Beam Pointing Pointing is the area of greatest uncertainty in terms of identifying specific solutions. However, there is reason to believe that the problem is not as severe as could be thought. To meet the requirement defined in Section 4.5 of a pointing precision of one-tenth the rectenna’s diameter, this would imply a pointing tolerance of a little under 0.10 at a transmission distance of 1 km. If a solid reflector has to be used and a transmission distance of only 250 m is achievable, this would lead to a pointing precision of about 0.3°. These are not considered particularly difficult levels to achieve. However, the one unknown is the dynamic behaviour or “pendulum motion” of the rectenna on the end of a long tether. Very approximately, for a 1 km long tether, the frequency of each swing should be less than 0.001 Hz. There are a number of options available for acquiring the rectenna. Since the tether will ensure the rectenna is within a reasonably narrow tolerance, one approach might be to scan the beam at a low power setting until the rectenna electronics register the beam. By determining the shape of the beam on the rectenna surface, it should be possible to adjust the beam position until it is centred on the rectenna. A feedback loop would then maintain the beam firmly locked onto the target Beam directional control could utilise a feed horn cluster acting as a simple phased array. Alternatively, a 2-degrees of freedom mechanism could be used to adjust the angle of a single horn. Power Module The choice of lithium batteries was considered to be the most convenient and least expensive power source and eliminates the need to incorporate a solar array and recharging circuitry - none of which the ASAP demonstrator needs to prove at this early stage. In addition, the short mission duration of just a few days does not justify the high cost of a more conventional solar array approach.
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