DESIGN OF A SPACEBORNE LIGHTNING SENSOR

Abstract

The design of BOLTS (Broad Area Lightning Telescope Sensor) is presented. This sensor will provide full-time (day/night) coverage of the continental U.S. from a geosynchronous orbit. The average ground resolution will be 8 km and the system will be able to detect ≃ 10⁷ watt strokes during nighttime and ≃ 4 x 10⁷ watt strokes during daytime with a probability of detection of 0.9. We present the system's requirements and projected performance, together with the design rationale. Contrast enhancement is achieved using a narrow band interference filter deposited on a curved surface inside the F/2.5, 101.7 mm optical system. Deposition of the interference layers on the curved surface reduce the passband wandering caused by off-axis bundles. The focal plane constitutes an 800 x 800 element virtual phase CCD array with a multiple outputs option. The central 800 x 400 elements are used for imaging while the outer 2x (800 x 200) elements serve as buffer memory for one frame storage. An additional 2x (800 x 200) array serves for storing a second frame. Signal detection is achieved via a frame-to-frame subtraction algorithm that is hardware implemented immediately following the CCD arrays. An integration time of 5 msec is used, which stems from SNR optimization requirements and from the fact that lightning strokes occur randomly in time and space. The data obtained after frame-to-frame subtraction is subjected to a threshold test and the resulting positive events are digitized and stored in an on-board digital memory using 48 bits/event. Each record contains intensity information over a dynamic range of 4000, location information and time of occurrence information. A prototype instrument built to perform measurements from aboard a U-2 plane is described. The purpose of this instrument is to refine some of the lightning data used in defining the system's parameters. A short discussion about the changes required to expand the design to either a global coverage instrument or a high resolution, smaller field instrument is presented.