SATELLITE COMMUNICATION PRESENTATION ON GPS(2).ppt

K6123- SATELLITE COMMUNICATION 1
KT6123 – SATELLITE
COMMUNICATION
PROJECT/CASE STUDY
M.ENG COMPUTER COMMUNICATION
RESENT BY : HASANAH BT SAFEIN @ SHAFIE
(G71894)
13 MARCH 2006
UNIVERSITY KEBANGSAAN MALAYSIA 43600 BANGI SELANGOR, MALAYSIA
GPS

GLOBAL POSITIONING
SYSTEM (GPS)

What is GPS?
 The Global Positioning System (GPS) is a
satellite-based navigation system made up of a
network of 24 satellites placed into orbit by the
U.S. Department of Defense. GPS was originally
intended for military applications, but in the 1980s,
the government made the system available for
civilian use. GPS works in any weather conditions,
anywhere in the world, 24 hours a day. There are
no subscription fees or setup charges to use GPS.

How it works
 GPS satellites circle the earth twice a day in a very
precise orbit and transmit signal information to earth.
GPS receivers take this information and use
triangulation to calculate the user's exact location.
Essentially, the GPS receiver compares the time a
signal was transmitted by a satellite with the time it
was received. The time difference tells the GPS
receiver how far away the satellite is. Now, with
distance measurements from a few more satellites,
the receiver can determine the user's position and
display it on the unit's electronic map.

The GPS satellite system
 The 24 satellites that make up the GPS space segment are orbiting the earth
about 12,000 miles above us. They are constantly moving, making two complete
orbits in less than 24 hours. These satellites are travelling at speeds of roughly
7,000 miles an hour.
 GPS satellites are powered by solar energy. They have backup batteries
onboard to keep them running in the event of a solar eclipse, when there's no
solar power. Small rocket boosters on each satellite keep them flying in the
correct path.
 Here are some other interesting facts about the GPS satellites (also called
NAVSTAR, the official U.S. Department of Defense name for GPS):
 The first GPS satellite was launched in 1978.
 A full constellation of 24 satellites was achieved in 1994.
 Each satellite is built to last about 10 years. Replacements are constantly being
built and launched into orbit.
 A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across
with the solar panels extended.
 Transmitter power is only 50 watts or less.

Sources of GPS signal errors
 Factors that can degrade the GPS signal and thus affect accuracy include the following:
 Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere.
The GPS system uses a built-in model that calculates an average amount of delay to partially correct for
this type of error.
 Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or
large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby
causing errors.
 Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the
GPS satellites. Therefore, it may have very slight timing errors.
 Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported
location.
 Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy.
Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception,
causing position errors or possibly no position reading at all. GPS units typically will not work indoors,
underwater or underground.
 Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal
satellite geometry exists when the satellites are located at wide angles relative to each other. Poor
geometry results when the satellites are located in a line or in a tight grouping.
 Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation
of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military
adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000,
which significantly improved the accuracy of civilian GPS receivers.


 A GPS receiver calculates its position by measuring the distance
between itself and three or more GPS satellite.Measuring the
time delay between transmission and reception of each GPS
radio signal gives the distance to each satellite, since the signal
travels at a known speed. The signals also carry information
about the satellites' location. By determining the position of, and
distance to, at least three satellites, the receiver can compute its
position using trilateration. Receivers typically do not have
perfectly accurate clocks and therefore track one or more
additional satellites to correct the receiver's clock error.
How its work

How its work
 Global Positioning System satellites transmit signals to
equipment on the ground. GPS receivers passively
receive satellite signals; they do not transmit. GPS
receivers require an unobstructed view of the sky, so
they are used only outdoors and they often do not
perform well within forested areas or near tall buildings.
GPS operations depend on a very accurate time
reference, which is provided by atomic clocks at the U.S.
Naval Observatory. Each GPS satellite has atomic clocks
on board.

How its work
 Each GPS satellite transmits data that
indicates its location and the current time.
All GPS satellites synchronize operations so
that these repeating signals are transmitted
at the same instant. The signals, moving at
the speed of light, arrive at a GPS receiver
at slightly different times because some
satellites are farther away than others. The
distance to the GPS satellites can be
determined by estimating the amount of
time it takes for their signals to reach the
receiver. When the receiver estimates the
distance to at least four GPS satellites, it
can calculate its position in three
dimensions. There are at least 24
operational GPS satellites at all times. The
satellites, operated by the U.S. Air Force,
orbit with a period of 12 hours. Ground
stations are used to precisely track each
satellite's orbit.

Determining Position
 A GPS receiver "knows" the location of the
satellites, because that information is
included in satellite transmissions. By
estimating how far away a satellite is, the
receiver also "knows" it is located somewhere
on the surface of an imaginary sphere
centered at the satellite. It then determines
the sizes of several spheres, one for each
satellite. The receiver is located where these
spheres intersect.

Satellite Communications-III
Navstar GPS Segments
 Space Segment-1
The Space Segment of the system consists of the 24
GPS satellites (21 in Operation, 3 as spare)
 These space vehicles (SVs) send radio signals from
space
 GPS Satellites orbit the earth in 12 hours
The satellite orbits repeat almost the same ground
track (as the earth turns beneath them) once each day
The orbit altitude (20, 200 km) is such that the
satellites repeat the same track and configuration over
any point approximately each 12 hours (4 minutes
earlier each day)
Six orbital planes (with nominally four SVs in each),
equally spaced (60 degrees apart), and inclined at
about fifty-five (55) degrees with respect to the
equatorial plane
Five and eight SVs are visible from any point on the
earth

 Space Segment-2
 Satellite Relative Positions

 Space Segment-3
The Mercator Projection of Navstar GPS Satellite Orbits: 3 GPS satellites provide horizontal
(two-dimensional) location of a GPS Rx where as four GPS satellites provide its 3D position
(including altitude)

 Control Segment
 The Control Segment consists of a system of tracking stations located around the world
 The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado
 These monitor stations measure signals from the SVs which are incorporated into orbital models for each
satellites
 The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite
 The Master Control station uploads ephemeris and clock data to the SVs
 The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals

 User Segment
 Navigation in three dimensions is the primary function of
GPS
 GPS User Segment consists of the GPS receivers and the
user community such as aircrafts, ships, ground
vehicles, and for hand carrying by individuals
 GPS receivers convert SV signals into position, velocity,
and time estimates
 Four satellites are required to compute the four
dimensions of X, Y, Z (position) and Time
 GPS receivers are used for navigation, positioning, time
dissemination, and other research projects
 Precise positioning is possible using GPS receivers at
reference locations providing corrections and relative
positioning data for remote receivers - Surveying, geodetic
control, and plate tectonic studies are examples
 Time and frequency dissemination, based on the precise
clocks on board the SVs and controlled by the monitor
stations, is another use for GPS - Astronomical
observatories, telecommunications facilities, and
laboratory standards can be set to precise time signals or
controlled to accurate frequencies by special purpose
GPS receivers
 Research projects have used GPS signals to measure
atmospheric parameters

GPS Accuracy
 The accuracy of a position determined with
GPS depends on the type of receiver. Most
hand-held GPS units have about 10-20 meter
accuracy. Other types of receivers use a
method called Differential GPS (DGPS) to
obtain much higher accuracy. DGPS requires
an additional receiver fixed at a known location
nearby. Observations made by the stationary
receiver are used to correct positions recorded
by the roving units, producing an accuracy
greater than 1 meter.

Global Positioning Satellite (GPS)
Receiver
 The Global Positioning Satellite (GPS) Receiver reuses a
system of 24 earth orbiting satellites to triangulate a point on
the earth with extreme accuracy.
 When the system was created, timing errors were inserted into
GPS transmissions to limit the accuracy of non-military GPS
receivers to about 100 meters. This part of GPS operations,
called Selective Availability, was eliminated in May 2000.

Gps Receiver Block Diagram

Design Considerations
 The Global Positioning System (GPS) works on the
principle that if you know your distance from several
locations, then you can calculate your location. The
known locations are the 24 satellites located in six
orbital planes at an altitude of 20,200Km. These
satellites circle the Earth every 12 hours and
broadcast a data stream at the primary frequency L1
of 1.575GHz which carries the coarse-acquisition
(C/A) encoded signal to the ground. The GPS
receiver measures the time of arrival of the C/A
code to a fraction of a millisecond, and thus
determines the distance to the satellite.

The Core Subsystems include
 Front End - the GPS L1 signals (Maximum = 24 signals) at 1.575GHz are
received at the antenna and amplified by the Low-Noise-Amplifier (LNA). The
RF front-end further filters, mixes, and amplifies (AGC) the signal down to the IF
frequency where it is digitally sampled by a ADC. '
 Baseband Processor/CPU - the ADC samples of GPS C/A code signals are
correlated by the DSP and then formulated to make range measurements to the
GPS satellites. The DSP is interfaced with a general-purpose CPU, which
handles tracking channels and controls user interfaces. TI OMAP integrates both
DSP and ARM processor on the same chip.'
 Memory - the processor runs applications stored in memory. The OS is stored in
non-volatile memory such as EE/FLASH/ROM. Applications may be loaded in
FLASH or DRAM. '
 User Interface - allows user to input/output data from the receiver using input
commands via microphone, touch screen, and output MP3 to the earplug.'
 Connectivity - allows the receiver to connect to the USB port.'
 Power Conversion - converts input power (battery or wall plug) to run various
functional blocks.

System segmentation
 GPS receivers come in a variety of formats,
from devices integrated into cars, phones,
and watches, to dedicated devices such
those shown here from manufacturers
Trimble, Garmin and Leica (left to right).

GPS time
 Atomic clocks on the satellites are set to "GPS time", similar to most time standards
, but not corrected to the rotation of the Earth, ignoring leap seconds and other
corrections. GPS time was set to match Coordinated Universal Time (UTC) in 1980,
but has since diverged as leap seconds were added to UTC.
 The current date is expressed in the GPS signal as a week number and a day-of-
week number. GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on
January 6, 1980. The week number is transmitted in a ten-bit field, and so it wraps
round every 1,024 weeks, (19.7 years). The transmitted week number rolled over to
zero at 00:00:19 TAI on August 22, 1999 (23:59:47 UTC on August 21, 1999). GPS
receivers thus need to know the time to within 3,584 days in order to correctly
interpret the GPS date signal. A new field is being added to the GPS navigation
message that specifies the calendar year number exactly, in a sixteen-bit field.
[citation needed]
 The GPS navigation message includes the difference between GPS time and UTC,
which is 14 seconds as of 2006. Receivers subtract this offset from GPS time to
calculate UTC and 'local' time. New GPS units may not show the correct UTC time,
or not attempt to show UTC time at all, until after receiving the UTC offset message
for the first time. This is usually within 15 minutes after the unit achieves GPS lock.
The GPS-UTC offset field is only eight bits, and so it wraps round every 256 leap
seconds. At the current rate of change of the earth's rotation, the first wraparound
of this field is projected to occur in the year 2330.

TDM-IN SATELLITE
TELEPHONY
 TDMA is not only used globally for satellite communications, but
has become virtually a household acronym as a result of
deployment in cellular communications systems. Of particular
significance in Dr. Sekimoto's work in TDMA was the world's first
successful demonstration of a TDMA satellite signal transmission
between the Andover, Maine, USA, and Mill Village, Canada
earth stations, in August 1966. Another innovation was the
commercialization of the SPADE system (single channel per
carrier PCM multiple access demand assignment equipment) in
the INTELSAT network. Other related contributions include digital
band width compression transmission of television signals, digital
echo cancellers, low bit rate speech source coders, digital
speech interpolation and digital channel multiplication. These
pioneering innovations contributed to the digital methods used in
today's satellite communications.

Conclutions
 In GPS, a constellation of 24 satellite circle
the earth in near circular inclined orbits.
 The receiver potition
(latitude,longitude,altitude) can be determine
accurately at receiving signal
 With GPS, time marker is needed getting
simultaneous measurement from our
satellites

REFERENCE
 1999 Federal Radionavigation Plan, February 2000. Washington, DC: U.S. Department of Transportation
and Department of Defense. Available on line from United States Coast Guard Navigation Center
 Global Positioning System Standard Positioning Service Specification, 2nd Edition, June2, 1995.
Available on line from United States Coast Guard Navigation Center
 NAVSTAR GPS User Equipment Introduction. 1996. Available on line from United States Coast Guard
Navigation Center
 GPS Joint Program Office. 1997. ICD-GPS-200: GPS Interface Control Document. ARINC
Research.Available on line from United States Coast Guard Navigation Center
 Hoffmann-Wellenhof, B. H. Lichtenegger, and J. Collins. 1994. GPS: Theory and Practice. 3rd ed.New
York: Springer-Verlag.
 Institute of Navigation. 1980, 1884, 1986, 1993. Global Positioning System monographs. Washington,
DC: The Institute of Navigation.
 Kaplan, Elliott D. ed. 1996. Understanding GPS: Principles and Applications. Boston: Artech House
Publishers.
 Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New York: John Wiley & Sons.
 National Imagery and Mapping Agency. 1997. Department of Defense World Geodetic System 1984: Its
Definition and Relationship with Local Geodetic Systems. NIMA TR8350.2 Third Edition. 4 July 1997.
Bethesda, MD: National Imagery and Mapping Agency. Available on line from National Imagery and
Mapping Agency
 Parkinson, Bradford W. and James J. Spilker. eds. 1996. Global Positioning System: Theory and Practice.
Volumes I and II. Washington, DC: American Institute of Aeronautics and Astronautics, Inc.
 Wells, David, ed. 1989. Guide to GPS positioning. Fredericton, NB, Canada: Canadian GPS Associates.
 (These and other references are available from Navtech Seminars and GPS Supply, 6121 Lincolnia Rd.
Suite 400, Arlington, VA 22312-2707 USA - (800) 628-0885 or (703) 256-8900). Fax: (703) 256-8988

Present by:
HASANAH BT SHAFIE @SAFEIN
G71894
M.Eng. Communication And Computer
16 APRIL 2007

SATELLITE COMMUNICATION PRESENTATION ON GPS(2).ppt

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