Introduction
This paper describes an innovative concept using tethered
Aerostats as a platform for raising wireless communication
payload, which overcomes the two main limitations of high
towers listed above. Tethered aerostats are an outcome of
Lighter-Than-Air Technology, where static lift production
mechanism is based on the Archimedes Principle [1]. An
aerostat does not require any additional energy to reach to a
certain height. For a given volume of envelope that contains
the lighter than air gas, displaced weight of air creates a
vertically upward buoyant force that leads to the lift. One or
more Ballonets are provided inside the envelope to adjust the
buoyancy. The envelope volume is large enough to ensure that
the displaced air should be able to produce sufficient lift,
under the entire range of operating conditions, to balance all
the weight groups of the aerostat system, viz., envelope, fin,
nose battens, ballonets, pivot mechanism, payload, tether,
recovery system, gas filling ports, and safety valves.
Aerostats are used all over the globe as a platform to house
high-resolution sensors for applications such as aerial
surveillance, regional atmospheric data collection and balloonbarrage
system. Depending on the payload, range of
surveillance, and operational time, these aerostats can be
launched to any desired altitude from a few meters above
ground level to as high as 5000 m above ground level. Of
course, the payload carrying capacity of an aerostat is reduced
as the operational height is increased.
Aerostats can easily be deployed at high altitudes, ensuring
disturbance free LOS for the communications payload. Once
they are deployed, there is very little recurring additional
expenditure to keep them afloat, except in the form of small
amounts of lighter-than-air gas, just to top-up for the leakages
through the fabric over a period of time. Due to its
aerodynamic shape as well as provision of fins, an aerostat can
remain fairly steady even in strong winds and hence can
provide stable line of sight connectivity. An omni-directional
antenna mounted below the aerostat, leads to a relaxation in
the antenna direction alignment requirement.
802.11b [3] uses the ISM (Industrial Scientific Medical)
band from 2.400 to 2.495GHz. Due to the ubiquity of
equipment and unlicensed nature of the 2.4 GHz ISM band,
our work is focused on building a network using 802.11b. It
makes use of Direct Sequence Spread Spectrum (DSSS)
modulation and has a maximum rate of 11 Mbps, with actual
usable data speeds up to about 5 Mbps. 802.11b can be used
in a point-to-multipoint configuration, wherein an access point
communicates via an omni-directional antenna with one or
more clients that are located in the neighborhood of the access
point. Typical indoor range is 30 m (100 ft) at 11 Mbit/s and
90 m (300 ft) at 1 Mbit/s. The overall bandwidth is
dynamically shared across all the users on a channel depending
on the individual demands. The protocol with few
modifications can also be used to achieve a range of several
kilometers by using high-gain directional antennas when line
of sight connectivity is available in fixed point-to-point
arrangements
Depending on the payload requirements, operating altitude,
temperature variation and other atmospheric input parameters,
the envelope volume is assumed at the start, using a thumb
rule. The surface area and other parameters like weight of
envelope, tether and the fins are then estimated. Once the
weight breakup is obtained, the volume and hence mass of the
ballonets are calculated. Since the value of net lift available is
known, the payload capacity of the aerostat can be estimated.
The envelope volume is iteratively adjusted till the payload
capacity of the aerostat matches the requirement specified by
the user.
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