2- Control Segment 1- Satellite/Space Segment 3- User Segment 3
Main Segments to any GNSS Monitor Stations Ground Antennas Master
Station
Slide 3
The US developed and operated NAVSTAR Global Positioning System
(GPS) was the first Global Navigation Satellite System (GNSS). GPS
provides specially coded satellite signals that can be processed in
a GPS receiver to compute Position, Velocity and Time. First
satellite launched in 1978. Full constellation achieved in
1994.
Slide 4
1. Position and coordinates (latitude, longitude, altitude) 2.
Direction of travel between any two points. 3. Travel velocity 4.
Accurate time of the day
Slide 5
GPS Operational Constellation consists of 24 + satellites
orbiting at a height of 20,000 KM(Medium Earth Orbit) Constellation
has spares 6 orbital planes (with 4 to 5 Satellites in each)
equally spaced. 4 to 8 active Satellites visible from any point on
the earth. GPS Satellites send Radio Signals. Every satellite
orbits the Earth twice daily (one revolution in approx 12 hrs)
Slide 6
Control Segment 5 Monitor stations Track GPS signals and send
them to master control station 1 Master Control Station located at
Falcon AFB (air force base) Colorado Correct orbit and clock errors
Create new navigation messages Ground Antenna (upload station) 4
ground antennas
Slide 7
Kwajalein Atoll US Space Command Control Segment Hawaii
Ascension Island Diego Garcia Cape Canaveral Ground Antenna Master
Control Station Monitor Station
Slide 8
Consists of GPS receivers and user community. GPS receivers
convert Radio Signals into Position, Velocity, and Time estimates.
3 satellites are required to compute Position (X, Y, Z). 4 th
satellite is used to recalibrate receiver clock (Time). Navigation
in three dimensions is the primary function of GPS.
Slide 9
Each satellite has a unique pseudo-random code which helps for
determining time difference (Travel Time) by comparing how late the
satellite's pseudo- random code appears compared to our receiver's
code. There are two types of pseudo-random codes: The C/A (Coarse
Acquisition) code for timing for civilian GPS users and the status
message are modulated on L1 frequency (1575.42 MHz). The more
precise and complicated pseudo-random code (P-code) for military
users is modulated on L1 and L2 (1227.60 MHz) frequencies. After
encryption, it is called Y-code. Pseudo-Random Codes and Carriers
Frequencies Ephemeris data including information about the
satellite's orbits, their clock corrections and other system
status. GPS Signal Navigation Message
Slide 10
GPS positioning based on Range Distance or Range This is the
distance between a satellite in space and a receiver on or above
the Earths surface Range distance is calculated as follows: Range =
speed of light * Travel time Coded signals are used to calculate
signal travel time by matching sections of the code Difference
between transmission and reception times is the travel time A range
measurement from a single satellite restricts the receiver to a
location somewhere on the surface of a sphere centered on the
satellite
Slide 11
Slide 12
How GPS Works? The measurement principle of GPS is based on 3D
Trilateration (A mathematical method of determining the relative
positions of objects using the geometry of triangles) from
Satellites To triangulate (loosely speaking), a GPS receiver
measures distance using the travel time of radio signals coming
from GPS Satellites. To measure travel time, GPS needs very
accurate timing
Slide 13
Fig A Position Calculation - Trilateration Surfaces of two
spheres intersect at a circle (Fig A) Intersection of third sphere
with the first two will intersect that circle at two points (Fig B)
For GPS carriers near Earth surface the position will be at the
intersecting point closest to the Earth Fig B
Slide 14
Fourth Satellite Positioning and Correcting the GPS Clock
Correct position can also be determined by fourth intersecting
sphere Four satellites are usually required to reduce receiver
clock errors (recalibrate) Correcting Clock Bias It's geometrically
impossible for four mutually intersecting spheres to merge at the
same point unless the clock is spot on A simple routine is run by
receiver to adjust or reset its clock so that all four lines of
position intersect the same point
Slide 15
4 th satellite range is required for confirmatory test as well
as Time.
Slide 16
GPS Error Sources 1. Atmospheric delays - The satellite signal
slows down 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. 2. Signal multi-path -
This occurs when the GPS signal is reflected off objects before it
reaches the receiver. This increases the travel time of the signal,
thereby causing errors. 3. 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.
4. Orbital errors - Also known as ephemeris errors, these are
inaccuracies of the satellite's reported location. 5. 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. 6. 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. 7.
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.
Slide 17
Differential GPS We discussed position measurements collected
with a single receivers known as autonomous GNSS positioning
Differential GPS involves the cooperation of two receivers:
Stationary (with known coordinate points) Rover When two receivers
are fairly close to each other, the signals that reach both of them
will have traveled through virtually the same slice of atmosphere,
and will have similar type/amount of errors, and: Differential GPS
can eliminate all errors that are common to both. These include
everything except multi-path errors and any receiver errors.
Stationary (Known Position) Rover (Un- known Position)
Slide 18
Differential GPS
Slide 19
DGPS Base-stations
Slide 20
Wide Area Augmentation System (WAAS) Based on a network of
ground reference stations scattered about North America (works only
with NAVSTAR GPS) To provide accurate, dependable aircraft
navigation. To provide real-time accuracies to reduce individual
errors less than 7 meters (tests shows errors less than 7 meters
95% of the time) Signals from satellites received at each station
and errors calculated Correction calculated and transmitted to a
geo-stationary satellite. Correction signals collected by WAAS
compatible roving receivers. Substantial improvement over
uncorrected GPS (errors above 15 meters)
Slide 21
CountryAugmentationAbbreviation USA: Wide Area Augmentation
System WAAS Europe: European Global Navigation Overlay System EGNOS
Japan:MTSAT Satellite Based Augmentation System MSAS
Canada:Canadian Wide Area Augmentation System CWAAS China:Chinese
Sat. Navigation & Augmentation ServiceSNAS Indian:GPS and Geo
Augmented Navigation System GAGAN GNSS Augmentation
Slide 22
GPS Jamming Devices Source: http://www.ac11.org/gps1.htm Range:
Few meters to 200 Km
Slide 23
CountryNameNo. of SatellitesOperational Date USNAVSTAR / GPS
(Global)24+ Operational since 17 July 1995 Russian
FederationGLONASS (Global)3124 Operational European UnionGALILEO30
2013 (2 launched in Oct 2011 and 2 in Oct 2012) ChinaCOMPASS
(Beidou) Beidou 1 = (3+1) Beidou2 = 35 14 satellites operational
for Asia Pacific general users, (2020 global coverage) India IRNSS
(Indian Regional Navigational Satellite System) 7 2015 (First
Satellite to be launched in June 2013) Japan QZSS (Quasi-Zenith
Satellite System) 3 2013 (1 st launched in Sept. 2010) French DORIS
(Doppler Orbitography and Radio-positioning Integrated by
Satellite) -Operational Since 1990 GNSS - Worldwide - United States
NAVSTAR Global Positioning System (GPS) has been the only fully
operational GNSS till recently - GLONASS: In October 2011, the full
orbital constellation of 24 satellites was restored, enabling full
global coverage
Slide 24
GPS Uses/Applications Search and rescue Disaster relief
Surveying and Mapping Geographic Information Systems (GIS) Marine,
aeronautical and terrestrial navigation Remote controlled vehicle
and robot guidance Satellite positioning and tracking Shipping
Military Recreation
Slide 25
References Lecture notes of Paul Burgess, University of
Redlands, Redlands Institute. Introduction to GPS
http://www.spacetoday.org/Satellites/GPS.html
http://www.astronautix.com/project/navstar.htm
http://en.wikipedia.org/wiki/Global_Positioning_System