Published on Nov 30, 2023
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.
