Lec8 GPS Intro

8/22/2019 Lec8 GPS Intro

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AE 4003

AIRCRAFT NAVIGATION AND FLIGHTCOMPUTERS

By Frederik Blumrich

Lecture Topic:Introduction of GPS


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Example

Consider where the PSD ofF(t) is

constant S0

:( )mx cx kx F t

fx


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Example) The first order low-pass filter with unit white

noise input, we have G(s)=1/(1+Ts), Sf(f)= .

Example


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Exercise

1)

2)

3)


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Exercise


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Exercise


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1)

2)F.T

Exercise


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Radio Navigation System

Radio navigation system: position is determined by

measuring the travel time of radio wave as it moves from

transmitter to receiver

Terrestrial Systems Decca, Omega, Loran etc. Satell ite Systems GPS, Gali leo, GLONASS etc.


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Navigation System with Time and Ranging & GlobalPositioning System

Navstar GPS is commonly referred to simply as GPS

Satellite based Navigation, 3D positioning, and Time-Distribution

System based on the concept of one-way time of arrival (TOA)ranging measurements

Originated from early (1969) effort on the development of the

Defense Navigation Satellite System (DNSS) by the Office of the

Secretary of Defense (OSD) from which the Navstar GPSconcept was evolved and the system developed by the GPS

Joint Program Office (JPO)

Owned by USA DoD (and maintained by US Air Force) since

1994 (formally declared operational in 1995) Provides continuous and precise position, velocity, and time

(UTC) information to an unlimited number of users equipped with

received-only GPS Receivers, at any place (land, sea, air,

space) and time under all weather conditions

Navstar GPS


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SPS

Highly accurate atomic frequency standards are used on-board

the satellites in synchronism with ground based standards which

maintain the precise GPS System Time Base

GPS offers two levels of services (different precisions)comprising Standard Positioning Service (SPS) and Precise

Positioning Service (PPS)


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PPS

System Architecture

GPS consists of three segments; Space Segment, Control

Segment, and User Segment


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Differential GPS

Maritime Differential GPS (DGPS) managed by the U.S.Coast Guard (USCG)

Employs ground stations along the coasts with known

fixed locations. Corrections are transmitted from ground stations at low

frequencies (200-500kHz).

Requires an additional Differential Beacon Receiver

(DBR) and an additional antenna.

Accuracy is a function of the distance from the ground

station.


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Wide Area Augmentation System(WAAS) Managed by the FAA

Communicates with several

ground stations.

Provides atmospheric corrections.

Early warning of GPS failures.

Same frequency as GPS

Higher data rate 250 Hz Satellites are in geostationary orbits


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Objectives of WAAS

Accuracy To provide a position accuracy of 7.6 metres (25 ft) or better (for both

lateral and vertical measurements), at least 95% of the time. Actual

performance measurements of the system at specific locations have shown

it typically provides better than 1.0 metre (3 ft 3 in) laterally and 1.5 metres

(4 ft 11 in) vertically throughout most of the U.S

Integrity

To provide timely warnings when its signal is providing misleading data that

could potentially create hazards and detect errors in the GPS or WAAS

network and notify users within 6.2 seconds the probability of failure is stated as 1107, and is equivalent to no more

than 3 seconds of bad data per year.

Availability

the probability that a navigation system meets the accuracy and integrityrequirements. Before the advent of WAAS, GPS could be unavailable for

up to a total time of 4 days/year. The WAAS specification mandates

availability as 99.999% equivalent to a downtime of just over 5

mins/year


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Copyright 2001

Todd Walter

Primary Means of Navigation - Take-Off,

En Route, Approach and Landing

More Direct Routes - Not Restricted ByLocation of Ground-Based Equipment

Precision Approach Capability -At AnyQualified Airport

Decommission of Older, Expensive Ground-Based Navigation Equipment

Reduced/Simplified Equipment On Board

Aircraft

Increased Capacity - Reduced SeparationDue to Improved Accuracy

Courtesy: FAA

WAAS applications


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Space Segment

31 satellites active (9/2007)


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GPS Satellite Vehicle

Four atomic clocks

Three nickel-cadmiumbatteries

Two solar panels Battery charging

Power generation

1136 watts

S band antennasatellitecontrol

12 element L band

antennausercommunication

Block IIF satellite vehicle

(fourth generation)

Weight 2370 pounds

Height

16.25 feet Width

38.025 feet including wingspan

Design life10 yearsBlock IIR satellite vehicleassembly at LockheedMartin, Valley Forge, PA


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GPS satellite


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Satellites and Orbits


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Satellites and Orbits


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Control Segment


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User Segment - receiver


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Receiver Outputs

Typically receivers provide two different formats. NMEA (Nation Marine Electronics Association)

ASCII Format

Defines a set of standard messages.

Proprietary Format

Typically Binary

No limit on information transmitted

Position, Velocity, Time (PVT) Latitude ddmm.mmmm

Longitude dddmm.mmmm

Altitude m Speed knots

Heading degrees

Date dd/mm/yy

Time hh/mm/ss.sss

Position

Velocity

Time (UTC)


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Receiver Outputs

Satellite information Satellite ID or PRN

Azimuth

Elevation Signal Strength

Dilution of precision(DOP)

PDOP ; HDOP; VDOP


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Signals from GPS Satellites

The SVs transmit two microwave carrier signals: The L1 frequency (f

L1=1575.42 MHz) carries the navigation

message and the SPS code signals

The L2 frequency (fL2=1227.60 MHz) is used to measure the

ionospheric delay by PPS equipped receivers

Signal consists of three components:

Carrier : RF sinusoidal signal with frequency fL1 orfL2

Ranging code : a unique sequence of 0s and 1s (zeroes andones) assigned to each satellite which allows the receiver to

determine the signal transit time instantaneously. The sequence

(called pseudo-random noise (PRN) sequences or PRN code)

are generated with great mathematical care and have special

properties (autocorelated and crosscorelated).

These sequences also allow precise range measurements. Each

satellite transmits two different codes: The C/A Code (Coarse /

Acquisition) and The P-Code (Precise).


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GPS signals


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C/A code

The C/A Code (Coarse / Acquisition) modulates the

L1 carrier phase

The C/A code is a repeating 1.023MHz Pseudo Random Noise

(PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading"

the spectrum over a 1 MHz bandwidth

The C/A code repeats every 1023 bits or chips(per 1millisecond).

The duration is about 1 s. There is a different C/A code PRN foreach SV. GPS satellites are often identified by their PRN

number, the unique identifier for each pseudo-random-noise

code.

The C/A code that modulates the L1 carrier is the basis for thecivil SPS

Chip width or wave length is about 300m


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P-code

The P-Code (Precise) modulates both the L1 and L2

carrier phases

The P-Code is a very long (seven days) 10.23MHz PRN code

(10 times that for a C/A-code) Chip width is about 30m (more accurate)

In the Anti-Spoofing (AS) mode of operation, the P-Code is

encrypted into the Y-Code

The encrypted Y-Code requires a classified AS Module for eachreceiver channel and is for use only by authorized users with

cryptographic keys

P codes repeat after one week

P(Y)-Code is the basis for the PPS


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Navigation data

Binary-codes message consisting of data bits thatdescribe the GPS satellite orbits, clock corrections, and

other system parameters.

A data bit frame consists of 1500 bits divided into five sub-

frames each carrying 300 bits

Data bit sub-frames (300 bits transmitted over six seconds)

contain parity bits that allow for data checking and limited error

correction

SV Clock corrections are sent in sub-frame one

SV are sent in sub-frames two and three sub-frames contain

orbital and clock data. Precise SV orbital data sets (ephemeris

data parameters) for the transmitting

Sub-frames four and five are used to transmit different pages of

system data (Almanac)


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Navigation data

A data frame is transmitted every thirty seconds

An entire set of twenty-five frames (125 sub-frames) makes up

the complete Navigation Message that is sent over a 12.5 minute

period

Clock Data parameters describe the SV clock and its relationship

to GPS time

Ephemeris data parameters describe SV orbits for short sections

of the satellite orbits

Normally, a receiver gathers new ephemeris data each hour, but

can use old data for up to four hours without much error

The ephemeris parameters are used with an algorithm that

computes the SV position for any time within the period of the

orbit described by the ephemeris parameter set

Transmitted at 50 Hz signal (50 bits per second (bps))

A bit duration of 20ms; It takes 12.5 mins for the entire message

to be received.


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Navigation data

Each code is combined with the binary navigation data

using modulo-2 addition: If the code chip and the data bit

are same (both are 0s or 1s), the result is 0; if different,

the result is 1. The composite binary signal is thenimpressed upon the carrier in a process called

modulation. The specific form is called binary phase shift

keying (BPSK). Detailed later !!


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Navigation data +C/A code

Navigation message is modulo 2 added to C/A code.

20 C/A codes per Navigation Bit.


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Navigation data


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Navigation data format

Subframe Data


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Modulation (wavelength)


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Modulation

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Lec8 GPS Intro