An Overview of Global Positioning System and Other ...ecee.colorado.edu/~ecen4242/GPS/gps_digital_coms_incorp.pdf · An Overview of Global Positioning System and Other Navigation

Aerospace Engineering Sciences

University of ColoradoUniversity of Colorado

An Overview of Global Positioning Systemand Other Navigation System Standards

11 March 2008Principles of Digital Communication

Steve LimburgAaron Nielsen

Chris Treib



Outline

• GPS Overview

• Communications/RF Problem

• Message Formats and Content

• Signal Structure and Generation: Physical Layer

• Acquisition and Tracking Techniques

• Modernized and Competing Signal Structures

• Applications and Users

• Standards Committees



GPS Overview

• Operated by USAF• 24 Satellites/6 Planes• Inclined 55o

• Semi-Sidereal Day• Orbital Rad 26,560 km• L1 1575.42 MHz

– C/A Code, P(Y) Code• 32 codes• 1023 chips long• 1.023 Mcps

• L2 1227.60 MHz– P(Y) Code

• 32 codes• ~1014 chips long• 10.23 Mcps

Courtesy USAF



GPS Operation

• 100 m horizontal (2drms)• 156 m horizontal (2 sigma)• 300 m vertical (99.99%)• 340 nanoseconds (95%)

Courtesy Wisconsin State Cartographer’s OfficeCourtesy Canadian CG

Courtesy FAA



Communications Problem

• Signal received on Earth > -158.5 dB (C/A)• Distance (satellite to receiver): 20000-25000 km• Path Loss =• Atmospheric Loss = 0.5 dB• Total Path Loss ≈ -160 dB

Courtesy Misra




• Power transmitted = 27 Watts (14.3 dB)• Satellite Antenna Gain = 12.1 dB• Effective Radiated Power = 26.4 dB• Received Power Density ≈ -133 dB/m2

• Effective Area of Antenna ( ) = 25 dBm2

• Received Power ≈ -158.5 dB




• Received Power ≈ -158.5 dB• Thermal Noise Level ≈≈ -144 dB

• As a result of low SNR, use of GPS indoors is verylimited

• Typical Accuracy (outdoors): 10 feet• Typical Accuracy (indoors): 100 feet or worse



Message Format

• Message contents:• Date and Time• Status and Accuracy• Ephemeris parameters• Almanac

• Each frame contains 300 bits• Transmit rate: 50 bit/s• 37 seconds needed to

download all dataCourtesy EuroControl



Orbital Parameters

Courtesy GPS ICD



Orbital Parameters

Courtesy GPS ICD



Error Correction Scheme

• CNAV bit train uses Forward Error Correction(FEC) in a rate ½ convolutional code

• CNAV message date rate: 25 bit/s• Transmitted signal: 50 bit/s

Courtesy GPS ICD



Outline

• GPS Overview










GPS Signal Structure

• C carrier amplitude• D Data Stream (50 bps)• x CDMA (c/a) code (1.023 Mcps)• fIF Intermediate mixing frequency• fD Doppler Frequency• N White Noise

!

s( t) = CD(t"# )x(t"# )cos(2$ ( fIF + fD )t+%& )+ n( t)



GPS Physical Layer

Courtesy Akos, Vinande

Courtesy gpsreview.net



Software Defined Radio Processing

0 5 10 15 20 25 300

0.5

1

1.5

2

2.5

3

3.5

4Acquisition results

PRN number (no bar - SV is not in the acquisition list)

Acquisition Metric

Not acquired signals

Acquired signalsRaw IF data file

from textbook DVD

Plot raw IF data(diagnostic check)

Acquire (compare tothreshold)

Track (all channelsindependently)

Compute PositionSolution

Decode navigationdata

10 20 30 40 50 60 70

-20

-15

-10

-5

0

5

10

15

20

25

Coordinates variations in UTM system

Measurement period: 500ms

Variations (m)

-10

0

10 -10

0

10

-30

-20

-10

0

10

20

30

North (m)

Positions in UTM system (3D plot)

East (m)

Upping (m)

30

210

60

240

90270

120

300

150

330

180

001530

45

60

75

90

6

26

24

29

7

10

21

18

Sky plot (mean PDOP: 2.3201)

E

N

U

Measurements

Mean Position

Lat: 40°0!27.5862!!

Lng: -105°15!42.4209!!

Hgt: +1500.2




C/A Code Generation



C/A Acquisition

Top: AkosBottom: Treib



C/A Tracking




Other GNSS Signals

Code C/A L2CM L2CL L5 WAAS Galileo GLONASS Access CDMA CDMA CDMA CDMA CDMA CDMA-

BOC(1,1) FDMA

Freq (MHz)

1575.42 1227.60 1227.60 1176.45 1575.42 1575.42 1278.75 1191.795

1602 1246

Length 1023 10,230 767,250 10,230 1023 4092 10230 10230

511

Chip Rate (Mcps)

1.023 0.5115 0.5115 10.23 1.023 1.023 5.115

0.511

Integration Time (ms)

1 20 1500 1 1 4 20 100

1

Data Rate (bps)

50 25 N/A 50 250 125 500 25

50

Signal (dBW)

-158.5 -164.5 -164.5 -157.9 -161 -157 -155 -155

-161 -167

Error Correction

Parity Rate Viterbi

N/A Rate Viterbi

Rate Viterbi

Rate Viterbi

Hamming



L2C Acquisition Techniques

• CM Only– CM 0 CM 0 CM 0 …– Data Symbol Transition– 20 ms integration– 1:32 PRN in 69 sec

• CM pilot to CL– CM CL CM CL CM CL…– Data Symbol Transition– 20 ms integration– 1:32 PRN in 5250 sec

• CL Only– 0 CL 0 CL 0 CL …– No Data transitions– No Serial Search for CL start– Variable integration times– 4M FFT x 0.67 Hz step– 1 PRN in 40 Hours for full 1.5 s– 40/75 Hour for 20 ms integration

20 ms

20 ms

20 ms

20 ms

1500 ms

zeros

zeros



Increasing Integration Time



BOC (1,1) Modulation

Courtesy Akos, Vinande, Li



Galileo Challenges - Ambiguity in Signal Tracking

Main Peak

Side Peak Side Peak

Autocorrelation of BPSK Code andBOC Code

Ambiguity in Galileo Signal Tracking

• Due to the introduction ofBOC code, Galileo signaltracking suffers frompotential ambiguity and thecorrect track rangedecreases from [-1/2, 1/2] forGPS to [-1/6,1/6] for Galileo

• False lock on the side peak(at 0.5 chips) results in aranging error of 147 meters



Outline

• GPS Overview










Applications

• The GPS system has been found to have many practical applications

• By calculating phase delay between satellites, refractive index, temperature,and pressure can all be measured in the atmosphere

• By monitoring the phase delay from satellites to ground receivers, water vaporcontent can be measured.



Other applications

• Thickness of sea ice can be followed by sensing reflected GPS signals

• Also reflected GPS signals can be used to study the soil moisture content, thismethod is only good for the top few centimeters of soil though

• With the ability to monitor temp., pressure, and water vapor content, globalweather prediction has increased accuracy

• Moreover, GPS is used to predict space weather at NOAA



Other applications

GPS is also a major tool for cartography

GPS and GIS have been combined and used for statistical studies abouteverything from agriculture to industrial pollution

• Also, GPS has been used to accurately survey and map developing countriesthat until recently haven’t had accurate maps



Other applications

• Tectonic movement can be monitored to a high level of accuracy using GPS

Global Velocities



Standards Committees

• GPS is controlled by the USAF and the Department of Defense

• The civil GPS is controlled by the US Coast Guard and the Department ofTransportation

• The Civil GPS Service Interface Committee is in charge of monitoring civil usersneeds



Standards Committees

• The Radio Technical Commission for Maritime Navigation sets the maritimeGPS standards

• The Radio Technical Commission for Aeronautics recommends the standardsfor the FAA

• The RTCA has over 335 members including over 100 international associates

• The Institute of Navigation(ION) also has a satellite division to advancenavigation and position determining



Global Timing

• A major application of GPS is the transmission of Universal Time

• GPS time is off of UTC by an integer number of seconds(leap seconds) but thisis contained in the message and receiver automatically corrects for it

• GPS time is synced with UTC daily, in the last few years GPS time is within afew hundred ns of UTC modulo 1 s

• GPS time is regulated at Schriever Air Force Base

• Cell phone companies, also a major distributor or time, use GPS time as theirstandard



Conclusions

• GPS can be desribed as a communications system– Weak signal, dominated by noise limits data rate– Data contains information to construct satellite orbits– Tracking 4 or more satellites required to determine position

• GPS Signal Processing– Migration of Hardware to Software reduces cost– New codes are longer, faster, and/or sharper correlation peak– Correlation is key to accurate ranges and positions

• GPS Applications and Standards– Wide range of users and applications makes developing a standard difficult– Competing navigation systems sought as means of increasing national sovereignty

Documents

An Overview of Global Positioning System and Other ...ecee.colorado.edu/~ecen4242/GPS/gps_digital_coms_incorp.pdf · An Overview of Global Positioning System and Other Navigation