Space observation with GESTRA
Due to the increased utilization of near-Earth space, radar-based space observation is a highly topical and important issue – the aerospace management of the German Aerospace Center commissioned Fraunhofer FHR with the construction of a high-performance sensor.
Current threat situation
Modern society relies to a growing extent on the utilization of near-Earth space, with the result that this area now accommodates a large number of satellites with payloads for telecommunications, navigation and scientific Earth observation. Added to this is the dramatic increase in the number of non-functional objects, such as decommissioned satellites, burnt out rocket stages and debris particles, that are inherently connected with the ongoing expansion of the infrastructure. The security of the infrastructure in near-Earth space has now entered a critical phase as the likelihood of operational systems being destroyed as a result of a collision is now acute.
It is estimated that there are currently around 20,000 objects with a minimum diameter of ten centimeters and over 700.000 objects with a minimum diameter of one centimeter. Due to the high orbital speeds of up to 28,000 kilometers per hour, these are extremely dangerous projectiles that are capable of seriously damaging or even completely destroying any satellite they encounter. Moreover, the population of the space debris can grow rapidly due to a potential snowball effect caused by debris-on debris collisions with the result that the utilization of near-Earth space would not be possible for a very long period.
As the active reduction of the space debris population still poses major technological and economic challenges, the production of an orbital data catalog listing all dangerous objects is an extremely important measure to maintain the operating ability of the infrastructure in near-Earth space. Based on a precise knowledge of this orbital data, satellite operators will, in individual cases, be in a position to protect their infrastructure from collisions through the implementation of evasive maneuvers.
The creation and maintenance of such a catalog requires continuous monitoring of the constantly changing debris population at a global level. In addition to the optical observation, particularly of objects in geostationary orbit, a radar-based sensor technology is the method of choice for the monitoring of objects and debris particles in lower orbits, e.g. the LEO region (Low Earth Orbit).
States, state partnerships and organizations all over the world have taken up this task at different levels of intensity. The French bistatic system GRAVES, which still maintains a relatively small catalog, the English phased-array system Fylingdales, which is integrated into the Allied Space Surveillance Network and, above all, the United States Space Surveillance Network are worthy of a special mention in this respect. This powerful network, which comprises several sensors, embraces, inter alia, the AN/FPS-85 phased-array radar system in Florida with an antenna aperture of several hundred square meters. This will soon be complimented by the Space Fence on the Marshall Islands which will list well over 100,000 objects in the catalog.
Within the framework of a preparatory program and with a view to contributing to this global task, the European Space Agency (ESA) commissioned the construction of a phased array-based demonstrator for space observation in 2009. This was successfully constructed by the Spanish company INDRA and Fraunhofer FHR and presented to ESA in 2012. FHR's share of the order came to € 1.4 million.
Germany has also recognized the importance of this theme: in accordance with the space strategy of the federal government, the establishment of national competence for the documentation and evaluation of the current situation in space is of major importance. This is also demonstrated by the intensive work on the construction and expansion of the space situational awareness center in Uedem that was carried out over the last few years.
The space situational awareness center, which is jointly operated by the German Air Force and the aerospace management of the German Aerospace Center (DLR-RFM), does not yet have access to a proprietary sensor for the ground-based observation of near-Earth space with radar sensors. The establishment and development of operational abilities on the basis of observation data provided by third parties does, however, make little sense with the result that the development and realization of national radar sensor assets would appear to the next logical step.
Based on the institute's outstanding expertise in the areas of phased array radar systems and ground-based air and space surveillance systems, the aerospace management of the German Aerospace Center commissioned Fraunhofer FHR with the construction of a high-performance experimental sensor for the surveillance of near-Earth space at the end of 2014.
The sensor GESTRA (German Experimental Surveillance and Tracking Radar) is expected to be completed by mid-2018 and subsequently used by the space situational awareness center for remote operations at a location of the German Armed Forces. The project volume for Fraunhofer FHR amounts to € 24.9 million.
Realization of GESTRA at Fraunhofer FHR
The experimental sensor GESTRA will have a quasi-monostatic design, i.e. one transmitter unit and a receiver unit which is located nearby. These units will be integrated semi-stationary into two containers with dimensions of 18 x 4 x 4 cubic meters with the result that GESTRA can be moved to a new location with relative ease in the event of changes in the operational conditions.
The sensor, which works in pulsed operation in L-band, will observe space objects in orbital heights of between 300 and 3,000 km. A number of flexible and innovative surveillance and orbit tracking modes can be used.
These modes are uniquely and equally suitable for mechanical as well as electronic beam scanning. For this reason, the transmitter and receiver units are equipped with phased array antennas that are mounted on 3D positioners. The 3D positioners also facilitate the selection of the sky area that is currently the subject of investigation. The phased array antennas facilitate the electronic and hence inertia free rotation of the antenna beams in any required direction within milliseconds. The simultaneous combination of both aspects, e.g. in the track-while-scan mode, is also conceivable.
The realization of GESTRA is based, to a large extent, on the software defined radar approach. Here, the essential characteristics of radar can be adapted to changed operating conditions at software level thus guaranteeing a sustainable rise in flexibility. Particularly noteworthy is the fact that each of the 256 antenna elements in the receive aperture are scanned directly. Using sophisticated algorithms and high-performance processor units, this high-rate digital data stream can be used to simultaneously synthesize several antenna beams in real time, with the result that the radar can look in different directions at the same time – this is known as the digital multiple beamforming method.
The fact that GESTRA is referred to as an experimental sensor, suggests that it will later be used in an environment where operational expertise is still being developed.
The sensor itself will meet the high requirements with regard to quality management, quality and product assurance as well as system verification that have been placed by DLR-RFM – which has commissioned several professional space-related programs and missions – for this project. While processing the project, Fraunhofer FHR also takes account of numerous normative boundary conditions which are derived from internal DLR regulations and the ECSS standards of the European space agencies.
Following the successful acceptance of the Preliminary Design Review (PDR) in mid-2015, the focus is now directed towards the Critical Design Review (CDR). The project is running according to schedule and within the budget. On completion in 2018, the space situational awareness center will have access to a high-performance radar sensor and will be in a position to provide other research institutes in Germany with data for scientific processing. This creates great potential for intensive cooperation and networking within the scientific community.
With regard to the further development of national and international space observation, it can be concluded that the GESTRA concept is – in terms of its surveillance capacity – inherently scalable with the result that it is excellently suited for integration into further expansion stages of the radar sensor network.