CS 128/ES 228 - Lecture 11a1 GPS. CS 128/ES 228 - Lecture 11a2 Global Positioning System .

CS 128/ES 228 - Lecture 11a 1

GPS


Global Positioning System

www.usace.army.mil


GPS in practice.

Recreational

GIS inputs

Surveying

Transportation

and of course, the military


Why study GPS theory… (just turn it on& use it,

right?)


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


Early GPS specs

http://gigaom.com/2006/12/08/telenav-gps/

Military criteria for a 1970s positioning system:

1. Able to direct 5 bombs to the same crater

2. Light enough to carry on soldier’s back

3. Cost under $20,000!

Bob Rennard, now Chief Technology Officer at TeleNav


They’ve gotten smaller!

http://www.engadget.com/2006/01/12/new-zealands-rakon-

develops-worlds-smallest-gps-receiver/

Posted Jan 12th 2006 10:45PM by Marc PertonFiled under: GPS

So you think today's handheld, SDIO and cellphone-based GPS units are small? Get ready for a new generation of even smaller GPS devices, ranging from wristwatches to slim cellphones to -- and we just know this is coming -- implants. That's the promise held forth by what is being billed as the world's smallest GPS receiver, which was developed by New Zealand-based Rakon, a company affiliated with GPS-make Navman. According to Rakon, the chip is about the size of a baby's fingernail, and should be available in a range of devices within the next two years.

http://www.engadget.com/bloggers/marc-perton

http://gps.engadget.com/

http://www.rakon.com/whatsnew/display?article_id=-80&template=news


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

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


But wait: for $19.95 you get …

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


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

Survey-grade receivers – mucho $$


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 <<1 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 but not troposphere errors


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 1-2

Differential 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

Advanced 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

ground stations collect DGPS data & uplink to 2 geostationary satellites, 1 over east coast, 1 over the west coast

www.garmin.com/aboutGPS/waas.html


Post-processed differential GPS

Opened just this year:- Set of NYS DOT DGPS stations, along major highways

- Locally: new stations in Friendship, Salamanca

http://www.ngs.noaa.gov/CORS/Maps2005/ny.html

National Geodetic Survey:CORS (Continuously Operating Refer. Stations)

http://www.ngs.noaa.gov/CORS/


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 < 1 cm

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??

???

http://www.flytip.com/blogs/mobility/

archives/location_based/

index.shtml


Using GPS with GIS

The GolfPS™ Personal GPS System provides instant and continuously updated distance to … any feature on the course, such as bunkers, water hazards and pin placements.

http://4h.missouri.edu/go/programs/mapping/gps1.ppt#275,47,Using GPS with Golf

http://www.golfps.com/trackyourgame.htm


A driver has blamed his sat nav for leaving his car teetering on the edge of a cliff after he followed its instructions

Driver Led To 100ft Cliff Edge By Sat Nav 3:40pm UK, Wednesday March 25, 2009

CS 128/ES 228 - Lecture 11a1 GPS. CS 128/ES 228 - Lecture 11a2 Global Positioning System .

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