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    Aircraft Navigation and Flight Computers: Introduction of GPS

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    Lec8 GPS Intro

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    Aircraft Navigation and Flight Computers: Introduction of GPS

    Uploaded by

    Jean Chan
    26 views36 pages
    Lec8 GPS Intro

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    Lec8 GPS Intro

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    Lec8 GPS Intro

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    26 views36 pages

    Aircraft Navigation and Flight Computers: Introduction of GPS

    Uploaded by

    Jean Chan
    Lec8 GPS Intro

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 36

    AE 4003 AIRCRAFT NAVIGATION AND FLIGHT COMPUTERS

    By Frederik Blumrich

    Lecture Topic: Introduction of GPS

    Example
    cx kx F (t ) where the PSD of F(t) is Consider mx constant S0 :

    Example

    Example) The first order low-pass filter with unit white noise input, we have G(s)=1/(1+Ts), Sf(f)= .

    Exercise

    1)

    3)

    2)

    Exercise

    Exercise

    Exercise

    1)

    2)

    F.T

    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. Satellite Systems GPS, Galileo, GLONASS etc.

    Navstar GPS
    Navigation System with Time and Ranging & Global Positioning 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 GPS concept 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

    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)

    PPS

    System Architecture
    GPS consists of three segments; Space Segment, Control Segment, and User Segment

    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.

    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

    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 integrity requirements. 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

    WAAS applications
    Primary Means of Navigation - Take-Off, En Route, Approach and Landing More Direct Routes - Not Restricted By Location of Ground-Based Equipment Precision Approach Capability - At Any Qualified Airport Decommission of Older, Expensive GroundBased Navigation Equipment Reduced/Simplified Equipment On Board Aircraft Increased Capacity - Reduced Separation Due to Improved Accuracy
    Copyright 2001 Todd Walter

    Courtesy: FAA

    Space Segment

    31 satellites active (9/2007)

    GPS Satellite Vehicle


    Four atomic clocks Three nickel-cadmium batteries Two solar panels
    Battery charging Power generation 1136 watts

    Weight
    2370 pounds

    Height
    16.25 feet

    Width
    38.025 feet including wing span

    S band antennasatellite Design life10 years Block IIR satellite vehicle control assembly at Lockheed 12 element L band Martin, Valley Forge, PA antennauser communication Block IIF satellite vehicle (fourth generation)

    GPS satellite

    Satellites and Orbits

    Satellites and Orbits

    Control Segment

    User Segment - receiver

    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)

    Receiver Outputs Satellite information


    Satellite ID or PRN Azimuth Elevation Signal Strength

    Dilution of precision (DOP)


    PDOP ; HDOP; VDOP

    Signals from GPS Satellites


    The SVs transmit two microwave carrier signals:
    The L1 frequency (fL1=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 or fL2 Ranging code : a unique sequence of 0s and 1s (zeroes and ones) 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).

    GPS signals

    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 for each 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 the civil SPS Chip width or wave length is about 300m

    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 each receiver 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

    Navigation data
    Binary-codes message consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.
    A data bit frame consists of 1500 bits divided into five subframes 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)

    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.

    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 then impressed upon the carrier in a process called modulation. The specific form is called binary phase shift keying (BPSK). Detailed later !!

    Navigation data +C/A code


    Navigation message is modulo 2 added to C/A code. 20 C/A codes per Navigation Bit.

    Navigation data

    Navigation data format


    Subframe Data

    Modulation (wavelength)

    Modulation

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