PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "14-05-2004: First Draft" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = MEX INSTRUMENT_ID = PFS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "PLANETARY FOURIER SPECTROMETER" INSTRUMENT_TYPE = "INFRARED INTERFEROMETER" INSTRUMENT_DESC = " 1.Introduction ============== PFS is a double pendulum interferometer working in two wavelength ranges (1.2- 5um and 5-45um, Table 1). Martian radiation is divided into two beams by a dichroic mirror. The two ranges correspond to two planes (one on top of the other) containing the two interferometers, so that the same motor can simultaneously move the two pendulums and the two channels are sampled simultaneously and independently. The pendulum motion is accurately controlled via a laser diode reference channel using the same optics as the martian radiation. The same laser diode also generates the sampling signal for the analogue- digital converter (ADC), measuring the 600 nm displacements of the double pendulum mirror. The measurements are double-sided interferograms, so that the onboard FFT can be computed without needing 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 of measurements for PFS 07 in SW. (2) In peak of the spectral responsivity curve (near 4.8 m and for the modulation frequency 1000 Hz. (3) By the modulation frequency 200 Hz. NEB and sensitivity are given for the output of the preamplifier. (4) Radiative cooling. (5) Linearized deviation from the position, corresponding to zero path 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 wavelengths 2.5 and 15 microns. 1.1.Instrument organization =========================== PFS is a Fourier spectrometer produced by the combined efforts of several groups from Italy, Russia, Poland, Germany, France and Spain. The flight hardware was built in Italy (the Interferometer Block with its controlling electronics, the digital electronics controlling the experiment, and the Ground Support Equipment with the spacecraft simulator) and Poland (power supply and pointing system). Special flight 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-bit ADCs. 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 separates the radiation of the SW channel from that of the LW channel and directs each into the appropriate interferometer channel. The PFS-S in front of the interferometer allows the FOV to be pointed along and across the projection of the flight path onto the martian surface. It also directs the FOV at the internal blackbody sources diffusers and to open space for inflight calibration. Each PFS channel is equipped with a pair of retroreflectors attached by brackets to an axle rotated by a torque motor. The axle and drive mechanism are used for both channels, which are vertically separated. The optical path difference is generated by the rotation of the retroreflectors. The motor controller uses the outputs of two reference channels, which are equipped with laser diodes. This interferometer design is very robust against misalignment in a harsh environment, in comparison with the classical Michelson-type interferometer. The detectors are in the centre of the parabolic mirrors. The optical path is changed by rotating the shaft of the double pendulum along its axis. In this way, the optical path is four times that provided by a single cube-corner displacement because two mirrors move at the same time. The dichroic mirror acts as a fork that divides the two spectral ranges. Indeed, it reflects all the wavelengths below 5um and remains more or less transparent for longer wavelengths. The band stop for wavelengths below 1.2um is provided by the silicon window, with its cutoff 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 to the source is not partially reflected on the detector. The double- pendulum axis is rotated by a brushless, frictionless motor (two for redundancy). The shaft of the double pendulum is held only by two preloaded ball bearings so additional mechanical friction is required for stabilizing the pendulum speed. Double-sided interferograms are acquired by placing the zero optical path difference in the centre of the mirror displacement. A double-sided interferogram has several advantages, including a relative insensitivity to phase errors. Bilateral operation is adopted in order to reduce the time-cycle of each measurement, but separate calibration for each direction is recommended in order to maintain the radiometric accuracy. The spectral reference beam is a diode laser (InGaAsPat 1.2um); its detector is an IR photodiode with maximum response at about 1.2um. The beam of the reference channel is processed like the input signal so that its optical path coincides with that of the signal being studied. Each channel has its own reference beam and the different lengths of the double-pendulum arms are fully compensated for. Because the LW beam splitter is not transparent at the wavelength of the corresponding reference diode laser, a special small window was added in order to keep the attenuation of the laser beams negligible through the beam splitter itself. The unused output beams of 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 separates the radiation of the SW channel from that of the LW channel and directs each into the appropriate interferometer channel. The PFS-S in front of the interferometer allows the FOV to be pointed along and across the projection of the flight path onto the martian surface. It also directs the FOV at the internal blackbody sources diffusers and to open space for inflight calibration. Each PFS channel is equipped with a pair of retroreflectors attached by brackets to an axle rotated by a torque motor. The axle and drive mechanism are used for both channels, which are vertically separated. The optical path difference is generated by the rotation of the retroreflectors. The motor controller uses the outputs of two reference channels, which are equipped with laser diodes. This interferometer design is very robust against misalignment in a harsh environment, in comparison with the classical Michelson-type interferometer. The detectors are in the centre of the parabolic mirrors. The optical path is changed by rotating the shaft of the double pendulum along its axis. In this way, the optical path is four times that provided by a single cube- corner displacement because two mirrors move at the same time. The dichroic mirror acts as a fork that divides the two spectral ranges. Indeed, it reflects all the wavelengths below 5um and remains more or less transparent for longer wavelengths. The band stop for wavelengths below 1.2um is provided by the silicon window, with its cutoff 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 to the source is not partially reflected on the detector. The double- pendulum axis is rotated by a brushless, frictionless motor (two for redundancy). The shaft of the double pendulum is held only by two preloaded ball bearings so additional mechanical friction is required for stabilizing the pendulum speed. Double-sided interferograms are acquired by placing the zero optical path difference in the centre of the mirror displacement. A double-sided interferogram has several advantages, including a relative insensitivity to phase errors. Bilateral operation is adopted in order to reduce the time-cycle of each measurement, but separate calibration for each direction is recommended in order to maintain the radiometric accuracy. The spectral reference beam is a diode laser (InGaAsPat 1.2um); its detector is an IR photodiode with maximum response at about 1.2um. The beam of the reference channel is processed like the input signal so that its optical path coincides with that of the signal being studied. Each channel has its own reference beam and the different lengths of the double-pendulum arms are fully compensated for. Because the LW beam splitter is not transparent at the wavelength of the corresponding reference diode laser, a special small window was added in order to keep the attenuation of the laser beams negligible through the beam splitter itself. The unused output beams of the two reference channels terminate into optical traps. 1.3.2.Electronics of PFS-O ========================== Most of the electronics inside PFS-O are analogue but the microprocessor-based On- Board Data Management (OBDM) board controls all the complex procedures during acquisition of the interferogram, including communication with PFS-E. It includes 32 kb word of EPROM for software storage and 96 kb word for data. The most important electronics block is the speed controller. The zero crossing of an interferogram of a monochromatic source that is very stable in wavelength can be used for sampling the interferogram of the source under study. Ideally, the interferogram of the monochromatic source should be a pure sine wave but it is not simply because its interferogram is limited in time. The shorter the wavelength of the reference source means better sampling accuracy. For PFS, 1.2um is the reference source because of the limited variety of diode lasers and it simplifies the optical design. The wavelength of a diode laser depends on its temperature and power, so great care has to be taken in their control. The speed of the double pendulum is such that a frequency of 2 kHz is generated for the SW channel, so a train of 4 kHz pulses is produced from the electronics of the SW reference channel. Thermal control is also very important for an IR interferometer; heaters and thermometers are positioned at eight locations. A locking system blocks the double pendulum during launch and maneuvering for orbital insertion and correction. The procedure of locking and unlocking takes a minimum of 3 minutes but using a paraffin actuator means it can be repeated hundreds of times. The launch acceleration vector will be along the axis of the double pendulum for maximum robustness. The photoconductor SW channel detector can work at temperatures down to 200K. It is passively cooled through a radiator and its holder is partially insulated from the rest of the IB. For the LW channel, the pyroelectric detector can operate without performance degradation even at ambient temperatures. 1.4.Module-E (PFS-E) ==================== PFS-E controls all the PFS modules: the communications to and from the spacecraft, memorizing and executing the command words, and operating PFS and sending back the data words to the spacecraft. Moreover, it synchronizes all the procedures according to the time schedule and to the clock time from the spacecraft. 1.5.Module-P (PFS-P) ==================== PFS combines many kinds of electrical energy consumers: standard digital 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 be electrically isolated (to ensure extremely high stability) and/or individually controlled by Module E's processor. This is why PFS-P is more complicated than a simple DC/DC converter: there are three independent converters, six different power outputs (totaling 13 independent voltages), one common input interface to satellite and one interface to DAM. All converters have cold redundancy. Switching between main/reserve +5 Volt is controlled by the spacecraft, while the 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 was rather heavy (8.5 kg for the system and 2.3 kg for the controlling electronics). The pointing system is certainly necessary for generating a complete set of measurements, since we need to measure not only the martian radiation but also the calibration blackbody and empty space. Mars Express provides nadir pointing so PFS itself needs only one degree of rotation, simplifying the PFS-S design and reducing 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, while PFS-E coordinates operations of the other modules by sending commands and receiving messages. During measurements, PFS-S must be motionless while PFS-O acquires data. This is the only synchronization point in the data-acquisition cycle. Upon completion of acquisition, all the modules 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 to start new acquisition. After each data-acquisition cycle, PFS checks whether new telecommands have been received and, if any, executes them. Telemetry can be sent at any time on request from the spacecraft. 1.8.Inflight calibration ======================== During observation sessions, PFS periodically performs calibrations by sending commands to PFS-S to point sequentially at the calibration sources: deep space, internal blackbody and calibration lamp. The housekeeping information obtained from PFS-O after each calibration measurement contains, in particular, the temperatures of the sensors and the blackbody. These data are used for the computation of the absolute 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 data that PFS must select and store in the mass memory to be sent to Earth. 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 in the autonomous test mode; the others are obtained in the science mode, where PFS acquires both LW and SW interferograms. If spectra are required (DTM 9, 10, 15, 16), PFS makes Fast Fourier Transforms of the interferograms. Then, depending on the DTM, PFS selects the required data. Interferograms can be selected completely or partially. Of the 15 DTMs, 10 provide interferograms and four spectra: -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. PFS will wake up from its sleep mode about 1h before and follow the scheme: -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 are calibrations. This corresponds to 1200 measurements per day (the third orbit per day being for downlink) and 823440 spectra in a martian year. The footprint from 4000km is of the order of 109km for the SW channel and 188km for the LW channel; at pericentre (250km) they become respectively 6.8km and 11.8km (perpendicular to the ground track, but 20km along it)." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "N/A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END