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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

GPS basics

  • Figure out where you are and where you’re going
  • Navigation and positioning are crucial to many activities

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

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


• 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.

how it works
How it works?

1. Triangulating

2. Distance measure

3. Getting perfect timing

4. Satellite position


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



Three distances = two points

  • Intersection of Four spheres = one point
  • Note:
  • 4th measurement not needed
  • Used for timing purposes instead

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


Satellite signal

Receiver signal


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

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

System as a whole consists of three segments

  • satellites (space segment)
  • receivers (user segment)
  • ground stations (control segment)

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

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

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

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.

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.

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



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


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

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



GPS Long

Lat. error

Long. error































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).


Garmin’s iQue 3600 PDA:


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.