Transponders relay multiple vessel moves simultaneously
- Inside this black box is the equipment for the new R3 GP&C transponder.
- Illustration shows a mixed maritime and aviation scenario using the GP&C Transponder system.
Swedish Space Corp has formed a new company, GP&C Sweden, specifically to handle its new generation 4S Transponder (Ship to Ship and Ship to Shore) which has been developed to complement maritime radar.
The transponder, also known as GP&C (Global Positioning and Communication) or STDMA-Transponder, uses existing satellite navigation to make automatic position reporting from a variety of vessels or aircraft in a reliable and cost-efficient way using VHF radio. By synchronizing position reports, one single radio channel can transmit these reports from a large number of ships in such a way that even time-critical messages can be distributed safely on the same radio channel.
Up to 2,250 position reports per minute can be handled by the system at time intervals selected to suit the traffic situation. For example, ships at anchor transmit position reports every minute, while moving ships transmit their position at time intervals measured in seconds.
Connecting an ordinary global positioning system (GPS) receiver to a computer already allows a vessel to see its own position on a digitized map. GP&C, however, has taken the technology a step further. Its concept involves equipping aircraft, ships and ground vehicles with a GP&C transponder comprising a GPS receiver, a communication processor and a radio transceiver.
The transponder receives position data from the GPS satellites and, using a predetermined VHF radio band, transmits the data to all aircraft, ships and base stations located within the radio range. By equipping each transponder with a serial data port for user data which is connected to a display system, the position of each aircraft or vessel, along with its identity and symbol designating course and speed, is displayed at the same time.
The secret behind all the transponders being able to transmit over the same radio channel lies in the high precision global time synchronization offered by GPS satellites. This makes it possible to efficiently divide time into slots, allowing each user's transponder to transmit in its own slot in accordance with the Time Division Multiple Access (TDMA) principle.
The GP&C system divides every minute into slots measured in milliseconds - ample time for users to transmit their identity, position, speed and course, as well as ordinary text messages. The interval between transmissions is adapted dynamically, depending on the traffic situation. Finally, a base station transmits DGPS corrections on the data link so that all transponders within radio range can navigate with high accuracy.
Troll trials
GP&C Sweden sees great potential for the 4S Transponder system in the offshore industry and is currently in discussions with Statoil which has already installed the system on one of its vessels and on the Troll platform, with a further installation on the Fedje VTS station on the west coast of Norway.
GP&C Sweden's marketing and sales director Lars Holmstrom recognizes there is still some way to go before the 4S Transponder system can become fully effective. "To fully appreciate the befits offshore, the system would need to be installed on all vessels, helicopters and offshore installations. We are conducting a number of trials around the world in the maritime sector and users have already responded positively to its benefits."
As well as monitoring normal maritime traffic, the system has shown particular benefits in emergency situations. Trials were performed in Kattegat in Sweden last June where an aircraft was downed in a simulated accident. Numerous ships and helicopters participated in the exercise, some equipped with the 4S Transponder system to allow the search and rescue operation to be monitored from a maritime rescue coordination centre. A large group of observers followed the exercise and could study the system's air-sea coordination capability, which allowed progress of both seagoing and airborne units to be followed on the same display.
For full future application, international cooperation is required to develop a worldwide accepted standard. The future of transponder systems was discussed at the IMO NAV-41 Conference in 1995 where the GP&C 4S Transponder was demonstrated. As a result, standardization work for maritime applications is in progress.
As a follow-on, Swedish Space Corp C has developed the R3 GP&C Transponder with a design optimized for mobile aviation and maritime users. The group station versions for STC-VTS centrals include a DGPS broadcast function.
For more information contact Lars Holmstrom, GP&C Sweden: telephone +46 8 627 6434, fax +46 8 627 6449, Email: lho(a)ssc.se or http://www.ssc.se/
