International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
Since any amount of fuel that is carried will severely limit the
aircraft’s endurance to a few days at most, the only source of
energy available to very long endurance platforms is solar
energy.
Nuclear power is also technologically possible (Graham-Rowe,
2003) but it is not an acceptable technology due to the risks
involved in case of a mishap. Due to its solar character, the
platform will not contribute to atmospheric pollution at all.
Most high-altitude UAV systems are aerodynamic, generating
lift by moving. The total lift force is determined by the craft's
speed, its wing area, and the density of the surrounding air. As
the air density is not varying and because the optimum air speed
for a lightweight aircraft at stratospheric heights is about 20
m/s, to carry a substantial payload, the wing area needs to be
increased. The total weight of the platform will thereby also
increase. An extreme example of this is the Helios prototype
which weighs about 930 kg (including a total payload of about
300 kg) and has a wing span of 75 meters, i.e. exceeding that of
a Boeing 747. Apart from practical problems of finding suitable
runways and hangars, these large UAVs are very expensive.
Therefore, we favour using small UAVs (wingspans of 15 — 20
m) that carry small payloads (2 — 5 kg), batteries and electronics
excluded.
Aerostatic systems (blimps) for high-altitude flight have been
proposed, but are not operational yet. They rely on very large
volumes (200 000 m? or more) of helium gas to provide lift. In
principle, this allows very large payload mass to be carried (up
to a ton or more). On the other hand, controlling such large
volumes and steering them requires a large amount of power, to
be provided by solar cells draped over the envelope of the
blimp after inflation. These systems are likely to be an order of
magnitude more expensive than aerodynamic systems (Küke,
2000)
2.2 Stratospheric environment
The stratosphere is characterized by an almost complete lack of
water vapour, relatively low wind speeds (10 m/s on average),
only limited turbulence and low temperatures (-50 to -70? C).
30000 ep
25000 .
: — average wind speed
= 20000 _ * maximum wind speed
=
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10000 €
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0 P
0 20 40 60 80 100
Wind speed (nvs)
Average and maximum wind speeds for the years
2002-2003 for elevations up to 30 km (data for Den
Helder, Netherlands, March-September)
Figure 1.
Figure 1 shows that an aircraft moving at 20 m/s (airspeed) is
able to overcome the average wind speeds. In extreme cases,
the platform will have to change flying height to avoid being
"blown away".
Air traffic is controlled up to 14 km altitude; above that height,
an aircraft is not limited in its movements air traffic control, and
there is virtually no air traffic. This allows for efficient mission
planning.
2.3 Solar energy
At the top of the atmosphere, about 1 368 W/m° of energy is
provided by solar radiation (BGC, 1994). Due to atmospheric
absorption and depending on the time of year and the
geographic latitude, this amount is reduced at lower altitudes.
Figure 2 shows the typical solar irradiance (in Wh/m?/day) as a
function of the month in the year for a horizontal surface at 55?
latitude. In order to fly continuously, the UAV needs at least
2500 Wh/m?/day, which is available for 7 months (March —
September) at latitudes up to 55°.
=
m
6000
5000 |
4000
3000
2000
1000
0
=
Jan E
Feb MEN
vov MN
Dec |
Figure 2. Typical solar irradiance (Wh/m°/day) as a function of
the month of the year for a location at 55? N latitude
During the day, batteries or reversible fuel cells are charged
with the excess power, to provide sufficient energy during night
time. The UAV can also glide downwards, using some of its
potential energy. In emergency situations during winter time,
the craft can be launched with non-regenerative fuel cells and a
limited amount of fuel, so allowing short (several days)
dedicated missions.
Whenever it is flying, the UAV platform is always available
for earth observation, efficiently covering large areas (100 000
km2 in one flying season of 7 months, taking cloud cover into
account). Unlike manned aircraft, it can take advantage of even
very small windows in the cloud cover. Indeed, at a modest
speed of 20 m/s, the platform can be transferred over large
distances (over 1700 km) in 24 hours, even if there are no
favourable winds.
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