IMAGING FOUIER TRANSFORM SPECTROMETERS-IFTS
AND THE PROPOSAL
SOLAR PHYSICS ELEMENT IN COLUMBUS/SPACE STATION-SPECS

SIGNAL PROCESSING IN INSTRUMENTS BASED ON INTERFEROMETRY

Norsk Elektro Optikk A/S has been involved in two feasibility studies for ESA concerning imaging spectroscopy using Fourier Transform Spectrometers. In this class of instruments the signal processing hardware is of importance since the spectral information should be extracted from the interferograms by on board calculations. A properly designed signal processing system will reduce the amount of data that have to be transmitted to ground.

Norsk Elektro Optikk A/S has proposed a new concept for signal processing circuitry very well suited for doing Fourier Transforms in applications where only a part of the possible spectral components is of interest. The selected spectral components are freely programmable among all possible spectral components.

The Discrete or Direct Fourier Transform (DFT) is the selected signal processing method. The DFT is advantageous compared to the often used Fast Fourier Transform (FFT) because it do not need buffering of input data and because the number of samples in the interferogram can be different from 2^n. The DFT also allows you to compute only a selected number of spectral components. The FFT, on the contrary, requires computation of all spectral components. Using the FFT all input samples must have arrived before a calculation can start. The DFT can be calculated "on the fly" and therefore lends itself to pipeline realization.

No data buffering and possibilities for pipeline realization are features of great importance for hardware implementation. Norsk Elektro Optikk A/S has proposed to use Application Specific Integrated Circuits (ASICs) in the signal processing system. A set of identical ASICs is operating in parallel and doing the Discrete Fourier Transform.

The use of Application Specific Integrated Circuits gives a solution with the lowest power and volume requirements. An ASIC solution also have the lowest chip count. Norsk Elektro Optikk A/S has proposed to use the residue number system internally in the ASICs. The residue number system gives small, but fast arithmetic blocks operating in parallel.

Silicon Compiler technology could dramatically reduce the total design time and the total time necessary for space qualification of the ASIC component. A Silicon Compiler is a program that generates the layout of an ASIC from a high level description of the wanted function. The layout generated by the Silicon Compiler is from a functional point of view "correct by construction". When it comes to space qualification the use of a Silicon Compiler could be favourable. The process/technology itself could be space qualified and the layout could be in accordance with design guidelines for space applications.

The signal processing concept is described in further detail in the following publications:

• "Very High Performance Signal Processing using the Residue Number System",
  by Oddvar Søråsen, Yngvar Lundh and Jon Kristian Hagene.
  Paper accepted for presentation at the Sixth MIT conference on Advanced Research in VLSI held at Massachusetts Institute of Technology, April 1990.
• Feasibility study of a High Resolution Imaging Interferometer (HRII),
  Estec Contract Number 7943/88/NL/JS.
  Prepared by Norsk Elektro Optikk A/S.
• Feasibility study of a high spectral resolution sounder (HSRS).
  Estec Contract Number 8575/89/NL/JS
  Prepared by Aerospatiale, Officine Galileo and Norsk Elektro Optikk A/S.
  Norsk Elektro Optikk responsible for signal processing.

Norsk Elektro Optikk A/S has proposed to ESA that a breadboarding of the basic ASIC building block in the HSRS-instrument should be done. This bredboarding could be useful also for other interferometry projects.

Back to Publications