Lecture and lab schedule
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Lecture and lab schedule. Lecture: GPS, remote sensing, spatial analysis and applications Labs: 1. GPS/RS lab 2. Fire Fuel Mapping and Modeling in a Forested Environment 3. Your lab. Why GPS. GPS basics. Figure out where you are and where you’re going

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Lecture and lab schedule

Lecture and lab schedule

  • Lecture: GPS, remote sensing, spatial analysis and applications

  • Labs:

    1. GPS/RS lab

    2. Fire Fuel Mapping and Modeling in a Forested Environment

    3. Your lab


Why gps

Why GPS

GPS basics

  • Figure out where you are and where you’re going

  • Navigation and positioning are crucial to many activities


Lecture and lab schedule

  • Generating mapped data for GIS databases

    • “traditional” GIS analysts & data developers

    • travel to field and capture location & attribute information cheaply (instead of surveying)

  • Other uses (many in real time):

    • 911/firefighter/police/ambulance dispatch

    • car navigation

    • roadside assistance

    • mineral/resource exploration


Lecture and lab schedule

GPS Basics

What is GPS?

GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites

• Created and Maintained by the US Dept. of Defense and the US Air

Force

• System as a whole consists of three segments

satellites (space segment)

receivers (user segment)

ground stations (control segment)

Note: Russia and a European consortium are implementing similar systems.


Lecture and lab schedule

Satellites


How it works

How it works?

1. Triangulating

2. Distance measure

3. Getting perfect timing

4. Satellite position


Lecture and lab schedule

How It Works

1. Triangulating

Start by determining distance between a GPS satellite and your position

Adding more distance measurements to satellites narrows down your possible positions


Lecture and lab schedule

Triangulating

Three distances = two points

  • Intersection of Four spheres = one point

  • Note:

  • 4th measurement not needed

  • Used for timing purposes instead


Lecture and lab schedule

2. Distance measure

  • Distance between satellites and receivers

    • determined by timing how long it takes the signal to travel from satellite to receiver

  • How?

    • Radio signals travel at speed of light: 186,000 miles/second

    • Satellites and receivers generate exactly the same signal at exactly the same time

    • Signal travel time = delay of satellite signal relative to the receiver signal

    • Distance from satellite to receiver =

      • signal travel time * 186,000 miles/second

1sec

Satellite signal

Receiver signal


Lecture and lab schedule

3. Getting perfect timing

  • How do we know that satellites and receivers generate the same signal at the same time?

    • satellites have atomic clocks, so we know they are accurate

  • Receivers don't -- so can we ensure they are exactly accurate? No!

    • But if the receiver's timing is off, the location in 3-D space will be off slightly...

  • So: Use 4th satellite to resolve any signal timing error instead

    • determine a correction factor using 4th satellite


Lecture and lab schedule

4. Satellite position

  • In order to make use of the distance measurements from the satellites, we must know their exact locations such that we

  • can match our signals with the right satellite.

    • satellites are placed into high orbits -- makes their orbits very predictable

    • receivers have almanacs that tell them where satellites should be

    • minor variations in orbit are monitored -- correction factors transmitted along with the signals


Lecture and lab schedule

  • System as a whole consists of three segments

  • satellites (space segment)

  • receivers (user segment)

  • ground stations (control segment)


Lecture and lab schedule

Satellites (space segment)

  • 24 NAVSTAR satellites

  • orbit the Earth every 12 hours

    • ~11,000 miles altitude

    • positioned in 6 orbital planes

    • orbital period/planes designed to keep 4-6 above the horizon at any time

    • controlled by five ground stations around the globe


Lecture and lab schedule

Receivers (User Segment)

  • Ground-based devices

    • can read and interpret the radio signal from several of the NAVSTAR satellites at once.

  • Use timing of radio signals to calculate position on the Earth's surface

  • Calculations result in varying degrees of accuracy -- depending on:

    • quality of the receiver

    • user operation of the receiver

    • local & atmospheric conditions

    • current status of system


Lecture and lab schedule

Ground stations (control segment)

Ground Stations (control segment)

Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

  • Five control stations

    • master station at Falcon (Schriever) AFB, Colorado

    • monitor satellite orbits & clocks

    • broadcast orbital data and clock corrections to satellites


Lecture and lab schedule

Error Sources

  • Satellite errors

    • satellite position error

    • atomic clock, though very accurate, not perfect.

  • Atmosphere

    • Electro-magnetic waves travels at light speed only in vacuum.

    • The ionosphere and atmospheric molecules change the signal speed.

  • Multi-path distortion

    • signal may "bounce" off structures nearby before reaching receiver – the reflected signal arrives a little later.


Lecture and lab schedule

Error Sources (cont’d.)

  • Receiver error: Due to internal noise.

  • Selective Availability

  • intentional error introduced by the military for national security reasons

  • Pres. Clinton cancelled May 2, 2000.


Lecture and lab schedule

Selective Availability (SA)


Lecture and lab schedule

Error Breakdown (typical case):

  • satellite clock:

  • satellite orbit:

  • ionosphere/troposphere:

  • multipath distortion:

  • receiver errors:

1.5 meters

2.5 meters

5.5 meters

0.6 meters

0.3 meters


Gps error correction

GPS - Error Correction

  • 2 Methods:

    • Point Averaging

    • Differential Correction


Gps point averaging

Averaged

Location

GPS - Point Averaging

  • This figure shows a successive series of positions taken using a receiver kept at the same location, and then averaged


Lecture and lab schedule

GPS

Differential Correction

Any errors in a GPS signal are likely to be the same among all receivers within 300 miles of each other.

Error signals

Reference Receiver sits over

Precisely surveyed point

Note: differential correction can be applied in "real time" or after the fact (post-processing)


Gps differential correction

~ 300 miles (~ 480 km) or less

Base station (known location)

Rover receiver

GPS - Differential Correction

  • Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver)

  • Any errors in a GPS signal are likely to be the same among all receivers within 300 miles of each other


Lecture and lab schedule

Differential correction

How it works:

• use a base stationat a known position base station calculate

its own position & compares to its known position

• determines correction factors that can be applied to

receiver-calculated positions

Differential correction will reduce horizontal position Error to 1 - 3 meters with standard receiver

much GPS fieldwork for GIS/mapping purposes will require differential correction!

National Differential GPS Network (NDGPS) being created


Gps differential correction1

GPS - Differential Correction

  • The base station knows its own location

  • It compares this location with its location at that moment obtained using GPS satellites, and computes error

  • This known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment

Example: Base Station File

Time

GPS Lat

GPS Long

Lat. error

Long. error

3:12.5

3:13.0

3:13.5

3:14.0

3:14.5

3:15.0

35.50

35.05

34.95

36.00

35.35

35.20

79.05

78.65

79.55

80.45

79.30

79.35

.5

.05

-.05

1.0

.35

.20

.5

-.35

.55

1.45

.30

.35


L11 5 the issue of gps datums

L11.5 The issue of GPS datums

Datums, or so called “reference globe” in map projections, need to be defined for GPS.

The WGS 84 is defined and maintained by the US National Imagery and Mapping Agency (NIMA) as a global geodetic datum. It is the datum to which all GPS positioning information is referred by virtue of being the reference system of the broadcast GPS satellite ephemerides.


Garmin s cheapest receivers

Garmin’s cheapest receivers

Garmin’s Forerunner 201: A watch that uses GPS to determine current speed, average speed, exact distance traveled, etc. ( ) Basic features also available in the Forerunner 101 ($115).

http://www.garmin.com/products/forerunner201/

Garmin’s iQue 3600 PDA:

http://www.garmin.com/products/iQue3600/


Garmin s outdoor gps receivers

Garmin’s Outdoor GPS Receivers:

Etrex Legend C ($375)

“Along with the Etrex Vista C, is one of Garmin's smallest, least expensive products to combine a color TFT display and advanced GPS routing capabilities in a waterproof design.”

--is WAAS enabled

--has USB port for downloading maps from Garmin’s MapSource CD library

Etrex Vista C ($430)

--has a TFT (thin-film transistor, with 1-4 tranistors controlling each pixel; it is the highest-definition flat-panel technique) display

--WAAS enabled

--has USB port for downloading maps from Garmin’s MapSource library


Choosing a gps receiver

  • The 2000 Receiver Survey in the GPS World magazine lists 495 receivers from 58 manufactures (GPS World, January 2001). Why are there so many GPS receivers on the market?

  • There are so many different applications of GPS

  • New uses spring up every day.

  • Dual-frequency or single-frequency GPS receiver

  • Smart antennas/integrated receivers – For the lower end of the accuracy requirements, handheld GPS receivers operate at the single-point accuracy level (<10 m without Selective Availability).

Choosing a GPS receiver


Choosing a gps receiver cont

Choosing a GPS receiver (Cont.)

12XL from Garmin –

City point database, $309.07 , area calculation

Etrex from Garmin – 500 points, $145.71


Choosing a gps receiver cont1

Choosing a GPS receiver (Cont.)

(2)GIS/Mapping receivers - Receivers used for mapping and GIS data Collection typically requires a positioning accuracy in the range of sub-meter to a few meters.

Both (1) and (2) are single-frequency units, designed to operate in real-time. (2) are distinguished from (1) by having both a LCD display/command unit through which instructions and user-centered data is input, and a Differential GPS (DGPS) signal decoder.

(3) Dual-frequency receiver, collecting data for post-processing, has the highest accuracy, and are often used for surveying/geodetic-type applications. These are typically the most expensive class of GPS receiver.


Choosing a gps receiver cont2

Choosing a GPS receiver (Cont.)

  • To meet military objectives, the department of Defense can degrade the accuracy with which positions can be determined using GPS. This can be done by:

  • Deliberate introduction of errors in the satellite clocks, called selective availability. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

  • Encrypting the measurement signals such that only military authorized users can receive them, called antispoofing.


Choosing a gps receiver cont3

Choosing a GPS receiver (Cont.)

  • Other sources of GPS signal errors (garmin.com)

  • Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

  • Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.


Choosing a gps receiver cont4

Choosing a GPS receiver (Cont.)

  • Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

  • Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.


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