There are many conditions that can make it tricky to land a helicopter including strong, unpredictable (shear) winds, confined landing spaces, and building structures. One of the worst conditions, though, is lack of ground sight. Landing a helicopter is a stressful situation for the pilot anyway. Bringing it down safely in the middle of nowhere and in areas that are notorious for dust and hence poor sight is one of the most challenging scenarios. It does not help that tactical missions can be deliberately timed in the early morning hours or at dusk, when the poor daylight conditions add to the problem.
Confronted with the need for a pilot-support system, the German Federal Office of Defense Technology and Procurement (BWB) contacted Elektroniksystem- und Logistik-GmbH (ESG), a Munich-based avionics, electronics, and human-machine interface (HMI) specialist. They had a history of working together since ESG had just equipped a special helicopter for the German Armed Forces (Bundeswehr). The Mission Avionics Test helicopter (MAT) serves as a flying experimental test bed for military and civilian purposes. It is based on a Bell UH-1D, an established model of the Bundeswehr, and is owned by its testing facility WTD61 located at Manching.
The MAT facilitates testing of avionics systems under operational conditions even though the system itself may still be within the development phase. Secondly, the MAT serves to evaluate test data. ESG had conceived the MAT, designed and integrated the full avionics system as well as the mission and test equipment for the helicopter. ESG is also responsible for the utilization of the MAT.
As the MAT’s basic helicopter equipment is completely separate from the test equipment, new systems can be tested without the need for certification, which speeds up development time and limits cost. The fully instrumented test helicopter was delivered in September 2006. This MAT served as test bed for the new Sensor-based aid to Land (SeLa), developed and designed to make helicopter landing safer despite poor or no ground sight.
System development started with defining the requirements to the SeLa system. Due to the urgent need for a landing aid for helicopters flying in Afghanistan, ESG responded quickly with a technology concept, based on radar sensors. The BWB had asked for a support system that was capable of guiding the helicopter descent during the critical last 10 m by measuring and displaying an exact altitude reading. To add more safety, ESG increased the desired system reach up to 30 m. It turned out that no available radar sensors offered the required precision for either spec, so ESG began to cooperate with the Research Institute for High Frequency Physics and Radar Techniques at Wachtberg near Bonn on improving radar sensors and with the Fraunhofer Institute for Applied Solid State Physics at Freiburg on the complete sensor modules.
Offering the best properties for penetrating varying types of dust (or snow) with differing density and to achieve the required accuracy, 94-GHz radar technology was chosen. Five of the resulting new radar sensors were combined with a highly precise drift sensor to measure the speed over ground and with two low-light-level cameras. The full system advanced to laboratory stage in August 2007 and was flight-tested between April 2008 and March 2009. Starting in July 2009, ESG will test an upgraded SeLa system in the MAT until March 2010.
The first helicopter to be equipped with SeLa will be the Bundeswehr CH53GS, the German variant of the Sikorsky S-65’s military version—a midsize transport helicopter powered by two General Electric T 64-GE-7 turbines.
According to Dietrich Pertsch, project manager of the MAT at ESG, the SeLa system is a worldwide first. Pertsch says that the radar sensors measure altitude to an accuracy of 30 to 40 mm. The drift sensor, technically based on Doppler frequency shift, is also more precise by a factor of 10 when compared to standard solutions that typically use inertia measurement, he said, adding that currently the system reach is considerably greater than originally specified. “We have reached 50 to 60 mm.”
The sensor-based landing aid is a modular system that comprises sensors and software. Depending on the depth of integration, it can either be installed with a dedicated mission computer or can be fully integrated in the onboard avionics system. In any case, the HMI requires a multifunctional display or a helmet-mounted display in the cockpit. As soon as the pilot activates SeLa, the display switches to the landing mode. AEM had the opportunity to have a closer look at the current version of the landing display. Its clear and straightforward graphics with highly targeted usage of color are clearly optimized to provide a pilot with the required minimum of easy-to-compute visual information. This landing display was developed with pilots of the German Army at ESG’s simulation center at Fürstenfeldbruck.While the current SeLa design as a stand-alone version with mission computer is oriented toward the CH53, ESG is already developing the system further to prepare its integration into other types of helicopters. “Our ultimate goal is a fully autonomous landing system fit for brownout conditions,” said Pertsch.