26 June 2014
Ship detection with the AISat satellite
The AISat satellite will receive Automatic Identification System (AIS) signals transmitted by ships while it orbits at an altitude of 660 kilometres. Its helical antenna will be pointed towards Earth and will locate ships, especially in high traffic areas like the German Bight.
DLR (CC-BY 3.0).
An antenna for ship detection
Until now, satellites equipped with non-directional antennas have monitored maritime traffic, and the individual signals cannot easily be distinguished in areas with large numbers of vessels. For the AISat satellite, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has developed a four-metre-long helical antenna and a satellite receiver. The spacecraft will be controlled and operated by the DLR Institute of Space Systems in Bremen.
Construction of the AISat satellite
The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has developed the satellite, receiver and helical antenna for the AISat mission. The satellite will receive signals from ships while orbiting at an altitude of 660 kilometres, using a directional antenna.
'Listening' with a helical antenna
Signals from ships will be received by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) AISat satellite, using a four-metre-long helical antenna.
Ship detection from space: AISat
In the future, the satellite AISat will use its 4-metre helix antenna to receive ship signals from the numerous vessels travelling along busy routes.
At first, the AISat satellite will be spinning rapidly after it has been carried into orbit by a launch vehicle that will depart from the Satish Dhawan Space Centre at Sriharikota, 80 kilometres north of Chennai, India, at 06:19 CEST on 30 June 2014. Once the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) satellite has stabilised, its main component will be deployed and brought into service – a four-metre-long helical antenna directed towards Earth will receive Automatic Identification System (AIS) signals from maritime traffic. DLR researchers expect their first contact with the satellite at around 11:00 CEST on 30 June. The experimental satellite will identify and track vessels from an altitude of 660 kilometres – paying particular attention to areas with high traffic density, such as the German Bight, an area in the southeast of the North Sea. Existing commercial satellites reach their operational limits with such large numbers of vessels.
Like the beam of a flashlight
Researchers at the DLR Institute of Space Systems have found well over 1000 vessels during an overflight along the German coast from Holland to Sylt. Conventional satellites cover an area with a radius of at least 5000 to 6000 kilometres with their non-directional antennas. "They receive everything at once – which would be equivalent to listening to 100 radio stations at the same time," says Project Manager Jörg Behrens. "The ships all broadcast their signals at once." In low traffic areas, further away from the coastal regions, this is not a problem. In waters such as the North Sea, the Mediterranean Sea or the North American Atlantic coast and around major ports such as Beijing, Tokyo and Singapore, however, the AIS signals with their information about position, identification, length and width, load, speed and direction can no longer be assigned to individual vessels. The DLR researcher and his team have therefore chosen an alternative technique – their helical antenna is oriented directly towards Earth and receives AIS signals from a smaller area, with a diameter of only 750 kilometres. "It is focused like the beam of a flashlight," says Behrens.
To test the performance of the AISat satellite, the researchers will compare its data with information received from the ships by remotely operated ground stations. Currently, DLR has six terrestrial receiving stations on the German North Sea coast, each of which has a range of between 40 and 60 kilometres. Four other stations are expected to be added. In addition, smaller, non-directional antennas are installed on AISat and will also provide comparative data. These control data from space and Earth will be used to determine the performance of the helical antenna and receiver. "There is a lot of room for improvement in high-traffic regions – and our antenna could be a good way to do this." The unusual antenna was developed and tested in microgravity conditions during parabolic flights in cooperation with the DLR Institute of Composite Structures and Adaptive Systems. The satellite and receiver were developed, built and tested at the DLR Institute of Space Systems.
Connection between Bremen and space
Two to three times a day, researchers in Bremen will receive data from space as the satellite passes over northern Germany. AISat will be controlled and monitored from DLR Bremen. Another receiving station is to be installed at Inuvik, in Canada, to extend the opportunities to receive data. If the helical antenna proves to be successful, it could be an alternative for future use on commercial satellite systems.
Before all this happens, the satellite has to withstand the stress of the launch. Exactly 1113.7 seconds after launch, AISat will be injected into orbit and then begin charging its batteries. In the first overpasses, only the health of the satellite and its receiver will be checked. Once this has been accomplished, the command will be sent to deploy the helical antenna. Initially pointing towards space, instead of Earth, the satellite will employ small electromagnets to achieve the correct orientation. "We rely, as it were, on the magnetic field of Earth," explains Behrens. Finally, the phase that is most exciting for the researchers at DLR will follow – AISat will listen to the radio signals sent from Earth's surface, and determine which ships it can 'hear' from orbit.
Last modified:27/06/2014 13:23:21