1.Introduction
==============

PFS is a double pendulum interferometer working in two wavelengthranges (1.2- 5um and 5-45um, Table 1). Martian radiation is dividedinto two beams by a dichroic mirror. The two ranges correspond to twoplanes (one on top of the other) containing the two interferometers,so that the same motor can simultaneously move the two pendulums andthe two channels are sampled simultaneously and independently. Thependulum motion is accurately controlled via a laser diode referencechannel using the same optics as the martian radiation. The samelaser diode also generates the sampling signal for the analogue-digital converter (ADC), measuring the 600 nm displacements of thedouble pendulum mirror. The measurements are double-sidedinterferograms, so that the onboard FFT can be computed withoutneeding the zero optical path difference location.

TABLE 1 - DETAILED PFS SCIENTIFICS PARAMETERS
-----------------------------------------------

PARAMETERS SW LW UNITS

SPECTRAL RANGE 1.2 - 5.0 5.5 - 45 [m]SPECTRAL RANGE 2000 - 8200 230 - 1750 [cm-1]SPECTRAL RESOLUTION 1.5 1.5 [cm-1](by triangular apodization)FOV FWHM 1.6 2.7 (deg)NEB 5 10-9 4 10-8 (1) [W cm- 2 sr -1]
MEASUREMENT CYCLE DURATION 10.0 10.0 [s]DETECTOR TYPE Photoconductor Pyroelectric -MATERIAL PbSe LiTaO3 -SHAPE/SIZE Square/0.7x0.7 Round/r=1.4 [mm]NEP 1*10^-12 (2) 4*10^-10(3) [W/Hz.5]SENSITIVITY 90 (2) 30 (3) [KV/W]
TEMPERATURE 220 (4) 290 [K]
INTERFEROMETER TYPE Double Pendulum -REFLECTING ELEMENTS Cubic corner reflectors -BEAMSPLITTER CaF2 CsI -REFL. ELEMENTS MOTION +/- 1.5 +/- 1.5 (5) [mm]MAX OPTIC PATH DIFFER. 5 5 [mm]TIME FOR MOTIONS 5 5 [s]TIME FOR MEASUREMENTS 4.5 4.5 [s]REFERENCE SOURCE Laser diode Laser diode -REF. SOURCE WAVELENGTH 1216 1216 [nm]COLLECTOR OPTICS Parabolic mirror -DIAMETER 49 38 [mm]FOCAL LENGTH 20 20 [mm]COATING Gold Gold -SW/LW SEPARATION KRS-5 with a multilayer coating reflecting SW radiation -OPTICS TRANSMISSION 0.64 0.78 (6) -
MODULATION FACTOR 0.87 0.98 (7) -
INTERFEROGRAM TWO-SIDED TWO-SIDED -
SAMPLINGS NUMBER 16384 4096(16384) -
SAMPLING STEP 608 2432 [nm]DYNAMICAL RANGE +/- 215 +/- 215 -

SPECTRA (from on-board FFT) -QUANTITY OF POINT 8192 2048 -
DYNAMICAL RANGE 6000 6000 -

ELECTRONICS
MODULATION FREQUENCY RANGE 423-1600 50-400 [Hz]ONB FFT COMPUTATION TIME 3.35 0.83 [s]BUFFER MEMORY VOLUME 32 [MBits]
Note---------------------------------------------------------------------
(1) Values are given for wavelengths 2.5 and 15 m. Results ofmeasurements for PFS 07 in SW.
(2) In peak of the spectral responsivity curve (near 4.8 mand for the modulation frequency 1000 Hz.
(3) By the modulation frequency 200 Hz. NEB and sensitivity aregiven for the output of the preamplifier.
(4) Radiative cooling.
(5) Linearized deviation from the position, corresponding to zeropath difference.
(6) From measurements of reflection and transmission of optical elements. Values are given for wavelengths 2.5 and 15 m(7) From estimates of tolerances. Values are given for wavelengths2.5 and 15 microns.


1.1.Instrument organization
===========================

PFS is a Fourier spectrometer produced by the combined efforts ofseveral groups from Italy, Russia, Poland, Germany, France and Spain.
The flight hardware was built in Italy (the Interferometer Block withits controlling electronics, the digital electronics controlling theexperiment, and the Ground Support Equipment with the spacecraftsimulator) and Poland (power supply and pointing system). Specialflight parts and subassemblies were built in Russia and Germany.

1.2.Technical description
=========================

The flight hardware, totaling 31.2 kg, is divided into four modules,with connecting cables (0.8 kg):-Module-O (PFS-O): the interferometer, with its optics and proximity electronics, is the core of PFS. 21.5 kg;-Module-S (PFS-S): the pointing device, which allows PFS to receive radiation from Mars or from the inflight calibration sources, 3.7 kg-Module-E (PFS-E): the digital electronics, including a 32-Mbit mass memory and a realtime FFT. 3.0 kg;-Module-P(PFS-P): the power supply, with the DC/DC converter, redundancies and the separate power supplies for the 16-bitADCs. 2.2 kg.

Power requirements are:- 5 Watt thermal control-10 Watt in sleep mode-35 Watt full operational mode-44 Wpeak.

1.3.Module-O (PFS-O)
====================

The incident IR beam falls onto the entrance filter that separatesthe radiation of the SW channel from that of the LW channel anddirects each into the appropriate interferometer channel. The PFS-Sin front of the interferometer allows the FOV to be pointed along andacross the projection of the flight path onto the martian surface.
It also directs the FOV at the internal blackbody sources diffusersand to open space for inflight calibration. Each PFS channel isequipped with a pair of retroreflectors attached by brackets to anaxle rotated by a torque motor. The axle and drive mechanism are usedfor both channels, which are vertically separated. The optical pathdifference is generated by the rotation of the retroreflectors. Themotor controller uses the outputs of two reference channels, whichare equipped with laser diodes. This interferometer design is veryrobust against misalignment in a harsh environment, in comparisonwith the classical Michelson-type interferometer. The detectors arein the centre of the parabolic mirrors. The optical path is changedby rotating the shaft of the double pendulum along its axis. In thisway, the optical path is four times that provided by a singlecube-corner displacement because two mirrors move at the same time.
The dichroic mirror acts as a fork that divides the two spectralranges. Indeed, it reflects all the wavelengths below 5um and remainsmore or less transparent for longer wavelengths. The band stop forwavelengths below 1.2um is provided by the silicon window, with itscutoff at 1.24um and placed in the optical inlet of the SW channel.
This filter is tilted by 1.5 deg. so that the radiation returning tothe source is not partially reflected on the detector. The double-pendulum axis is rotated by a brushless, frictionless motor (two forredundancy). The shaft of the double pendulum is held only by twopreloaded ball bearings so additional mechanical friction is requiredfor stabilizing the pendulum speed. Double-sided interferograms areacquired by placing the zero optical path difference in the centre ofthe mirror displacement. A double-sided interferogram has severaladvantages, including a relative insensitivity to phase errors.
Bilateral operation is adopted in order to reduce the time-cycle ofeach measurement, but separate calibration for each direction isrecommended in order to maintain the radiometric accuracy. Thespectral reference beam is a diode laser (InGaAsPat 1.2um); itsdetector is an IR photodiode with maximum response at about 1.2um.
The beam of the reference channel is processed like the input signalso that its optical path coincides with that of the signal beingstudied. Each channel has its own reference beam and the differentlengths of the double-pendulum arms are fully compensated for.
Because the LW beam splitter is not transparent at the wavelengthof the corresponding reference diode laser, a special small windowwas added in order to keep the attenuation of the laser beamsnegligible through the beam splitter itself. The unused output beamsof the two reference channels terminate into optical traps.

1.3.1.Optical scheme of PFS-O
=============================

The incident IR beam falls onto the entrance filter that separatesthe radiation of the SW channel from that of the LW channel anddirects each into the appropriate interferometer channel. The PFS-Sin front of the interferometer allows the FOV to be pointed along andacross the projection of the flight path onto the martian surface.
It also directs the FOV at the internal blackbody sources diffusersand to open space for inflight calibration. Each PFS channel isequipped with a pair of retroreflectors attached by brackets to anaxle rotated by a torque motor. The axle and drive mechanism are usedfor both channels, which are vertically separated. The optical pathdifference is generated by the rotation of the retroreflectors. Themotor controller uses the outputs of two reference channels, whichare equipped with laser diodes. This interferometer design is veryrobust against misalignment in a harsh environment, in comparisonwith the classical Michelson-type interferometer. The detectors arein the centre of the parabolic mirrors. The optical path is changedby rotating the shaft of the double pendulum along its axis. In thisway, the optical path is four times that provided by a single cube-corner displacement because two mirrors move at the same time. Thedichroic mirror acts as a fork that divides the two spectral ranges.
Indeed, it reflects all the wavelengths below 5um and remains more orless transparent for longer wavelengths. The band stop forwavelengths below 1.2um is provided by the silicon window, with itscutoff at 1.24um and placed in the optical inlet of the SW channel.
This filter is tilted by 1.5 deg so that the radiation returning tothe source is not partially reflected on the detector. The double-pendulum axis is rotated by a brushless, frictionless motor (two forredundancy). The shaft of the double pendulum is held only by twopreloaded ball bearings so additional mechanical friction is requiredfor stabilizing the pendulum speed. Double-sided interferograms areacquired by placing the zero optical path difference in the centre ofthe mirror displacement. A double-sided interferogram has severaladvantages, including a relative insensitivity to phase errors.
Bilateral operation is adopted in order to reduce the time-cycle ofeach measurement, but separate calibration for each direction isrecommended in order to maintain the radiometric accuracy. Thespectral reference beam is a diode laser (InGaAsPat 1.2um); itsdetector is an IR photodiode with maximum response at about 1.2um.
The beam of the reference channel is processed like the input signalso that its optical path coincides with that of the signal beingstudied. Each channel has its own reference beam and the differentlengths of the double-pendulum arms are fully compensated for.
Because the LW beam splitter is not transparent at the wavelength ofthe corresponding reference diode laser, a special small window wasadded in order to keep the attenuation of the laser beams negligiblethrough the beam splitter itself. The unused output beams of the tworeference channels terminate into optical traps.


1.3.2.Electronics of PFS-O
==========================

Most of the electronics inside PFS-O are analogue but themicroprocessor-based On- Board Data Management (OBDM) board controlsall the complex procedures during acquisition of the interferogram,including communication with PFS-E. It includes 32 kb word of EPROMfor software storage and 96 kb word for data. The most importantelectronics block is the speed controller. The zero crossing of aninterferogram of a monochromatic source that is very stable inwavelength can be used for sampling the interferogram of the sourceunder study. Ideally, the interferogram of the monochromatic sourceshould be a pure sine wave but it is not simply because itsinterferogram is limited in time. The shorter the wavelength of thereference source means better sampling accuracy. For PFS, 1.2um isthe reference source because of the limited variety of diode lasersand it simplifies the optical design. The wavelength of a diode laserdepends on its temperature and power, so great care has to be takenin their control. The speed of the double pendulum is such that afrequency of 2 kHz is generated for the SW channel, so a train of 4kHz pulses is produced from the electronics of the SW referencechannel. Thermal control is also very important for an IRinterferometer; heaters and thermometers are positioned at eightlocations. A locking system blocks the double pendulum duringlaunch and maneuvering for orbital insertion and correction. Theprocedure of locking and unlocking takes a minimum of 3 minutes butusing a paraffin actuator means it can be repeated hundreds of times.
The launch acceleration vector will be along the axis of the doublependulum for maximum robustness. The photoconductor SW channeldetector can work at temperatures down to 200K. It is passivelycooled through a radiator and its holder is partially insulated fromthe rest of the IB. For the LW channel, the pyroelectric detector canoperate without performance degradation even at ambient temperatures.

1.4.Module-E (PFS-E)
====================

PFS-E controls all the PFS modules: the communications to and fromthe spacecraft, memorizing and executing the command words, andoperating PFS and sending back the data words to the spacecraft.
Moreover, it synchronizes all the procedures according to the timeschedule and to the clock time from the spacecraft.

1.5.Module-P (PFS-P)
====================

PFS combines many kinds of electrical energy consumers: standarddigital and analogue electronics, sensitive preamplifiers and ADCs,light sources and electromechanical devices (motors and relays).
All of them have different supply requirements and some need to beelectrically isolated (to ensure extremely high stability) and/orindividually controlled by Module E's processor. This is why PFS-Pis more complicated than a simple DC/DC converter: there are threeindependent converters, six different power outputs (totaling 13independent voltages), one common input interface to satellite andone interface to DAM. All converters have cold redundancy. Switchingbetween main/reserve +5 Volt is controlled by the spacecraft, whilethe other main/reserve converters are controlled by PFS itself.

1.6.Module-S (PFS-S)
====================

The previous version of the pointing system, for the Mars-96 mission,had two degrees-of-freedom in pointing (two rotation axes), but wasrather heavy (8.5 kg for the system and 2.3 kg for the controllingelectronics). The pointing system is certainly necessary forgenerating a complete set of measurements, since we need to measurenot only the martian radiation but also the calibration blackbody andempty space. Mars Express provides nadir pointing so PFS itself needsonly one degree of rotation, simplifying the PFS-S design andreducing mass considerably (to 3.7 kg).

1.7.Modes of operation, data-acquisition cycle
==============================================

PFS-S and PFS-O work in parallel during an observation session, whilePFS-E coordinates operations of the other modules by sending commandsand receiving messages. During measurements, PFS-S must be motionlesswhile PFS-O acquires data. This is the only synchronization point inthe data-acquisition cycle. Upon completion of acquisition, all themodules work asynchronously while PFS-E coordinates their operations:-starts rotation of PFS-S;-receives LW and SW interferograms from PFS-O;-if spectra are required, PFS uploads previously acquired LW and SW interferograms into the Fast Fourier Transform processor and downloads computed spectra;-prepares the telemetry data pack i.e. splits information into frames and stores them in the mass memory;-upon completion of the PFS-S rotation gives a command to PFS-O tostart new acquisition.

After each data-acquisition cycle, PFS checks whether newtelecommands have been received and, if any, executes them. Telemetrycan be sent at any time on request from the spacecraft.

1.8.Inflight calibration
========================

During observation sessions, PFS periodically performs calibrationsby sending commands to PFS-S to point sequentially at the calibrationsources: deep space, internal blackbody and calibration lamp. Thehousekeeping information obtained from PFS-O after each calibrationmeasurement contains, in particular, the temperatures of the sensorsand the blackbody. These data are used for the computation of theabsolute spectra for the LW channel.


2.Data-taking Along the Orbit
=============================

2.1.Data-transmission modes
===========================

The data-transmission mode (DTM) defines the kind of scientific datathat PFS must select and store in the mass memory to be sent toEarth. PFS has 15 DTMs, numbered for historical reasons, 0, 2, 4, 5,6, 7, 8, 17, 18, 27, 28, 9, 10, 15, 16. DTM0 is for PFS operating inthe autonomous test mode; the others are obtained in the sciencemode, where PFS acquires both LW and SW interferograms. If spectraare required (DTM 9, 10, 15, 16), PFS makes Fast Fourier Transformsof the interferograms. Then, depending on the DTM, PFS selects therequired data. Interferograms can be selected completely orpartially. Of the 15 DTMs, 10 provide interferograms and fourspectra:
-MODE 0: autotest of the interferometer (4096 points in the LW channel and 16384 points in the SW channel provide the sine wave shape and the monitoring of the speed during the double pendulum motion);-MODE 2: full LW interferograms;-MODE 4: half-resolution interferograms, SW and LW;-MODE 5: half-resolution LW interferograms;-MODE 6: half-resolution SW interferograms;-MODE 7: full LW interferogram + one-sided SW interferogram ( including the zero optical path difference and right side);-MODE 8: one-sided LW and SW interferograms (right side);-MODE 17: full LW and SW interferograms;-MODE 18: full SW interferograms;-MODE 27: full LW interferogram + one-sided SW interferogram (including the zero optical path difference and left side);-MODE 28: one-sided LW and SW interferograms (left side);-MODE 9: modules of LW and SW spectra;-MODE 10: modules of LW spectra;-MODE 15: full modules of LW spectra and SW spectra with reduced range(2000 points in the SW channel between 2000 and 4000 cm-1);-MODE 16: modules of SW spectra (6144 points).

2.2.Data-taking along the orbit
===============================

PFS will perform measurement when the spacecraft is below 4000km. PFSwill wake up from its sleep mode about 1h before and follow thescheme:-apocentre: PFS is in sleep mode, telecommands can be received;-pericentre minus 60min: wake-up, wait for warm-up, start autonomous test, calibration LW, calibration SW, calibration deep space. PFS-S in nadir direction. Give data to the spacecraft;-pericentre minus 40min: start martian observations. Give data to the spacecraft;-pericentre plus 48min: stop martian observations. Give data to the spacecraft;-pericentre plus 53min: calibration LW, calibration SW, calibration deep space, autonomous test. Give data to the spacecraft. Go into sleep mode;-up to apocentre in sleep mode.

In total, 600 measurements per orbit are taken, of which 60 arecalibrations. This corresponds to 1200 measurements per day (thethird orbit per day being for downlink) and 823440 spectra in amartian year. The footprint from 4000km is of the order of 109km forthe SW channel and 188km for the LW channel; at pericentre (250km)they become respectively 6.8km and 11.8km (perpendicular to theground track, but 20km along it).