The HALO project is based on an initiative of the German atmospheric science community. It was found that the acquisition of a modern research aircraft is essential to continue the successful research and maintain the leading position of German scientists for the next decades in this area. DLR (Deutsches Zentrum für Luft- und Raumfahrt) and MPG (Max-Planck-Gesellschaft) took the lead and submitted the application for this major research facility investment. This proposal was supported by scientists from 31 institutes. After an evaluation, the scientific advisory body of the Federal Government (Wissenschaftsrat) recommended the realisation of HALO without restrictions.
In September 2004 the German ministry of education and research approved the construction and modification of this high-performance research aircraft. The purchase of HALO and its initial development into a scientitic platform was funded jointly by the German Ministry of Science and Education, the Helmholtz Association (HGF) and the Max-Planck-Gesellschaft.
HALO is based on a GulfstreamG550 – a large business jet. However, it underwent significant modifications, e.g. the insertion of openings, in order to maximise its capabilities as a research platform.
The concept behind HALO is to provide an optimal platform for airborne atmospheric science and Earth observation, a well-equipped flying laboratory that allows the scientists onboard to completely focus on their own experiment. A large set of jet aircraft candidates was investigated for its suitability. Finally the business jet aircraft G550 of Gulfstream was found to meet best the essential requirements of the HALO users:
- range well above 10000 km or more than 10 flight hours for transcontinental experiments and long duration measurements;
- certified ceiling of more than 15 km,
- maximum payload of 3 tons,
- a large usable cabin area of 20-30 m2 for simultaneous operation of several complementary instruments and scientific personnel from several groups (for multidisciplinary and international projects),
- potential for quick modifications for a wide variety of applications and for flexible use as research aircraft with different instrument configurations for various research projects.
Of course, the G550 underwent significant modifications, e.g. the insertion of openings, in order to maximise its capabilities as a research platform. The main strengths of the proposed HALO aircraft are its long range and endurance, high ceiling altitude and large instrument load capacities, which are not available in such combination on any other research aircraft in Europe. Thus, HALO provides the opportunity to carry out measurements in areas that can currently not be probed by other platforms. This includes nearly the full altitude range of the troposphere and lowermost stratosphere and remote parts of the Earth.
HALO represents a major improvement in the airborne research capability for research institutes and universities to study atmospheric phenomena and their interactions from local to global scales. The observed spatial variability in clouds, aerosols, water vapour and ozone, for example, ranges from less than 100 m (turbulence) to more than 1000 km (synoptic weather systems). Because of its relatively large size, HALO facilitates the deployment of comprehensive sets of instrumentation, to simultaneously measure physical and chemical parameters to characterise transport, radiation and chemical processes. Improved process understanding from aircraft measurements contributes to the development of models. These models serve to study complex interactions and feedbacks, and to perform sensitivity studies and scenario-based predictions. Confidence in models can only be established by showing that processes are well reproduced at all relevant scales. Aircraft measurements provide the required three-dimensional resolution, and they can be linked with satellite images to construct a more complete view at regional to global scales.