A high-resolution super-spectral camera is being developed by Elbit Systems in Israel for the joint CNES- Israel Space Agency satellite, VENμS (Vegetation and Environment monitoring on a new Micro-Satellite). This camera will have 12 narrow spectral bands in the Visible/NIR region and will give images with 5.3 m resolution from an altitude of 720 km, with an orbit which allows a two-day revisit interval for a number of selected sites distributed over some two-thirds of the earth's surface. The swath width will be 27 km at this altitude. To ensure the high radiometric and geometric accuracy needed to fully exploit such multiple data sampling, careful attention is given in the design to maximize characteristics such as signal-to-noise ratio (SNR), spectral band accuracy, stray light rejection, inter- band pixel-to-pixel registration, etc. For the same reasons, accurate calibration of all the principle characteristics is essential, and this presents some major challenges. The methods planned to achieve the required level of calibration are presented following a brief description of the system design. A fuller description of the system design is given in ,  and .
A 5m GSD satellite camera with 12 narrow spectral bands in the VNIR region is being developed by El-Op, Israel, for a cooperative project between CNES (France) and the Israel Space Agency. The satellite, called "VENμS" (Vegetation and Environment monitoring on a New Micro-Satellite) will enable evaluation of the use of high-resolution, high repetitivity, super-spectral imaging data for vegetation and environmental monitoring. The camera will image a limited number of selected sites around the globe with a two-day revisit interval. Highly demanding requirements for signal-to-noise ratio, radiometric accuracy, band-to-band registration and precise location on the ground will ensure the validity of the data. It will also help to define the optimal set of bands and the image processing algorithms of future instruments in the framework of the GMES program. The satellite bus will be built by Israel Aircraft Industries and will also carry an experimental ion propulsion system developed by Rafael (Israel).
Cameras for remote sensing and for long range reconnaissance on airborne and space platforms in the 0.4-1.0 μm region exist, but there is a growing need for images in longer IR wavelengths. In those wavelength regions, large apertures are essential to achieve satisfactory ground resolution but this conflicts with the limitations of weight often associated with long-range reconnaissance platforms. The high costs of the system itself and of its installation on the platform make it advantageous to provide both wavelength bands in a single instrument. A dual-waveband airborne telescope and camera system has been developed by El-Op Ltd. which will provide high-resolution images in the thermal IR at night and in the visible in daylight, or both simultaneously. It offers low weight, compact size and high dimensional stability over a wide range of environmental conditions. The two detector arrays are mounted in a compact focal plane assembly with a spectral beam-splitting system that optimizes the transmittance of each band. A cooled IR detector is essential to achieve the necessary sensitivity and a special detector array with a high reliability, lightweight, low power cooling unit has been developed.
High-resolution IR scanning systems able to scan large areas quickly require linear detector arrays with more than 1000 elements and high sensitivity, achieved by TDI. ELOP initiated the development of such a long detector array in the 3-5μm spectral region. The architecture of the detector is based on several sub-segments butted together in a staggered configuration to achieve the desired detector length. One problem is the large non-uniformity of the detector, which is exacerbated by the cos<sup>4</sup>α optical effect. With the entrance pupil imaged on the cold shield aperture to enhance efficiency, the angle a becomes large. This imposes significant additional non-uniformity that has to be compensated and affects the dynamic range of the electronics. A way to overcome this problem is suggested, based on de-selecting specific pixels in any TDI channel.
Another problem is that while higher TDI levels increase the SNR, they increase the smear (blur) due to vibrations, drift etc. The optimal TDI level depends on the specific conditions of the system, namely: signal level and vibrations. Using superfluous pixels in the overlap between segments, several TDI levels can be operated simultaneously, allowing a decision to be made automatically as to the optimal TDI level for operation.
A new era in commercial remote sensing from satellites is beginning, with the emergence of high-resolution cameras that approach the capabilities of aerial photography. The first satellite of the EROS constellation will be launched in a few months and will provide panchromatic images of the Earth at a resolution of 1.8 m. Subsequent units will follow with one meter class panchromatic systems and 3.2 m multi-spectral channels. The constellation will allow high revisit rates and large data collection capacity over most of the Earth. The paper will describe the payloads planned for the series with emphasis on the technological features of the cameras.
Earth horizon sensors, used on satellites for determining orientation, usually operate on the principle of detecting the discontinuity in the infrared radiance at the limb. Most such sensors make use of radiant emission from the upper atmosphere in the 15 micrometers CO<SUB>2</SUB> absorption band and locate the 50% point on the horizon radiance profile, normalized to the maximum radiance value. Their accuracy is affected by errors due to seasonal, latitudinal, and random climatic variations in the shape of this profile. A method is presented for reducing the errors due to these variations, based on determining a specific profile shape which best fits the data obtained, and using this shape to estimate the position on the profile of a chosen tangent height. This method has been developed especially for a new type of static horizon sensor to be used in the Techsat-1 micro-satellite. This sensor gives a multi-point characterization of the profile, from which the shape can be ascertained and compared with a selection of representative profiles stored in the on-board computer. The profile best matching the data is then selected and used in the computation. The method could also be applied to those types of dynamic sensor which scan the profile with adequate resolution. The theory and application of the method is described together with computer analysis showing the degree of improvement in accuracy which can be achieved compared to traditional methods. It is shown that accuracies better than +/- 0.1 degree(s) can be obtained in low earth orbits.
The TAUVEX space astronomy experiment to image wide sky areas in the 140 - 280 nm spectral region is part of the SODART telescope complex on SRG, and functions as a separate scientific instrument and as a service system for the spacecraft. The experiment consists of three bore-sighted telescopes with 20 cm diameter Ritchey-Chretien optics. Each telescope is equipped with a four-position filter wheel and can select one of six UV bands in the spectral region of operation. The photon-counting, imaging detectors cover a field of view of 0 degree(s).9, with 80% of the energy from a point source within about 10 arcsec. The image is sampled at 3 arcsec intervals. The sensitivity is such that stars of 10 - 11 mag in the UV are detected in 2 sec, and in a typical SRG pointing of 5 hours stars as faint as 20 mag are detectable. TAUVEX provides off-line aspect solutions for the SODART focal plane instruments and on-line fine pointing information to the SRG attitude and control system. The experiment is constructed by El-Op, Electro-Optical Industries Ltd., and is financially supported by the Government of Israel, through the Israel Space Agency and the Ministry of Science and Arts. By mid-1994 four models of TAUVEX had been produced and supplied to the SRG integrators: size and mass models in 1992, a thermal model in early 1993 and an engineering model in spring 1994. A qualification model is being tested intensively at El-Op these days and the flight model will be ready, after testing, burn-in and calibration, by the end of 1994. TAUVEX is a light-weight, low-power, versatile UV imaging experiment with significant redundancy, which is not limited to operations on-board SRG. The system may operate on other platforms, including small satellites, if such an opportunity occurs.
The astronomical ultra-violet space telescope, TAUVEX, being developed in Israel by EL-OP Ltd., in conjunction with Tel Aviv University's Dept. of Astronomy and Astrophysics, has three co-aligned 20 cm diameter telescopes, each with an imaging photon-counting detector of the Wedge & Strip Anode type. The geometric and radiometric parameters of the system must be calibrated before launch in order that the image data acquired by the detector and signal processing sub-system can be converted into accurate maps of the UV sources in the sky. We describe the calibration philosophy and methodology involved in the TAUVEX system and sub-system calibration process. Also presented are the facilities and equipment specially designed and adapted for this purpose.
An athermalized objective has been designed for a compact, lightweight push-broom camera which is under development at El-Op Ltd. for use in small remote-sensing satellites. The high performance objective has a fixed focus setting, but maintains focus passively over the full range of temperatures encountered in small satellites. The lens is an F/5.0, 320 mm focal length Tessar type, operating over the range 0.5 - 0.9 micrometers . It has a 16 degree(s) field of view and accommodates various state-of-the-art silicon detector arrays. The design and performance of the objective is described in this paper.
A compact horizon sensor has been developed for use on TECHSAT-1 and other small satellites. The stringent requirements for low weight and power have been met by designing a static sensor with a new type of thermal detector array. The sensor uses four identical telescopes mounted at an optimal angle for the satellite altitude, and a microprocessor to convert the data from the detectors into an accurate measure of the angle to the nadir.
An astronomical UV space telescope, TAUVEX (Tel Aviv University Ultra-violet Explorer), is being built by EL-OP in conjunction with the Tel Aviv University Wise Observatory. It will be launched in 1995 on the SRG satellite to act as the optical monitor for the Danish X-Ray Telescope, SODART, and to survey the sky simultaneously in three UV wavelength bands. This paper describes the imaging aspects of the system.
The TAUVEX UV Space Telescope currently under construction by El-Op Ltd. in Israel is designed both for recording images of the sky in the UV region and to serve as the optical monitor for the SODART X-Ray Telescope being built by the Danish Space Research Institute. The two systems, together with several other experiments, will be flown on the S-R-G satellite to be launched by the CIS in 1995. TAUVEX will image a field of about 1 deg simultaneously in three spectral bands. In addition, it will record a selected object in a high-speed time-resolved mode in these bands. The concept and design of TAUVEX is described in this paper.