gps 31 english

8/8/2019 Gps 31 English

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GPSWhat is it?

How does it work?

Errors and Accuracy

Ways to maximize

accuracySystem components

From Lecture by Robert Long, University of VermontFrom Lecture by Robert Long, University of Vermont

Many materials for this lecture adapted from Trimble Navigation Ltds GPS Web tutorialMany materials for this lecture adapted from Trimble Navigation Ltds GPS Web tutorialatat http://trimble.com/gps/index.htmlhttp://trimble.com/gps/index.html


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GPSStands for Global Positioning System

GPS is used to get an exact location on or above thesurface of the earth (1cm to 100m accuracy).

Developed by DoD and made available to public in1983.


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GPSGPS is a worldwide radio-navigationsystem formed from 24 satellites andtheir ground stations.

Satellites orbit earth every 12 hours atapproximately 20,200 km

GPS uses satellites in space asreference points for locations here on

earthGround stations help satellites

determine their exact location in space.Hawaii

Ascension IslandDiego Garcia

Kwajalein

Colorado Springs


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The Global Positioning System (GPS) is a worldwideradio-navigation system formed from a constellation of 24 satellites and their ground stations .

GPS uses these sattelites as reference points totriangulate positions accurate to a matter of meters. Infact, with advanced forms of GPS you can makemeasurements to better than a centimeter!

http://www.trimble.com/gps/index.html


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Three Segments


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H ere's how GPS works in five logical steps:

1. The basis of GPS is " triangulation " from satellites.

2. To "triangulate," a GPS receiver measures distanceusing the travel time of radio signals.

3. To measure travel time, GPS needs very accuratetiming which it achieves with some tricks.

4. Along with distance, you need to know exactlywhere the satellites are in space. High orbits and

careful monitoring are the secret.5. Finally you must correct for any delays the signal

experiences as it travels through the atmosphere.


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GPS UsesTrimble Navigation Ltd., breaks GPS uses intofive categories:

Location positioning things in space

Navigation getting from point a to point b

Tracking - monitoring movements

Mapping creating maps based on those positions

Timing precision global timing


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How does GPS work?GPS derives position relative to satellitereference points, using triangulation

The GPS unit on the ground figures out itsdistance to each of several satellites

11,500 km

12,500 km

11,200 km


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How Does GPS Work?

We need at least 3 satellites as reference points totriangulate our position.


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How Does GPS Work?Sphere Concept

Source: Trimble Navigation Ltd.

A fourth satellite narrows it from 2 possible points to 1 point


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This method assumes we can find exactdistance from our GPS receiver to a satellite.HOW???

Simple answer: see how long it takes for aradio signal to get from the satellite to thereceiver.

We know speed of light, but we also need toknow: 1. When the signal left the satellite

2. When the signal arrived at the receiver

How Does GPS Work?

Distance = Velocity * Time


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The difficult part is measuring travel time

This gets complicated when you think about theneed to perfectly synchronize satellite andreceiver.

How Does GPS Work?


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To do this requires comparing lag in exactly similar patterns, one from satellite and one from receiver.

This code has to be extremely complex (hencealmost random), so that patterns are not linked up atthe wrong place on the code.

Source: Trimble Navigation Ltd.

How Does GPS Work?


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A fourth satellite allows a correction factor to becalculated that makes all circles meet in oneplace.

This correction is used to update the receiversclock.

How Does GPS Work?


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The receiver then knows the difference betweenits clocks time and universal time and can applythat to future measurements.

Of course, the receiver clock will have to beresynchronized often , because it will lose or gaintime

How Does GPS Work?


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Accuracy Depends On:Time spent on measurementsLocationDesign of receiver Relative positions of satellitesUse of differential techniques


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Sources of Error Gravitational effectsAtmospheric effectsObstructionMultipathSatellite geometrySelective Availability


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Errors and Accuracy (cont.)Obstruction - Signal blocked or strength reduced when

passing through objects or water.Weather MetalTree canopyGlass or plasticMicrowave transmitters

Multipath Bouncing of signals may confuse the receiver.


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Errors and Accuracy (cont.)Satellite Constellation Geometry

Number of satellites availableElevations or azimuths over time

(P.D.O.P.)


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Errors and Accuracy (cont.)PDOP

Indicator of satellite geometry Accounts for location of each satellite relative to

others

Optimal accuracy when PDOP is LOW

Low PDOP

Satellite 1

Satellite 2

High PDOP

Satellite 1

Satellite 2


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Locating SatellitesWe know how far we are from the satellites, buthow do we know where the satellites are?

Because the satellites are 20,000 km up, theyoperate according to the well understood laws of physics, and are subject to few random, unknownforces.

This allows us to know where a satelliteshould be at any given moment.Also tracked by radar to measure slight

deviations from predicted orbits.


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Locating Satellites (cont.)This location information (ephemeris) is relayed

to the satellite, which transmits the info when itsends its pseudo random code.

There is also a digital almanac on each GPSreceiver that tells it where a given satellite issupposed to be at any given moment.

Other information is relayed along with the radiosignal: time-of-day, quality control info.


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Errors and Accuracy (cont.)Selective Availability (S.A.)

Until May of 2000, the DoD intentionally introduced asmall amount of error into the signal for all civilianusers.

SA resulted in about 100 m error most of the time

Turning off SA reduced error to about 30 m radius


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Ensuring Accurate Locations

Adequate satellites

Low PDOP ( 4 excellent, 5-8 acceptable) Averaging

Clear weather Minimize multipath error

Use open sitesAppropriate planning (ephemeris, skyplots)


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Differential GPSIncrease accuracy dramaticallyThis was used in the past to overcome Selective

Availability (100m to 4-5m)DGPS uses one stationary and one moving

receiver to help overcome the various errors inthe signal

By using two receivers that are nearby eachother, within a few dozen km, they are gettingessentially the same errors (except receiver

errors)


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Differential GPSDGPS improves accuracy much more than

disabling of SA does

This table shows typical errorthese may vary

Source: http://www.furuno.com/news/saoff.html

IN TRODUCT ION TO GPS


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How does DGPS work?The stationary receiver must be located on a

known control point

The stationary unit works backwardsinstead of using timing to calculate position, it uses itsposition to calculate timing


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How does DGPS work?Can do this because, precise location of stationary receiver is known, and hence, so islocation of satellite

Once it knows error, it determines a correctionfactor and sends it to the other receiver.


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A A SimplifiedSimplified Conceptual Look at Differential CorrectionConceptual Look at Differential Correction

Corrected Point1-5 Meter Error

Uncorrected Point with 50-100 Meter Error

Differential Correction (DGPS)Differential Correction (DGPS)of GPS Jobof GPS Job

Base Station at aBase Station at aKnown LocationKnown Location

Calculates GPS ErrorsCalculates GPS Errors(Optiona l) RTCMReal Time Corrections Broadcast Service

A GPS position can be calculated when the MC-GPS tracks4 or more satellites :

Latitude

Longitude

Altitude

TimeEphemeris & Almanac Data Sent

SLIDE COURTESY CMTSLIDE COURTESY CMT


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How does DGPS work?Message sent to rover with correction factor for

all satellites.

More reference stations becoming available.


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Other DGPS ConceptsReal-time vs. Post-processing

Wide Area Augmentation System (WAAS)


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System ComponentsReceiver

Receives satellite signalsCompiles location info, ephemeris info, clock calibration,constellation configuration (PDOP)Calculates position, velocity, heading, etc

Data Collector Stores positions (x,y,z,t)

Attribute data tagged to position

SoftwareFacilitates file transfer to PC and backPerforms differential correction (post-processing)Displays data and permits file editing.


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System Components- Receivers

Course/Acquisition (C/A) Code ReceiversCivilian grade

Use info in satellite signals to calculate position12-40m CEP* without differential correction


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System Components

- ReceiversCarrier Phase (P-Code) Receivers

Military or survey grade

Uses actual radio signal to calculate position1cm SEP* (50% of locations within sphere of this radius)Must record positions continuously from at least 4satellites for at least 10 minutes requires clear view

Number of Channels4 satellites for accurate 3D positions, 5 or more for highestaccuracy9-12 satellites required to track all visible satellites at givenmoment


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The Wide Area Augmentation System (WAAS) is a GPS-basednavigation and landing system that provides precision guidanceto aircraft at thousands of airports and airstrips where there iscurrently no precision landing capability

Systems such as WAAS are known as satellite-basedaugmentation systems (SBAS). WAAS is designed to improvethe accuracy and ensure the integrity of information comingfrom GPS satellites.

The FAA is using WAAS to provide a Lateral Navigation/VerticalNavigation (LNAV/VNAV) capability with commissioning in2003.

WAAS testing in September 2002 confirmed accuracyperformance of 1 2 meters horizontal and 2 3 meters verticalthroughout the majority of the continental U.S. and portions of

Alaska.


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H ow it Works

WAAS consists of approximately 25 ground referencestations that monitor GPS satellite data.

Two master stations, located on either coast, collect datafrom the reference stations and create a GPS correctionmessage.

The corrected differential message is then broadcast through

one of two geostationary satellites.The information is compatible with the basic GPS signal

structure, which means any WAAS-enabled GPS receiver canread the signal.

The Wide Area Augmentation System


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Network of Reference Stations

Master Stations

Network of Reference Stations

Master Stations

Network of Reference Stations Geostationary SatellitesMaster Stations Geo Uplink Stations


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Copyright 2001Todd Walter

WAAS Reference Stations


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Copyright 2001Todd Walter

Benefits of WAASPrimary Means of Navigation - Take-Off,En Route, Approach and Landing

More Direct Routes - N ot Restricted ByLocation of Ground-Based Equipment

Precision Approach Capability - At Any

Qualified Airport

Decommission of Older, Expensive Ground-Based Navigation Equipment

Reduced/Simplified Equipment On Board Aircraft

Increased Capacity - Reduced Separation Dueto Improved Accuracy

Courtesy: FAA


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100 meters: Accuracy of the original GPS system, which wassubject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) positionaccuracy.

< 3 meters: Typical WAAS position accuracy

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