28.03.
2014
Global Positioning System
GPS
Chapter Ten
GPS
Lecture by:
Alemu Nebebe
What is GPS Why Use GPS?
• The Global Positioning System, or GPS, can show you your exact
position on Earth any time, anywhere, in any weather. The system
consists of a constellation of 24 satellites (with about 3 to 6 "spares") Location - to determine a basic position.
that orbit 11,000 nautical miles above Earth’s surface and
continuously send signals to ground stations that monitor and
Navigation – to get from one location to another.
control GPS operations
• GPS satellite signals can also be detected by GPS receivers, which
calculate their locations anywhere on Earth within less than a meter Tracking – to monitor the movement of people and things.
by determining distances from at least three GPS satellites. No other
navigation system has ever been so global or so accurate.
Mapping – to create maps of the world.
Timing – to bring precise timing to the world.
GPS antennas point their transmission
to the center of the Earth
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Principle of GPS Elements of GPS
• GPS has three parts: the space segment, the user segment, and the
control segment.
The space segment consists of a constellation of 24 satellites (and
about six "spares"), each in its own orbit 11,000 nautical miles
above Earth. The user segment consists of receivers, which you
can hold in your hand or mount in a vehicle, like your car. The
control segment consists of ground stations (six of them, located
around the world) that make sure the satellites are working properly.
The master control station at Schriever Air Force Base,, near
Colorado Springs, Colorado, runs the system.
Methods of GPS Data Calculating a GPS Position (continued)
The pseudorange of satellite with respect to a receiver
Acquisition is its apparent distance to the receiver, computed from
the time delay with which its radio signal is received.
1. Static
2. Rapid-Static
3. Real-Time Kinematic(RTK)
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GPS positioning: A simple principle
Principle of GPS positioning: Satellite 1 sends a signal at time te
Receiver position
Ground receiver receives it signal at time tr
The range measurement ρ1 to satellite 1 is: ρ1 = (tr-te) x speed of light General procedure:
We are therefore located on a sphere with Radius ρ1 centered on satellite 1 1. Acquire one satellite to get time and almanac
In simple mathematical terms:
GPS receivers:– 2. Acquire 2 other satellites to get 2-D position
Measure tr
Decode te 3. Acquire 4th satellite to get 3-D position, synchronization bias of
Compute ρrs receiver clock (x,y,z,∆t)
– If the position of the satellites in an Earth-fixed frame (Xs, Ys, Zs) is known,
– Then one can solve for (Xr, Yr, Xr) (if at least 3 simultaneous
4. Acquire any other visible satellite
range measurements)
c = 299 792 458 m/s
Satellite Dilution of Precision (DOP)
GPS errors
@ The Dilution of Precision is a measure of the strength of satellite geometry and is Examples include:
related to the spacing and position of the satellites in the sky.
1. Receiver Clock Errors – receiver clocks are not as accurate as the atomic
@ Four types of DOP Dimensions: clocks onboard the GPS satellites and therefore may have slight timing
errors.
1. HDOP –Horizontal Only
2. VDOP –Vertical Only 2. Ephemeris Errors – satellite health status, current date and time.
3. PDOP –Positional in 3D
4 . GDOP –Geometric in 3D and Time 3. Receiver Noise - from other radios broadcasting on the same or similar
frequencies in the area, etc.
@ Th
The most useful
f l DOP is
i the
h GDOP,
GDOP since
i this
hi a combination
bi i off allll the
h
factors…vertical, horizontal, 3D and time. 4. Ionosphere and Atmospheric delays – occurs as the satellite frequencies travel
through the atmosphere to the GPS receivers.
@ The lower the DOP value, the better the satellite geometry. 5. Multipath – occurs when the GPs signal is reflected off of objects such as tall
buildings.
Good Geometric 6. Poor DOP – poor satellite geometry and visibility.
Dilution of
Precision 7. Selective Availability (SA) – occurred when the DoD intentionally degraded the
Poor Geometric accuracy of GPS positions by around 300 feet (!) by introducing artificial clock
Dilution of Precision and code errors into the broadcasted radio frequencies.
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GPS Limitations
GPS cannot be used indoors, around a bunch of tall
buildings, or in heavily wooded areas.
GPS requires a clear view to at least 4 satellites (5 for sub meter work).
Remove one or more satellites out of y
your triangulation
g equation
q and yyour
results (solutions) change.
A clear view to at least 4 satellites
2. Differently Corrected GPS (DGPS) DGPS Correction Options
(Single Frequency)
Several options are available for obtaining differential GPS corrections:
DGPS requires that your GPS unit receives “corrected” positional information 1.Use another GPS receiver as a stationary base that is mounted on a
that is broadcasted from another stationary receiver and/or geostationary known point, yet can be moved to other known points as needed.
satellite (i.e. a known point), in addition to the information already being
received from the NAVSTAR satellites. 2.Use a CORS site (Continuously Operating Reference Station) which
is a permanently mounted GPS receiver/transmitter.
Since this secondary stationary receiver/satellite “knows” where it is, it can compute
the errors in its position and apply them to any number of roving receivers in the same 3.U.S. Coast Guard Beacon Ground Network –a network of land-
general area. based correctional broadcast towers,
towers usually near major navigable
bodies of water. Sends out a single frequency signal only.
* Advantage: Free of Charge
* Disadvantage: Requires a beacon receiver and antenna;
coverage is limited to within ~60 miles of
the beacon station.
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DGPS Correction Options, continued Reference Stations Network
4.Use other satellite systems (in addition to NAVSTAR) to receive position corrections.
Examples include:
a. WAAS–(Wide Area Augmentation System) –established by the Federal Aviation
Administration (FAA) and is free of charge. Although not officially complete, many GPS
manufacturers are incorporating WAAS capabilities into newer, lower-cost receivers and is
currently available for civilian use within the United States.
WAAS is made up of two geostationary satellites that broadcast corrected GPS data and 25
ground monitoring stations that are positioned across the United States to monitor the
satellite data and upload corrections to the satellites.
A good WAAS information website: www.garmin.com/aboutGPS/waas
b. RACAL Land-Star-commercially owned geostationary satellite system with ground
monitoring
stations; available in over 40 countries; requires an approximate $800/year subscription and
a proprietary Racal receiver and GPS antenna.
Racal information website: www.navtechgps.com/supply/racal
c. Omni-Star–essentially the same as the Land-Star but just a different manufacturer.
CORS Reference Stations Network
(cont..)
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Examples of RTK GPS
Examples of GPS Base Stations for DGPS Corrections
measurement
GPS base receiver CORS U.S. Coast Guard
mounted on a known permanently lighthouse with
position, but capable mounted beacon antenna.
of moving from point GPS base
to point as needed. receiver.
3. Differential Phase Position GPS
(Dual Frequency)
GPS Limitations
@ Requires the use of at least two dual-frequency L1/L2
receivers…a base and a rover.
GPS cannot be used indoors, around a bunch of tall
@ The stationary base is mounted on a known point and buildings, or in heavily wooded areas.
sends corrections to the rover via radios, cell phones, internet,
etc.
GPS requires a clear view to at least 4 satellites (5 for sub meter work).
@ The dual-frequency base not only sends corrections based
on the calculations from the L1 and L2 frequencies, but is Remove one or more satellites out of your triangulation equation and your
usually a lot closer than a beacon station and/or a results
lt ((solutions)
l ti ) change.
h
geostationary satellite.
@ Same basic techniques as DGPS methods, except in the
way satellite ranges are calculated due to the L1 and L2 dual-
frequency channels.
@ Accuracies range from 0.0066 ft (static measurements) to
less than 1 ft (real-time).
@ Used mainly for surveying, machine control and etc.
A clear view to at least 4 satellites
@ Costs range from $20 000 to around $50 000 with software
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In General: Benefits of a CORS Network
GPS receivers vary greatly in accuracy, data processing capabilities,
communication and interfacing options, software….and price.
Think of a GPS receiver (either single or dual-frequency) as a computer
with an antenna. The more powerful and efficient the receiver processor
and the better the tracking and filtering capabilities of the antenna, the
faster and more pprecise the GPS system.
y
The GPS satellites are referenced to the World Geodetic System of 1984
(WGS84) ellipsoid. Therefore, all GPS receivers collect WGS84
coordinates (latitude and longitude).
For surveying and mapping purposes, the WGS84 coordinate system
must be converted (i.e. transformed) to a user-defined datum
