I. ABSTRACT

Electronic clocks control critical functions in many applications. However, clocks are often designed for low cost rather than for keeping accurate time. Even fairly accurate computer clocks are likely to vary due to manufacturing defects, changes in temperature, electric and magnetic interference, the age of the quartz crystal, or even system load. Additionally, even the smallest errors in keeping time can significantly add up over a long period

The time synchronization system based on GPS is used for developing a universal synchronize clock using GPS. The GPS based time synchronizer provides an accurate, traceable low-cost method. To have accurate time in all the systems are as important as having a high security in a system. The system based on GPS receives information from satellites and gives out information to the application processor. Thus the information obtained is applied to the processor and its output is used to synchronously trigger all clocks to obtain time synchronization

II. INTRODUCTION

A GPS based time source, or time synchronization system produces frequency, accuracy and stability similar to that of atomic frequency standards. The GPS receiver used here is coupled to the application processor. The GPS receiver receives and produces GPS information with high accuracy and stability. The information is processed using the application processor. The FlexCon (MCF 5272 development platform) has to be interfaced with GPS module as processor. The GPS system provides the time stability to 300 ns. The GPS receiver uses its own free running local oscillator to receive transmissions from GPS satellite so as to produce the 1 pulse per second time output .The received time signal is transmitted to all sections in the facility to obtain the synchronized clocks to the exact second.

III.BLOCK DIAGRAM



IV. Implementation

The FlexCon (MCF5272 development platform) has to be interfaced with the GPS module. GPS module provides longitude, latitude and time. GPS Receiver captures the time signal from the global positioning system (GPS) satellites and sends it to the Transmitter. The Transmitter then broadcasts the time to every clock in our facility. As a result, all of the clocks are synchronized to the exact second. The firmware will be developed using embedded C and the user interface will be done using simple HTML and java applet.

V. GLOBAL POSITIONING SYSTEM (GPS)

What is GPS?

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works?

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

The GPS satellite system

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):

  • The first GPS satellite was launched in 1978.
  • A full constellation of 24 satellites was achieved in 1994.
  • Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
  • A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
  • Transmitter power is only 50 watts or less.

What's the signal?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS (Precise Positioning Service) equipped receivers. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

A GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information.

Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.

Almanac data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

How to access?

The GPS positioning services are of two types. They are Precise Positioning Service (PPS) and Standard Positioning Service (SPS)

Precise Positioning Service (PPS)

  • Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS.
  • PPS Predictable Accuracy
    • 22 meter Horizontal accuracy
    • 27.7 meter vertical accuracy
    • 200 nanosecond time (UTC) accuracy

Standard Positioning Service (SPS)

  • Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal.
  • SPS Predictable Accuracy
    • 100 meter horizontal accuracy
    • 156 meter vertical accuracy
    • 340 nanoseconds time accuracy

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

  • Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
  • Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
  • Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
  • Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
  • Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
  • Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
  • Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Synchronization to GPS

The GPS system (also called NAVSTAR) includes 24 satellites each with three or four onboard atomic clocks. The US Naval Observatory monitors the satellite’s clocks and sends control signals to minimize the differences between their atomic clocks and a master atomic clock for accuracy and traceable to national and international standards (known as UTC). For time synchronizing a clock, the GPS signal is distributed by a master clock, time server, or primary reference source to a device, system, or network so the local clocks are synchronized to UTC. Typical accuracies range from better than 500 nanoseconds to 1 millisecond anywhere on earth. The GPS clock synchronization eliminates the need for manual clock setting (an error-prone process). The benefits are numerous and include: legally validated time stamps, regulatory compliance, secure networking, and operational efficiency.

Differential GPS (DGPS) Techniques

  • The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal.
  • Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. These corrections are passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-range corrections to each SV used in the navigation solution. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges because both receivers would have to be using the same set of SVs in their navigation solutions and have identical GDOP terms (not possible at different locations) to be identically affected by bias errors.

VI. GPS Receiver (Jupiter 12)

Navman’s Jupiter Global Positioning System (GPS) module is a singleboard,12 parallel-channel receiver intended as a component for an Original Equipment Manufacturer (OEM) product. The receiver continuously tracks all satellites in view and provides accurate satellite positioning data. Jupiter is designed for high performance and maximum flexibility in a wide range of OEM configurations including handhelds, panel mounts, sensors, and in vehicle automotive products. The highly integrated digital receiver uses the Zodiac chipset composed of two custom SiRF devices: the CX74051 RF Front-End and the CX11577 Scorpio Baseband Processor (BP). These two custom chips, together with memory devices and a minimum of external components, form a complete low-power, high-performance, high reliability GPS receiver solution for OEMs. Configurations allow the OEM to design for multi-voltage operation and/or dead reckoning navigation that uses vehicle sensors in the absence of GPS signals. Each configuration provides up to four options for different types of antenna connectors. The Jupiter receiver decodes and processes signals from all visible GPS satellites. These satellites, in various orbits around the Earth, broadcast radio frequency (RF) ranging codes, timing information, and navigation data messages. The receiver uses all available signals to produce a highly accurate navigation solution that can be used in a wide variety of end product applications. The all-in-view tracking of the Jupiter receiver provides robust performance in applications that require high vehicle dynamics and in applications that operate in areas of high signal blockage such as dense urban centers. The Jupiter receiver shown in Figures below is packaged on a miniature printed circuit board with a metallic RF enclosure on one side. The receiver is available in several configurations. The configuration and type of antenna connector must be selected at the time of ordering and is not available for field retrofitting.

VII. FLEXCON 3000

FlexCon-3000 is a versatile, programmable, web-enabled, application-independent process controller that can provide solutions to a wide range of process control applications. Easy-to-use web user interface allows setting up multi-loop process control and programmable logic control with minimum level of expertise. The FlexCon-3000 uses a modular architecture and self-stacking hardware design that enables users to customize its feature sets to develop an application-specific process controller easily. It is very compact and offers many sophisticated features which are:

· Scalable and modular hardware architecture

· Low power consumption

· Configurable analog and digital input/output

· Supports voltage, current and different thermocouple inputs

· Configurable full-featured alarms

· Selectable number of control loops with dual output (heat and cool)

· Manual/Automatic/Cascade/Ratio/Differential loop capability

· Selectable Bumpless transfer modes

· PID Control with auto-tuning option

· Programmable Ramp/soak

· Independently selectable ON/OFF, Time-proportionate, Distributed Zero Crossing and DAC control output types

· Easy-to-use web-based interface (10/100BaseT) to monitor, configure and control

· No software installation needed

· Real-time graphical representation of loop variables

· Optional alarm notification via Email

· Optional FTP support

· Remote firmware updating

· 20-25% less costly than currently available products

VIII. applications

· Computer clocks (servers and workstations)

· Network devices (routers, switches, firewalls)

· Telecommunications networks (PBXs, MUXs, SONET networks, wireless systems)

· Critical devices and networks (9-1-1 centers, command and control operations, military test ranges, radar systems, time displays)

· Physical security systems (video, building access controls, networks)

· IT security systems (cryptography, authentication, encryption)

IX. CONCLUSION

The time synchronizer based on GPS can provide an accurate and low cost method. The GPS clock synchronization eliminates the need for manual clock setting (an error-prone process). The benefits are numerous and include: legally validated time stamps, regulatory compliance, secure networking, and operational efficiency. Typical accuracies range from better than 500 nanoseconds to 1 millisecond anywhere on earth. The application processor takes the required information (time) from the received GPS signal and the information obtained is used for several applications such as medical, industrial and also in space shuttle launches that may require synchronous operations.

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