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
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)
cm-1 (double-sided interferogram)
| Throughput ottico
| 0.01 cm2 sr
| Measurement time
|| 1 mW/(m2 sr cm-1)
at 400 cm-1
|62 cm diam. X 26 cm
- 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.
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
bread-board for a FTS to remotely sense the rotational water vapour
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.