When the communication equipment EDRS-A on board the Eutelsat 98 communication satellite is finally operative, it will brighten every day for users of earth observation data. Today, the users often wait for data from one of the main ground stations, however, with the new technology data in very near real time from the whole of Europe will now be accessible. The ground breaking technology is based on use of the laser technology. The technology is not new, but the new type of use is ground-breaking.
This is the first terminal in the European Data Relay System (EDRS), a system that will revolutionize the data transmission between earth observation satellites in low earth orbit and users on the ground. Until recently, near real time data has been transferred to the user in the short period of a day the satellite has been visible from the ground station. As for the remaining time, only stored data has been accessible. With EDRS operational, near real time date will be accessible over a much longer period.
The fully operational system will have two communication nodes in geostationary orbit. The first, EDRS-A, is a terminal in a commercial communication satellite, Eutelsat 98, while the second nodes, EDRS-C, are in a dedicated satellite using the European SmallGEO platform.
Both Sentinel-1 and 2 have laser communication terminals on board and can communicate with the two nodes with very high speed, up to 1800 Mbit/s with the laser beams. The EDRS terminals can transfer the same quantity of data to earth stations over the Ka-band with the same speed. Both Sentinel 1 and 2 consist of two satellites, A and B, so for the time being, four satellites will have the terminals. But, that is only the beginning.
Laser Communication – Supplement or Threat to the Established Ground Station
Satellites that circle the earth generates enormous quantities of data. A large part of a satellite project is to receive, process, interpret and store data from sensors on Board. The preferred method is to receive data at a large ground station, transmit these to dedicated processing centre, distribute data to the users and finally, store all data for later use.
To receive the data three main methods are in use.
One method is the direct transmission to a ground station while the satellite passes by the station. That is an ideal method if one wants nearly real time data from near the same place as the ground station. The disadvantage is the short period the satellite is in sight of the ground station. All users of satellite data can have such a small station, but they will receive data from very few orbits a day.
The second method in use is to store received data on board and transmit data when the satellite passes a dedicated ground station. Such ground stations are often established as near as possible to the poles. Satellites in polar orbits will pass near the poles at every orbit, and near pole ground stations can “see” the satellites in nearly all orbits. The best known station is Svalsat at the archipelago of Svalbard, only 12 degrees from the North Pole. Large organisations like ESA, NASA, NOAA and EUMETSAT use this station. From the station data can be transmitted via satellite communication or through the terrestrial network. The disadvantage is that the data is stored and is not in near real time. However, if real time data not is necessary, then this method can transmit large amount of data in an effective way.
The third, and possibly the future method, is transmitting in near real time via communication satellites in geostationary orbits. That will be a very effective method for regions of the world. A geostationary satellite over Central Europe will cover the whole of Europe and Africa. Data from satellites that pass by the region can transmit real time data to the user over a third of the earth’s surface.
The European Data Relay System (EDRS) is designed to transmit data between low-orbiting satellites and the EDRS payloads in geostationary orbit using innovative laser communication technology and send them down to Earth via radio link. The system is based at technology and experience from the European geostationary Artemis satellite, launched in 2001. Among the payloads at the Artemis satellite, is the SILEX (Semiconductor-laser Intersatellite Link Experiment) used to communicate with the SPOT-4 remote-sensing satellite.
The EDRS system will dramatically increase the speed of data transmission for satellites in lower orbit to users on the ground. This will greatly benefit the monitoring and response for natural disasters like forest fires and flooding as well as environmental monitoring.
With their shorter wavelength, laser based data transmission offers several advantages over conventional radio frequencies (RF), including the ability to achieve higher data rates than radio signals for the same aperture. Laser terminals tend to be lighter than their RF counterparts, and laser beams require less power for data transmission. Due to the higher efficiency and low beam divergence of a laser, the link is a secure point to point connection. Laser optics also eliminate the need to coordinate RF spectrum allocation with regulators.
The downside of laser communication is that the beams cannot penetrate clouds, and transmissions are easily disrupted or terminated by dust or other atmospheric elements, making optical communications best suited to the vacuum of space. The main area of application will therefore be between the remote sensing satellite and the communication satellite, not between the communication satellite and the ground stations.
Norway and Sweden have large ground stations to receive data from polar orbiting satellites. These stations are modern, have large receiving capacity and can receive and distribute large amounts of data. When it comes to the future and whether or not future polar stations may or may not become redundant, remains to be seen. It all depends on the future technology, market shares and activities on the poles.
Not New Technique
Optical communication, or free space communication are not new invention. In various forms, they have been used for thousands of years. The use of light in the form of a fire, the use of the flag for semaphoring and similar methods have been well known for centuries. The Use of artificial light and optics improved the techniques further, but it was the invention of lasers in the 1960s that revolutionized free space optics.
Free-space point-to-point optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using light-emitting diode (LED).
Not Only for Earth Observation
Using laser communication is possibly the future communication tool for several more areas. NASA’s Lunar Laser Communication Demonstration (LLCD) has transmitting data from lunar orbit to the Earth at a rate of 622 Mbit/s. This is six times faster than previous state-of-the-art radio systems from the moon. LLCD is flown by the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite. A critical part of laser communication is being able to point the narrow laser beam at a very small target over a great distance.
Another NASA experiment is the Laser Communication Relay Demonstration project that will demonstrate bi-directional optical demonstrations from geosynchronous orbit using lasers to encode and transmit data significantly faster than today’s fastest radio-frequency systems, using comparable mass and Power.
In 2017 NASA will launch the Laser Communication Relay Demonstration mission. This mission will demonstrate laser relay communication capability for Earth – orbiting satellites, similar to the European system.