Lighterthanair Platforms

Lighter-than-air platforms comprise various balloons and blimps that may be manned or unmanned, tethered or free flying, and powered or unpowered. Contrasting with the spherical shape of a balloon, a blimp has an elongated, aerodynamic shape with fins. Strictly speaking, the term blimp refers to a free-flying airship without internal frame structure, which is kept in shape by the overpressure of the lifting gas. However, the term blimp is often, as in this book, also used for tethered or moored airships, including those where the lifting medium is hot air.

Balloons and blimps are widely employed nowadays for meteorological sounding, commercial advertising, sport flying, and other purposes. Manned, hot-air balloons are, in fact, quite popular (Fig. 8-6), but lack positional control unless tethered. Near-calm conditions are necessary for launching and landing, which often restrict flights to dawn and dusk times of day with consequences for lighting conditions (Fig. 8-7).

Owing to their comparatively low level of high-tech components, unmanned, tethered lighter-than-air platforms have been employed for SFAP for many decades. Their suitability for aerial surveys using various instruments from compact cameras to multi-sensor systems is documented by a wide range of publications. Among the most basic versions, a simple plastic bag containing the lifting gas was employed by Ullmann (1971) for SFAP of bogs in Austria. Examples for the use of helium balloons include the coastal and periglacial geomorphology studies by Preu et al. (1987) and Scheritz et al. (2008), the photogrammetric documentation of archaeological sites by Altan et al. (2004) and

FIGURE 8-6 Hot-air balloon, Dragon Egg, of Jane English lifting off from a field near Mt. Shasta, northern California, United States. This is a relatively small balloon that can carry four people. Photo by JSA, May 1999.
FIGURE 8-7 Early morning picture looking south toward Mt. Shasta taken from the hot-air balloon (in previous figure) about 150 m high above Shasta Valley, northern California, United States. Note morning fog in the distance and heavy shadowing of ground. Photo by JSA, May 1999.

Bitelli et al. (2004), and vegetation studies in wetlands and semi-arid shrubland conducted by Baker et al. (2004) and Lesschen et al. (2008).

For an investigation on salt-marsh eco-geomorphological patterns in the Venice lagoon spanning different spatial scales, Marani et al. (2006) suspended a double-camera system (VIS and NIR) from a helium balloon operating at low heights of around 20 m, and a similar system is used by Jensen et al. (2007) for wheat crop monitoring. In order to sidestep costly satellite imagery and conventional aerial photography with a low-cost remote-sensing method applicable in developing countries, Seang and Mund (2006) have used SFAP taken from a hydrogen balloon for various applications in regional and urban planning, monitoring of degraded forest, and basemap compilation for infrastructure projects in Cambodia.

Tethered spherical balloons are, unfortunately, highly susceptible to rotational and spinning movements (Preu et al., 1987). For SFAP, more aerodynamically shaped airships, namely blimps, are preferable because they align themselves with the wind direction. As an alternative to spherical balloons, various researchers have employed helium blimps: Pitt and Glover (1993) for the assessment of vegetation-management research plots, Inoue et al. (2000) for monitoring various vegetation parameters on agricultural fields, Jia et al. (2004) for evaluating nitrate concentrations in agricultural crops, and Gomez Lahoz and Gonzalez Aguilera (2009) for 3D virtual modelling of archaeological sites. The suitability of blimps even for heavy high-tech sensors at greater flying heights (up to 2000 m) was proven by Vierling et al. (2006), who used a 12-m helium blimp as a multi-sensor platform for hyperspectral and thermal remote sensing of ecosystem level trace fluxes.

Hot-air blimps as camera platforms have been employed for many years by two of the authors for investigations on soil erosion and vegetation cover (Marzolff and Ries, 1997, 2007; Marzolff, 1999, 2003; Ries and Marzolff, 2003). The same hot-air blimp and its prototype predecessor were used for archaeological applications documented by Hornschuch and Lechtenborger (2004) and Busemeyer (1994).

Because of their lightweight and comparatively large surface area, lighter-than-air platforms, especially balloons, are relatively difficult or impossible to fly in windy conditions. This wind susceptibility is even increased and thus positively exploited by the so-called Helikite, a hybrid of helium balloon and kite combining the advantages of both platforms. Examples for investigations conducted with this rather unusual platform are given by Verhoeven et al. (2009) for aerial archaeology and Vericat et al. (2009) for monitoring river systems.

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