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EE 570: Location and Navigation: Theory & Practice The Global Positioning System (GPS) Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice Slide 1 of 23

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EE 570: Location and Navigation: Theory & Practice. The Global Positioning System (GPS). The Global Positioning System (GPS ) Dead Reckoning vs Position Fixing. Navigation can be accomplished via “position fixing” or “dead reckoning” - PowerPoint PPT Presentation

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Page 1: EE 570: Location and Navigation: Theory & Practice

EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice Slide 1 of 23

Page 2: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Dead Reckoning vs Position Fixing

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Navigation can be accomplished via “position fixing” or “dead reckoning” Dead Reckoning - Measures changes in position and/or

attitudeo Inertial sensors provide relative position (and attitude)

Position Fixing - Directly measuring locationo GPS provides absolute position (and velocity)

• How does GPS work? Effectively via Multilateration

o If I can measure my distance to three (or more) satellites at known locations, then, own location can be resolved

– Measure distance via “time-of-flight” (speed of light)

Slide 2 of 23

Page 3: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)An Overview

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The GPS is a Space-Based Global Navigation Satellite System (GNSS)1. Space segment (satellites)

o First satellites launched in 19892. Control segment (ground station(s))

o Master control segment, alternate, and monitors 3. User segment (receivers)

o Both military and civilian

• Other GPS-like systems exist GLONASS – Russian COMPASS/BeiDou – China Galileo - European Union (EU)

wikipedia

Slide 3 of 23

Page 4: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Overview – The Space Segment

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The Space Segment A constellation of 24 satellites

in 6 orbital planes Four satellites in each plane 20,200 km altitude at 55

inclinationo Each satellite’s orbital

period is ~12 hourso >6 satellites visible in each

hemisphere

Courtesy of MATLABSlide 4 of 23

Page 5: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Overview – The Control segment

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The Control segment Tracking stations around

the worldo 1 Master control station

– Command & Controlo 1 Alternate control station

– Backupo 16 Monitor stations

– Orbit monitoringo 4 dedicated ground antenna

– Communication

gps.gov

Slide 5 of 23

Page 6: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Overview – The User Segment

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The User Segment Military receivers can receive encrypted

GPS signals to realize higher performance– E.g. Selectively Available Anti-Spoofing Module (SAASM ) and

Precise Positioning System (PPS) encrypted key based systems Civilian receivers

o Commercial handheld– e.g. Gamrin Montana 650

o OEM chipsets– ublox

» Multi-GNSS engine for GPS, GLONASS, Galileo and QZSS

– Vectornav» VN-200 OEM GPS-Aided Inertial Navigation System

NavAssure® 100

Slide 6 of 23

Page 7: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Multilateration - Intersection of Spheres

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

1 satellite – A sphere2 s

atell

ites –

A ci

rcle

3 satellites – Two points

Slide 7 of 23

Page 8: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Multilateration - Intersection of Spheres

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Only one of the two points will be feasible E.g. on the surface of the Earth

3 satellites – Two points

Slide 8 of 23

Page 9: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Multilateration – Basic Idea

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Multilateration – The Basic Idea Determine range to a given satellite via time-of-flight of an

RF signal (i.e. speed of light) Requires very precise

time baseso Receiver clock bias

Slide 9 of 23

Page 10: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Modulation Scheme

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Position is determined by the travel time of a signal from four or more satellites to the receiving antenna Three satellites for X, Y, Z position, one satellite to solve for

clock biases in the receiver

Image Source: NASA

Transmission Time

Receiver

Time delay

Satellite

Slide 10 of 23

Page 11: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Modulation Scheme

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The GPS employs quadrature Binary Phase Shift Keying (BPSK) modulation at two frequencies (CDMA) L1 = 1,575.42 MHz

o 1 = 19 cm L2 = 1,227.6 MHz

o 1 = 24 cm

• Two main PRN codes C/A: Course acquisition

o 10-bit 1 MHz P: Precise

o 40 bit 10 MHzo Encrypted P(Y) code

Slide 11 of 23

Page 12: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Modulation Scheme

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Quadrature BPSK modulation

Ref: JNC 2010 GPS 101 Short Course by Jacob Campbell

Slide 12 of 23

Page 13: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Signal Processing

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Code and Carrier Phase Processing Code used to determine user’s gross position Carrier phase difference can be used to gain more accurate

positiono Timing of signals must be known to within one carrier cycle

Slide 13 of 23

Page 14: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Pseudorange

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

1 23

n

Sat 1

(x 1,y 1,z 1)

Sat2(x2 ,y2 ,z2 )

Sat3

(x3,y3,z3)

Sat n

(x n,y n,z n)

GPS receiver(x,y,z)

All measurements in ECEF coordinates

2 2 2i i i ix x y y z z

2 2 2 2 2

2 2

2

2 2i i i i

i i i

x x x x y y

y y z z z z

2 2 2 2 2 2 2 2 2 2i i i i i i ix y z x y z x x y y z z

2 2 2 21 1 1 1

1 1 12 2 2 22 2 2 2 2 2 2

2 2 2 2

2 2 22 2 2

2 2 2

e

e

n n nn n n n e

x y z r x y zx

x y z r x y zyz

x y zx y z r

..

.

- pseudorangere - Earth’s radius

Slide 14 of 23

Page 15: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Pseudorange

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• A more realistic model is

• Can perturb this model to form

• This can be solved via least-squares or Kalman filter

2 2 2i i i i ix x y y z z c T n

1

2

11

1n

xy

nz

c t

Slide 15 of 23

Page 16: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Sources Of Error - GDOP

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Geometry of satellite constellation wrt to receiver• Good GDOP occurs when

Satellites just above the horizon spaced and one satellite directly overhead

• Bad GDOP when pseodurange vectors are almostlinearly dependent

Slide 16 of 23

Page 17: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Other Sources Of Error

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Selective Availability Intentional errors in PRN Discontinued in 5/1/2000

• Atmospheric Effects Ionospheric Tropospheric

• Multipath• Ephemeris Error

(satellite position data)• Satellite Clock Error• Receiver Clock Error

Slide 17 of 23

Page 18: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Error Mitigation Techniques

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Carriers at L1 = 1,575.42 MHz & L2 = 1,227.6 MHz Ionospheric error is frequency dependent so using two

frequencies helps to limit error• Differential GPS

Post-Process user measurements using measured error values

• Space Based Augmentation Systems(SBAS) Examples are U.S. Wide Area Augmentation System (WAAS),

European Geostationary Navigational Overlay Service (EGNOS)

SBAS provides atmospheric, ephemeris and satellite clock error correction values in real time

Slide 18 of 23

Page 19: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Error Mitigation Techniques – Differential GPS

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Uses a GPS receiver at a fixed, surveyed location to measure error in pseudorange signals from satellites Pseudorange error for each satellite is subtracted from

mobile receiver before calculating position (typically post processed)

Slide 19 of 23

Page 20: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Error Mitigation Techniques - WAAS/EGNOS

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Provide corrections based on user position• Assumes

atmosphericerror is locally correlated

Slide 20 of 23

Page 21: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)Summary of the Sources of Error

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• Single satellite pseudorange measurement

• GPS error summary SPS: L1 C/A with S/A off PPS: Dual frequency P/Y code

Ref: Navigation System Design by Eduardo Nebot, Centre of Excellence for Autonomous Systems, The University of Sydney

Slide 21 of 23

measured true ionospheric tropospheric ephemeris satelite clock receiver clock multipath

Table 1. GPS Error sources: SPS vs PPS

ERROR SOURCE

DETERMINISTIC RANDOM TOTAL (RMSE) DGPS

SPS PPS SPS PPS SPS PPS SPS PPS Ephemeris 2.1 2.1 0.0 0.0 2.1 2.1 0.0 0.0

Satellite Clock 2.0 2.0 0.7 0.7 2.1 2.1 0.0 0.0 Ionosphere 4.0 1.0 0.5 0.5 4.0 1.2 0.4 0.1

Troposphere 0.5 1.0 0.5 1.0 0.7 1.4 0.2 1.4 Multipath 1.0 1.0 1.0 1.0 1.4 1.4 1.4 1.4

UERE 5.1 3.3 1.4 1.5 5.3 3.6 1.6 1.5

Vertical 1 12.8 8.6 3.9 3.7 Horizontal 1 10.2 6.6 3.1 3.0

Page 22: EE 570: Location and Navigation: Theory & Practice

II/IIA IIR IIR M‐ IIF III (A,B,C)

Number SV’s 28 13 8 12 30*

First Launch 1989 1997 2005 2010* 2014*

Satellite Weight (Kg)

900 1,100 1,100 844 TBD

Power (W) 1,100 1,700 1,700 NA TBD

Design Life (Years)

7.5 10 10 15* TBD

Unit Cost ($M) 43 30 30 39* TBD

In Use (2010) 11 12 7 0 0

II/IIA IIR IIR M‐ IIF III (A,B,C)

Number SV’s 28 13 8 12 30*

First Launch 1989 1997 2005 2010* 2014*

Satellite Weight (Kg)

900 1,100 1,100 844 TBD

Power (W) 1,100 1,700 1,700 NA TBD

Design Life (Years)

7.5 10 10 15* TBD

Unit Cost ($M) 43 30 30 39* TBD

In Use (2010) 11 12 7 0 0

II/IIA IIR IIR M‐ IIF III (A,B,C)

Number SV’s 28 13 8 12 30*

First Launch 1989 1997 2005 2010* 2014*

Satellite Weight (Kg)

900 1,100 1,100 844 TBD

Power (W) 1,100 1,700 1,700 NA TBD

Design Life (Years)

7.5 10 10 15* TBD

Unit Cost ($M) 43 30 30 39* TBD

In Use (2010) 11 12 7 0 0

II/IIA IIR IIR M‐ IIF III (A,B,C)

Number SV’s 28 13 8 12 30*

First Launch 1989 1997 2005 2010* 2014*

Satellite Weight (Kg)

900 1,100 1,100 844 TBD

Power (W) 1,100 1,700 1,700 NA TBD

Design Life (Years)

7.5 10 10 15* TBD

Unit Cost ($M) 43 30 30 39* TBD

In Use (2010) 11 12 7 0 0

The Global Positioning System (GPS)The Future of GPS

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice

• The Future of GPS

II/IIA IIR IIR M‐ IIF III (A,B,C)

Number SV’s 28 13 8 12 30*

First Launch 1989 1997 2005 2010* 2014*

Satellite Weight (Kg)

900 1,100 1,100 844 TBD

Power (W) 1,100 1,700 1,700 NA TBD

Design Life (Years)

7.5 10 10 15* TBD

Unit Cost ($M) 43 30 30 39* TBD

In Use (2010) 11 12 7 0 0

Slide 22 of 23

Page 23: EE 570: Location and Navigation: Theory & Practice

The Global Positioning System (GPS)The Future of GPS

Thursday 11 April 2013 NMT EE 570: Location and Navigation: Theory & Practice Slide 23 of 23