Satellite

Newest approaches to AIS data collections involve the usage of satellite mounted AIS antenna’s: S-AIS, satellite AIS.

The VHF signal of an AIS sender travells in a straight line. On earth, it therefore is limited by the line of sight (up to 200km). However, the signal also travells into spac on a straight line and is therefore receivable by satellites.

 
satellite

Satellite mounted AIS receivers provide signal coverage on remote places and off-shore. S-AIS therefore fills the gaps left by terrestrial AIS networks.

Satellite AIS comes with a number of limitations:
– The transmission protocol of AIS is build around the limited range if VHF senders. Satellites however cover an area which is a multiple of the VHF range. Whereas in normal AIS usage, collision between transmissions are rare, they regularly occur when the signals are received by satellites. Especially in crowded regions satellites have dificulties distinguishing the signals of individual vessels. Problematic areas are e.g. the english channel/north sea area, the mediteranean and the south east asia.
– Currently, AIS equiped satellites do not cover the whole earth. Depending on their orbit and the geometry of their satellite constelation they need several passes of several hours to cover the whole surface of the earth.
– Satellites are not continously online. They are collecting information when passing over vessels but they have to store the information until they are passing over a groudn station again to transmit the data to a data center. This leads to a time daly of up to multiple hours.

 

Satelllite data providers