Topic for the class: Anti Spoofing (AS), Selective Availability(SA),
Difference Between GPS and GALILEO Satellite Construction.
Module II: GPS Signals
Dr. Rajasekhar Chukka
Assistant Professor
Department of EECE
GITAM Institute of Technology (GIT)
Visakhapatnam – 530045
Email: rchukka@gitam.edu
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Learning Outcomes
At the end of this session Students will be able to
1. Understand the Signal structure, anti spoofing (AS) and
selective availability.
2. Explain the difference between GPS and GALILEO satellite
construction.
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Syllabus
Module I
Overview of GPS: Basic concept, system architecture, space
segment, user segment, GPS aided Geo-augmented navigation
(GAGAN) architecture.
Module II
GPS Signals: Signal structure, anti spoofing (AS), selective
availability, difference between GPS and GALILEO satellite
construction.
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Syllabus
Module III
GPS Coordinate Frames and Time References: Geodetic and geo
centric coordinate systems, ECEF coordinate world geodetic 1984
(WGS 84), GPS time.
Module IV
GPS Orbits and Satellite Position Determination : GPS orbital
parameters, description of receiver independent exchange format
(RINEX), observation data and navigation message data
parameters, GPS position determination.
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Syllabus
Module V
GPS Errors: GPS error sources – clock error, ionospheric error,
tropospheric error, multipath, ionospheric error estimation using
dual frequency GPS receiver.
Text Book(s)
1. B. Hoffman-Wellenhof, H. Liehtenegger, J. Collins, GPS: Theory
and Practice, Springer-Wien, 2001.
2. Gottapu Sasibhushana Rao, Global Navigation Satellite Systems,
Tata McGraw Hill Education, 2010.
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Syllabus
References
1. B.Parkinson, J.Spilker, Jr., GPS:Theory and Applications, Vol. I, II,
AIAA, 370 L’Enfant Promenade SW, Washington, DC 20024, 1996.
2. James Ba, Yen Tsui, Fundamentals of GPS Receivers: A Software
Approach, John Wiley and Sons, 2001
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Contents
1. GPS Signals
 Signal Structure
 Anti Spoofing (AS)
 Selective Availability(SA)
 Difference between GPS and GALILEO satellite construction
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GPS Signals
GPS Signal Structure
GPS Signal: a GPS signal is composed of 3 components
1. Navigation message data
2. C/A code(Coarse /Acquisition)
3. Carrier signal
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• Each GPS satellite transmits a MW radio signal composed of two
carrier frequencies (or sine waves) modulated by two digital
codes and a navigation message
• The two carrier frequencies are generated at 1,575.42 MHz
(referred to as the L1 carrier) and 1,227.60 MHz (referred to as
the L2 carrier).
• The availability of the two carrier frequencies allows for
correcting a major GPS error, known as the ionospheric delay
• All of the GPS satellites transmit the same L1 and L2 carrier
frequencies.
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• The code modulation, however, is different for each satellite,
which significantly minimizes the signal interference.
• The two GPS codes are called coarse acquisition (or C/A-code)
and precision (or P-code).
• Each code consists of a stream of binary digits, zeros and ones,
known as bits or chips.
• The codes are commonly known as PRN(Pseudorandom Noise)
• The C/A-code is modulated onto the L1 carrier only, and the Pcode is modulated onto both the L1 and the L2 carriers.
• This modulation is called biphase modulation,(BPSK)
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• C/A-code is a stream of 1,023 binary digits (i.e., 1,023 zeros and
ones) that repeats itself every millisecond
• This means that the chipping rate of the C/A-code is 1.023
Mbps.
• Each satellite is assigned a unique C/A-code, which enables the
GPS receivers to identify which satellite is transmitting a
particular code.
• The C/A-code range measurement is relatively less precise
compared with that of the P-code.
• It is, however, less complex and is available to all users.
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• P-code is a very long sequence of binary digits that repeats itself
after 266 days.
• It is also 10 times faster than the C/A-code (i.e., its rate is 10.23
Mbps).
• The 266-day-long code is divided into 38 segments; each is 1
week long.
• Out of these, 32 segments are assigned to the various GPS
satellites.
• That is, each satellite transmits a unique 1-week segment of the
P-code,
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• The P-code is designed primarily for military purposes.
• It was available to all users until January 31, 1994.
• At that time, the P-code was encrypted by adding to it an
unknown W-code.
• The resulting encrypted code is called the Y-code, which has the
same chipping rate as the P-code.
• This encryption is known as the anti-spoofing (AS).
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• The GPS navigation message is a data stream added to both the
L1 and the L2 carriers as binary biphase(BPSK) modulation at a
low rate of 50 bps.
• It consists of 25 frames of 1,500 bits each, or 37,500 bits in total.
• This means that the transmission of the complete navigation
message takes 750 seconds, or 12.5 minutes.
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GPS Satellite Signal Generation
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GPS Signal: a GPS signal is composed of 3 components
1. Navigation message data
2. C/A code(Coarse /Acquisition
3. Carrier signal
1. Navigation data:
• Is a binary data pulse sequence of 50 Hz
• It consists of data regarding satellite health
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2. C/A code(Coarse /Acquisition):
• It is a randomly generated unique PN code for satellite
identification
3. Carrier signal:
• Is of frequency 1575.42MHz(for L1) or 1227.60MHz (for L2)
• Satellites transmit the signals on two these L1 &L2
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 C/A code and P(Y) code
 All SVs transmit at 1575.42MHz
 Each SV modulates using a unique 1023-bit
pseudorandom (PRN) code sent at 1.023Mcps
(chips per second)
 PRN allows spread-spectrum CDMA management
of GPS transmit frequency
 Receiver’s distance to the SV can be determined
by measuring the PRN time skew between the
transmitted and received signals
 GPS system data (ephemeris, clock, and
atmospheric parameters) are transmitted by
further modulating the PRN code at 50bps
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• Each code modulates the navigation data using modulo-2
(Exclusive-OR) operation spreading the data over 2MHz
bandwidth.
• The resultant output after the modulo-2 operation in turn
modulates the carrier signal using Binary Phase Shift Keying
(BPSK).
• With every change in modulated data, there is a 1800 change in
the carrier phase.
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GPS Signal Parameters
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• Navigation data transmitted at a rate of 50bits/s includes
information such as
• Satellite time and synchronization signals
• Precise orbital data (ephemeris)
• Time-correction information to determine the exact satellite time
• Approximation orbital data for all satellites (almanac)
• Correction signals to calculate signal transit time
• Data on the ionosphere
• Information on the operating status (health) of the satellite
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• The time required to transmit all this information is 12.5 minutes.
• By using the navigation message, the receiver is able to
determine the transmit of each satellite signal and the exact
position of the satellite at the time of transmission
Selective Availability:
• To ensure national security, some security features, known as
selective availability (SA) and anti-spoofing(AS), were added to
Block II/IIA satellites
• Selective Availability (SA) was an intentional degradation of
public GPS signals implemented for national security reasons.
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• U.S DOD only implemented the Selective Availability (SA) on Block-II
GPS satellites to deny accurate real time autonomous positioning to
unauthorized users
• (SA) was officially activated on March25,1990
• Selective Availability introduces two types of errors
1.Delta error(δ): results from dithering the satellite clock and is
common to all users in the worldwide
2.Epsilon error(ε): is an additional slowly varying orbital error
• The US Govt. controls the satellites and uses SA to confuse the GPS
receiver so it can’t find the exact position user
• SA makes the satellite to send the wrong time
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Anti-Spoofing(AS):
• The design of GPS includes the ability to essentially "turn off" the
P-code or invoke an encrypted code as a means of denying
access to the P-code to all but authorized users.
• The rationale for doing this is to keep adversaries from sending
out false signals with the GPS signature to create confusion and
cause users to misposition themselves.
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• AS is accomplished by the modulo-2 sum of the P-code and an
encrypting W-code.
• The resulting code is denoted as the Y-code.
• Thus, when AS is active, the P-code on the L1 and the L2 carrier
is replaced by the unknown Y-code.
• P-code +W-code = Y-code
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Signal Power
All satellite use onboard atomic clocks and transmit on the same
frequency (1575.42MHz and 1227.60 MHz).
The minimum signal strength received on earth is approximately158 dB W to-160 dBW.
According to the specifications, the maximum strength is
approximately-153dBW
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Link Budget Analysis between a satellite and a user
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Link Budget Analysis between a satellite and a user
• The link budget analysis between a satellite and a user is suitable
for establishing the required level of satellite transmission
power.
• According to the specifications, the minimum amount of power
at a GPS receiver must not fall below-160dBm.
• In order to maintain this level, the satellite L1 carrier
transmission power, modulated with the C/A code, must be
21.9 W.
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Link Budget Analysis between a satellite and a user
• The maximum of the power spectral density of the received
signal is around -190 dBm/Hz.
• Spectral power density of the thermal background noise is
about-174 dBm/Hz (at a temperature of 2900K).
• Thus, the maximum received signal power is approximately
16dB below the thermal background noise level.
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Galileo
Galileo:
• Galileo is the global navigation satellite system that is currently
being created by the European Union (EU) and the European
Space Agency (ESA).
• Is Europe’ own global navigation satellite systems(GNSS)
• It provides a highly accurate GPS service under civilian control.
• Galileo configuration is slightly different from GPS.
Features:
30 Medium Earth Orbit(MEO)-27 operational +3 spare
9 satellites in each of three orbital plane
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Galileo
Life span 15 years
Altitude:23,616km
Inclination angle :560
Orbital Period :14hrs21.6 minutes
Modulation: BOC &Alt BOC
8 GEO satellites on the equator
Galileo interoperable with GPS & GLONASS
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Galileo
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Galileo Signals
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Galileo Services
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• The Galileo system provides four services:
Open service (OS):
• Open and free-of-charge service for basic positioning and timing
services.
Commercial service (CS):
• Improved service with improved levels of guaranteed
performance. CS offers the possibility for signal encryption in
order to reduce the risk of active signals being disturbed or
fabricated.
Public regulated service (PRS):
• Service restricted to government-authorised users for sensitive
applications which require a high level of service continuity. This
service is intended for use by approved authorities only.
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Search and rescue service (SAR):
• European contribution to the international search and
rescue distress alert detection system (Cospas-Sarsat).
• An initial fifth service, the ‘safety of life’ service, was reprofiled in 2010 and abandoned in 2013.
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Major advantages of the Galileo programme
• The Galileo navigation system creates multiple benefits both
within Europe and throughout the world.
• Galileo provides socio-economic and strategic advantages and
offers several unique features which are not found in other,
competing GNSS constellations.
The key advantages are
• Civil governance, open communication policy
• Strategic autonomy of the EU
• Key enabler of various added socio-economic values
• Envisaged to provide signal authentication for open signals
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• Promoter of new GNSS technologies and of the development of
associated downstream markets within the EU
• Improved accuracy
• European contribution to worldwide Cospas-Sarsat system,
provides a unique technical feature to announce that rescue
forces are on their way
• Galileo is the first satellite positioning system specifically
designed for civil use.
• It provides outstanding accuracy, availability, integrity and
guarantee.
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• Novel modulation schemes BOC and Alt BOC.
• Galileo transmits signals in four frequency bands: E5a, E5b, E6
and L1.
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Carrier Frequencies and Receiver Bandwidth
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GALILEO Signal Structure
• The signal structure serves the needs of 4 Galileo positioning
services and one Search And Rescue Service (SAR) :
• Open Service (OS)Free of charge and provides positioning and
timing services. OS data transmitted on E5a, E5b, E6 and L1.
• Safety-of-Life (SOL) Provides safety to critical users such as
maritime, civil aviation and trains. E5a+E5b and L1.
• Commercial Service (CS) Access to Commercial data is provided
directly to users by service providers.E5b, E6 and E2-L1-E1
• Publicity Regulated Services (PRS) Improve the probability of
continuous availability of signals in space. E6 and L1
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Difference between GPS and GALILEO
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FLASH QUIZ
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Session Quiz (5-10 MCQ/True false questions)
(marks to be counted for continuous evaluation)
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THANK YOU
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