REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development), is the prototype of the wideband Fourier transform spectrometer designed within the REFIR proposal for a space mission, see REFIR project. The instrument uses a new optical scheme that is capable of resolving the upwelling radiation from the Earth, seen from the orbit, with sufficient spectral resolution (0.5 cm-1) and signal-to-noise ratio (SNR>100) in wideband operations (100-1400 cm-1).

REFIR-PAD is a compact FTS, developed at CNR since 2003, with double-input/double-output port configuration, which maximizes the reliability of the spectrometer in particular for long lifetime space applications and optimizes its performances for room temperature operations (including detectors). The wide spectral range is covered by using wiregrid polarisers (Martin-Puplet configuration) or amplitude multilayer beamplitter (Mach-Zehnder configuration). The instrument has simple mechanical and electrical interfaces for an easy integration on board different stratospheric platforms for field observations. No telemetry is required, the acquired data are stored on board using pressurised hard disks.

Picture of REFIR-PAD instrument

REFIR-PAD instrument

Main parameters of the spectrometer

Interferometer type
Martin-Puplet or Mach-Zehender interferometer with double input/output ports
Spectral bandwidth
100-1400 cm-1
Resolution (max) 0.25 cm-1 (double-sided interferogram)
Throughput ottico
0.01 cm2 sr
IFOV (Field-of-view) 100 mrad
Measurement time
10-120 s.
NESR 1 mW/(m2 sr cm-1) at 400 cm-1
62 cm diam. X 26 cm
55 kg
50 W
Operational temperature

Operational products

  • Raw measurement data (level 0);
  • Non apodized and apodized calibrated spectra in the 100-1100 cm-1 spectral range for both upwelling and down-welling radiation in the upper troposphere/lower stratosphere (level 1);
  • Geophysical products (level 2): water vapour and temperature vertical profiles below flight altitude.

Instrument development

The instrument concept was designed within the REFIR project from January 1997 to April 2000. The development of the prototype, called REFIR-BB, was funded in 2000-01 by ASI with the project called “R&D bread-board for a FTS to remotely sense the rotational water vapour band (100-1000 cm-1)”. One of the main concern of the design of the REFIR instrument is the beam splitter (BS) efficiency. Currently there are not available space-qualified BS components able to meet the instrument requirements. To solve this problem, a study, called REFIR Beam Splitter Pre-development was funded in 2001-03 by the European Space Agency (ESA) for the development of polarising BS covering the spectral range from 100 to 1100 cm-1. REFIR-PAD is the second prototype that was developed for stratospheric balloon and ground-based field applications. The first flight was performed on June, 30th 2005 from the CNES base located near Teresina in Brazil. The results confirm experimentally the measurement capabilities and supply the first spectral measurement with wide band coverage of the Earth's atmospheric emission toward space, see REFIR-PAD results.