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Data input 1: GPS


... are five monitor stations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, ... Mapping creating maps based on those positions. Timing precision ... – PowerPoint PPT presentation

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Title: Data input 1: GPS

------Using GIS--
Introduction to GIS
  • Lecture 14
  • Data input 1 GPS
  • By Austin Troy
  • University of Vermont
  • Many materials for this lecture adapted from
    Trimble Navigation Ltds GPS Web tutorial at

Data input
Introduction to GIS
  • Over the next several lectures well talk about
    data input sources and methods for GIS. This will
  • GPS (today)
  • Scanning and digitizing
  • Geocoding
  • Publicly available GIS data sources
  • Remote Sensing

Introduction to GIS
  • Stands for Global Positioning System
  • GPS is used to get an exact location on the
    surface of the earth, in three dimensions.
  • GPS is a very important data input source, used
    for surveying, military operations, engineering,
    vehicle tracking, flight navigation, car
    navigation, ship navigation, unmanned vehicle
    guidance, agriculture, and of course, mapping
  • For mapping, a GPS tells us where and allows us
    to input what

Introduction to GIS
  • GPS is a worldwide radio-navigation system formed
    from 24 satellites and their ground stations.
  • Uses satellites in space as reference points for
    locations here on earth
  • Ground stations help satellites determine their
    exact location in space. There are five monitor
    stations Hawaii, Ascension Island, Diego Garcia,
    Kwajalein, and Colorado Springs.

How does GPS work?
Introduction to GIS
  • GPS derives position relative to satellite
    reference points, using triangulation
  • The GPS unit on the ground figures its out
    distance to each of several satellites using the
    time it takes for a radio signal to travel to the
  • To do this, the exact position of the satellites
    at a given time, must be known otherwise they
    cant serve as reference points

How does GPS work?
Introduction to GIS
12,500 km
11,200 km
11,500 km
How does GPS work?
Introduction to GIS
  • We need at least 3 satellites as reference points
    to triangulate our position.
  • Based on the principle that where we know our
    exact distance from a satellite in space, we know
    we are somewhere on the surface of an imaginary
    sphere with radius equal to the distance to the
  • With two satellites we know we are in the plane
    where the two intersect. With three or more, we
    can get two possible points, and one of those is
    usually impossible from a practical standpoint
    and can be discarded

How does GPS work?
Introduction to GIS
  • Heres how the sphere concept works
  • A fourth satellite narrows it from 2 possible
    points to 1 point

Source Trimble Navigation Ltd.
How does GPS work?
Introduction to GIS
  • This method assumes we can find exact distance
    from our GPS receiver to a satellite. How does
    that work?
  • Simple answer see how long it takes for a radio
    signal to get from the satellite to the receiver.
  • Since we know speed of light, we can answer this
  • This gets complicated when you think about the
    need to perfectly synchronize satellite and
  • A tiny error in synchronization can result in
    hundreds of meters of positional error

How does GPS work?
Introduction to GIS
  • The difficult part is measuring travel time,
    because the amount of time elapsed is tiny (about
    .06 seconds for an overhead satellite), and we
    require a way to know precisely WHEN the signal
    left the satellite
  • To do this requires comparing lag in exactly
    similar patterns, one from satellite and one from
  • Analogy, going to a stadium, sitting 1000 feet
    from the speaker and pressing play on your
    handheld tape player containing REO Speedwagon at
    exactly the same time as the guy in the sound
    booth presses play for that same song.

How does GPS work?
Introduction to GIS
delayedI cant fight this feeling any more,
Local I cant fight this feeling any more,
  • Only, instead of using cheesy eighties rock power
    ballads, GPS uses something called pseudo-random
  • This code has to be extremely complex (hence
    almost random), so that patterns are not linked
    up at the wrong place on the codethat would
    generate the wrong
    time delay
    and hence the wrong distance

Source Trimble Navigation Ltd.
How does GPS work?
Introduction to GIS
  • So how do we know that the two Speedwagon fans
    are pressing play at exactly the same time? Do
    Speedwagon fans all think alike? Hardly.
  • We must assume that satellite and receiver
    generate signal at exactly the same time if
    theyre off by 1/1000th of a second, that means
    200 m of error
  • The satellites have expensive atomic clocks that
    keep perfect timethat takes care of their end.
  • But what about the ground receiver?

How does GPS work?
Introduction to GIS
  • Here is where the fourth satellite signal comes
  • While 3 perfect satellite signals can give a
    perfect location, 3 imperfect signals cant, but
    4 can
  • Imagine time to receiver as distance, with each
    distance from each satellite defining a circle
    around each satellite of that radius
  • If receiver clock is correct, 4 circles should
    meet at one point. If they dont meet, the
    computer knows there is an error in the clock
    They dont add up

How does GPS work?
Introduction to GIS
  • Dotted lines represent real distance, and solid
    lines represent erroneous distance, based on
    clock errorthey dont meet. Notice here we used
    three circles, because were looking in 2D, but
    in reality (3D) this represents four satellites,
    or four circles
  • Assuming the clock error affects all measurements
    equally, the computer can then simply apply a
    correction factor that makes circles meet in one

Source Trimble Navigation Ltd.
How does GPS work?
Introduction to GIS
  • The receiver then knows the difference between
    its clocks time and universal time and can apply
    that to future measurements.
  • Of course, the receiver clock will have to be
    resynchronized often , because it will lose or
    gain time
  • This is one reason why a GPS receiver needs at
    least four channels to get four signals

How does GPS work?
Introduction to GIS
  • So now we know how far we are from the
    satellites, but how do we know where the
    satellites are?? We cant use them as a reference
  • Because the satellites are 11,000 ft up, they
    operate according to the well understood laws of
    physics, and are subject to few random, unknown
  • This allows us to know where a satellite should
    be at any given moment.

How does GPS work?
Introduction to GIS
  • There is a digital almanac on each GPS receiver
    that tells it where a given satellite is supposed
    to be at any given moment.
  • While the positions can be predicted very
    accurately based on simple mathematics, the DOD
    does monitor them using precise radar, just to
    make sure.
  • These errors are called ephemeris and are
    caused by gravitational pull of other celestial
  • That info is relayed to the satellite, which
    transmits the info when it sends its pseudo
    random code.

How does GPS work?
Introduction to GIS
  • Even after all this, there are still many factors
    that can generate errors and reduce positional
  • One of the biggest error sources is the fact that
    the radio signal does not travel at the
    exact speed of light in
    parts if the atmosphere as it does
    in the vacuum of space.
  • This can be partly dealt with using predictive
    models of known atmospheric behavior

Source Trimble Navigation Ltd.
How does GPS work?
Introduction to GIS
  • Signals also can bounce off features, like tall
    buildings, cliffs and mountains, resulting in
    multipath error, where a direct signal hits,
    followed by a bunch of bounced signals which
    can confuse the receiver.
  • Good receivers have algorithms that can deal with
    this by determining what counts as a multi-path
    signal and choosing the first one as the signal
    to use
  • There are other errors as well, resulting from
    things like ionospheric distortions and satellite

How does GPS work?
Introduction to GIS
  • Until May of 2000, the DoD intentionally
    introduced a small amount of error into the
    signal for all civilian users, calling it
    selective availability, so non- US military
    users would not have the same positional accuracy
    as the US military.
  • SA resulted in about 100 m error most of the time
  • Turning off SA reduced error to about 30 m radius
  • Here is Clintons letter http//

Differential GPS
Introduction to GIS
  • This is a way to dramatically increase the
    accuracy of GPS positioning to a matter of a few
    meters, using basic concepts of geometry
  • This was used in the past to overcome SA, but
    with that gone, is now used for reducing the 30m
  • DGPS uses one stationary and one moving receiver
    to help overcome the various errors in the signal
  • By using two receivers that are nearby each
    other, within a few dozen km, they are getting
    essentially the same errors (except receiver

Differential GPS
Introduction to GIS
  • DGPS improves accuracy much more than disabling
    of SA does
  • This table shows typical errorthese may vary

Source http//
How does DGPS work?
Introduction to GIS
  • The stationary receiver must be located on a
    control point whose position has been accurately
    surveyed eg. USGS benchmarks
  • The stationary unit works backwardsinstead of
    using timing to calculate position, it uses its
    position to calculate timing
  • It determines what the GPS signal travel time
    should be and compares it with what it actually
  • Can do this because, precise location of
    stationary receiver is known, and hence, so is
    location of satellite

How does DGPS work?
Introduction to GIS
  • Can do this because, precise location of
    stationary receiver is known, and hence, so is
    location of satellite
  • Once it knows error, it determines a correction
    factor and sends it to the other receiver.

How does DGPS work?
Introduction to GIS
  • Since the reference receiver does not know which
    satellites the mobile receiver is using, it sends
    a message to it telling the correction factor for
  • It used to be that only big companies and
    governments could use DGPS because they had to
    set up their own reference receiver station
  • Now there are many public agencies that maintain
    them, especially the Coast guard these stations
    broadcast on a radio frequency, which GPS
    receivers with a radio receiver can pick up

How does GPS work?
Introduction to GIS
  • DGPS now or later?
  • If you dont need the exact DGPS measurement at
    the moment you take a GPS measurement in the
    field, you can correct your data later.
  • All you need to know is the roving receivers
    measured positions and the exact times they were
  • At a later time, the reference receivers
    corrections can then be integrated with the
    roving GPS data based on the time and location
    no radio link needed for this

Surveyor DGPS
Introduction to GIS
  • There are even more accurate types of DGPS that
    surveyors use
  • These are accurate to a matter of millimeters
  • This uses a very involved method that wont be
    discussed here
  • One of the techniques they use though,
    carrier-phase GPS is beginning to make its way
    into consumer GPS
  • Use carrier-phase signal, which is much smaller
    cycle widths than the standard code phase signal

Aviation DGPS
Introduction to GIS
  • FAA is implementing DGPS for the continent, so
    all planes can get extremely accurate GPS
    navigation, called Wide Area Augmentation System
  • They have installed 25 ground reference stations
    as well as a master ground station that almost
    instantaneously processes and sends out satellite
  • Improves error to 7 m and, when finished, will
    allow GPS to be used as primary navigational tool
    for Category I landings, where there is some
  • Soon, it will allow zero-visibility landing

GPS Uses
Introduction to GIS
  • Trimble Navigation Ltd., breaks GPS uses into
    five 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
  • You can learn about all these applications at
    these web links, but we mainly care about mapping

GPS Uses
Introduction to GIS
  • The uses for GPS mapping are enormous. Here are
    just a few examples
  • Centerlines of roads
  • Hydrologic features (over time)
  • Bird nest/colony locations (over time)
  • Fire perimeters
  • Trail maps
  • Geologic/mining maps
  • Vegetation and habitat