Global Positioning Systems (GPS)
Finding your position – Historical Efforts
– Greek philosopher, Erotoshenes (276 – 194 B.C) had dug
wells in a place called Syene to observe the sun’s rays and
tried to measure the size of the earth. He is credited with
devising a system of latitudes and longitudes.
– Ptolemy, had made use of latitudes and longitudes in his
conception of the world.
– As late as the 18th Century there was no international
standard for an accepted latitude and longitude network.
– The British has their own system, with the prime meridian
running through the Greenwich Observatory
– The French, as influenced by the Cassini Family, erected its
own Prime Meridian running through Paris.
– So, pinpointing a position had no real standards to base
from.
Erosthosthene’s method of measuring the size of the earth at Syene
The documentation
of the French Prime
Meridian that passes
through Paris.
Global Positioning Systems (GPS)
Finding your position – Historical Efforts
– Determining latitude at sea presents no great problem.
– The use of a sextant helps. It is an instrument for measuring
the angle of stars above the horizon. When the angle and
the time are measured, they can be used to calculate a
positional line on a sea chart.
– The sextant can also be used to take a sighting of the noon
sun to determine the degree of latitude.
– Since longitudes are not parallel lines, and they converge on
the poles, measuring longitude will need a different
approach.
– Determining longitude over land was not a problem if you
have a chronometer (the forerunner of a clock), but
determining longitude on a ship was much more difficult
because earlier chronometers will not work at sea because
of the motion of the ship.
Global Positioning Systems (GPS)
Finding your position – Historical Efforts
– Measuring longitude over land would simply be reading off
the difference in time from two different east west locations
because we do know that the globe is made up of 360
degrees of longitude and it takes 24 hours for the globe to
rotate 360 degrees.
– So, the difference in time can proportionally represent the
number of degrees.
– The problem was that chronometers do not work well at sea.
– Thus, sailors had no way of measuring which longitude they
are located at sea.
– To solve this problem, the British Parliament offered 20,000
pounds to anyone who can accurately measure longitudes
while at sea.
– A British chronometer maker, by the name of John Harrison,
invented a chronometer by using a counter-balancing dual
pendulum to compensate for the wave motion at sea.
– It worked, but Harrison was not given the award and died
without it; ultimately, it was awarded to his daughter.
John Harrison’s first
chronometer with its dual
pendulum mechanism to
counterbalance wave
actions. This
chronometer is now kept
at the Greenwich
Observatory.
John Harrison
1693-1776
John Harrison’s second
chronometer. Again, the
dual pendulum can still
be seen.
John Harrison’s third
chronometer.
John Harrison’s 4th
chronometer, which is
more compact and
resemble the modern
day pocket watch.
Fast forward to today and how we measure
latitude and longitude with GPS.
Global Positioning System (GPS)
• How GPS work
– Global Positioning Systems (GPS) is a navigation system
based on signals received from satellites initially launched
by the Department of Defense in 1978.
– It was originally called NAVSTAR (Navigation System with
Timing and Ranging).
– It was initially designed for use by the US military.
– It grew to a full constellation of 24 satellites in 1994.
– But in 1983 when President Reagan mentioned GPS at the
slip of his tongue, civilian mapmakers demanded the use of
these signals for mapping purposes.
– It was then GPS was opened up for use by civilian
mapmakers.
– The initial impact was that a better and more accurate
surveying and mapping datum was developed (this is the
North American Datum of 1983, or NAD83)
– As the technology of utilizing GPS signals developed and
matured, today’s society is reaping the endless list of
benefits that GPS has made possible.
Global Positioning Systems (GPS)
• How GPS work
– The 24 GPS
satellites circle
the earth twice a
day in three
groups of two
very near orbits
(totals 6) that
have been
precisely
determined to
ensure total
coverage
globally at any
one time.
Global Positioning Systems (GPS)
• How GPS work
– Each satellite is built to last 10 years; replacement satellites
are constantly built and launched into orbit
– Each satellite has a wing-span of solar panels and operates
with solar energy. But there are back-up batteries on board
to ensure continuous operation should there be no solar
power, as in solar eclipses or on the shadow side.
– Very accurate atomic clocks are carried on board these
satellites (estimated to be one second off in a million years).
– GPS signals are broadcasted every half second 24 hours a
day, under any weather conditions, covering the entire
surface of the earth, and with no subscription fee. (compare
with OnStar!)
– GPS receivers on the ground compares the time signal sent
by the satellites with the time signals received by the GPS
receiver. This difference is calculated by the GPS receiver
unit to determine how far away is the GPS satellite.
Global Positioning Systems (GPS)
• How GPS work
– The GPS receiver
must lock into position
with at least three
satellites to be able to
calculate 2-
dimensional position,
and at least four
satellites to calculate
3-D (including
elevation) position.
This process, based
on trigonometry, is
called trilateration.
http://electronics.howstuffworks.com/gps.htm
http://www.trimble.com/gps/index.shtml
YouTube video explaining how GPS works (3:35 min.)
http://www.youtube.com/watch?v=wi_3XwkA8cQ&feature=related
Global Positioning Systems (GPS)
• Accuracy Aspects of GPS
– Early civilian GPS accuracy were good to 50 meters. While
this accuracy is not good enough for mapping, it is good
enough for several applications, e.g. accidents with airbag
deployment and when ambulances arrive, they can certainly
see the accident within 50 meters (or 165 feet).
– During the Clinton administration, Selective Availability was
taken off, and GPS accuracy was then active to about 3
meters (slightly over 10 feet).
– In the meantime, the European Union launched its own GPS
satellites, called Galileo, and the Russians has its own GPS
satellites.
– Today, most GPS units have an accuracy of 1 meter.
– Survey grade GPS base stations, with built in differential
calculations can be as accurate as 1 cm.
– As accurate as these data can be, there is also error due to
atmospheric scatters and delay of time signals.
International collaboration in the development of GPS signal plan.
Global Positioning Systems (GPS)
• Accuracy Aspects of GPS
– These errors can be corrected and may be done by
professionals who needed superior accuracy.
– Garmin, a GPS receiver manufacturer, also has the option of
WAAS (Wide Area Augmentation System), an automatic
self-correction system built into some high end receivers.
– For most of the uses by the average citizen, I meter
resolution is accurate enough.
– For mapmakers, the process of Differential GPS is
important.
– Differential GPS is based on the use of a base station that
has a known and well-established position; data will be
broadcasted from the base station to be used by a GPS
receiver to verify the accuracy of signals received from the
satellites; somewhat like a dual process just to verify the
signal accuracies.
Global Positioning Systems (GPS)
• Uses of GPS
– One can certainly think of many different uses of GPS.
– It ranges from the most common use in automobile, aerial,
and marine navigation to keeping track of hiking trails to
locating restaurants and gas stations.
– But there are many more innovative uses of GPS simply
because GPS units are portable and works in real time.
– Today, GPS units can be integrated with hand-held
computers, blackberries, and even cell phones.
– The portability of GPS units makes it ideal for fieldwork
applications, e.g. fighting brush fires, ecological mapping,
archaeological finds, etc.
– But there are also advanced scientific developments that are
possible simply because of GPS.
Experiments with autonomous
(no drivers) navigation. The
team of winners of an annual
competition successfully
designed an automobile that
traveled over 130 miles of an
obstacle course in a desert
terrain, without a driver.
Some Resources:
http://www.navcen.uscg.gov/gps/
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
http://www.gpsy.com/gpsinfo/ (GPS Resource Library)
Using GPS with Google Earth – A User Guide:
http://earth.google.com/userguide/v4/ug_gps.html
GPS Visulaizer http://www.gpsvisualizer.com/
NASA GPS applications exchange: http://gpshome.ssc.nasa.gov/
VW google-based GPS http://www.cnet.com/gps.html