10-11-2012, 01:17 PM
Paper Presentation on GLOBAL POSITIONING SYSTEMS
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INTRODUCTION:
Determining once position on the earth either from an aircraft or a ship or on land becomes an essential requirement when there are no terrestrial landmarks. Over the ages numerous methods and tools have been used for the same. The increased requirement of accurate data along with great strides in satellite science has given birth to a precise system called the Global Positioning System (GPS).
GPS is a satellite-based navigation system installed and controlled by the U.S. Department of Defense. It was originally intended for military applications, but in the 1980s it was made available for civilian use without any subscription fees or setup charges.
GPS provides specially coded satellite signals that can be processed in a GPS receiver of the user. Four GPS satellite signals are required by a single user to compute position, velocity and time.
User Segment
The GPS User Segment consists of the GPS receivers used by the users of GPS. GPS receivers convert satellite-transmitted signals into position, velocity, and time. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.
Control Segment
The Control Segment consists of a system of tracking stations located around the world to monitor the GPS satellites, checking their operational health and exact position in space. These monitor stations measure signals from the satellites, which are incorporated into orbital models for each satellite. These models compute precise orbital data for location (ephemeris) and clock corrections for individual satellites.
The Master Ground Station then transmits these correction data to these. The satellites can then incorporate these updates in the signals they send to GPS receivers of the users.
GPS SATELLITE SIGNALS
The Satellites transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers. There are three binary codes which shift the L1 and/or L2 carrier phase.
C/A Code (Coarse Acquisition)
The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz 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 (one millisecond). There is a different C/A code PRN for each satellite. 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.
P-Code (Precise code)
The P-Code (Precise code) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. 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. The P (Y)-Code is the basis for the PPS.
Navigation Message
The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. An entire set of twenty-five frames (125 sub frames) making up the complete Navigation Message is sent over a period of 12.5 min.
COMPUTATION OF VELOCITY AND TIME IN GPS
The velocity is computed from change in position over time, the Satellite Doppler frequencies, or both. Time is computed in Satellite Time, GPS Time, and UTC(Universal Time Coordinated)
Satellite Time is the time maintained by each satellite. Each Satellite contains four atomic clocks (two cesium and two rubidium). Satellite clocks are monitored by ground control stations and occasionally reset to maintain time to within one-millisecond of GPS time. Clock correction data bits reflect the offset of each Satellite from GPS time.
Satellite Time is set in the receiver from the GPS signals.. Multiple Satellites and a navigation solution (or a known position for a timing receiver) permit Satellite Time to be set to an accuracy limited by the position error and the pseudo-range error for each Satellite. Satellite Time is converted to GPS Time in the receiver.
THE GEODETIC DATUM AND COORDINATE SYSTEMS
Through the ages earth has been understood as having a flat, then spherical, and since maritime navigational adventures as a Geoidal surface. The spherical models fail to model the actual shape of the earth as the slight flattening of the earth at the poles results in about a twenty kilometer difference at the poles between an average spherical radius and the measured polar radius of the earth. This lead to understanding the shape of the earth as a Geoid .
For this geoidal earth, Ellipsoidal earth models are required for accurate range and bearing calculations over long distances. Loran-C, and GPS navigation receivers use ellipsoidal earth models to compute position and waypoint information. Ellipsoidal models define an ellipsoid with an equatorial radius and a polar radius
GPS TECHNIQUES AND PROJECT COSTS
Receiver costs vary depending on capabilities. Small civil SPS receivers can be purchased for under $200, some can accept differential corrections. Receivers that can store files for post-procesing with base station files cost more ($2000-5000). Military PPS receivers may cost more or be difficult to obtain. Other costs include the cost of multiple receivers when needed, post-processing software, and the cost of specially trained personnel. Project tasks can often be categorized by required accuracies which will determine equipment cost.