Introduction to GPS

1
Introduction to GPS

• GLY 560 GIS for Earth Scientists

2
What is GPS?

• The original intent of the Global Positioning
  System was to develop an all-weather, 24-hour,
  truly global navigation system to support the
  positioning requirements for the armed forces of
  the U.S. and its allies.
• First satellite launched in 1978
• The total investment by the U.S. military in the
  GPS system to date is well over 10
  BILLION!

3
What is GPS?

• Although the primary goal is to provide
  positioning capabilities to the U.S. armed forces
  and its allies, GPS is freely available to all
  users.
• The number of civilian users is already far
  greater than the military users, and the
  applications are growing rapidly.
• The U.S. military however still operates several
  "levers" with which they control the performance
  of GPS.

4
How GPS Works

• Consider two ways of determining ranges

One Clock
Two Clocks
5
Advantages of One-Way Ranging

• Receiver doesnt have to generate signal, which
  means
• We can build inexpensive portable receivers
• Receiver cannot be located (targeted)
• Receiver cannot be charged

6
Determining Range (Distance)

• Measure time it takes for radio signal to reach
  receiver, use speed of light to convert to
  distance.
• This requires
• Very good clocks
• Precise location of the satellite
• Signal processing over background

7
Determining Position

• To determine position in 3-D, we need 3
  satellites for triangulation

Once we have position narrowed to 2 possible
points, we can usually throw one away as
nonsense
8
The Clock Problem

• To measure distance from speed of light we need a
  VERY accurate clock(clock error of 1/100 sec
  distance error of 1820 miles!).
• GPS Satellites have very accurate atomic clocks.
• Our receivers do not have atomic clocks, so how
  can we measure time with necessary accuracy?

9
Psuedo-Random Code

• GPS satellites and receivers communicate via
  pseudo-random-code (PRC) signals.
• PRC has three advantages
• Enhances signal over background
• Allows synchronization of satellite and receiver
  clocks
• Military can change the code and switch of system
  if necessary

10
PRC Signal Amplification

• Uses correlation of peaks between generated
  random signal and truly random background noise
  to enhance signal
• Allows receiver to work without a big satellite
  dish!

11
PRC Synchronization

• GPS receiver generates the same PRC as satellite,
  i.e. they start counting at the same time.

• By determining how far off the satellite and
  receiver are in their counting, determines
  difference in time it took for signal to reach
  receiver.

12
PRC Synchronization

• How do we assure satellite and receiver start
  counting at same time, i.e. clocks are
  synchronized?

• The trick is to use a 4th satellite to
  over-specify position. This allow timing to be
  corrected by the receiver

13
Pseudo-Range (2-D Example)

• If clocks were perfect, 2 satellites would locate
  position
• If clocks are off, range is off

14
Pseudo-Range

• We add a 3rd satellite to over-specify position
• There is only one combination of wrong times
  for which all 3 ranges converge.
• Receiver varies clock times until all satellites
  agree

15
Satellite Position

• Must know position of satellite to determine
  receiver location
• Satellites are put in precise orbit
• Satellite's orbit or "ephemeris is monitored by
  DOD and transmitted to satellite

16
Atmospheric Correction

• GPS signal slowed down through the charged
  particles of the ionosphere and then through the
  water vapor in the troposphere

• Must correct for atmospheric effects with modeling

17
GPS Constellation

• Must have at least 4 satellites overhead to
  determine position

18
NAVSTAR

• Current GPS System is NAVSTAR. There are 4 GPS
  satellite constellations in existence
• Block I satellites were the experimental
  satellites launched between 1978 and 1985 used to
  test the system. Eleven (11) were launched, none
  functioning.
• Block II satellites comprise the first nine
  spacecraft of the operational series.
• The Block IIA satellites comprise the second 19
  spacecraft of the operational series.
• The Block IIR satellites comprise the replacement
  series.

19
Where are the Satellites?

• Orbit is high enough to avoid earth gravity
  perturbations, low enough to pass correction
  stations 1 per day.
• Orbital period of GPS satellites is 12 hours

20
Satellite Overhead Schedule
21
Orbital Period and Altitude
length of a sidereal day
http//liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JT
rack3D.html
22
GPS Accuracy
"The decision to discontinue Selective
Availability is the latest measure in an ongoing
effort to make GPS more responsive to civil and
commercial users worldwideThis increase in
accuracy will allow new GPS applications to
emerge and continue to enhance the lives of
people around the world.


President Bill Clinton, May 1, 2000

• Between 1st and 3rd May 2000, the National
  Geodetic Survey/NOAA compared the accuracy of GPS
  determined navigation positions at its Continuous
  Reference Station with and without Selective
  Availability.
• With SA turned on, 95 of solutions were within a
  radius of 45 meters
• With SA turned off, 95 of the estimated
  (horizontal) positions were within 6.3 meters.

23
Accuracy of GPS
Autonomous Accuracy 15 - 100 meters 
Differential GPS (DGPS)  Accuracy 0.5 - 5 meters
Real-Time Kinematic Float (RTK Float)  Accuracy  20cm - 1 meter
Real-Time Kinematic Fixed (RTK Fixed) Accuracy 1cm - 5 cm

24
GPS Signals
GPS satellites broadcast on three different
frequencies, and each frequency (or career wave)
has some information or codes on it. You can
think of it as three different radio stations
broadcasting several different programs. The
table below lists the signals and the contents
L1 Career L2 Career L3 Career
19 cm wavelength 24 cm wavelength Data not available
1575.42 M Hz 1227.6 M Hz Data not available
C/A Code P Code Data not available
Navigation Navigation Message Data not available

• P Code Reserved for direct use only by the
  military
• C/A Code Used for rougher positioning
• For Single frequency use only L1 career is used
• For Double frequency, L1/L2/L3 career is used
• The navigation message (usually referred to as
  the ephemeris) tells us where the satellites are
  located, in WGS-84.

25
Different types GPS locations

• Autonomous Positions(C/A signal, 5-15 m
  accuracy)
• Real-Time Differential GPS(C/A signal, 0.5-5 )
• Real-Time Kinematic (RTK) Float(C/A and Carrier,
  0.2-1 m)
• Real-Time Kinematic (RTK) Fixed(C/A and Carrier,
  1-5 cm)

26
Differential GPS (DGPS)

• Error due to signal transmission through the
  atmosphere can be corrected using DGPS
• Atmospheric errors are the same over short
  distances.
• Error in base station, can be removed from remote
  (roving) receiver position, and code phase signal.

27
Code vs. Carrier Phase

• Satellites generate Code Phase and Carrier Phase
  signals.
• Code phase is used by hand-held GPS
• Carrier phase used by surveying instruments,
  navigational systems

28
Where to Get Differential Corrections

• The United States Coast Guard and other
  international agencies are establishing reference
  stations all over especially around popular
  harbors and waterways.
• Anyone in the area can receive these corrections
  and radically improve the accuracy of their GPS
  measurements. Most ships
  already have radios capable of tuning the
  direction finding beacons, so adding DGPS will be
  quite easy.
• Many new GPS receivers are being designed to
  accept corrections, and some are even equipped
  with built-in radio receivers.

29
Differential Code GPS (Navigation)

• Differential corrections may be used in real-time
  or later, with post-processing techniques.
• Real-time corrections can be transmitted by radio
  link. The U.S. Coast Guard maintains a network of
  differential monitors and transmits DGPS
  corrections over radio beacons covering much of
  the U.S. coastline.

30
RTK (Differential Carrier GPS)

• RTK is based on using many (5 satellites) to
  resolve timing.
• Produces very accurate measurements because using
  carrier phase.
• Requires advance tracking of satellites, and
  better signal resolution (bigger antennae and
  more power)

31
Future of GPS

• Soon, the U.S. Federal Communications Commission
  will require location determination technology in
  cellular phones for use in emergencies as part of
  their enhanced 911 service.
• Future plans for improving the accuracy of GPS
  include the launching of eighteen additional
  satellites that are awaiting launch or are
  currently in production.
• Two new signals will be broadcast from the
  satellites by 2005, to help bypass any distortion
  from the ionosphere.