Large Diffractive Optics for GEo-Based Earth Surveillance

Hyde, R A

Lawrence Livermore National Laboratory

关键词: Laser Weapons;    Apertures;    Targets;    Resolution;    74 Atomic And Molecular Physics;   

DOI  :  10.2172/15005467
RP-ID  :  UCRL-CR-155379
RP-ID  :  W-7405-ENG-48
RP-ID  :  15005467

美国|英语

来源: UNT Digital Library

【 摘 要 】

The natural vantage point for performing Earth-centric operations from space is geosynchronous orbit (GEO); a platform there moves at the same rate as the Earth's surface, so appears to continually ''hover'' over a fixed site on the Earth. Unlike spacecraft in other orbits, which rapidly fly-over targets, a GEO-based platform remains in-position all the time. In order to insure continual access to sites using low earth orbit (LEO) platforms, one needs a large enough constellation ({approx} 50) of spacecraft so that one is always overhead; in contrast, a single GEO platform provides continuous coverage over sites throughout Euro-Asia. This permanent coverage comes, unfortunately, with a stiff price-tag; geosynchronous orbit is 36,000 km high, so space platforms there must operate at ranges roughly 100 times greater than ones located in LEO. For optical-based applications, this extreme range is difficult to deal with; for surveillance the price is a 100-fold loss of resolution, for laser weapons it is a 10,000-fold loss in flux-on-target. These huge performance penalties are almost always unacceptable, preventing us from successfully using GEO-based platforms. In practice, we are forced to either settle for brief, infrequent access to targets, or, if we demand continuous coverage, to invest in large, many-satellite, constellations. There is, fortunately, a way to use GEO-based optical platforms without incurring the huge, range-dependent, performance penalties; one must simply use bigger optics. As long as the aperture of a platform's optics increases as much as its operating range, then its performance (resolution and/or flux) does not suffer; the price for operating from GEO is simply 100-fold larger optics. This is, of course, a very stiff price; while meter-class optics may suffice for many low-earth-orbit applications, 100 meter apertures are needed in order to achieve similar performance from GEO. Since even the largest Earth-based telescope is only 10 meters in diameter, building ten-fold larger ones for GEO applications (let alone delivering and operating them there) presents major difficulties. However, since the challenges of fielding large platforms in GEO are matched by the benefits of continuous coverage, we propose a program to develop such optical platforms. In this section, we will examine a particular form of large aperture optic, using a flat diffractive lens instead of the more conventional curved reflectors considered elsewhere in this report. We will discuss both the development of this type of large aperture optics, as well as the steps necessary to use it for GEO-based Earth surveillance. In a later section of this report we will discuss another use for large diffractive optics, their application for global-reach laser weapons.