Introduction to
Global Navigation Satellite System (GNSS)
How It Works?
Dinesh ManandharCenter for Spatial Information Science
The University of Tokyo
Contact Information: dinesh@iis.u-Tokyo.ac.jp

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Fundamental Problem• How to know my location precisely ?
• In any condition• At any time
• Everywhere on earth (at least outdoors!)

Where am I
on the Earth?
350 40’ 41”
1390 39’ 40”• How to navigate to the destination? ?
• Guidance or Navigation

How far ?
Which Route ?
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Navigation Types• Landmark-based Navigation• Stones, Trees, Monuments• Limited Local use• Radio-based Navigation• LORAN, OMEGA• Subject to Radio Interference, Jamming,
Limited Coverage• Celestial-based Navigation• Stars, Moon
• Complicated, Works only at Clear Night• Sensors-based Navigation• Dead Reckoning• Satellite-based Navigation or GNSS
• TRANSIT, GPS, GLONASS, GALILEO, QZSS,BEIDOU (COMPASS), IRNSS• Global, Difficult to Interfere or Jam, HighAccuracy & Reliability• Gyroscope, Accelerometer, Compass,Odometer• Complicated, Errors accumulate quickly
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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What is GNSS?
Global Navigation Satellite System (GNSS) is the standard generic term for allnavigation satellites systems like GPS, GLONASS, GALILEO, BeiDou, QZSS, NAVIC.
• Global Constellation• GPS USA
• GLONASS, Russia• Galileo, Europe• BeiDou (COMPASS), China• Regional Constellation• QZSS, Japan• NAVIC (IRNSS), India

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 4Satellite Based Augmentation System (SBAS)• Satellite Based Augmentation System (SBAS) are used to augment
GNSS Data• Provide Higher Accuracy, Integrity, Continuity and Availability• Some correction data like satellite orbit, satellite clock and atmospheric data
are broadcasted from communication satellites• Used by ICAO for Aviation• Different Types of SBAS
••
•
•
•

WAAS, USA
MSAS, JapanEGNOS, Europe
GAGAN, India
SDCM, Russia
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Determine the Distance using Radio Wave
0ms

Satellite Transmits Signal at 0ms.
25ms

Receiver Receives
the Same Signal
after 67ms.
0ms

50ms

Satellite with a known positiontransmit a regular time signal.

75ms

Speed of Light
300,000 km/s

25ms
50ms

Distance = (Transmission time – Reception time)× Speed of light
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Principle of Satellite-based Navigation(xk,yk,zk)(x,y,z) kx  x   y  y   z  z   bk2k2

kIf k ≥ 4, solve for x, y, z and clock bias, b2∆tCorrelation between Incoming Signal andReceiver Generated Signal

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Characteristics of GNSS Signals• GNSS Signals have basically three types of signals• Carrier Signal• PRN Code (C/A Code)• Navigation Data

• All GNSS Signals except GLONASS are based on CDMA
• Only GLONASS use FDMA
• Future Signals of GLONASS will also use CDMA• The modulation scheme of GNSS signals are BPSK andvarious versions of BOC

CDMA: Code Division Multiple Access
FDMA: Frequency Division Multiple Access
BPSK : Binary Phase Shift Keying
BOC: Binary Offset Carrier

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Method of GPS L1C/A Signal Generation
900 Phase Reversex154

L1 Carrier, 1575.42Mhz∑x1/10Clock10.23Mhz

C / A Code, 1.023Mhz

x1/204600

Navigation Data, 50Hz

L1 Band GPS Signal

P Code, 10.23Mhz

si (t )  2 Pi (t )  CAt   i (t )   D t   i (t )   cos2  f L  f L,i (t ) t  i (t )   ni (t )Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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PRN (Pseudo Random Noise) Code• PRN Code is a sequence of randomly distributed zeros and ones that is one millisecondlong.
• This random distribution follows a specific code generation pattern called Gold Code.
• There are 1023 zeros or ones in one millisecond.

• Each GPS satellite transmits a unique PRN Code.
• GPS receiver identifies satellites by its unique PRN code or ID.

• It is continually repeated every millisecond and serves for signal transit time measurement.
• The receiver can measure where the PRN code terminated or repeated.

1ms / 1023

0 1 0 1 0 0 11 0 1 0 0
1ms
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS L1C/A PRN Code Generator
G1 Polynomial: [3,10]1234567

8910Output

1234

56

78910

G2 Polynomial: [2,3,6,8,9,10]

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Characteristics of PRN CodeAuto-correlation: Only four values:
1023, 1, 63 or 65 (Ideal case)

Cross-correlation: Only three values:
1, 63 or 65 (Ideal Case)• PRN codes are very uniquely designed.
• GPS and other GNSS use CDMA
• One PRN code is assigned to one satellite.
• In case of GPS, PRN code is 1023 bits long.
• GLONASS is different. It uses FDMA. The same code for all
satellites but different frequencies.
• Some new signals of GLONASS also uses CDMA signals• Maximum Cross-correlation Value is -23dB.
• If any signal above this power enters a GPS receiver, itwill totally block all GPS signals.
• If longer PRN code is used, receiver becomes moreresistive to Jamming signal• But, signal processing is more complex

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS signal structure
Carrier Wave

GPS Signal

2P sin 2ft 2P xt Dt sin 2ft PRN

Navigation Messagext Dt +1

+1-1

-1

1.023Mbps

50bps

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 13BPSK (Binary Phase Shift Keying)Phase shift keying is a digital modulation scheme that conveys
data by changing, or modulating, the phase of the carrier
wave. BPSK uses two phases which are separated by a halfcycle.
Carrier Wave
+11

100

1011 Digital Bit Stream-1
Binary Phase Shift Keying

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Modulation
Modulation is the process of conveying a message signal, for example a digital bit
stream, into a radio frequency signal that can be physically transmitted.
1100101

1

You want to transmit
this binary code

Amplitude Shift Keying

Frequency Shift Keying

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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CDMA vs. FDMA
CDMA
[GPS, QZSS, Galileo, BeiDou,
IRNSS, Future GLONASS
Satellites]

FDMA[GLONASS]PRN CodeDifferent PRN Code for eachsatellite
Satellites are identified by PRN
CodeOne PRN Code for all satellites
Satellites are identified by center
frequency

Frequency

One Frequency for all satellites

Different frequency for eachsatellite

Merits &Demerits

Receiver design is simpler
No Inter-Channel Bias
More susceptible to Jamming

Receiver design is complex
Inter-channel bias problem
Less susceptible to Jamming

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Navigation Data
• Navigation Data or Message is a continuous stream of digital
data transmitted at 50 bit per second. Each satellite
broadcasts the following information to users.
• Its own highly accurate orbit and clock correction (ephemeris)• Approximate orbital correction for all other satellites (almanac)• System health, etc.

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS L1C/A Signal NAV MSG

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Pseudorange (1/2)
Transmission
Time

Pseudorange = (Transmission time – Reception time)× Speed of light

Signal propagation at the speed of light

20,200 km

Reception
TimeTransit time

A GPS receiver measures the signal transmission timefrom the code phase at signal reception time.
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Pseudorange (2/2)• Essential GNSS observable• Full distance between the satellite and the receiver• Provides a position accuracy of approximately a few meters2m
20,200 km
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Carrier phase (1/2)• PRN repeats every 1ms, which corresponds 300 km in distance at thespeed of light, but pseudorange accuracy is about 1 m.
• Carrier phase provides millimeter range accuracy, but repeats everycycle, which correspond 19 cm in distance at a GPS signal carrier
frequency of 1575.42 MHz.
Pseudo random numberCarrier wave
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Carrier phase (2/2)• Fractional carrier phase of the received signal• Therefore there is an unknown integer number of full carrier
cycles between the satellite and the receiver• Provide “survey-grade” accuracy of 1-2 cm once theunknown number of full carrier cycles are resolved

19 cm

1 cm

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS Signal Power: How Strong or How Weak?
• GPS satellites are about 22,000km away
• Transmit power is about 30W• This power when received at the receiver isreduced by 1016 times.

30Watt• The power reduces by 1/distance2• This is similar to seeing a 30W bulb 22,000Km far• GPS signals in the receiver is about 10-16 Watt,
which is below the thermal noise
10-16 Watt

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS Signal Power: How Strong or How Weak?
• GPS Signal Power at Receiver• -130dBm or -160dBW• Thermal Noise Power• Defined by kTeffB, where• K = 1.380658e-23JK-1, Boltzman Constant• Teff = 362.95, for Room temperature in Kelvin at 290• Teff is effective Temperature based on Frii’s formula• B = 2.046MHz, Signal bandwidth• Thermal Noise Power = -110dBm for 2MHz bandwidth• If Bandwidth is narrow, 50Hz
• Noise Power = -156dBm
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS Signal Structure
900 Phase Reversex154

L1 Carrier, 1575.42Mhz∑x1/10

X1, Clock10.23Mhz

C / A Code, 1.023Mhz

x1/204600

Navigation Data, 50Hz

L1 Band GPS Signal

P Code, 10.23Mhz
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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GPS Signal Power
Noise PowerAny Signal below thisnoise level can’t bemeasured in a
Spectrum Analyzer

GPS Signal Power at
Antenna, -130dBm

Mobile phone, WiFi,
BT etc have power
level above -110dBm,
much higher than GPSSignal Power

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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Power of GPS Signal vs. Other Signals
Signal TypePower (based on calculations, not measured)

Below Noise

Above Noise

Watt
Mobile Phone
Handset TX Power *RX Power at Mobile
Phone Handset*dBWdBm

1W

0dBW

30dBm

100e-6W-40dBW-70dBmZigBee316e-16W-115dBW-85dBm

VHF

200e-16W-137dBW-107dBm

Thermal Noise79e-16W-141dBW-111dBm

GPS**

1e-16W-160dBW-130dBm• * Actual power values will differ. These are just for comparison purpose• ** GPS Signals are hidden under the noise. Thus, it can’t be measured directlye.g. using a Spectrum AnalyzerDinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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