On 28 November 2014 for the first time satellite images could be transferred in a very few seconds with the help of laser beams from an earth observation satellite to a relay satellite, before being conveyed to earth for reception at EOC in Oberpfaffenhofen.
The data came from the European earth observation satellite Sentinel 1A, which orbits the earth from an altitude of some 700 kilometres. Six to seven times daily the satellite can be seen for a few minutes by DFD’s antennas in Neustrelitz. During this interval data can be directly acquired from the satellite before it again disappears beyond the horizon. Any subsequently collected images have to be stored on board until the next transmission opportunity. That takes a while.
Not this time, however. Sentinel used a detour to send data to its destination in order to test a laser communication terminal. The satellite transmitted its data via a laser beam to the European communication satellite Alphasat I-XL, which orbits the earth from an altitude of 36,000 kilometres. From there the data were sent by microwave to DFD in Oberpfaffenhofen. The acquisition equipment in Oberpfaffenhofen can constantly acquire data from Alphasat since it keeps this satellite in view around the clock. The satellite is geostationary, meaning that its orbit is linked to the earth’s rotational period. Viewed from the earth it thus appears to be stationary over one particular location.
Thanks to laser communication and relay satellites, this kind of live transmission of satellite images becomes possible. What is daily routine in the case of television was long inconceivable in the world of satellite remote sensing. The amount of data to be transmitted was simply too vast. Now with the laser technology developed in Germany, up to 1.8 gigabits per second can be transported over distances up to 45,000 kilometres. By comparison, extremely rapid terrestrial VDSL links currently offer just 100 megabits per second.
New technologies were also used to transmit the data to earth. “The antennas installed for this purpose exploit Ka-band microwaves with a frequency of 26 gigahertz, which is significantly higher than the customary frequency for this type of transmission“ explains Erhard Diedrich, who is responsible for the Alphasat ground station at DLR. In contrast to laser beams, they pass through Earth’s atmosphere almost without distortion and permit reception independent of the weather situation.
The technology tested is to be used for the European data relay system EDRS. This system will initially consist of two geostationary communication satellites with data relay stations. This data freeway in space is important for time-critical applications, such as rapid mapping during major natural disasters. Most satellite data will, however, continue to take the direct path to acquisition stations. Although data transmission via laser is impressively rapid, the capacity will not for a long time be adequate for the quantities of measurement data that earth observation satellites collect daily about our environment.