Research Area (F):  Geodesy and Geophysics

HALO opens the excellent opportunity to conduct not only atmospheric research but also geoscientific research. There is a great number of issues in geodesy and geophysics where the utilization of such an aircraft is of utmost advantage. Utilizing measurements of the various geodetic-geophysical sensors aboard HALO aims on different applications in geodesy (e.g., regional and global geoid improvement, unification of geodetic reference systems), geophysics (e.g., lithospheric magnetic field, tectonics, structure of the Earth), oceanography (mean seasurface topography, properties of upper water layers), glaciology (e.g., ice-sheet dynamics and properties, ice-mass balance), and, last but not least, atmospheric sciences (e.g., atmosperic remote sensing for tropospheric temperature and humidity profiles).

The long-range capability of HALO is the crucial key to all of these applications, enabling investigations which are not possible otherwise: Only HALO enables to cover continental-wide, yet inaccessible, remote regions like Antarctica in one survey, thus getting homogeneous and consistent measurements over large areas. Only HALO enables to survey structures of longwavelength extent like the East African Rift system (which is being connected northwards up to the Jordan Rift Valley) or the Nubian sandstone aquifer system. Only HALO enables by long-range survey profiles to connect existing different, locally isolated surveys in order to clarify inconsistencies (e.g offsets or reference system issues like gravimetric datum). In addition, with its long range and its capability to fly up to high altitudes, HALO is the perfect tool to provide a link between (low resolution) satellite measurements and (high resolution) terrestrial measurements, to close gaps both in areal extent and in resolution as well between terrestrial and marine environments and, thus, also for the validation and calibration of satellite missions (e.g., SWARM, see below).

Moreover, geoscientific research using HALO benefits from a synergy effect, which cannot be rated highly enough: aircraft parameters like size and maximum payload allow to equip HALO with a complete ensemble of geodetic-geophysical sensors and instruments. These sensors canbe chosen to be partly redundant – which follows a major principle in geodesy to reach a high level of redundancy of observables – and complementary to get best results out of joint analyses and cross-comparisons. These sensors include:

• special accelerometers – normally spring-type gravity meters – to measure the gravity field of the Earth;
• scalar and vector magnetometers to measure (different quantities of) the magnetic field of the Earth;
• top-mounted GNSS antennas to determine the flight trajectory and, subsequently, kinematic velocity and kinematic acceleration of the aircraft;
• sideward-looking and nadir-looking GNSS antennas to conduct measurements of the novel technology of reflectometry, scatterometry and occultation;
• laser distance meter (or laser scanner) to measure the altimetric height and, thus, to infer topography information, occasionally combined with waveform analysis to yield small-scale features (like surface roughness);
• radio echo sounding (RES) to determine ice surface and bedrock topography as well as internal ice layers in ice-covered areas like Antarctica or Greenland;
• thermal, multi- or hyperspectral camera systems.

Gravity Field: The determination of the (exterior) gravity field is one of the major tasks of geodesy. Gravity field related quantities play an important role. First of all, the geoid – a special equipotential surface of the gravity potential – forms the best approximation of the physical figure of the Earth and provides a reference surface for height systems. Besides its importance in geodesy, it also provides reference for further applications, e.g., in oceanography (mean sea-surface topography) or in glaciology (thickness of floating ice from freeboard height and equilibrium condition).                    For higher resolutions satellite measurements have to be combined with terrestrial (ground–based or near–surface) gravity measurements. In this respect, the only large gap in terrestrial gravity data remains in Antarctica, Scheinert (2012a) for details. 

Magnetic Field: A measurement of the Earth magnetic field is made of several contributions: the fields generated in the Earth’s outer core, in the lithosphere and by systems of electric currents flowing in the ionosphere and magnetosphere. Between the fields of internal origins the core field dominates the long wavelengths, whereas the field generated in the lithosphere is predominant for the intermediate and small wavelengths. An accurate mapping and describing of this lithospheric field is important as it carries valuable information on the past of our planet, its evolution in terms of tectonic, magnetic and thermal history.

GNSS Reflectometry, Scatterometry and Occultation Measurements: GNSS provides the major technology to determine the flight trajectory of the aircraft. Besides the HALO GNSS user antenna (provided in the framework of BAHAMAS) the scientific users may install further GNSS antennas on top (see e.g., the GEOHALO mission). A completely different field of applications opens when mounting GNSS antennas with nadir- or sideward-looking directions.                                                                                First, using nadir-looking (occasionally combined with sideward-looking) GNSS antennas enable GNSS reflectometry and scatterometry (GNSS-R/S) observations. From these, altimetric heights can be inferred as well as wind speed and direction or further properties of land, ocean and ice surfaces (e.g., roughness or waves, soil moisture, snow layers).                                      Second, using sideward-looking GNSS antennas the GNSS radio occultation technique (GNSS-RO) will be used to extract the information content of phase and amplitude observations at low-elevation angles with respect to atmospheric refractivity, dry temperature and humidity. These days atmospheric remote sensing by GNSS-RO is a well-established technique. GNSS-RO data from several space-borne platforms (COSMIC, MetOp, GRACE, TerraSARX/ TanDEM-X) are presently being used for operational weather forecasts by many meteorological agencies.
In this way, GNSS reflectometry, scatterometry and occultation observations are valuable, on the one hand, to retreive information in regions which are not covered by other systems (especially by dedicated satellite missions), e.g., in Antarctica. On the other hand, they allow to validate, to densify and/or to complement data of other sources.

Specific Geophysical Applications: Large-scale geoscientific surveys utilizing HALO have the potential to provide homogeneously recorded data for geophysical applications. In contrast to small-scale geophysics, which aims on the exploration or the survey of interior structures of the Earth at high resolution, HALO can substantially contribute to the investigation of structures of long-wavelength extent.