Feasibility of High Altitude Platforms for weather radar

High Altitude Platforms (HAPs) were conceived as a platform for the provision of broadband services to the home; an alternative to satellite and terrestrial fixed wireless access. HAPs are intended to operate over urban regions at an altitude of 20-22km. HAPs have been proposed using both airship technology (Sky Station International www.skystation.com) and high altitude aircraft (Angel Technologies www.angeltechnologies.com). Here, we assess the feasibility of using such a system as a platform for operating a weather radar. HAP based radars offer a number advantages over ground and satellite borne rainfall radars, particularly in urban or mountainous regions. Rainfall estimation in urban catchments are notoriously problematic. Rain gauges provide only a point measurement, have a limited dynamic range, and are difficult to site in urban areas. Techniques have been proposed that employ microwave links to derive path integrated rainfall from differential (frequency) attenuation measurements. Ground based weather radars may not provide adequate spatial resolution and will almost certainly suffer from increased clutter and beam-blockage problems.

Given the low operating altitude a weather radar operating from a HAP has the potential to match or better the spatial resolution of a ground based radar. In urban areas due to the near-nadir incidence angles the problems of beam blockage would be greatly reduced thus providing the opportunity for improved rainfall rate estimates. HAP weather radar provides the benefits (short-range, low power) of airborne weather radar from a quasi-stationary platform. This paper considers the key system requirements for a HAP weather radar including the likely (radar) power budget, sensitivity, choice of operating frequency and antenna scanning mechanism (e.g. steerable reflector or phased array).

Consideration is also given to the likely attainable areal coverage, spatial resolution (with and without pulse compression) and temporal resolution. The similarity in the geometry with airborne and spaceborne radar systems permits the use of mature rainfall rate estimation algorithms, including the use of mirror echo and surface reference techniques. Consideration is given to the applicability of such techniques, and the problems of non-uniform beam filling particularly at non-nadir incidence. Calibration of HAP weather radars using ground-based targets will also be considered.