Introduction :

As the volume of data traffic over the public network increases, so does the need for precise synchronization of network equipment. Also there is a need to display identical Time information across the organization / institution to achieve better management of time. There was indeed a need for an accurate timing source for achieving the Time synchronization. However until the GPS constellation became operational, such a cost effective solution was never thought of.

As an answer to the requirement for an accurate Time source, Global Positioning System (GPS) constellation of 24 satellites parked in geo-synchronous orbit 10,900 miles above the equator, provides a free source of accurate timing that could enable all telecommunication networks to function off the same “big clock in the sky”.

The beauty is that everybody will be operating off the same constellation and the same pulse. GPS maintains extremely accurate system clock and communicates the same to all receivers. Instead of relying on a single main source for timing, as it exists, GPS correlates signals from a number of satellites.

Internally, the GPS receivers maintain time better than a nanosecond and the time at the dedicated 1 PPS time port is better than 1 micro second.

Today, GPS based timing is a preferred solution whenever an absolute error of more than few milliseconds is a critical issue. Many synchronization systems use GPS as a source of accurate time. One of the most common applications is wherein GPS is used as a reference clock for Time Code generator. With GPS synchronization, computers can share exactly the same time in the world.

GPS as a time source has following advantages,

•    Extremely good accuracy & traceability
•    Available reliably 24 hours in a day at any part of the world.
•    Provision for position information as well.
•    Expected to be stable for decades to come.

•    Network Time Displays (Referenced Digital Clocks),


•    Network Time Synchronization Servers,


•    Frequency and Pulse Synchronization (for Control Applications) and


•    Stand Alone Computer Time Synchronization (with GPS on PCI Bus).

In addition to this, GPS synchronous timing can be effectively applicable in,

•    Power Industry
•    As identical Time Display units in the following areas,
  •    Public Utilities like Railway Stations, Bus terminus
  •    Universities
  •    Hospitals & Health Care centers
  •    Government Organisations
  •    Financial Institutions
  •    Large Corporate houses & Office complexes
  •    Manufacturing Plants
  •    Broadcasting centers
  •    Air Traffic Control Towers
  •    Control Rooms of security agencies etc,.

Importance of Network Time synchronization :

Our network of workstations and servers, each with their own clock, are time stamping files, email, transactions etc.. At some point during the day it is quite likely that automatic processes such as archiving, directory synchronization, etc., execute and alter files based on time stamps. Fundamental to all these is the belief that the time is correct. Even if the time is not absolutely correct there is often a belief that the time is “close enough”. But this close enough time is no substitute for accurate network time.

Also the network operations cannot depend on the computer clocks which are based on inexpensive oscillators and are known for drifting seconds per day. With increasing distributed computing and interdependence on network infrastructure, having many drifting clocks puts the applications that run on it at risk. In particular, network operations and applications related activities are most susceptible to problems related to lack of time synchronization. Network time synchronization is critically important to ensure optimal network performance. In some cases network processes will not function without time synchronization.

Key Areas where time synchronization directly affects network operations are,

•    Log file accuracy, auditing & monitoring.
•    Network fault diagnosis and recovery.
•    File time stamps.
•    Directory services.
•    Access security and authentication.
•    Distributed computing.
•    Scheduled operations.
•    Real world time values

Typical Network Time Synchronisation System

Among the GPS time synchronization products, the Network based systems are popular. This is represented by the following block diagram.

Network Time Protocol

NTP is the defacto standard for network time synchronization. The synchronization protocol determines the time offset of the server clock relative to the client clock and sends the appropriate corrections.
NTP protocols work in one or more association modes, viz Client /Server, Broadcast/ Multicast etc. The Broadcast mode allows many clients to synchronize to one or few servers, thereby reducing network traffic when large numbers of clients are involved. Multicast is used when the subnet spans across multiple networks.
By implementing appropriate clock disciplining algorithms in conjunction with NTP, reliable Network Time Synchronization can be achieved with accuracies of 1 to 100 milliseconds.

Importance of Time synchronization in Power Industry :

Besides tracing outages, power companies also need synchronized time keeping to prevent outages. To-day power companies buy and sell electricity to each other several times each day and schedule delivery across a network for interconnected grids several power stations and utilities. Since electricity cannot be stored, supply and demand must always be kept in balance everywhere.

Energy needed by the customer must be provided in real time. Time lining events, scheduling power flows, and monitoring line frequencies – these are all applications that demand time synchronized to UTC. Another application is power line fault detection. To determine exactly where the fault exists, precise time keeping is required.

GPS Synchronized Time Display Units :

The need to have several Time display units indicating identical and accurate time information at least to a second in the organizations like Hospitals, Railway stations, ATC Towers, control rooms, Large corporate houses etc, for better time management, for better productivity was felt and ASL team started working towards this requirement.

As a result, ASL has currently designed and developed GPS Synchronous Display units, to cater for the areas which need moderate accuracy (To the extent of milli seconds).

ASL’s GeoTime 2511 is a GPS based clock, to provide accurate time information by synchronizing with the Navstar constellation of GPS satellites. Built as a low cost solution compared to the expensive sources for accurate timing, GeoTime 2511 can be configured either as a stand alone GPS synchronous time clock or as a master clock which provides Time information across several slave display units in a network, to display time information, thus providing a cost effective time distribution network .

To cater for temporary non-availability of GPS signals, GeoTime 2511 has been provided with an internal Real Time Clock, which gets synchronized to the GPS Time at pre-set intervals.

The information flow from the Master clock to the slave clocks is through RS 232 / RS 422 hard wired link. However further development of establishing this link through Blue Tooth interface is also in progress to establish wireless connectivity between Master and slave clocks.

Size of the characters, colour of the characters can be customized depending on the customer requirement , ambience in which the clocks are installed.

ASL team is also geared up to take up development of high end timing solution products to cater for applications that require high accuracy time synchronization, which will be very cost effective compared to the products available internationally.

Conclusion :

GPS constellation is a reliable source of accurate time, which can find many areas of application.

ASL as a GPS company is in the area of development of GPS based Timing solutions at an affordable cost. As a first step towards this, ASL has come out with GPS Synchronised clock display units, one version of which is already in use by Indian Railways.

ASL is also working towards bringing out cost effective timing solution for high end applications which require higher timing accuracy.