HAPS through the Lens of Satellites and UAVs: A Function-Level Perspective on the Emerging High Altitude Economy
High-Altitude Platform Stations (HAPS) operate in the lower stratosphere at 17-27 km, between satellites and Unmanned Aerial Vehicles (UAVs). For this third tier the architectural case has long outpaced the flight evidence, but a wave of 2020-2026 stratospheric flights now permits a direct comparison. We evaluate HAPS function by function across sensing, navigation, and communication, taking operational satellite and UAV implementations as the reference. We define a strict evidence rule, counting a function as flight-validated only on operationally relevant stratospheric data return at or above 18 km, and apply it to nineteen functions. The resulting count is lower than the literature implies: five functions have credibly crossed over (optical Earth observation, hyperspectral imaging, methane imaging, RF/SIGINT, and broadband relay), yet these rest on only four flight programs, with at most one carrying peer-reviewed flight evidence. One function is ground-demonstrated, two are partially demonstrated, three are conceptual, and eight remain unflown. Four engineering domains (size, weight, and power; station-keeping; aperture; and viewing geometry), bounded by an operational envelope of platform stability and payload operability, explain the pattern. The governing advantage is persistence at close range, not altitude. Eight use cases, supported by same-sensor forward simulations, translate the pattern into missions, led by resilient public-safety mission-critical services (MCX). On this evidence, HAPS fits as a persistent regional tier in a multi-tier non-terrestrial network and as the seed of an emerging High Altitude Economy. Carrier-grade service, station-keeping precision, and regulation remain the principal open problems, and we pose the persistent-tier reading as a testable hypothesis with dated 2030 markers.