5 GHz

Can achieve accuracies of < 1cm

Expensive, complex, and requires long periods of data collection at each station



The Future of GPS Everyday utilities

(cell phones, car navigation systems)

Integrated GIS/GPS units

GPS stalking??

???

www.garmin.com/aboutGPS/waas.html

CS 128/ES 228 - Lecture 10b1 GPS. CS 128/ES 228 - Lecture 10b2 A guide to GPS theory … .

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