Introduction toGlobal Navigation Satellite System (GNSS)
Signal Structure
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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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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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

Slide : 3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]
123

4

5

678

910Output

123

4

5

6

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, it
will 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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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
+1

1

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.
11001011You 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 satellitesSatellites 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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GPS L1C/A Signal NAV MSG

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

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Navigation Message, Sub-frame 1

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

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GPS L1C/A Signal NAV MSG, Sub-frame 2

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

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GPS L1C/A Signal NAV MSG, Sub-frame 3

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

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GPS L1C/A Signal NAV MSG, Sub-frame 4 Page 1,6,11,16,21

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

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GPS L1C/A Signal NAV MSG, Sub-frame 4 Page 12,19,20,22,23,24

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

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GPS L1C/A Signal NAV MSG, Sub-frame 4, Page 14, 15

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

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GPS L1C/A Signal NAV MSG, Sub-frame 4, Page 17

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

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GPS L1C/A Signal NAV MSG, Sub-frame 5

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 aSpectrum 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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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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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*

dBW

dBm

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 directly
e.g. using a Spectrum Analyzer
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

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