Introduction to GPS and GNSS
Vivek Srivastava
viveksrivastava09@gmail.com

Early Space-Based Radio Navigation System
Launch of Sputnik – Tracking? -------------Doppler Shift.
Altitude: 985km; revolution period: 98 min
Frank McClure, of the Applied Physics Laboratory, made
a suggestion: would it be possible to invert this problem?
– given rise to TRANSIT in late 1950’s (US- 6 sat; Altitude:
1100km; revolution period: 108 min) / TSYKLON(USSR-10
sat; 6- PARUS: Military; 4- TSIKADA-commercial/civilian;
Altitude: 1000km)
The Navy Navigational Satellite System or TRANSIT, used
observed measurements in Doppler shift to calculate
distance and position to satellites (till 31-12-96).
A fix requires 40 minutes for a static user-2D.

“ It is an all-weather, space based navigation system
development by the U.S. DOD to satisfy the
requirements for the military forces to accurately
determine their position, velocity, and time in a
common reference system, anywhere on or near
the Earth on a continuous basis. ”
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# In 1973 the U.S. DOD decided to establish,
develop, test, acquire, and deploy a spaceborne
Global Positioning System (GPS), resulting in the
NAVSTARGPS (NAVigation Satellite Timing And
Ranging Global Positioning System).
NAVSTAR Global Positioning System

GPS General CharacteristicsGPS General Characteristics
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Global Positioning System
Developed by the US DOD
Provides
Accurate Navigation
10 - 20 m
Worldwide Coverage
24 hour access
Common Coordinate System
Designed to replace existing
navigation systems
Accessible by Civil and Military

Introduction – GNSS
The theoretical definition:
“GNSS. A worldwide position and time determination system that
includes one or more satellite constellations, aircraft receivers and
system integrity monitoring, augmented as necessary to support the
required navigation performance for the intended operation.” [from ICAO
Annex 10, Volume I]
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• USA’s GPS (+ WAAS)
• Russian GLONASS(+SBAS)
• EU’s Galileo (+ EGNOS);
• Japan’s QZSS (+ MSAS);
• India’s IRNSS (+ GAGAN);
• China’s Beidou; Compass;
• Australian Augmentations
GNSS is the result of a recognition by the
civilian community of the benefits that can
be derived from the development of a 'true'
civilian global positioning system that is:
Multimodal (air, sea and land users),
Capable of meeting future navigation &
timing requirements, Global standard, Cost
effective, Easy to use, Fundamentally based
around the integration and augmentation of
technologies.
GNSS elements:
GPS and augmentation systems
(based on aircraft /satellite/ground inst.)

How Well Does It Work?
Navigation Accuracy Comparisons
Omega- 2 km
Inertial- 1 km
Tactical Air Navigation
(TACAN)- 400m
Transit- 200m
LORAN C- 180m
GPS- 15m
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GPS Constellation
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Space Segment
( Initial Operational Capability(IOC)-1993)
(Full Operational Capability(FOC)-1995)
Block I
First Launch: 22 Feb 78(78-85)
On-Orbit: None, Total=11
Block IIR / IIR-M(L2C civil signal &
new military code M on both L1& L2)
Block II/IIA
First Launch: 14 Apr 89(89-97)
Total: 28
First Launch: 2009
Acquiring up to 19 SV’s
Block IIF
First Launch: 22 Jul 1997/25Sep2005
Total=21/8
(R: Replenishment; M: Modernized)
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Block III

GPS System ComponentsGPS System Components
24 Satellites
4 satellites in 6 Orbital
Planes inclined at 55
Degrees
20200 Km above the Earth
• 12 Hourly orbits
– In view for 4-5 hours
• Designed to last 7.5 years
• Different Classifications
– Block 1, 2, 2A, 2R & 2 F
EquatorEquator
55
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Managed by the US National Space-Based Positioning,
Navigation, and Timing (PNT) Executive Committee
SpaceSpace SegmentSegment

Master Control Station
Responsible for collecting
tracking data from the monitoring stations and
calculating satellite orbits & clock parameters
5 Monitoring Stations
Responsible for measuring pseudorange data.
This orbital tracking network is used to
determine the broadcast ephemeris and
satellite clock modeling
Ground Control Stations
Responsible for upload of information to SV’s
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User Segment:User Segment: The most visible segment
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Everyone!
Merchant, Navy, Coast Guard vessels
Forget about the sextant, Loran, etc.
Commercial Airliners, Civil Pilots
Surveyors
Has completely revolutionized surveying
Commercial Truckers
Hikers, Mountain Climbers, Backpackers
Cars now being equipped
Communications and Imaging Satellites
Space-to-Space Navigation
Any system requiring accurate timing

Weather Independent
Does not require line of sight
Gives high Geodetic Accuracy
Can be operated day and night
Quicker and requires less manpower
Economical advantages
Common Coordinate System
Wide Range of Applications
Competitively Priced
Why GPS ?Why GPS ?
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How It Works (In 5 Easy Steps)
GPS is a ranging system (triangulation)
The “reference stations” are satellites moving at 4
km/s
1. A GPS receiver (“the user”) detects 1-way ranging signals
from several satellites
Each transmission is time-tagged
Each transmission contains the satellite’s position
1. The time-of-arrival is compared to time-of-transmission
2. The delta-T is multiplied by the speed of light to obtain
the range
3. Each range puts the user on a sphere about the satellite
4. Intersecting several of these yields a user position
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Xll
Vl
Xl
lll
l
ll
lV
V
Vll
Vlll
X
lX
Range = Time Taken x Speed of Light
Outline Principle : RangeOutline Principle : Range
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Point Positioning:oint Positioning:
Accuracy 10 - 100 mAccuracy 10 - 100 m
A receiver in autonomous mode provides navigation and
positioning accuracy of about 10 to 100 m due to the effects

Multi-Satellite Ranging
1 range puts user
on the spherical
face of the cone.
Intersecting with
a 2nd range
restricts user to
the circular arcs.
A 3rd range
constrains user
to 1 of the 2
points.
Pictures courtesy http://giswww.pok.ibm.com/gps
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The satellites are like “Orbiting Control StationsOrbiting Control Stations”
Ranges (distances) are measured to each satellite using
time dependent codes
Typically GPS receivers use inexpensive clocks. They are
much less accurate than the clocks on board the satellites
A radio wave travels at the speed of light
(Distance = Velocity x Time)
Consider an error in the receiver clock
1/10 second error = 30,000 Km error
1/1,000,000 second error = 300 m error
Outline Principle : PositionOutline Principle : Position
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Timing
Accuracy of position is only as good as your clock
To know where you are, you must know when you
receive.
Receiver clock must match SV clock to compute delta-T
SVs carry atomic oscillators (2 rubidium, 2 cesium each)
Not practical for hand-held receiver
Accumulated drift of receiver clock is called clock bias
The erroneously measured range is called a pseudorange
To eliminate the bias, a 4th
SV is tracked
4 equations, 4 unknowns
Solution now generates X,Y,Z and b
If Doppler also tracked, Velocity can be computed
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GPS Signal StructureGPS Signal Structure
Each GPS satellite transmits a number of signals
The signal comprises two carrier waves (L1-19cm and L2-
23cm) and two codes (C/A on L1 and P or Y on both L1 and L2)
as well as a satellite orbit message
Bandwidth allocated for L1-24 MHz, L2-22 MHz, & L5-28 MHz
Fundamental
Frequency
10.23 MHz
Fundamental
Frequency
10.23 MHz
x 154
x 120
L1
1575.42 MHz
L1
1575.42 MHz
L2
1227.60 MHz
L2
1227.60 MHz
C/A Code
1.023 MHz
C/A Code
1.023 MHz
P (Y)-Code
10.23 MHz
P (Y)-Code
10.23 MHz
P (Y)-Code
10.23 MHz
P (Y)-Code
10.23 MHz
÷ 10
50 BPS50 BPS Satellite Message (Almanac & Ephemeris)Satellite Message (Almanac & Ephemeris)
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L3(1381.05MHz):L3(1381.05MHz): used only for a Nudet (Nuclear Detection)used only for a Nudet (Nuclear Detection)
Detection System (NDS).Detection System (NDS).

Code Modulation
Source: Peter Dana, http://www.colorado.Edu/geography/gcraft/notes/gps/gps_f.html
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Selective Availability (SA):To deny high-accuracy realtime
positioning to potential enemies, DoD reserves the right to
deliberately degrade GPS performance (on C/A code: deactivated
on 1 may, 2000).

Coarse Acquisition (C/A) Code
1023-bit Gold Code
Originally intended as simply an acquisition code for P-
code receivers
Modulates the L1 only
Chipping rate = 1.023 MHz (λ=290 meter)
Sequence Length = 1023 bits, thus Period = 1 millisec
Provides the data for Standard Positioning Service (SPS)
The usual position generated for most civilian receivers
Modulated by the Navigation/Timing Message code
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Precise (P) Code
Generally encrypted into the Y-code (A.S.)
Requires special chip to decode
Modulates both L1 & L2
Also modulated by Nav/Time data message
Chipping rate=10.23 MHz (λ=29.30m) i.e. 10 times faster
than C/A code ensuring improved time measurement.
Sequence Length = 2.35*1014
bits, thus Period = 266 days
P-code rate is the fundamental frequency (provides the
basis for all others)
P-Code (10.23 MHz) /10 = 1.023 MHz (C/A code)
P-Code (10.23 MHz) X 154 = 1575.42 MHz (L1).
P-Code (10.23 MHz) X 120 = 1227.60 MHz (L2).
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Navigation Message
In order to solve the user position equations, one must know where
the SV is:
The navigation and time code provides this
50 Hz signal modulated on L1 and L2
The SV’s own position information is transmitted in a 1500-bit data
frame
Pseudo-Keplerian orbital elements
Determined by control center via ground tracking
Receiver implements orbit-to-position algorithm
Also includes clock data and satellite status
And ionospheric / tropospheric corrections
The International Telecommunication Union (ITU) has reserved 1559-
1610MHz band for satellite based navigation through World Radio
Communication (WRC) conferences, held every three year.
GPS bands (US Federal Communication Commission): (1215-
1240MHz, 1559-1610 MHz, L5- 1164-1188MHz)
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The Almanac
In addition to its own nav data, each SV also
broadcasts info about ALL the other SV’s
In a reduced-accuracy format
Known as the Almanac
Permits receiver to predict, from a cold start,
“where to look” for SV’s when powered up
GPS orbits are so predictable, an almanac may be
valid for months
Almanac data is large
12.5 minutes to transfer in entirety
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GPS PositioningGPS Positioning
• Point Positioning Methods using stand alone receivers
provide 10 - 100 m accuracy
– Dependent on SA
– 1 Epoch solution
• Differential Positioning Methods using 2 receivers,
simultaneously tracking a minimum of 4 satellites
(preferably 5) will yield 0.5 cm to 5 m accuracy with
respect to a Reference Station
• Differential Techniques using CodeCode will give metermeter
accuracy
• Differential Techniques using PhasePhase will give
centimetercentimeter accuracy
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Topo and Locations
Mapping
Monitoring
Volumes
Photo control
Construction Control
and Stakeout
Boundaries
Seismic Stakeout
Profiles
Establishing Portable Control
Stations (sharing with Total
Stations)
Agriculture - Slope Staking
Tracking of people, vehicles
Plate movements
Sports (boating, hiking,…)
Archeology
Public Transport
Emergency services
ApplicationsApplications
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User requirements in positioning accuraciesUser requirements in positioning accuracies
Accuracy Geomatics Land Marine Airborne
Geodetic Infrastructure Earth Moving Dredging Cat
II/III
Fault Monitoring Road Grading Pylon Positioning Sensor positioning
< 20 cm Construction Surveys Agriculture (air- spaceborne)
Engineering Surveys Mining
Geodynamics Urban cadastral survey
Resource Mapping Facility Surveys
GIS Database Mapping/GIS Docking
0.2 - 1.5 m Utility Mapping Highway Surveys Buoy Position
Highway Surveys Rural cadastral survey
Legal Surveys Precision farming
GIS Data Collection Automobiles Channel Navigation Sensor navigation
1 - 5 m Site Specific Farming Emergency Cabling Oceanic
Navigation Public Transport Research
Tracking Harbor Entry
Mapping
10 - 100 m Reconnaissance Navigation Harbor Approach
Area Navigation Oceanic
# Accuracy requirement depending on application iirs
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GPS Opportunities in India
• Indian GPS market can be broadly divided in following segments
• National Survey Agencies(NRSC, SOI,Mil. Survey,NHO,)
• Land Records (Kerala,TN,J&K, Rajasthan,Gujrat, etc.)
• Oil and Energy( OIL, ONGC, BPCL, HPCL)
• Power(NHPC Assam, Himachal, J&K,Arunachal, Kaveri Basin)
• Defense, Remote Sensing , GIS, Forestry AND Education
• Scientific Institution(IIGM,CMMAC,WIHG, NGRI, SAC,GSI)
• Mining and Geology(CMPDIL,WCL,MCL, SECL, NCL,BCCL)
• Consultancy( CRRI, CBRI, NCB, CSMRS,WAPCOS)
• Engineering /Infrastructure(IRCON,RITES,Delhi Metro, NHAI)
• Private Surveying Companies (ICT, Theowell, Secon, etc. )
• Marine and Maritime Boards(Port Trust, MMB, NIO, etc.)
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Never get lost again
Earth deformation measurement is the main application of GNSS.
GPS is no longer primarily a military tool
Now a product with vast commercial potential
From aviation to outdoor recreational activities
Applications will change our lives and save money
GPS with computer mapping will help manage our natural
resources
Vehicle location and navigation lets us avoid congested freeways,
find more efficient routes and save fuel, & air pollution
Ships and aircraft are safer in all weather conditions
Businesses with large outside plant (railroads, utilities) will manage
resources more efficiently, reducing consumer costs
The GPS technology is evolving rapidly
More accurate and affordable TODAY than YESTERDAY.
Conclusion
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References
http://gps.gov/
http://www.glonass-ianc.rsa.ru/pls/htmldb/f?
p=202:1:15000421459964108253
http://igscb.jpl.nasa.gov/
http://www.navcen.uscg.gov/?pageName=GPS
Interface Control Documents:
http://www.navcen.uscg.gov
http://www.Glonass-ianc.ras.ru
http://www.Galileoju.com
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Communications
Surveying &
Mapping
Fishing &
Boating
Off shore
Drilling
Recreation
Trucking &
Shipping
Personal Navigation
Aviation
Railroads
Power Grid
Interfaces
Use of GPS
Precision farming
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