gis

1. Navigation and the Global Positioning
System (GPS)

2. Few changes of great importance to economics and safety
have had more immediate impact and less fanfare than
GPS.
The Global Positioning System:
• GPS has quietly changed everything about how we locate objects
and people on the Earth.
• GPS is (almost) the final step toward solving one of the great
questions of human history.
Where the heck are we, anyway?

3. In the Beginning…….
• To understand what GPS has meant to navigation it is necessary
to go back to the beginning.
• A quick look at a ‘precision’ map of the world in the 18th century tells
one a lot about how accurate our navigation was.
• People have been trying to measure latitude and longitude for a long
time, with varying success.

4. Tools of the Trade:
• Navigator’s early tools could only crudely estimate location.
• A Sextant can
measure the elevation
of something above the
horizon (Polaris, the
Sun...) This can be
used to find Latitude.
• The Compass
could provide you
with a
measurement of
your direction
• Distance could sort
of be measured, by
counting steps (or
wheel rotations) or
estimating speed and
time traveled
• Longitude could only be estimated by dead reckoning.

5. • To determine one’s East-West position, the accepted method was
called Dead Reckoning (perhaps aptly named).
• Dead Reckoning has many sources of error that add up over a
long journey. Even 95% accuracy in crossing from New York to
London will accumulate to 175 MILES of error at the end of the trip.
Dead Reckoning:
• These kinds of error were a Serious problem for ships approaching
rocky coasts or areas with submerged shoals and seamounts!

6. The early tools had many levels of uncertainty that were
cumulative in producing poor maps of the world.
Plenty of Weaknesses:
• Step or wheel rotation counting has an obvious built-in uncertainty.
• The compass relies on the assumption that the North
Magnetic pole is coincident with North Rotational pole (it’s
not!) and that it is a perfect dipole (nope…).
Navigation on land was helped by the availability of landmarks, places
that could put context to a map and help calibrate a journey.
Such a technique is worthless at Sea….

7. Without any question THE most important maritime dilemma of
the Renaissance world was how to determine Longitude.
Navigation at Sea:
• A sextant can be used to give latitude very effectively at sea.
• A compass can give you a good idea of your direction.
• But unless you can determine how far East/West you’ve gone….
This Will Eventually Turn into THIS!

8. On October 22, 1707, a dead reckoning error by the fleet of
Admiral Sir Clowdisley Shovel led to the death of 2000 sailors.
Longitude!
• In 1717, Queen Anne authorized a prize of 20,000 £ to anyone who
could maintain knowledge of longitude to ½ degree (the equivalent of
30 miles on the equator).
• Almost all of the methods proposed for solving this problem centered
on the 4th element of navigation we haven’t talked about
much…….TIME.
Why is Time so important?

9. • Your Meridian is nothing more than a circle on the Earth that
goes through the North and South Poles and your position.
Meridians and Longitude:
• The Prime Meridian is the meridian that
goes through an agreed upon zero point.
• The angle going west from Greenwich to
your meridian is your LONGITUDE!
• The Prime is located
today in Greenwich,
England.

10. Longitude and Time:
• It turns out that while there may be no landmarks on the ocean,
there are fixed reference points…the stars.
• As the Earth turns, the stars pass by overhead. Each star crosses
every meridian on Earth exactly once each day.
So how do longitude and time relate?
• So the difference between the time a
star crosses the prime and your
meridian is your longitude.
• The problem then comes down to
knowing what time it is at the prime
meridian…exactly.
• Every 4 minutes of error equals 1
degree or 60 miles on the equator.

11. To win the longitude prize one had to be able to maintain accurate
time to within 2 minutes over a several months at sea.
Telling Time:
• Galileo couldn’t win the prize (he was dead), but he had devised a
way of determining the time using the moons of Jupiter.
People found several ways to do this.
Galileo’s Notebook
Galileo Galilei, 1564-1642

12. • The four readily visible (Galilean) moons of
Jupiter have a (complex) predictable cycle
which can be used to tell time
Telling Time With The Moons of Jupiter
• This actually worked well, but only for that part of the
year when Jupiter was visible at night!

13. Benefits of Longitude

14. The Lunar Method:
• The astronomers Tobias Mayer and Nevil Maskelyne proposed
using the predictable changes in the distance to the moon.
• This also worked, but was VERY hard to do correctly
and didn’t work when the moon was less than ½ full.
• “They had all used the procedure, many
times, they said, just as it was outlined by
Maskelyne in The British Mariner’s Guide,
and they always managed to compute their
longitude in a matter of a mere four hours.”

15. John Harrison’s Clock:
• John Harrison went after the prize by building accurate
clocks that could survive the weather extremes and motion
of ship travel.

16. John Harrison’s Clocks:
• Between 1736 and 1764 John Harrison
produced 4 clocks for the Board of
Longitude (the group that held the prize).
• Because Nevil Maskelyne was chair of the Board of Longitude…
• Each clock was smaller and more
accurate than the previous one. And they
ALL met the condition for the longitude
prize. None were accepted!
• So why were they locked in an observatory instead of saving lives on
ships during this time?
• It would take an act of King George III to break the logjam and put
chronometers into wide use.

17. John Harrison’s 4th Clock

18. Harrison’s clock changed navigation in a fundamental way.
The (FIRST) Global Positioning System:
• Anyone with a sextant and a chronometer could find their position
to within a few miles on the Earth.
• The clocks were incredibly expensive and in fairly short supply.
They weren’t perfect though
• Since they relied on Greenwich time, they had to be re-calibrated
to Greenwich: Usually in Greenwich.
• Positions could only be determined at sunrise or sunset when both
a star and the horizon could be seen.
• It didn’t work at all if the weather was cloudy…..

19. Harrison’s clock (and its successors) made navigation
possible for commercial shipping and the well to do, but it
wasn’t for the masses.
A Modern Solution:
• A universal navigation system would need the following.
• A way to tell time that isn’t expensive.
Enter the MODERN Global Positioning System (GPS):
• A set of references that didn’t disappear whenever it was
cloudy
• A clock that can be calibrated anywhere, not just in
Greenwich.

20. For all its complexity, GPS still comes down to the same
set of requirements that Harrison faced.
The Global Positioning System:
• Find out the time.
• The reference points also serve as the clock.
GPS adds a pair of twists:
• Find the reference points.
• Use the output to determine Longitude and Latitude to high
precision.
• Everyone uses the same system. The GPS network is effectively
a single device, like Harrison’s clock.

21. The GPS system consists of 3 elements.
Parts of the GPS Network:
• GPS ground support.
• GPS satellites.
• GPS receivers.

22. The first GPS Satellite was launched in 1978.
The GPS Satellite System:
• To function properly the network of satellites must contain 24 units.
With GPS it is all about coverage.
• Each satellite has a 12 hour orbit, which means it passes over the
same place twice each day.
• There are 6 orbit ‘planes’ inclined by
55° to the equator and rotated for a 60°
spacing between them.
• Each plane contains 4
satellites.

23. The first GPS Satellite was launched in 1978.
The GPS Satellite System:
• A Ground Track map shows
how this scheme covers the
Earth.

24. The GPS satellites are nothing more than atomic clocks that
work because we know where and when they can be found.
The GPS Ground Support:
• To maintain the satellites requires a ground support network (called
the control segment - CS) that uplink time and radar tracking position
data to the satellites (called the space vehicles – SV).
• There are 5 CS components that update satellites and also
communicate data to some advanced GPS reveivers

25. The receiver units are the backbone of the mass market
GPS. The receivers serve 3 purposes.
The GPS Receiver:
• Small handheld units receive signals from 4 or more SVs. The
receivers are called the User Segment -US. Decoded signals provide
X, Y, Z, and T. Everyone can find out where they are.
• Via SV-US communication, the exact time can be synchronized
worldwide in a matter of a few seconds.
• By combining signals from nearby
receivers, very accurate navigation
and surveying data can be obtained.

26. The Method of GPS:
• How does GPS work?.
• Your GPS receiver gets signals from satellites that are doing nothing
more than repeating the time over and over.
• Since light travels at a finite speed, there will be a difference
between the instantaneous time on your receiver and the time you
get from the satellite.
Time Difference = (Speed of Light) x (Distance)
• The satellites know exactly where they are in X, Y, Z. So, if you
have a bearing to three of them, then you know as well.
• The time differences are small. 1000 feet of distance translates to
only a 1,000,000th of a second!

28. But wait!
• There’s a caveat to this. YOU don’t carry an atomic clock. They
are expensive and heavy. YOUR receiver clock is going to be off a
some random amount when it compares time with the satellites.
How do we get around this?

29. An atomic clock in your pocket
We add a 4th measurement!
• If we knew the time, a 4th
satellite would be redundant.
• However, the extra satellite
can ONLY match up with the
other three at the CORRECT
time. It removes the error and
calibrates your receiver at the
same time!!!
• This technique lets your GPS
unit know the time to within ~100
ns (0.0000001 seconds).

30. • ESS205 flights use
the postage-stamp
sized Trimble
Lassen-SQ GPS
• How much for this
incredibly
advanced
technology? $50!
GPS is amazing