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Muhammad Nisar
Yousuf
Divisional Engineer (Codevtel)
Telecommunication Staff College(TSC), BTCL
Gazipur
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INTRODUCTION:
GPS is a
constellation of satellites owned and managed by the US DoD. There are over 30
SVs (space vehicles) in 55 degree orbits on 6 different planes (spaced 30
degrees apart). They are constantly broadcasting a number of different signals,
only one of which is used by consumer GPS
receivers (the L1). Your GPS receiver does its best to determine how
long it took that signal to travel to it from the satellite. By determining time of travel it
then determines how far away it is from the satellite. By doing this for
multiple SVs it can trilaterate (not triangulate) your location.
WORKING PRINCIPLE OF GPS
The Global Positioning
System consists of a network of 24 broadcasting satellites orbiting the earth
at a height of 20,200km. GPS also consists of receivers on the ground, which
listen to and interpret the transmissions of the satellites. Stations (the Ascension Island, Diego Garcia and Kwajalein
monitor stations) on the earth carefully monitor the orbit of each satellite,
maintaining a highly accurate record of the satellites instantaneous position.
The knowledge of the precise position of the satellites allows them to be used
as reference points, from which GPS receivers on earth can determine their
position. This technique of determining the position of an object is called ranging.
RANGING principles:
The concept of ranging is
best illustrated by example. Consider one satellite that is a distance of
25,000 kilometers from a person holding a GPS receiver. Then the person's
position is known to be somewhere on a sphere 25,000 km in radius, centered on
the satellite. However, the exact location of the person on that sphere is yet
unknown. If, at the same time, the distance from the person to a second satellite
can be discovered to be 20,000 km, then a second sphere of radius 20,000 km on
which the person is positioned can be determined. Thus the person must be on
the circle formed by the intersection of the two spheres of position. A third
satellite provides yet a third sphere, which narrows down the location of the
person to exactly two points. One of these points is often an impossible
solution, frequently several thousand kilometers off in space, thus three
satellite ranges can determine the precise position of the person. Three
satellites provide enough information to find the x, y, and z coordinates
(measured from the center of mass of the earth). However, in practice, four
satellites are required to pinpoint a position, for reasons that will soon
become clear.
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Resecting places using
Satellites
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Intersection of three
imaginary spheres: sphere concept
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In the above model of
ranging, the distance between one satellite and the person on earth is given to
be 20,000 km. However, no mention was made as to how the distance was
determined. The Global Positioning System works by having each of the 21 active
satellites constantly radiate microwaves. These microwaves are received by the
GPS receiver, which can use the method of ranging to locate its position. The
distance from the receiver to one satellite is measured in the following way.
The satellite and receiver are controlled by separate clocks. The satellites
are set as accurately as possible with an atomic clock, and are assumed to be
synchronized with one another. At some known time a satellite emits a signal in
the form of microwaves. This signal reaches the receiver after a certain
interval has passed. Since microwaves travel at the speed of light, a known
velocity and a known time allows the receiver to determine the distance to the
satellite. Thus it is important that the time be measured precisely in order to
accurately measure distance, as an error of the synchronization of the two
clocks of one microsecond creates an error of 300 meters. This requires a fourth
satellite, since a fourth variable, time, has been added to the unknowns
who previously included only the x, y, and z distances.
Basic
Functions of Monitor Stations
l These
stations are the eyes and ears of GPS, monitoring satellites as they pass
overhead by measuring distances to them every 1.5 seconds
l This data is then smoothed using ionospheric
and meteorological information and sent
to Master Control Station at Colorado
Springs.
l The
ionospheric and meteorological data is
needed to get more accurate delay measurements, which in turn improve location
estimation.
l Master
control station estimates parameters describing satellites' orbit and clock
performance,. It also assesses health
status of the satellites and determines if any re-positioning may be required.
l This information is then returned to three
uplink stations (collocated at the Ascension Island,
Diego Garcia and Kwajalein monitor stations)
which transmit the information to satellites.
Triangulation Requirements
l To
triangulate, a GPS receiver measures distance using the travel time of radio
signals.
l To measure
travel time, GPS receiver needs very accurate timing.
l Along with
distance, receiver needs accurate data on where satellites are in space.
l System will
also need to correct for any delays the signal experiences as it travels
through atmosphere.
Space Segment
l Space
segment is the satellite constellation.
l 24
satellites with a minimum of 21 operating 98% of the time
l 6 Orbital
planes
l Circular
orbits
l 20-200 km
above the Earth's surface
l 11 hours 58
minute orbital period
l Visible for
approximately 5 hours above the horizon
l Orbits of
GPS satellites need to be updated every once in a while because orbit does not
stay circular without adjustments.
l Adjustments
needed because:
l Other
objects exert gravitational force on each satellite (e.g. sun, moon)
l Effect
of gravity is non-uniform during orbit.
l Radiation
pressure (due to solar radiation).
l Atmospheric
drag
l Other
effects
User Segment
l User
segment comprises receivers that have been designed to decode signals
transmitted from satellites for purposes of determining position, velocity or
time.
l Receiver
must perform the following tasks:
(a) select
one or more satellites in view (b) acquire GPS signals (c) measure and track
signal (d) recover navigational data
Important Terminology
l Satellite transmits Ephemeris and Almanac Data to GPS receivers.
l Ephemeris data contains important information about status of satellite
(healthy or unhealthy), current date and time. This part of signal is essential
for determining a position.
l Almanac data tells GPS receiver where each GPS satellite should be at any
time throughout day. Each satellite transmits almanac data showing orbital
information for that satellite and for every other satellite in the system.
Measuring Time Of Arrival (TOA) in
GPS
TOA Concept
l GPS uses concept of time of arrival (TOA) of signals to determine user
position.
l This involves measuring time it takes for a signal transmitted by an
emitter (satellite) at a known location to reach a user receiver.
l Time interval is basically signal
propagation time.
l Time interval
(signal propagation time) is multiplied by speed of signal (speed of light) to
obtain satellite to receiver distance.
l By measuring propagation time of signals
broadcast from multiple satellites at known locations, receiver can determine
its position.
l Assuming we have
precise clocks, how do we measure signal travel time?
Measuring Distance using a PRC
Signal
l At a
particular time (let's say midnight),
the satellite begins transmitting a long, digital pattern called a pseudo-random
code (PRC).
l The
receiver begins running the same digital pattern also exactly at midnight.
l When the
satellite's signal reaches the receiver, its transmission of the pattern will
lag a bit behind the receiver's playing of the pattern.
Measuring Distance
l The length
of the delay is equal to the signal's travel time.
l The
receiver multiplies this time by the speed of light to determine how far the
signal traveled.
l Assuming
the signal traveled in a straight line, this is the distance from receiver to
satellite.
Synchronizing Clocks
l In order to
make this measurement, the receiver and satellite both need clocks that can be
synchronized down to the nanosecond.
l Accurate
time measurements are required. If we
are off by a thousandth of a second, at the speed of light, that translates
into almost 200 miles of error.
l To make a
satellite positioning system using only synchronized clocks, you would need to
have atomic clocks not only on all the satellites, but also in the receiver
itself.
l But atomic clocks
cost somewhere between $50,000 and $100,000, which makes them a just a bit too
expensive for everyday consumer use.
l The Global
Positioning System has a clever, solution to this problem. Every satellite
contains an expensive atomic clock, but the receiver itself uses an ordinary
quartz clock, which it constantly resets.
l The Global
Positioning System has a clever, effective solution to this problem.
l Every
satellite contains an expensive atomic clock, but the receiver itself uses an
ordinary quartz clock, which it constantly resets.
l In a
nutshell, the receiver looks at incoming signals from four or more satellites
and gauges its own inaccuracy.
l When you
measure the distance to four located satellites, you can draw four spheres that
all intersect at one point.
l Three
spheres will intersect even if your numbers are way off, but four
spheres will not intersect at one point if you've measured incorrectly.
l Since the
receiver makes all its distance measurements using its own built-in clock, the
distances will all be proportionally incorrect.
l The
receiver can easily calculate the necessary adjustment that will cause the four
spheres to intersect at one point.
l Based on
this, it resets its clock to be in sync with the satellite's atomic clock.
l The
receiver does this constantly whenever it's on, which means it is nearly as
accurate as the expensive atomic clocks in the satellites.
l The
receiver can easily calculate the necessary adjustment that will cause the four
spheres to intersect at one point.
l Based on
this, it resets its clock to be in sync with the satellite's atomic clock.
l The
receiver does this constantly whenever it's on, which means it is nearly as
accurate as the expensive atomic clocks in the satellites.
Knowing
Satellite Locations
l In order to
properly synchronize clocks and figure out which PRC signal to listen to, the
receiver has to know where the satellites actually are.
l This isn't
particularly difficult because the satellites travel in very high and
predictable orbits.
Using Almanac Information
l The GPS
receiver simply stores an almanac that tells it where every satellite
should be at any given time.
l Things like
the pull of the moon and the sun do change the satellites' orbits very
slightly.
l However,
the Department of Defense constantly monitors their exact positions and
transmits any adjustments to all GPS receivers as part of the satellites'
signals.
2 Types of Errors
l Errors can
be categorized as intentional and unintentional.
l (1)
Intentional errors: government can and
does degrade accuracy of GPS measurements.
This is done to prevent hostile forces from using GPS to full accuracy.
(2) Policy of inserting inaccuracies in GPS signals is called Selective Ability
(SA). SA was single biggest source of inaccuracy in GPS. SA was deactivated in 2000.
Sources
of Unintentional Timing Errors : (Typical
Errors )
Source of Error
Satellite Clocks
Orbit Errors
Ionosphere
Troposphere
Receiver Noise
Multipath
SA
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Typical Error in Meters
(per satellite)
1.5
2.5
5.0
0.5
0.3
0.6
30
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Differential
GPS
l Technique
called differential correction can yield accuracies within 1-5 meters, or even better, with
advanced equipment.
l Differential
correction requires a second GPS receiver, a base station, collecting
data at a stationary position on a precisely
known point.
l Because physical location of base station is
known, a correction factor can be computed by comparing known location with GPS
location determined by using satellites.
Improved
Offered by Differential GPS
Source Uncorrected With
Differential
Ionosphere 0-30 meters Mostly
Removed
Troposphere 0-30 meters All
Removed
Signal Noise 0-10 meters All Removed
Orbit Data 1-5 meters All
Removed
Clock Drift 0-1.5 meters All
Removed
Multipath 0-1 meters Not
Removed
Receiver Noise
~1 meter Not
Removed
SA 0-70
meters All Removed
Using GPS
Data
l A GPS
receiver essentially determines the receiver's position on Earth.
l Once the
receiver makes this calculation, it can tell you the latitude, longitude and
altitude of its current position. To make the navigation- more userfriendly,
most receivers plug this raw data into map files stored
in memory.
l You can
l use maps
stored in the receiver's memory,
l connect the
receiver to a computer that can hold more detailed maps in its memory, or
l simply buy
a detailed map of your area and find your way using the receiver's latitude and
longitude readouts.
l Some
receivers let you download detailed maps into memory or supply detailed maps
with plug-in map cartridges.
l A standard
GPS receiver will not only place you on a map at any particular location, but
will also trace your path across a map as you move.
l If you
leave your receiver on, it can stay in constant communication with GPS
satellites to see how your location is changing.
l This is
what happens in cars equipped with GPS.
With this information and its built-in
clock, the receiver can give you several pieces of valuable information:
(A) How far
you've traveled (odometer) , (B) How
long you've been traveling, (C) Your
current speed (speedometer) , (D) Your
average speed , (E) A "bread crumb" trail showing you
exactly where you have traveled on the map , (F) The estimated time of arrival at your
destination if you maintain your current speed .
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