Optical downlink study for Low Earth Orbit satellite

Currently, the high increase of space systems capacity requires investigating a new generation of satellites capable of assuring a high data rate in the link between satellite and ground. In particular, in the Earth Observation domain, Optical Communications are being studied to ensure high throughput communication systems for the download of images taken by the satellite: they will play an important role in the near future as they offer potential advantages over microwaves. However, the impact of clouds delays its implementation for the time being. These atmospheric events vary as a function of the ground station location. Hence, this report describes a research study with a system engineering point of view on the feasibility of optical links. Employing the clouds databases provided by geostationary satellites and in-situ detectors, it aims at designing the network of ground stations and satellite features, which permit the complete download of Earth images acquired by the satellite during its period in orbit. Such a study is a step forward compared to the state of the art, which only considers cloud condition data from GEO imagery satellites. The comparison has been performed considering a 1-month simulation, due to the in-situ data unavailability: further studies will accomplish a 1-year (or more) simulation. It is shown that link availability due to Cloud Mask (CMa) derived from geostationary imagery has the 85% probability to correspond to an Optical Depth (OD - averaged in space and time) smaller than 0.7, a preselected threshold that defines the OD cloud blockage. In the same way, it is discovered that CMa cloud blockage might be pessimistic: 40% of the CMa cloud blockage time corresponds to an Optical Depth (averaged in space and time) that is smaller than 0.7. Moreover, an end-to-end simulator of data downloaded has been implemented in order to process the feasibility of optical links. It permits the analysis of the mean downloaded data volume and the evolution of the satellite on-board memory along the entire simulation as functions of the minimum elevation and the Concepts of Operations (Adaptive FEC rate, FEC=1/2, FEC=2/3 and FEC=4/5): this process permits to verify that all the images acquired by the satellite are transferred to the ground stations and to size the satellite memory. This simulation has been carried on for 7 days, due to the in-situ data unavailability: further studies will accomplish a 1-year (or more) simulation. The simulation that considers only one Optical Ground Station states the uselessness of an adaptive FEC rate when the minimum elevation is greater than 20°: the added complexity does not permit to have a great difference in downloaded data volume with respect to the case of FEC equal to 4/5. The results of a second simulation show the necessity of a network of optical ground stations and the use of a satellite on-board configuration that consists of at least 2 lasers: they are essential to satisfy the constraints imposed by all the minimum elevations and the Concepts of Operations. The variation in downloaded data considering in-situ and geostationary satellite data is also proven: in-situ data, thanks to their greater precision, permit to obtain a bigger quantity of transferred images. This information will encourage the use of in-situ detectors for many other stations. All these results validate the functioning of the simulator algorithm, but they should be confirmed by additional simulations.