How does a GPS signal look ?
MGA Webinar Series # 13
Dr. Dinesh Manandhar, Associate Professor (Project)
Center for Spatial Information Science, The University of Tokyo
14th MAY 2020, 18:00 – 19:00 (JST)ZOOM Link:Registration: https://gnss.peatix.com

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo1

Introduction
This webinar provides information on GPS L1C/A signal structure, signal generation and its properties like power spectrum,
signal power. A software generated signal will be used for demonstration to show its properties like modulation,power spectrum and measurement of these power values using a Tektronix RSA 306B spectrum analyzer.
This webinar will help you to understand and visualize GPS signal. GPS L1C/A signal is a base signal for many other GNSS
signals. If you are interested in signal processing, jamming, interference and spoofing (JIS) issues, then this webinar will
provide necessary information required to understand JIS mitigation techniques. We will conduct webinar on JIS in near
future.
DATE: 14th MAY 2020 (Thursday)
TIME: 18:00 - 19:00 JST (Japan Time)
Registration: https://gpsca.peatix.com
Main Link: https://gnss.peatix.com (Registration is required)Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

2

GPS L1C/A Signal Structure• Carrier Signal• It defines the frequency of the signal• For example:• GPS L1 is 1575.42MHz, L2 is 1227.60MHz and L5 is 1176.45MHz• PRN (Pseudo Random Noise) Code• Randomly generated codes following a particular rule
• Used to modulate the data and identify each satellite• Should have good auto-correlation and cross-correlation properties• Navigation Data

• Required to compute distance between the satellite and the receiver• Provides satellite orbit related data (ephemeris data)• Provides satellite clock related information (clock errors etc)
• Provides satellite health related information to receiver
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo3

GPS L1C/A Signal Structure• Carrier Signal• Frequency: 1575.42MHz• PRN Code• Chip Rate
• Code Period• No of Chips/Period: 1.023MHz: 1msec: 1023

• Navigation Data• Data Rate
• Bit Length• Modulation: 50bps (bits per sec)
: 20msec: BPSK(1)• BPSK (Binary Phase Shift Keying)Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

4

GPS Signals
Band

L1

L2

L5Frequency,
MHz1575.42(10.23*154)

1227.60(10.23*120)
1176.45(10.23*115)

Modulation
Type

Data /SymbolRate,bps/sps

Notes

1.023

BPSK

50Legacy Signal

101.023

BOC(1,1)

50 / 100

CPilot101.023TMBOC

No DataP(Y)

7 days

10.23

BPSKCM200.5115

CL15000.5115P(Y)

7days

10.23

I

1Q

1SignalType

CodeLengthmsec

Chip Rate,MHzC/A

1CData

10.23

BPSK

Restricted / Military

25 / 50
No Data

BPSK
BPSK

BOC(1,1) & BOC(6,1)*

Modulated by TDM of(L2CM xor Data) andL2CL

Restricted / Military

50 / 100

Provides Higher
Accuracy

No Data

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

5

GPS L1C/A, P Signals

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

6GPS L1C/A Signal Structure

Other Signal

900 Phase Reversex154

L1 Carrier, 1575.42Mhz
Carrier Signal

X1, Clock10.23Mhz∑x1/10C / A Code, 1.023Mhz
PRN Code

L1 Band GPS Signal

x1/204600

Navigation Data, 50 bps
Navigation Data
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

7

GPS L1 C/A PRN CODE

Source: GPS Interface Specification Document, IS-GPS-200,
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

8

Generation of GPS L1C/A PRN Code•••••••

Based on Gold Codes
Use two 10 bit registers, G1 and G2
LFSR (Linear Feed Shift Register)
All initial bits of registers are set at 1
Taps 3 and 10 are used for G1
Taps 2,3,6,8,9,10 are used for G2
Two additional taps are selectedbased on PRN ID. See GPS IS
document for the list of the taps.
Example, Taps 2 and 7 are used for
PRN ID 1.

G1 Polynomial: [3,10]
11

1

11

1

1

1

1

1

1234

5678

910

OutputTap 2 & 6 for PRN ID 112345678910

11111111

1

1

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

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

9PRN Code Output #1

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

10

PRN Code Output #3 and #25

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

11

Modulo – 2 Operation
Represent all signals by “+1” or “-1” and use multiplication for Software Approach
00=

01X

1=

10

1=

11X

-1

=-1

1

0=

1-1

X

1=-1

1

1=

0-1X-1=1• Either use “0” and “1” or “1” and “-1”

• Represent “0” by “1” and “1” by “-1”

• We need the code representation in• Then multiply for Modulo - 2 operation• Simpler for programming“1” and “-1” format• Digital signal can have only “+1” or “-1”
Example:PRN 1 : 1 1 0 0 1 0 0 0 0 0….. Shall be-1 -1 1 1 -1 1 1 1 1 1 ……………

+1• “1” or “0” means “ON” and “OFF”-1

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo12

GPS L1 C/A Code of PRN 1 to 32
PRN 1- [PRN 2- [C/A Codes for
the First 10bits
PRN 1 : 1100100000
PRN 29 : 1001010111
PRN 32 : 1111001010

PRN 32- [Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

13

Auto Correlation Property• Auto correlation Property of Gold Code when theshift is an integer multiple of a chip width is :• 1
• -1/N• -B(n)/N• [B(n)-2]/N• Where B(n) = 1+2(n+2)/2• n = 10 for Gold Code that generates C/A code

• Example :• N = 2n-1 = 1023 for n = 10
• Auto Correlation Values are• 1023/1023 = 1• -1/1023• -65/1023• 63/1023

or 20log10(1) = 0dB
or 20log10(1/1023) = -60dB
or 20log10(65/1023) = -24dB
or 20log10(63/1023) = -24dB

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

14

Auto-Correlation and Cross-Correlation Properties of C/A CodeAuto Correlation of C/A Code of Satellite No : 10

Auto Correlation of C/A Code of Satellite No : 10100010001023/1023 = 1or 20log10(1) = 0dB

900
800

Autocorrelation function Rc()Autocorrelation function Rc()

800

Zoom

600

400

200700600
500400300200
10000-1000-800-600-400-200

0Delay 

200

400

600

800

1000-424660

1000-1/1023-65/1023
63/1023

or 20log10(1/1023) = -60dB
or 20log10(65/1023) = -24dB
or 20log10(63/1023) = -24dB

40

Autocorrelation function Rc()Autocorrelation function Rc()0Delay 

Auto Correlation of C/A Code of Satellite No : 10 & 12

Auto Correlation of C/A Code of Satellite No : 10 & 12800-26004002000-20-40Zoom

0-100020-800-600-400-2000Delay 200400600800

1000-60-20-15-10-50Delay Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

510152015

Auto-Correlation and Cross-Correlation Properties of C/A Code0dB-24dB-60dB

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

16

Carrier, Code and Data Modulation Example
BPSK (Binary Phase Shift Keying)
DataRandom CodeSpread Spectrum CodeCarrier Signal

Carrier Signal

Modulated Spread Spectrum Signal

SSDSSSBPSK: Spread Spectrum: Direct Sequence Spread Spectrum: Binary Phase Shift Keying
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo17

Generate GPS L1C/A Signal• Carrier Signal• Generate Carrier Signal• C = A*cos(w*t) = A*cos(2*pi*fc*t)• fc = Carrier frequency or center frequency• A = amplitude or signal power• PRN Code• Generate as specified in the GPS IS Document• Navigation Data

• Generate as specified in the GPS IS Document• Data Rate: 50bps• 1 word: 30bits• 1 sub-frame: 300bits (6 sec)
• 10 words / sub-frame• 1 frame: 1500bits (30 sec)

• 5 sub-frames/frame

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

18

Generate GPS L1C/A Signal• Generate GPS L1C/A signal in computer• Digital IF signal• Signal is generated at IF level• For example:• IF (Intermediate Frequency) : 4MHz
• Sampling Frequency: 16MHz• A/D Resolution: 4bit

• Convert the digital IF signal to analog RF signal• Use Software Radio Device• Up-convert from IF to RF• Generate RF signal at 1575.42MHz (analog signal)• Transmit the signal from antenna
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

19

Low-Cost GPS Signal Generator
Ephemeris Data
RINEX File

User Motion
Data
Static or Dynamic

Transmit
SignalSelectVisibleSatellites

Simulation Date,Time andDuration

GenerateSignal

Adjust Signal
OutputPowerDigital Signal
Properties(Sampling Freq,
Bit Rate, IF etc)

Antenna
Gain
PatternDigital I/QSignal
OutputDigital
IQ Signal
fs = 26MHz
A/D = 12bit

Digital to Analog(D/A) Conversion

Hardware

Software
GPS L1C/A
f = 1575.42MHz
Signal Generation

Digital IQ
SignalAbout $400

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

20

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

21

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

22

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

23

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

24

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

25

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

26

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

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

27

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

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

28

29

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

30

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo31

SNR & C/No
• SNR

• Signal to Noise Ratio, unit is dB• S: Signal Power in dBm or dBW• N: Noise power in a given bandwidth in dBm or dBW• C/No• Carrier to Noise Density, unit is dB-Hz• C: Carrier power in dBm or dBW• No: Noise power density in dBm-Hz or dBW-Hz• SNR = C/No-BW (BW = bandwidth of the Front End)• If BW = 4Mhz = 10*log10(4000000) = 66dB
• If C/No = 48dB-Hz• SNR = C/No-BW = 45-66 = -21dB• C/No from Received Signal Power• Noise Density (No) at Room Temperature (290K): -204dBW/Hz• Received Power (GPS L1C/A Signal) at Antenna: -158.5dBW or -128.5dBm• C/No = -158.5-(-204) = 45.5dB-Hz
Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

32

Contact Information
Dinesh Manandhar
Associate Professor(Project)
Center for Spatial Information Science (CSIS)The University of Tokyoe-mail: dinesh@csis.u-tokyo.ac.jp
Web: https://home.csis.u-tokyo.ac.jp/~dinesh/Webinar: https://gnss.peatix.com
Skype: mobilemap
Facebook: https://www.facebook.com/gnss.lab

Contact: dinesh@csis.u-tokyo.ac.jp, Center for Spatial Information Science, The University of Tokyo

33