OPTICS FOR THE CANADIAN HYPERSPECTRAL MISSION (HERO)

University of, Victoria, BC, Canada, Email: harveyr@shaw.ca ABSTRACT The proposed Canadian Hyperspectral Environment and Resources Observer (HERO) mission is described. Preliminary, phase A, instrument design forms are discussed. 1. MISSION OBJECTIVE The HERO mission objective is to deliver high quality hyperspectral data for the Canadian and international users, covering wide range of applications such as: geological mapping of the Canadian north, improved species and bio-health assessment as part of a national forest inventory, environmental mapping, monitoring of carbon sources and sinks on land, assessment of productivity in coastal and littoral aquatic ecosystems. From a technological point of view, it is also expected that the mission will stimulate the development of new and better algorithms for the exploitation of data , and will advance space imaging spectrometry in general, as well as Canadian expertise and capabilities in that area. 2. HERO INSTRUMENT REQUIREMENTS The main HERO instrument requirements are summarized below. Table 1. Parameter Value Imager type pushbroom Altitude ~700 km Swath width t30 km Ground sampling distance (GSD) 30 m Spectral coverage 400-1000nm VNIR 1000 – 2500 nm SWIR Spectral sampling interval (SSI) 7.5 nm Spectral resolution (FWHM) ~10 nm (full width at half maximum) Smile distortion 0.3 @ Nyquist Signal to noise ratio (SNR) for typical signal (30% albedo, 30 deg SZA) 600:1 @ 650nm 400:1 @1650 nm 200:1 @2100 nm . From the point of view of the optical design, the most challenging parameters listed in Table 1 are: the SNR values requiring a fast system (F/2.2, aperture ~320 mm) and the very low keystone and smile distortions. Both distortions are characteristic for the pushbroom spectrometers and must be kept at small fractions of Ground Sampling Distance (GSD) and Spectral Sampling Interval (SSI), respectively. Reduction of keystone errors is especially important because it causes spectra combination from adjacent spatial locations; correction of these errors cannot be accomplished without an a priori knowledge of the form of the spectra to be recovered. Errors due to smile, on the other hand, are more easily corrected in the retrieval process and can be characterized in orbit, using for example the atmospheric features as described by Neville et al [1]. 2. OTHER PUSHBROOM INSTRUMENTS In Table 2 the main design characteristics of all pushbroom type instruments currently in space are listed and compared to HERO. It shows broad acceptance of the Offner scheme of design. One exception is MERIS, which employs the Dyson type optics. The table reveals also that the HERO design requirements are the most demanding of the critical optical design parameters - F/#, aperture size and distortions - particularly because they have to be met simultaneously. For example, MERIS has lower F/# but much smaller aperture and thus smaller aberrations; CHRIS has small distortions, but high F/# and smaller aperture.