Introduction to GPS

GLY 560: GIS for Earth Scientists

04/20/23 GLY560: GIS

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!

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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.

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How GPS Works

Consider two ways of determining ranges:

One Clock Two Clocks

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Advantages of One-Way Ranging

Receiver doesn’t have to generate signal, which means

•We can build inexpensive portable receivers

•Receiver cannot be located (targeted)

•Receiver cannot be charged

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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

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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”

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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?

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Psuedo-Random Code

• GPS satellites and receivers communicate via pseudo-random-code (PRC) signals.

• PRC has three advantages:1. Enhances signal over background

2. Allows synchronization of satellite and receiver clocks

3. Military can change the code and switch of system if necessary

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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!

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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.

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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

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Pseudo-Range (2-D Example)

• If clocks were perfect, 2 satellites would locate position

• If clocks are off, range is off

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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

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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

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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

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GPS Constellation

•Must have at least 4 satellites overhead to determine position

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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.

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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

ellipse orbital of axis major-semia

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2

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Satellite Overhead Schedule

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Orbital Period and Altitude

* length of a sidereal day

http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

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GPS Accuracy

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.

"The decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide…This 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

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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

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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

C/A Code P Code

Navigation Navigation Message

•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.

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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)

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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.

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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

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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.

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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.

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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)

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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.