lec8 gps intro
TRANSCRIPT
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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