Lecture and lab scheduleLecture: 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

Figure out where you are and where you’re going

Navigation and positioning are crucial to many activities

GPS basics

• 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 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.

Satellites

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

Triangulating

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

1sec

Receiver signal

Satellite signal

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

3. Getting perfect timing

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

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

Receivers (User Segment)

Ground stations (control segment)

Five control stationsmaster station at Falcon (Schriever) AFB, Coloradomonitor satellite orbits & clocksbroadcast orbital data and clock corrections to satellites

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

Ground Stations (control segment)

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.

Selective Availability (SA)

Error Breakdown (typical case):

satellite clock:satellite orbit:ionosphere/troposphere:multipath distortion:receiver errors:

1.5 meters2.5 meters5.5 meters0.6 meters0.3 meters

GPS - Error Correction

2 Methods:Point AveragingDifferential Correction

GPS - Point Averaging

AveragedLocation

•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.

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

GP

S

Reference Receiver sits overPrecisely surveyed point

Error signals

GPS - Differential CorrectionDifferential 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

~ 300 miles (~ 480 km) or less

Base station (known location) Rover receiver

How it works:

• use a base station at 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

Differential correction

GPS - Differential CorrectionThe base station knows its own locationIt compares this location with its location at that moment obtained using GPS satellites, and computes errorThis known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment

Time GPS Lat GPS Long Lat. error Long. error3:12.53:13.03:13.53:14.03:14.53:15.0

35.5035.0534.9536.0035.3535.20

79.0578.6579.5580.4579.3079.35

.5

.05-.051.0.35.20

.5-.35.551.45.30.35

Example: Base Station File

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 iQue 3600 PDA:

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

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 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 receiverThe 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

(1) 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 (Cont.)

Etrex from Garmin – 500 points, $145.71

12XL from Garmin –

City point database, $309.07 , area calculation

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 (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:

(1) 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.

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

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 (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.