Hakan Velioğlu
27.03.2012 Remote Sensing and Satellite Comminication
Instructor: Dr. Sedef Kent
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Short History
Transit Satellite System
Kosmos(Cosmos), Parus, Tsikada
Global Positioning System (GPS)
GLONASS
Satellite Navigation Techniques
Doppler Effect Based Navigation System
Trilateration Based Navigation System
GPS (NAVSTAR)
Space Segment
Control Segment
User Segment
GPS Signal Structure
GPS Pseudorange Measurements
From PRN code
From Carrier Phase measurement
GPS Position Calculation
GPS Positioning Services
GPS Positioning
Point Positioning
Relative Positioning (Differential Positioning -DGPS)
GPS Error Sources
Applications of Navigation
References
Short History
Definition of Navigation:
"the guidance of ships or airplanes from place to place"
(http://www.audioenglish.net/dictionary/navigation.htm)
What we actually need from Navigation is to measure our points/objects
POSITION and SPEED.
Early navigation for sailing and air travel -- LORAN (Long Range Navigation).
Military needs.
Short History
Transit Satellite System
• USA Navigation System
• The first navigation satellite system in the world
• Transit I launched at 13 April 1960 (~3 years after the first artificial satellite
Sputnik).
• Transit is operational for military at 1964 for civilian 1967
• Transit had used 6 satellites (3 active and 3 spare) at LEO orbit of altitude
~1000km.
• Transit System navigation working principle is Doppler effect.
• Transit System propogates signals in two frequencies (150Mhz, 400Mhz)
• Last Transit satellite launched 1988
• Transit System gives positions for two dimension (longtitude, latitude) not
height.
• Low altitude = Small coverage & Short service time
Short History
Kosmos(Cosmos), Parus, Tsikada
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Soviet Union (Russian)
First Kosmos navigation satellite is launched at 1967
Similar principles and frequency used with Transit system.
Operational until 1978
Kosmos system is superceded by Parus and Tsikada system.
• Parus is for military
• 6 orbital plane, 30◦ longitude intervals.
• 98 satellites (last one at 21 July 2009) were launched.
• Tsikada is for civilian use
• 4 orbital planes, 45◦ longitude intervals.
• 20 satellites (last one at 21 January 1995) were launched.
Short History
Global Positioning System (GPS)
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USA Navigation System
GPS formally known as NAVSTAR (Navigation Satellite Timing and Ranging)
First effort began in late 1972
First GPS satellite launched in 1978
System is fully functional on 17 July 1995 with 24 operational satellites.
Most common system for navigation.
Uses trilateration techniques.
There are 32 operational satellites in constellation
• We will dig into GPS in later sections.
Short History
GLONASS
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Soviet Union (Russian)
Firs satellite launched on 12 October 1982
MEO circular orbit of altitude ~19100km
Uses trilateration techniques
System fully operational on 23 September 1993.
Due to short life time of satellites GLONASS had 10 satellites in August 2000.
Now there are 24 satellites constellation in operation.
Satellite Navigation Techniques
• Doppler Effect Based Navigation System
• Trilateration Based Navigation System
Satellite Navigation Techniques
Doppler Effect Based Navigation System
• Christian Doppler found that if reciver and transmitter is moving accourding
to each other then, the transmitted signal frequency is changed according to
this movement (1842).
• It can be observed from a moving car. While a car is getting closer or away,
it's sound changes.
http://en.wikipedia.org/wiki/Doppler_effect
Satellite Navigation Techniques
Doppler Effect Based Navigation System
http://www.seaturtle.org/tracking/faq.shtml
http://www.aviso.oceanobs.com/en/doris/principle/index.html
Satellite Navigation Techniques
Doppler Effect Based Navigation System
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 511
Satellite Navigation Techniques
Doppler Effect Based Navigation System
• Transit and Kosmos navigation satellites used this system.
• Satellites transmit signal in two frequencies (150Mhz, 400Mhz)
• Signal contains the satellite path and timing information
• Receiver calculates doppler effects and satellite path to find its position
• 1 satellite is enough to calculate position
• Accuracy is 500m for single frequency, 25m for dual frequency
• Position can be calculated in two dimension (latitude, longitude)
Satellite Navigation Techniques
Trilateration Based Navigation System
• GPS and GLONASS use this system. Glileo will use this system.
• Trilateration system has better accuracy and wide coverage
• Receiver calculates its position by measuring its distence to three or four
satellites.
http://joem.hubpages.com/hub/How-Does-GPS-Work-In-Cell-Phones
Satellite Navigation Techniques
Trilateration Based Navigation System
Satellite Navigation Techniques
Trilateration Based Navigation System
http://pegasus.cc.ucf.edu/~jweisham/pcb5937/GPS/GPS.html
GPS (NAVSTAR)
GPS comprises of three segments.
1. Space Segment
• Satellites
2. Control Segment
• Control Stations
• Monitor Stations
• Antenna Stations
3. User Segment
• Military Users
• Civil Users
GPS (NAVSTAR)
Space Segment
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24? satellites in operation and 32 satellites in orbit.
Medium Earth Orbit (MEO), altitude of ~20200km
6 orbital planes
Satellites inclined at 55◦ to the equator
Orbital period 11 hours 58 minutes
5 hours view in horizon
All satellites have a Rubidium atomic clock (accuracy of 1 second in 300000
years, 3ns in a second)
Satellites use solar energy and rechargable battery for power
Small rocket boosters for station keeping.
Satellites also have nuclear blast detectors
Satellites transmit signals in two Microwave bands L1 (1575.42Mhz) and L2
(1227.60Mhz)
GPS (NAVSTAR)
Space Segment
http://www.defenseindustrydaily.com/the-gps-constellation-now-and-future-01069/
GPS (NAVSTAR)
Control Segment
The purpose of control segmen is to track all satllites position and health and
manage/control them.
• Ground Antenna Stations:
• Send commands to satellites using S band.
• Receive temeletry data
• Monitor Stations:
• Track satellite positions (they have very accurate GPS receiver and
Cesium oscilators)
• Sent track data to Master Control Station
• Master Control Station
• Get data from Monitor Stations
• Calculate this data in every 15 minutes.
• Upload path correction data to satellites with Ground Antenna Stations
ones or twice in a day.
GPS (NAVSTAR)
Control Segment
http://www.fas.org/spp/military/docops/army/ref_text/chap07c.htm
GPS (NAVSTAR)
Control Segment
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 519
GPS (NAVSTAR)
User Segment
User segment includes all receiver devices.
A receiver must do three main task
1. Get the radio signal correctly
• One or more antenna
• Filter
• Amplify
• Remove the carrier signal (down converter)
2. Process the signal (DSP).
• Calculate the Pseudorange code (PRN)
• Correlate signal with appropriate satellite
3. Position computation
• Compute position (according to WGS84), velocity, time, etc...
• Display this data (on a screen, on a map)
GPS (NAVSTAR)
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 519
GPS Signal Structure
GPS signal contains 3 types of information
1. Pseudorandom code (PRN): ID of satellites that transmits signal
• PRN Code is two type:
• C/A Code
• Civilian Access Code
• 1023 bits with bitrate of 1.023Mbps
• Code repeats itself in every milisecond
• Carried with L1 signal (1575.42 Mhz)
• P Code
• Military use and Encrypted
• 2.35 x 10^14 bits with bitrate of 10.23 Mbps
• Code repeat itself in every 266 days. 38 codes each of them is 7
days long.
• Carried with L1 and L2 (1227.60 Mhz) signal
• Could be further encrypted to Y code
2. Ephemeris data: Information about health of the satellite and current date
and time
3. Almanac data: Information of each satellites position during the day.
GPS Signal Structure
• Almanac and Ephemeris data is named as Navigation data
• Navigation data is transmitted at a bitrate of 50Kbps
• All information transfer is made with BPSK with 180 degree phase shifting for
1 and 0
• All GPS satellites uses same frequency bands
• GPS use CDMA ( Code Division Multiple Access) technique to discriminate
each satellites information from PRN code.
GPS Signal Structure
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 523
GPS Pseudorange Measurements
From PRN code
Receiver gets satellite signal which has PRN code in it.
At the same time receiver crates same PRN code
While receiver gets satellite signal, it contains a delay
This delay is calculated by compairing receivers code with detected signals
code.
• This delay is multiplied with the speed of electromagnetic wave to found the
distance to the satellite.
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• PROBLEM: Receiver clocsks are not as accurate as satellites clock.
This problem can be solved by getting signal from at least 4 satellite.
GPS Pseudorange Measurements
From PRN code
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 524
GPS Pseudorange Measurements
From PRN code
Accuracy of C/A code:
• C/A code is 1.023Mbps.
• Duraiton of 1 bit  1/(1.023 * 10^6) ~= 10^-6 sec.
• Distance = Velocity * Time
• Inaccuracy of C/A code  3*(10^8) * 10^-6 = 300m
Accuracy of P code:
• C/A code is 10.23Mbps.
• Duraiton of 1 bit  1/(10.23 * 10^6) ~= 10^-7 sec.
• Inaccuracy of C/A code  3*(10^8) * 10^-7 = 30m
GPS Pseudorange Measurements
From Carrier Phase measurement
• Range is the sum of carrier phase and some fraction.
• This sum is multiplied by wavelength 19cm (L1) or 24cm (L2)
• As the vawelengths are in cm, so inaccruacy is as short as milimeter.
• However, calculation of this technique requires two receivers and complex
signal processing.
• It is used in Diferential GPS.
GPS Position Calculation
(x1 − Ux)2 + (y1 − Uy)2 + (z1 − Uz)2 = (PR1 ± EC)2
(x2 − Ux)2 + (y2 − Uy)2 + (z2 − Uz)2 = (PR2 ± EC)2
(x3 − Ux)2 + (y3 − Uy)2 + (z3 − Uz)2 = (PR3 ± EC)2
(x4 − Ux)2 + (y4 − Uy)2 + (z4 − Uz)2 = (PR4 ± EC)2
xn, yn, zn =x, y and z coordinates of the nth satellite
Ux,Uy,Uz =x, y and z coordinates of the user receiver
PRn
= pseudorange of the user receiver from the nth satellite
EC
= error correction
GPS Positioning Services
Standart Positioning System (SPS)
• Available to all GPS receivers worldwide
• Calculations made with C/A code in L1 frequency (1575.42 Mhz)
• Horizontal accuracy 100-300m
• Vertical accuracy 140m
• Timing accuracy < 340ns.
• Selective Availability (befora 1 May 2000)
Precise Positioning System(PPS)
• Availably to only authorized users only
• Calculations made with P code in L1 and L2 frequencies
• Horizontal accuracy 16m
• Vertical accuracy 23m
GPS Positioning
Point Positioning
• Positioning with one GPS receiver
• Receiver calculates its position from three or four satellites by using PRN
codes.
• Low accuracy
GPS Positioning
Relative Positioning (Differential Positioning -DGPS)
• This is used for high accuracy applications
• Two GPS receivers used whileall of them sees same satellites
• Measurement is done with PRN code or from phase measurement
• Main principle is to calculate GPS error from a base locaiton and propogate
this correction data to other receivers
GPS Error Sources
Signal Propogation Error
• Delays in signal caused by the medium (ionosphere, troposphere)
• Change in signal propogation speed
• Lower the frequency greater the delay, so L2 is much more affected than L1
• This error can be eliminated by using DGPS or calculating P code from two
frequencies.
Multipath Reflection
• This error occurs when same signal comed from different paths.
• This multipath occurs when GPS signal is reflceted from objects like tall
buildings or large rock surfaces.
• This error can be eliminated by using ring antenna or selection of good
position for surveying.
Clock Errors
• Receiver clocks are not accurate as satellite clocks.
• This error can be reduced by calculation from different satellite.
GPS Error Sources
Ephemeris Errors
• Inaccurcies in the satellites reported popsition
• This error is about 2 to 5 m.
• Can be eliminated with DGPS
Number of visible satellites
• More visible satellites means uch more accurate positioning
• Buildings, mountains, rocks may affect GPS signal.
• GPS signal can’t propogate from objects like water, buildings
Satellite Distribution Geometry
• If satellites are distributed to wide angels than positioning is good
• If satellites are grouoped to one direction than positioning is not good
GPS Signal Interference
• GPs signal power is weak and for this reason it can be interfere from near
frequencies
GPS Error Source
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini and Varsha Agrawal , Page 532
Applications of Navigation
Military Applications
• Navigation for soldiers
• Navigation for military vehicles
• Navigation for missiles
Civilian Applicaitons
• Vehicle tracking and navigation
• Rescue operations
• Mapping and construcitons
• Geodesic sciences
• Precise timing
References
Main Reference:
Satellite Technology: Principles and Applications, Second Edition Anil K. Maini
and Varsha Agrawal © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66024-9
Useful links:
• http://news.stanford.edu/pr/95/950613Arc5183.html
• http://www.loadstone-gps.com/docs/articles/hist_nav.html
• http://en.wikipedia.org/wiki/LORAN
• http://www.vectorsite.net/ttgps_2.html
• http://www.zarya.info/Frequencies/FrequenciesAll.php
• http://www.glonass-ianc.rsa.ru/en/
• http://gps.gov/
• http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/323.htm
Questions?
Thanks for listening.