LFI design, qualification, and performance

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Instrument description

The Low-Frequency Instrument (see Fig. 1 in the LFI overview) consists of a 20 K focal plane unit hosting the corrugated feed horns, the orthomode transducers (OMTs) and the receiver front-end modules (FEMs). Forty-four composite waveguides [1] are interfaced with three conical thermal shields and connect the front-end modules to the warm (roughly 300 K) back-end unit (BEULFI warm electronics Back End Unit) containing a further radio frequency amplification stage, detector diodes, and all the electronics for data acquisition and bias supply.

Best LFI(Planck) Low Frequency Instrument noise performance is obtained with receivers based on InP High Electron Mobility Transistor (HEMTHigh Electron Mobility Transistor) low noise amplifiers (LNAs) for minimal power dissipation and best performance. To further minimize power consumption in the focal plane, the radiometers are split into two sub-assemblies connected by waveguides, one located at the telescope focal area, the other on the 300 K portion of the Planck satellite. These design features allow the entire front-end LNA dissipation to be <0.55 W, which enables the active cooling of the focal assembly. This is achieved with a vibration-less hydrogen sorption cooler, which also provides 18 K pre-cooling to the HFI(Planck) High Frequency Instrument helium J-T cooler. Two sorption cooler units are included in the flight hardware.

As shown schematically in Fig. 1 below, the LFI(Planck) Low Frequency Instrument consists of the following subsystems:

  • Radiometer Array Assembly (RAALFI Radiometer Array Assembly);
  • Sorption Cooler Subsystem (SCSSorption Cooler Subsystem (Planck));
  • Radiometer Electronics Box Assembly (REBALFI Radiometer Electronics Box Assembly).

The RAALFI Radiometer Array Assembly includes the Front End Unit (FEULFI cryogenic amplifying stage Front End Unit) and the Back End Unit (BEULFI warm electronics Back End Unit), connected via waveguides. The FEULFI cryogenic amplifying stage Front End Unit is located at the focus of the telescope, as one component of the joint LFI(Planck) Low Frequency Instrument/HFI(Planck) High Frequency Instrument focal assembly (see sections below). The BEULFI warm electronics Back End Unit is mounted on the top of the Planck service module (SVMService Module). The REBALFI Radiometer Electronics Box Assembly (Radiometer Electronics Box Assembly) and the warm parts of the Sorption Cooler System (SCSSorption Cooler Subsystem (Planck)) are located on one of the lateral panels of the SVMService Module. The FEULFI cryogenic amplifying stage Front End Unit and the Sorption Cooler Compressor (SCCSorption Cooler Compressor assembly) are connected by concentric stainless steel tubes. The smaller tube carries hydrogen at approximately 60 atmospheres from the cooler compressors to the FEULFI cryogenic amplifying stage Front End Unit, while the larger tube returns the hydrogen at about 0.3 atmospheres. These units are described in following sections and in the Annexes, while the SCSSorption Cooler Subsystem (Planck) is described in details in the Sorption Cooler section. All LFI(Planck) Low Frequency Instrument units are linked together by the LFI(Planck) Low Frequency Instrument harness, which also connects to the spacecraft interface.

Figure 1. Block Diagram of LFI(Planck) Low Frequency Instrument.

Radiometer Array Assembly (RAALFI Radiometer Array Assembly)

The Radiometer Array Assembly (RAALFI Radiometer Array Assembly) consists of two main units (the front end unit, FPUFocal Plane Unit and the Back End Unit, BEULFI warm electronics Back End Unit), connected by a set of waveguides. The Focal Plane Unit (FPUFocal Plane Unit) is the heart of the LFI(Planck) Low Frequency Instrument instrument; it contains the feed array and associated orthomode transducers (OMTs) and FEMs, all cooled to 20 K by the sorption cooler. The FPUFocal Plane Unit comprises a set of 11 modules, which are mounted on a mechanical support which meets the thermo-mechanical requirements of the instrument and adds thermal inertia. The BEULFI warm electronics Back End Unit comprises the radiometer Back End Modules (BEMLFI warm electronics Back End Module) and the Data Acquisition Electronics (DAELFI Data Acquisition Electronics), which are connected by an internal harness. The HFI(Planck) High Frequency Instrument Unit is located inside the LFI(Planck) Low Frequency Instrument FPUFocal Plane Unit and supported by the LFI(Planck) Low Frequency Instrument structure. The LFI(Planck) Low Frequency Instrument structure gives the mechanical and thermal interface to the HFI(Planck) High Frequency Instrument unit with the proper stiffness and thermal de-coupling. The LFI(Planck) Low Frequency Instrument structure also guarantees the proper alignment of the HFI(Planck) High Frequency Instrument detector with the telescope focal plane.

The timescale of the stability of the receiverS is driven by the 1 rpmrevolutions per minute rotation speed of the spacecraft, which requires a very low 1/f-noise or gain variation of the low noise amplifiers and other components. The LFI(Planck) Low Frequency Instrument uses a pseudo-correlation receiver concept (Fig. 2 below). This radiometer concept is chosen to maximize the stability of the instrument by reducing the effect of non-white noise generated in the radiometer itself. In this scheme, the difference between the inputs to each of the chains (the signal from the telescope and that from a reference blackbody, respectively) is continuously being observed. To remove the effect of instability in the back-end amplifiers and detector diodes, it is necessary to switch the signal detected at the diodes at a high rate. The signals from the sky and from a reference load are combined by a hybrid coupler, amplified in two independent amplifier chains, and separated out by another hybrid. The sky and the reference load power can then be measured and differenced. Since the reference signal has been subject to the same gain variations in the two amplifier chains as the sky signal, the true sky power can be recovered. The differencing receiver greatly improves the stability if the two input signals are almost equal, at a cost of a factor of √2 in sensitivity, compared to a perfectly stable total-power radiometer with the same noise temperature and bandwidth. This radiometer concept is also capable of greatly reducing the knee frequency. A single Radiometer Chain Assembly (RCALFI Radiometer Chain Assembly; see Fig. 2) consists of each functional unit from the feed horn to the BEMLFI warm electronics Back End Module. The RAALFI Radiometer Array Assembly therefore includes a set of 11 RCAs and the Data Acquisition Electronics (see also Fig. 1 above), all mounted on a suitable mechanical structure. Although there are differences in the details of the radiometer chains at different frequencies, their overall configuration is similar, and a general description of their design is provided in this section. Planck LFI(Planck) Low Frequency Instrument has 11 Radiometer Chain Assemblies (RCAs). Each RCALFI Radiometer Chain Assembly is constituted by a feed horn and FEMLFI cryogenic amplifying stage Front End Module in the FEULFI cryogenic amplifying stage Front End Unit (at 20 K), BEMLFI warm electronics Back End Module (at 300 K) in the BEULFI warm electronics Back End Unit, and four waveguides that connect each FEMLFI cryogenic amplifying stage Front End Module-BEMLFI warm electronics Back End Module couple. The frequency distribution of the RCALFI Radiometer Chain Assembly is the following:

  • 2 RCAs at 30 GHz;
  • 3 RCAs at 44 GHz;
  • 6 RCAs at 70 GHz.
Figure 2. A complete RCALFI Radiometer Chain Assembly from feed-horn to analogue voltage output. The insets show the OMTLFI Ortho Module Transducer, the details of the 20 K pseudo-correlator and of the back-end radio-frequency amplification, low-pass filtering, detection, and DC amplification.

Radiometer Chain Assembly (RCALFI Radiometer Chain Assembly)

Every RCALFI Radiometer Chain Assembly consists of two radiometers, each feeding two diode detectors (see Fig. 2 above), for a total of 44 detectors. The 11 RCAs are labelled by a number from 18 to 28, as outlined in Fig. 1 of the LFI overview, right panel.

Fig. 2 provides a more detailed description of each radiometric receiver. In each RCALFI Radiometer Chain Assembly, the two perpendicular linear polarization components split by the OMTLFI Ortho Module Transducer propagate through two independent pseudo-correlation differential radiometers, labelled as M or S, depending on the arm of the OMTLFI Ortho Module Transducer they are connected to (Main or Side, see lower-left inset of Fig. 2).

In each radiometer the sky signal coming from the OMTLFI Ortho Module Transducer output is continuously compared with a stable 4 K blackbody reference load mounted on the external shield of the HFI(Planck) High Frequency Instrument 4-K box (Valenziano et al. 2009[2]). After being summed by a first hybrid coupler, the two signals are amplified by approximately 30 dB (see upper-left inset of Fig. 2). The amplifiers were selected for best operation at low drain voltages and for gain and phase match between paired radiometer legs, which is crucial for good balance. Each amplifier is labelled with codes 1 or 2 so that the four outputs of the LNAs can be named with the sequence: M1, M2 (radiometer M) and S1, S2 (radiometer S). Tight mass and power constraints called for a simple design of the Data Acquisition Electronics (DAELFI Data Acquisition Electronics) box so that power bias lines were divided into five common-grounded power groups with no bias voltage readouts; only the total drain current flowing through the front-end amplifiers is measured and is available to the house-keeping telemetry (this design has important implications for front-end bias tuning, which depends critically on the satellite electrical and thermal configuration and was repeated at all integration stages during on-ground and in-flight satellite tests). A phase shift (or phase switch) alternating between 0° and 180° at the frequency of 4096 Hz is applied in one of the two amplification chains and then a second hybrid coupler restores the sky and reference load components, which are further amplified and detected in the warm BEULFI warm electronics Back End Unit, with a voltage output ranging from -2.5 V to +2.5 V.

Each radiometer has two output diodes which are labelled with binary codes 00, 01 (radiometer M) and 10, 11 (radiometer S), so that the four outputs of each radiometric chain can be named with the sequence M-00, M-01, S-10, S-11.

After detection, an analogue circuit in the DAELFI Data Acquisition Electronics box removes a programmable offset in order to obtain a nearly null DC output voltage and a programmable gain is applied to increase the signal dynamics and optimally exploit the ADCanalog to digital converter input range. After the ADCanalog to digital converter, data are digitally down-sampled, re-quantized and compressed in the REBALFI Radiometer Electronics Box Assembly according to a scheme described in (Herreros et al. 2009[3], Maris et al. 2009[4]), before being combined in telemetry packets. On the ground, telemetry packets are converted to time ordered data for both the sky and reference load, after calibrating the ADU (Analog-to-Digital Unit) samples into volts. The calibration takes into account the applied offset and gain factors.

To first order, the mean differential power output for each of the four receiver diodes can be written as follows (Seiffert et al. 2002[5], Mennella et al. 2003[6], Planck-PreLaunch-II[7]):

[math] \label{eq:power} P_{\rm out}^{\rm diode} = a\, G_{\rm tot}\,k\,\beta \left[ T_{\rm sky} + T_{\rm noise} - r\left( T_{\rm ref} + T_{\rm noise}\right) \right] [/math]

where Gtot is the total gain, k is the Boltzmann constant, β the receiver bandwidth, and a is the diode constant. Tsky and Tref are the average sky and reference load antenna temperatures at the inputs of the first hybrid and Tnoise is the receiver noise temperature.

The gain modulation factor (Mennella et al. 2003[6], Planck-Early-V[8]), r, is defined by

[math] \label{eq:erre1} r = \frac{T_{\rm sky} + T_{\rm noise}}{T_{\rm ref} + T_{\rm noise}} , [/math]

and is used to balance (in software) the temperature offset between the sky and reference load signals and minimize the residual 1/f noise in the differential data stream. This parameter is calculated from the average uncalibrated total power data using the relationship

[math] \label{eq:erre2} r = \langle V_{\rm sky} \rangle/ \langle V_{\rm ref}\rangle, [/math]

where <Vsky> and <Vref> are the average sky and reference voltages calculated in a defined time range. The white noise spectral density at the output of each diode is essentially independent of the reference-load absolute temperature and is given by

[math] \label{eq:dt1} \Delta T_0^{\rm diode} = \frac{2\,(T_{\rm sky}+T_{\rm noise})}{\sqrt{\beta}}. [/math]

If the front-end components are not perfectly balanced, then the separation of the sky and reference load signals after the second hybrid is not perfect and the outputs are mixed. First-order deviations in white noise sensitivity from the ideal behaviour are caused mainly by noise temperature and phase-switch amplitude mismatches. Following the notation used in (Seiffert et al. 2002 [5]), we define εTn, as the imbalance in front end noise temperature, and εA1 and εA2, the imbalance in signal attenuation in the two states of the phase switch. Equation above for the two diodes of a slightly imbalanced radiometer then becomes

[math] \label{eq:dt2} (\Delta T^{\rm diode} )^2 ≈ (\Delta T_0^{\rm diode})^2 ( 1± \frac{\epsilon_{A1}- \epsilon_{A2}}{2} + \alpha \epsilon_{T{\rm n}}), [/math]

which is identical for the two diodes apart from the sign of the term εA1 − εA2, representing the phase switch amplitude imbalance. This indicates that the isolation loss caused by this imbalance generates an anti-correlation between the white noise levels of the single-diode data streams. For this reason, the LFI(Planck) Low Frequency Instrument scientific data streams are obtained by averaging the voltage outputs from the two diodes in each radiometer:

[math] \label{eq:v1} V^{\rm rad}_{\rm out} = w_1 V^{\rm diode\,1}_{\rm out} +w_2 V^{\rm diode\,2}_{\rm out} [/math],

where ω1 and ω2 are inverse-variance weights calculated from the data as discussed in (Planck-Early-V[8]). This way, the diode-diode anti-correlation is cancelled, and the radiometer white noise becomes

[math] \label{eq:dt3} \Delta T^{\rm rad} ≈ \frac{ T_0^{\rm diode}}{\sqrt{2}} (1+\alpha \epsilon_{T{\rm n}})^{1/2}. [/math]

In the equations above, ε«1, while α (a term ≈1) is given by

[math] \label{eq:alpha} \alpha = \frac{ T_{\rm noise} (2 \; T_{\rm noise} + T_{\rm sky} + T_{\rm ref} } {2 (T_{\rm sky} + T_{\rm noise} )(T_{\rm ref} + T_{\rm noise})}. [/math]

See the Diode Combination section for the details of the diode combination procedure.

In Fig. 3 below we show a close-up of the two front end modules of an RCALFI Radiometer Chain Assembly with the four phase switches, which are labelled with the four letters A and B (main arm), and C and D (side arm). Each phase switch is characterized by two states: state 0 (no phase shift applied to the incoming wave); and state 1 (180° phase shift applied) and can either stay fixed in a state or switch at 4 kHz between the two states.

Phase switches are clocked and biased by the DAELFI Data Acquisition Electronics and their configuration can be programmed via telecommand. In order to simplify the instrument electronics, phase switches are configured and operated in pairs, by convention they are labelled A/C (corresponding to the first LNA both of main and side arm) and B/D (corresponding to the second LNA both of main and side arm). This means that if phase switches A and C are switching at 4 kHz then B and D are fixed, both in the same state (either 0 or 1); and viceversa. This simplification, required during the design phase to comply with mass and power budgets, comes at the price of losing some setup redundancy.

Figure 3. Close-up of the two front-end modules of an RCALFI Radiometer Chain Assembly. There are four phase switches, labelled A, B, C, and D. Each switch can be fixed in one of its two positions (labelled as 0, 1) or switched at 4 kHz between 0 and 1. Phase switches are clocked and biased by the DAELFI Data Acquisition Electronics in pairs: A/C and B/D.

Feed Horns (FHs)

Dual profiled corrugated horns have been selected at all LFI(Planck) Low Frequency Instrument frequencies as the best design in terms of the shape of the main lobe, level of the side lobes, control of the phase centre, and compactness. Details of the design, flight model and tests of Planck-LFI(Planck) Low Frequency Instrument feed horns can be found in (Villa et al. 2009[9]) and in the corresponding Annex section.

Ortho-Mode Transducers (OMTs)

The Ortho–Mode Transducer (OMTs) separates the radiation collected by the feedhorn into two orthogonal polarization components. It consists of a circular to square waveguide transition (directly connected to the FHFeed Horn), a square waveguide section and two separate rectangular waveguides (the main and side arms, which separate and pick up the orthogonal polarizations, connected with the FEULFI cryogenic amplifying stage Front End Unit). A 90° bend is always present on the side arm, while a twist is also necessary on the main (30 and 44 GHz) and side (70 GHz) arms, in order to match the FEULFI cryogenic amplifying stage Front End Unit polarization.

The details of the flight models and measurements of the Planck LFI(Planck) Low Frequency Instrument ortho-mode transducers can be found in (D'Arcangelo et al. 2009[10]) and in the corresponding Annex section.

Front End Modules (FEMs)

Front End Modules are located in the FPUFocal Plane Unit, just behind the Feed Horn and the Ortho Mode Transducers. The 70 GHz FEMs are mounted onto the inner wall of the main frame (the wall facing HFI(Planck) High Frequency Instrument instrument) from the HFI(Planck) High Frequency Instrument side. The 44 and 30 GHz FEMs are inserted into the main frame from the waveguide (WGLFI Waveguide) side and fixed to the bottom plate. Screws to the bottom plate are inserted from the WGLFI Waveguide side. The LFI(Planck) Low Frequency Instrument FEMs are the first active stage of amplification of the radiometer chain. Each FEMLFI cryogenic amplifying stage Front End Module contains four amplification paths, each of which is composed of several cascaded LNAs followed by a phase switch. Two passive hybrids, at the input and output of the FEMLFI cryogenic amplifying stage Front End Module, are used to mix pairs of signals of the same radiometer (see Fig. 3). This cuases the instabilities of each chain to be applied to both the sky and load signals.

The passive hybrid coupler ("magic-tee") combines the signals from the sky and cold load with a fixed phase offset of either 90° or 180° between them. It has a 20% bandwidth, low loss, and amplitude balance needed at the output to ensure adequate signal isolation.

The details of the design, development and verification of the 70 GHz front-end modules for the Planck Low Frequency Instrument can be found in (Varis et al. 2009[11]) and in the corresponding Annex section.

Waveguides (WGs)

The LFI(Planck) Low Frequency Instrument Front End Unit (FEULFI cryogenic amplifying stage Front End Unit) is connected to the Back End Unit (BEULFI warm electronics Back End Unit) by 44 rectangular waveguides approximately 1.5-2.0 m long. Each waveguide exhibits a low voltage standing wave ratio, low thermal conductivity, low insertion loss, and low mass. In addition, the waveguide path permits the LFI(Planck) Low Frequency Instrument/HFI(Planck) High Frequency Instrument integration and the electrical bonding between the FPUFocal Plane Unit and the BEULFI warm electronics Back End Unit. Because of the Focal Plane Unit arrangement, the waveguides are in general twisted and bent in different planes and with different angles, depending on the particular waveguide. From the thermal point of view the waveguides have to connect two systems (the BEMLFI warm electronics Back End Module and FEMLFI cryogenic amplifying stage Front End Module) that are at very different temperatures. At BEMLFI warm electronics Back End Module level the waveguides are at a temperature of 300 K, while at FEMLFI cryogenic amplifying stage Front End Module level the temperature is 20 K. The waveguides have to reduce the thermal flow from 300 K to 20 K. In Fig. 1 in the LFI overview (left panel) a conceptual sketch of the LFI(Planck) Low Frequency Instrument configuration is shown.

Details of the Planck-LFI(Planck) Low Frequency Instrument flight model of the composite waveguides can be found in (D'Arcangelo et al. 2009[1]) and in the corresponding Annex section.

Back End Modules (BEMs)

The BEMs are composed of four identical channels each made of Low Noise Amplifiers (LNAs), an RF bandpass Filter, RF to DC diode detector, and DC amplifiers. The FEMLFI cryogenic amplifying stage Front End Module output signals are connected by waveguides from the Focal Plane Unit (FPUFocal Plane Unit) assembly to the Back End Modules (BEMs) housed adjacent to the Data Acquisition Electronics (DAELFI Data Acquisition Electronics) assembly. To maintain compatibility with the FEMs, each BEMLFI warm electronics Back End Module accommodates four receiver channels from the four waveguide outputs of each FEMLFI cryogenic amplifying stage Front End Module. The BEMLFI warm electronics Back End Module internal signal routes are not cross-coupled and can be regarded as four identical parallel circuits. Each BEMLFI warm electronics Back End Module is constructed as two mirror halves. The two amplifier/detector assemblies each contain two amplifier/detector circuits. Each is supplied from one of a pair of printed circuit boards, which also house two DC output amplifiers.

The details of the design, development and verification of the 30 and 44 GHz back-end modules for the Planck Low Frequency Instrument can be found in (Artal et al. 2009[12]). Details of the design, development and verification of the 70 GHz back-end modules for the Planck Low Frequency Instrument can be found in (Varis et al. 2009[11]). Details are also reported in the corresponding Annex section.

4K Load

The purpose of the 4K reference load is to provide the radiometer with a stable reference signal. Reducing the input offset (the radiometric temperature difference between the sky and the reference load) reduces the minimum achievable radiometer 1/f noise knee frequency for a given amplifier fluctuation spectrum. A reference load temperature that matches the sky temperature (approximately 2.7 K) would be ideal.

Details of the design, characteristics, and performance of the LFI(Planck) Low Frequency Instrument 4-K reference load units are given in (Valenziano et al. 2009[2]) and in the corresponding section of the Annexes.

Naming Convention

The naming of all the LFI(Planck) Low Frequency Instrument elements has been described in the previous sections, but here is summarized again for clarity.

The 11 RCAs are labelled by numbers from 18 to 28, as outlined in Fig. 1 in the LFI overview (right panel). In each RCALFI Radiometer Chain Assembly, the two perpendicular linear polarization components are labelled as M or S according to the arm of the OMTLFI Ortho Module Transducer they are connected to (Main or Side, see lower-left inset of Fig. 2).

Each front-end amplifier (see upper-left inset of Fig. 2) is labelled with the codes 1, 2, so that the four outputs of the FEMLFI cryogenic amplifying stage Front End Module LNAs can be named with the sequence M1, M2 (radiometer M) and S1, S2 (radiometer S).

Each radiometer has two output diodes (see upper-right inset of Fig. 2), which are labelled with binary codes 00, 01 (radiometer M) and 10, 11 (radiometer S), so that the four outputs of each radiometric chain can be named with the sequence M-00, M-01, S-10, and S-11.

REBALFI Radiometer Electronics Box Assembly

The Radiometer Electronics Box Assembly (REBALFI Radiometer Electronics Box Assembly) is the electronics unit that processes the digitized scientific data and manages the overall instrument. It is also in charge of communication with the spacecraft. There are two REBALFI Radiometer Electronics Box Assembly boxes, one nominal and one redundant. The redundancy concept is cold, which means that both boxes are never ON at the same time; the operation of each unit is managed by the spacecraft switching on the corresponding unit. The REBALFI Radiometer Electronics Box Assembly ASW (Application SoftWare) is the same in each REBALFI Radiometer Electronics Box Assembly box.

A detailed description of the Planck LFI(Planck) Low Frequency Instrument REBALFI Radiometer Electronics Box Assembly can be found in (Herreros et al. 2009[3]) and in the corresponding section of the Annexes.

Instrument On-board Software

The REBALFI Radiometer Electronics Box Assembly software is the on-board software of LFI(Planck) Low Frequency Instrument. It is installed in the two computing subunits of REBALFI Radiometer Electronics Box Assembly: the DPUData Processing Unit (Digital Processing Unit), responsible of the control and monitoring of the instrument and the interface with the spacecraft; and the SPUSignal Processing Unit (Signal Processing Unit), responsible for the data reduction and compression.

Details can be found in the corresponding section of the Annexes.

Reduction and Compression of Science Data

To assess stability against 1/f noise, the Low Frequency Instrument (LFI(Planck) Low Frequency Instrument) on board the Planck mission acquired data at a rate much higher than the data rate allowed by the science telemetry bandwidth of 35.5 kbps. The data were processed by an on-board pipeline, followed on the ground by a decoding and reconstruction step, to reduce the volume of data to a level compatible with the bandwidth, while minimizing the loss of information. The on-board processing of the scientific data used by Planck/LFI(Planck) Low Frequency Instrument to fit the allowed data-rate is an intrinsically lossy process which distorts the signal in a manner that depends on a set of five free parameters (Naver, r1, <i,>r</i>2, q, O) for each of the 44 LFI(Planck) Low Frequency Instrument detectors. A brief description of the characteristics of this algorithm and the level of distortion introduced by the on-board processing as a function of these parameters can be found in the corresponding section of the Annexes, while a full description of the Planck LFI(Planck) Low Frequency Instrument on-board data handling system and the tuning and optimization method of the on-board processing chain can be found in (Maris et al. 2009[4]).

The strategy adopted to fit into the telemetry bandwidth relies on three on-board processing steps: downsampling; pre-processing the data to ensure loss-less compression; and loss-less compression itself. To demonstrate these steps, a model of the input signal was used. Note that while the compression is loss-less, the pre-processing is not, due to the need to rescale the data and convert them to integers (a process named data re-quantization). However, the whole strategy is designed to keep strict control over the way in which lossy operations are done, and to quantify the amount of information loss in order to assess the optimal compression rate with minimal information loss.

Instrument Operations

Operational Modes

The operations of the LFI(Planck) Low Frequency Instrument are designed to be automatic and require little if any intervention from the ground. A small number of commands are required for operating the instrument and eventually for diagnostic and reconfiguration purposes. Each sky survey is conducted by the LFI(Planck) Low Frequency Instrument with the instrument in the Normal Operations Mode. No deployable elements, or mechanically moving parts are included in the instrument. The scanning of the sky is achieved by progressive repointing of the satellite spin axis, with the Sun direction always within a cone 10° from the spin axis. Within the Normal Science Mode the instrument can be configured in order to fit with different science or diagnostic needs without changing the power consumption and thus the temperature in the FPUFocal Plane Unit.

A brief summary of the LFI(Planck) Low Frequency Instrument Operational Modes and the transitions between them is given in the corresponding section of the Annexes.


Ground Tests

During its development, the LFI(Planck) Low Frequency Instrument flight model was calibrated and tested at various integration levels from sub-systems (Davis et al. 2009[13]) to individual integrated receivers (Planck-PreLaunch-XIII[14]) and the whole receiver array (Planck-PreLaunch-VII[15]). In every campaign we performed tests according to the following classification:

  • functionality tests, performed to verify the instrument functionality;
  • tuning tests, to tune radiometer parameters (biases, DC electronics gain and offset, digital quantization and compression) for optimal performance in flight-like thermal conditions;
  • basic calibration and noise performance tests, to characterize instrument performance (photometric calibration[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period., isolation, linearity, noise and stability) in tuned conditions;
  • susceptibility tests, to characterize instrument susceptibility to thermal and electrical variations.

Where possible, the same tests were repeated in several test campaigns, in order to ensure enough redundancy and confidence in the repeatability of the instrument behaviour. A matrix showing the instrument parameters measured in the various test campaigns is provided in Table 1 of (Planck-PreLaunch-VII[15]).

The ground test campaign was developed in three main phases: cryogenic tests on the individual RCAs; cryogenic tests on the integrated receiver array (the so-called radiometer array assembly, RAALFI Radiometer Array Assembly); and system-level tests after the integration of the LFI(Planck) Low Frequency Instrument and HFI(Planck) High Frequency Instrument instruments onto the satellite. The first two phases were carried out at the Thales Alenia Space - Italia laboratories located in Vimodrone (Milano, Italy) (note that receiver tests on 70 GHz RCAs were carried out in Finland, at Yilinen laboratories), while system level tests (SLTs) were conducted in a dedicated cryofacility at the Centre Spatiale de Liège (CSLCentre Spatial de Liège) located in Liege, Belgium.

In Table 1 below we list the temperature of the main cold thermal stages during ground tests compared to in-flight nominal values. These values show that system-level tests were conducted in conditions that were as flight-representative as possible, while results obtained during RCALFI Radiometer Chain Assembly and RAALFI Radiometer Array Assembly tests need to be extrapolated to flight conditions to allow comparison. Details of the RCALFI Radiometer Chain Assembly test campaign are discussed in (Planck-PreLaunch-XIII[14]), while the RAALFI Radiometer Array Assembly tests and the extrapolation methods are presented in (Planck-PreLaunch-VII[15]).

Table 1. Temperatures of the main cold stages during the various ground test campaigns compared to in-flight nominal values.
Temperature Nominal RCALFI Radiometer Chain Assembly tests RAALFI Radiometer Array Assembly tests System-level
Sky ~ 3 K ≳ 8 K ≳ 18.5 K ~ 4 K
Ref. load ~ 4.5 K ≳ 8 K ≳ 18.5 K ~ 4.5 K
Front-end unit ~ 20 K ~ 20 K ~ 26 K ~ 20 K

During the various test campaigns the instrument was switched off and moved several times in a time period of about three years. A series of functional tests were always repeated at each location and also in-flight, in order to verify the instrument functionality and the response repeatability. No failures or major problems have been identified due to transport and integration procedures.

The expected Planck LFI(Planck) Low Frequency Instrument scientific performance, resulting mainly from cryogenic system level tests, are described in (Planck-PreLaunch-VII[15]).

In-flight Calibration

The LFI(Planck) Low Frequency Instrument Commissioning and Calibration and Performance Verification (CPVCalibration and Performance Verification) phases started on 4 June 2009 and lasted until 12 August 2009 when Planck started scanning the sky in nominal mode. At the onset of CPVCalibration and Performance Verification, the active cooling started when the radiating surfaces on the payload module reached their working temperatures (approximately 50 K on the third V-groove, and 40 K on the reflectors) by passive cooling. This was achieved during the transfer phase. Nominal temperatures were achieved on 3 July 2009, when the dilution cooler temperature reached 0.1 K (Planck-Early-II[16], Meinhold et al. 2009 [17]). The cooldown of the HFI(Planck) High Frequency Instrument 4-K stage (see Fig. 4 below), was key during CPVCalibration and Performance Verification for the LFI(Planck) Low Frequency Instrument, because it provided a variable input signal that was exploited during bias tuning.

The LFI(Planck) Low Frequency Instrument Commissioning and CPVCalibration and Performance Verification was carried out in four phases:

  • LFI(Planck) Low Frequency Instrument switch-on and basic functionality verification (Commissioning);
  • tuning of front-end biases and back-end electronics (CPVCalibration and Performance Verification);
  • preliminary calibration tests (CPVCalibration and Performance Verification);
  • thermal tests (CPVCalibration and Performance Verification).
Figure 4. Functional tests performed during CPVCalibration and Performance Verification against timeline. Curves from thermal sensors monitoring the 4-K stage and the FPUFocal Plane Unit temperature are superimposed.

Details of the LFI(Planck) Low Frequency Instrument Commissioning and CPVCalibration and Performance Verification test campaign are given in (Gregorio et al. 2013[18]).

Performance Summary

A summary of the LFI(Planck) Low Frequency Instrument performance parameters is given in Table 1 of the section titled Summary of LFI data characteristics.

Instrument scientific performance

Optical parameters

The most accurate measurements of the LFI(Planck) Low Frequency Instrument main beams were made with Jupiter, the most powerful unresolved (to Planck) celestial source in the LFI(Planck) Low Frequency Instrument frequency range. Since the LFI(Planck) Low Frequency Instrument feedhorns point to different positions on the sky, they detect the signal at different times. To map the beam, each sample in the selected timelines was projected in the (u, v) plane perpendicular to the nominal line-of-sight (LOSLine Of Sight) of the telescope (and at 85° to the satellite spin axis). The u and v coordinates are defined in terms of the usual spherical coordinates (θ, φ) :

[math] u = \sin θ \, \cos φ, [/math]
[math] v = \sin θ \, \sin φ. [/math]

To increase the signal-to-noise ratio, data were binned in an angular region of 2′ for the 70 GHz channels and 4′ for the 30 and 44 GHz channels. We recovered all beams down to −25 dB from the peak. An elliptical Gaussian was fit to each beam for both M and S radiometers. Differences between the M and S beams caused by optics and receiver non-idealities are inevitable at some level, but they appear to be well within the statistical uncertainties, and for the purposes of point-source extraction, the beams may be considered identical. For the details of the typical FWHMFull-Width-at-Half-Maximum and ellipticity averaged over each frequency channel, refer to (Planck-Early-III[19]) and (Planck-2015-A05[20]). Exhaustive details on all LFI(Planck) Low Frequency Instrument beam parameters are presented in the LFI(Planck) Low Frequency Instrument data processing section entitled Beams.

Photometric Calibration

Photometric calibration[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period., i.e., conversion from voltage to antenna temperature, is performed for each radiometer after total power data have been cleaned of 1 Hz frequency spikes (see the LFI(Planck) Low Frequency Instrument data processing section Spikes Removal page and (Planck-Early-V[8], Planck-2015-A03[21])), and differenced.

Here we report a brief overview of the photometric calibration[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period.; for details see the LFI(Planck) Low Frequency Instrument data processing section Photometric Calibration.

Our calibrator is the dipole signal induced by spacecraft motion with respect to the CMBCosmic Microwave background rest frame. The largest calibration uncertainty comes from the presence of the Galaxy and of the CMBCosmic Microwave background anisotropies in the measured signal. We therefore use an iterative calibration procedure in which the dipole is fitted and subtracted, producing a sky map that is then removed from the original data to enhance the dipole signal for the next iteration. Typically, convergence is obtained after a few tens of iterations.

We calibrate in two major steps. First, we find the absolute calibration using the motion of the spacecraft around the sun (the 'orbital dipole'). This process uses only the temperature of the CMBCosmic Microwave background and the very well known velocity of the spacecraft as inputs. From this calibration[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period., and comparing with HFI(Planck) High Frequency Instrument, we can find the Planck estimate of the solar system velocity relative to the CMBCosmic Microwave background (the 'solar dipole'). Second, We run another iterative calibration with this solar dipole as our signal. The larger signal gives us a better estimate of the time variation of our gain than that obtained from the orbital dipole.


Noise Properties

The noise characteristics of the LFI(Planck) Low Frequency Instrument data streams are closely reproduced by a simple (white + 1/f ) noise model,

[math] \label{eq:p1} P(f)= \sigma^2 \left[1+ \left(\frac{f}{f_{\rm knee}}\right)^\alpha \right] [/math],

where P(f) is the power spectrum and α≈−1. In this model, noise properties are characterized by three parameters, the white noise limit σ, the knee frequency fknee, and the exponent of the 1/f component α, also referred to as the slope. Here we give noise performance estimates based on one year of operations; details of the analysis are given in LFI(Planck) Low Frequency Instrument data processing section (Noise estimation) and in (Planck-Early-V[8], Planck-Early-III[19], Planck-2015-A03[21]).

Noise properties have been calculated following two different and complementary approaches: 1) fitting the equation above to time-ordered data for each radiometer; and 2) building normalized noise maps by differencing data from the first half of each pointing period with data from the second half of that pointing period to remove the sky signal (“jackknife” data sets).

Typical uncertainties are 0.5% for the white noise, between 5 and 10% for the slope, and between 10 and 20% for the knee frequency.

White Noise Sensitivity

Details of the white noise sensitivity can be found in (Planck-Early-III[19]).

Table 2 summarizes the sensitivity numbers calculated during the first year of operations using methods and procedures described in detail in (Planck-Early-V[8], Planck-2015-A03[21]), compared with scientific requirements. The measured sensitivity is in very good agreement with pre-launch expectations. While the white noise moderately exceeds the design specification, this performance is fully in line with the LFI(Planck) Low Frequency Instrument science objectives.

Table 2. White noise sensitivity of the LFI(Planck) Low Frequency Instrument frequency channels compared with requirements.
Channel Measurement [μKCMBCosmic Microwave background s1/2] Requirement [μKCMBCosmic Microwave background s1/2]
70 GHz 152.0 119
44 GHz 174.2 119
30 GHz 148.1 119

Instrument technical performance

Spectral response

The in-band receiver response was thoroughly modelled and measured for all the LFI(Planck) Low Frequency Instrument detectors during ground tests. The complete set of bandpass curves was published in (Zonca et al. 2009[22], Planck-2015-A06[23]) where all the details of the LFI(Planck) Low Frequency Instrument radiometer's spectral response are given. From each curve we have derived the effective centre frequency according to:

[math] \label{eq:spectr} \nu_0 = \frac{ \int_{\nu_{\rm min}}^{\nu_{\rm max}} \nu g(\nu) \; d \nu} {\int_{\nu_{\rm min}}^{\nu_{\rm max}} g(\nu)\; d\nu } [/math]

where Δν = νmax − νmin is the receiver bandwidth and g(ν) is the bandpass response. Details about colour corrections, C(α), needed to derive the brightness temperature of a source with a power-law spectral index α, are provided in

Some details are also given in the corresponding section of the Annexes.


Bandpass estimation

As detailed in (Zonca et al. 2009[22], Planck-2015-A06[23]), our most accurate method to measure the LFI(Planck) Low Frequency Instrument bandpasses is based on measurements of individual components integrated into the LFI(Planck) Low Frequency Instrument Advanced RF Model (LARFM) to yield a synthesized radiometer bandpass. The LARFM is a software tool based on the open-source Quasi Universal Circuit Simulator (QUCS). The measured frequency responses of the various subsystems (feed-OMTLFI Ortho Module Transducer, FEMLFI cryogenic amplifying stage Front End Module, BEMLFI warm electronics Back End Module) are considered as lumped S-parameter components. Measurements of single components are obtained with standard methods and provide highly reliable results, with precision of order 0.1-0.2 dB over the entire band. Waveguides are simulated with an analytical model, in order to reproduce the effect of their temperature gradient and the effect of standing waves caused by impedance mismatch at the interfaces between the FEMLFI cryogenic amplifying stage Front End Module and BEMLFI warm electronics Back End Module. This is because the 1.8-m long waveguides were not measured at unit level in cryogenic conditions. The model provides accurate agreement with the measured waveguide response in the conditions of the test measurements (300 K). The composite bandpasses are estimated to have a precision of about 1.5 to 2 dB.

Some details are also given in the corresponding section of the Annexes.

Stability

Thanks to its differential scheme, the LFI(Planck) Low Frequency Instrument is insensitive to many effects caused by 1/f noise, thermal fluctuations, or electrical instabilities. As detailed in (Planck-Early-III[19]), one effect detected during the first survey was the daily temperature fluctuation in the back-end unit induced by the downlink transponder, which was powered on each day for downlinks during the first 258 days of the mission. As expected, the effect is highly correlated between the sky and reference load signals. In the difference, the variation is reduced by a factor around (1 − r), where r is the gain modulation factor defined above (see r definition in RCA section).

A particular class of signal fluctuations occasionally observed during operations was due to electrical instabilities that appeared as abrupt increases in the measured drain current of the front-end amplifiers, with a relaxation time variable from few seconds to some hundreds of seconds. Typically, these events caused a simultaneous change in the sky and reference load signals. Because they are essentially common-mode, their residual on the differenced data is negligible, and the data are suitable for science production. In a few cases the residual fluctuation in the differential output was large enough (a few millikelvin in calibrated antenna temperature units) to be flagged, and the data were not used. The total amount of discarded data for all LFI(Planck) Low Frequency Instrument channels until Operational Day 389 was about 2000s per detector, or 0.008%.

A further peculiar effect appeared in the 44 GHz detectors, where single isolated samples, either on the sky or the reference voltage output, were clear outliers compared with the rest. Over a reference period of four months, 15 occurrences of single-sample spikes (out of 24 total anomaly events) were discarded, an insignificant loss of data.

Thermal susceptibility

As mentioned in the section LFI In-flight Calibration above, and detailed in (Gregorio et al. 2013[18]), during the CPVCalibration and Performance Verification campaign, susceptibility tests were performed in order to characterize the LFI(Planck) Low Frequency Instrument instrument susceptibility to thermal and electrical variations.

The effect of temperature fluctuations on the LFI(Planck) Low Frequency Instrument radiometers originates in the Planck cold end interface of the hydrogen sorption cooler to the instrument focal plane. The temperature is actively controlled through a dedicated stage, the Thermal Stabilization Assembly (TSAhermal Stabilization Assembly), providing a first reduction of the effect. The thermal mass of the focal plane strongly contributes to reduce residual fluctuations. The physical temperature fluctuations propagated at the front end modules cause a correlated fluctuation in the radiometer signal, degrading the quality of scientific data. The accurate characterization of this effect is crucial for attempts to remove it from raw data by exploiting the housekeeping information on thermal sensors.

The propagation of the temperature oscillations through the focal plane and the instrument response to thermal changes were characterized through two main tests:

  • the thermal dynamic response, aimed at measuring the dynamic thermal behaviour of the LFI(Planck) Low Frequency Instrument Focal Plane;
  • the thermal susceptibility of the radiometers.

Further details are also given in the corresponding section of the Annexes.

Instrument budgets

LFI(Planck) Low Frequency Instrument power, mass and telemetry budgets are given in the corresponding section of the Annexes.

Systematic Effects

The LFI(Planck) Low Frequency Instrument design was driven by the need to suppress systematic effects well below instrument white noise. The differential receiver scheme, with reference loads cooled to 4 K, greatly minimizes the effect of 1/f noise and common-mode fluctuations, such as thermal perturbations in the 20-K LFI(Planck) Low Frequency Instrument focal plane. The use of a gain modulation factor (see r definition in the RCA section above) largely compensates for spurious contributions from input offsets. Furthermore, diode averaging (see Vradout definition in the RCA section above) allows us to cancel second-order correlations, such as those originating from phase switch imbalances.

We have developed an error budget for systematic effects (Planck-PreLaunch-II[7], Planck-Early-III[19]Planck-2013-III[24] and Planck-2015-A04[25]) as a reference for both instrument design and data analysis. Our goal is to ensure that each systematic effect is rejected to the specified level, either by design or by robust removal in software. At this stage, the following effects are relevant:

Selection 258.png

For each of these effects we used flight data and information from ground tests to build timelines, maps, and angular power spectra that represent our best knowledge of their impact on the scientific analysis. The details of the systematic effect analysis are given in the section entitled LFI(Planck) Low Frequency Instrument data processing section (Systematic Effects uncertainties).

The Sorption Cooler

The Planck H2 Sorption Cooler is the first stage of the active cryogenic chain. Its task is to maintain the LFI(Planck) Low Frequency Instrument down to the operating temperature while providing a pre-cooling stage for the HFI(Planck) High Frequency Instrument refrigerators. The system performs a simple thermodynamic cycle based on hydrogen compression, gas pre-cooling by three passive radiators, further cooling due to the heat recovery by the cold low pressure gas stream, expansion through a J-T valve, and evaporation at the cold stage (Planck-Early-II[16]). A schematic of the Planck Sorption Cooler System (SCSSorption Cooler Subsystem (Planck)) is shown in Fig. 5. The engine of the cryocooler is the compressor. It serves two main functions: to produce the high-pressure hydrogen gas flow; and to maintain a stable gas recovery rate, which keeps the return pressure, hence the liquid temperature, constant. The high pressure gas flowing to the cold end is pre-cooled by exchanging heat with the three passive stages at the V-Grooves and with the evaporated cold gas returning back to the compressor. The gas then expands at the cold end through a J-T valve, producing approximately 1 Watt of cooling power at a temperature of <20K: most of this heat lift is used at the LVHX2 (Liquid Vapour Heat eXchanger 2, see Fig. 5) interface to absorb the LFI(Planck) Low Frequency Instrument heat load at a temperature around 20K. The remaining heat lift is used at the LVHX1 as a pre-cooling stage, at a temperature lower than 19 K, for the two HFI(Planck) High Frequency Instrument refrigerators. The cooler and its performance are described in detail in (Morgante et al. 2009[26]).

Figure 5. Planck Sorption Cooler CAD view with labels of the main sub-systems.

Both LVHXs provide a temperature of around 18 K, with fluctuations driven by the cooler instabilities (compressor element variations, cycling, two-phase flow dynamics, etc.). Stabilization of the HFI(Planck) High Frequency Instrument interface (LVHX1) temperature is not necessary, since thermal control of the subsequent colder stages is more efficient and very effective. To reduce cold end fluctuations directly transmitted to the LFI(Planck) Low Frequency Instrument radiometers, a copper block, designated as the Temperature Stabilization Assembly (TSAhermal Stabilization Assembly), is inserted, as an intermediate stage, between LVHX2 and the LFI(Planck) Low Frequency Instrument Focal Plane Unit (FPUFocal Plane Unit). The TSAhermal Stabilization Assembly comprises a temperature sensor and a heater, controlled by a PID-type feedback loop, working in combination with the passive thermal inertia. The set-point temperature of the TSAhermal Stabilization Assembly is an adjustable parameter of the sorption cooler system, chosen to provide dynamic range for control during mission: as the compressor elements age, the return gas pressure and thus the temperature of LVHX2 rise slowly. To keep the LFI(Planck) Low Frequency Instrument temperature reference stable during operations the set point must be periodically adjusted to maintain the level of oscillations within the required range (Fig. 6).

Figure 6. LVHX2 (black, HKHouse Keeping parameter name SD029540) and TSAhermal Stabilization Assembly (red, HKHouse Keeping parameter name SD030540) temperature profile from 125 to 563 days after launch.

Two sorption cooler units were integrated on-board the Planck spacecraft. Both units were used to cover the mission lifetime, with one cooler operated for the first two sky surveys. At the end of its lifetime a switchover operation, performed on August 11th 2010 (455 days after launch), activated the second unit that ran since then. The LVHX2 temperature change due to the new cooler start, and the subsequent re-adjustment of the LFI(Planck) Low Frequency Instrument temperature stabilization stage, is clear in Fig. 6.


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  15. 15.0 15.1 15.2 15.3 Planck pre-launch status: Low Frequency Instrument calibration and expected scientific performance, A. Mennella, M. Bersanelli, R. C. Butler, F. Cuttaia, O. D'Arcangelo, R. J. Davis, M. Frailis, S. Galeotta, A. Gregorio, C. R. Lawrence, R. Leonardi, S. R. Lowe, N. Mandolesi, M. Maris, P. Meinhold, L. Mendes, G. Morgante, M. Sandri, L. Stringhetti, L. Terenzi, M. Tomasi, L. Valenziano, F. Villa, A. Zacchei, A. Zonca, M. Balasini, C. Franceschet, P. Battaglia, P. M. Lapolla, P. Leutenegger, M. Miccolis, L. Pagan, R. Silvestri, B. Aja, E. Artal, G. Baldan, P. Bastia, T. Bernardino, L. Boschini, G. Cafagna, B. Cappellini, F. Cavaliere, F. Colombo, L. de La Fuente, J. Edgeley, M. C. Falvella, F. Ferrari, S. Fogliani, E. Franceschi, T. Gaier, F. Gomez, J. M. Herreros, S. Hildebrandt, R. Hoyland, N. Hughes, P. Jukkala, D. Kettle, M. Laaninen, D. Lawson, P. Leahy, S. Levin, P. B. Lilje, D. Maino, M. Malaspina, P. Manzato, J. Marti-Canales, E. Martinez-Gonzalez, A. Mediavilla, F. Pasian, J. P. Pascual, M. Pecora, L. Peres-Cuevas, P. Platania, M. Pospieszalsky, T. Poutanen, R. Rebolo, N. Roddis, M. Salmon, M. Seiffert, A. Simonetto, C. Sozzi, J. Tauber, J. Tuovinen, J. Varis, A. Wilkinson, F. Winder, A&A, 520, A5+, (2010).
  16. 16.0 16.1 Planck early results. II. The thermal performance of Planck, Planck Collaboration II, A&A, 536, A2, (2011).
  17. Noise properties of the Planck-LFI receivers, P. Meinhold, R. Leonardi, B. Aja, E. Artal, P. Battaglia, M. Bersanelli, E. Blackhurst, C. R. Butler, L. P. Cuevas, F. Cuttaia, O. D'Arcangelo, R. Davis, M. L. de la Fuente, M. Frailis, C. Franceschet, E. Franceschi, T. Gaier, S. Galeotta, A. Gregorio, R. Hoyland, N. Hughes, P. Jukkala, D. Kettle, M. Laaninen, P. Leutenegger, S. R. Lowe, M. Malaspina, R. Mandolesi, M. Maris, E. Martínez-González, L. Mendes, A. Mennella, M. Miccolis, G. Morgante, N. Roddis, M. Sandri, M. Seiffert, M. Salmón, L. Stringhetti, T. Poutanen, L. Terenzi, M. Tomasi, J. Tuovinen, J. Varis, L. Valenziano, F. Villa, A. Wilkinson, F. Winder, A. Zacchei, A. Zonca, Journal of Instrumentation, 4, 2009-+, (2009).
  18. 18.0 18.1 In-flight calibration and verification of the Planck-LFI instrument, A. Gregorio, F. Cuttaia, A. Mennella, M. Bersanelli, S. Maris, P. Meinhold, Submitted to Journal of Instrumentation, (2013).
  19. 19.0 19.1 19.2 19.3 19.4 Planck early results. III. First assessment of the Low Frequency Instrument in-flight performance, A. Mennella, R. C. Butler, A. Curto, F. Cuttaia, R. J. Davis, J. Dick, M. Frailis, S. Galeotta, A. Gregorio, H. Kurki-Suonio, C. R. Lawrence, S. Leach, J. P. Leahy, S. Lowe, D. Maino, N. Mandolesi, M. Maris, E. Martínez-González, P. R. Meinhold, G. Morgante, D. Pearson, F. Perrotta, G. Polenta, T. Poutanen, M. Sandri, M. D. Seiffert, A.-S. Suur-Uski, D. Tavagnacco, L. Terenzi, M. Tomasi, J. Valiviita, F. Villa, R. Watson, A. Wilkinson, A. Zacchei, A. Zonca, B. Aja, E. Artal, C. Baccigalupi, A. J. Banday, R. B. Barreiro, J. G. Bartlett, N. Bartolo, P. Battaglia, K. Bennett, A. Bonaldi, L. Bonavera, J. Borrill, F. R. Bouchet, C. Burigana, P. Cabella, B. Cappellini, X. Chen, L. Colombo, M. Cruz, L. Danese, O. D'Arcangelo, R. D. Davies, G. de Gasperis, A. de Rosa, G. de Zotti, C. Dickinson, J. M. Diego, S. Donzelli, G. Efstathiou, T. A. Enßlin, H. K. Eriksen, M. C. Falvella, F. Finelli, S. Foley, C. Franceschet, E. Franceschi, T. C. Gaier, R. T. Génova-Santos, D. George, F. Gómez, J. González-Nuevo, K. M. Górski, A. Gruppuso, F. K. Hansen, D. Herranz, J. M. Herreros, R. J. Hoyland, N. Hughes, J. Jewell, P. Jukkala, M. Juvela, P. Kangaslahti, E. Keihänen, R. Keskitalo, V.-H. Kilpia, T. S. Kisner, J. Knoche, L. Knox, M. Laaninen, A. Lähteenmäki, J.-M. Lamarre, R. Leonardi, J. León-Tavares, P. Leutenegger, P. B. Lilje, M. López-Caniego, P. M. Lubin, M. Malaspina, D. Marinucci, M. Massardi, S. Matarrese, F. Matthai, A. Melchiorri, L. Mendes, M. Miccolis, M. Migliaccio, S. Mitra, A. Moss, P. Natoli, R. Nesti, H. U. Nørgaard-Nielsen, L. Pagano, R. Paladini, D. Paoletti, B. Partridge, F. Pasian, V. Pettorino, D. Pietrobon, M. Pospieszalski, G. Prézeau, M. Prina, P. Procopio, J.-L. Puget, C. Quercellini, J. P. Rachen, R. Rebolo, M. Reinecke, S. Ricciardi, G. Robbers, G. Rocha, N. Roddis, J. A. Rubiño-Martín, M. Savelainen, D. Scott, R. Silvestri, A. Simonetto, P. Sjoman, G. F. Smoot, C. Sozzi, L. Stringhetti, J. A. Tauber, G. Tofani, L. Toffolatti, J. Tuovinen, M. Türler, G. Umana, L. Valenziano, J. Varis, P. Vielva, N. Vittorio, L. A. Wade, C. Watson, S. D. M. White, F. Winder, A&A, 536, A3, (2011).
  20. Planck 2015 results. IV. LFI beams and window functions, Planck Collaboration, 2016, A&A, 594, A4.
  21. 21.0 21.1 21.2 Planck 2015 results. II. LFI processing, Planck Collaboration, 2016, A&A, 594, A2.
  22. 22.0 22.1 Planck-LFI radiometers' spectral response, A. Zonca, C. Franceschet, P. Battaglia, F. Villa, A. Mennella, O. D'Arcangelo, R. Silvestri, M. Bersanelli, E. Artal, R. C. Butler, F. Cuttaia, R. J. Davis, S. Galeotta, N. Hughes, P. Jukkala, V.-H. Kilpiä, M. Laaninen, N. Mandolesi, M. Maris, L. Mendes, M. Sandri, L. Terenzi, J. Tuovinen, J. Varis, A. Wilkinson, Journal of Instrumentation, 4, 2010-+, (2009).
  23. 23.0 23.1 Planck 2015 results. V. LFI calibration, Planck Collaboration, 2016, A&A, 594, A5.
  24. Planck 2013 results. III. Low Frequency Instrument systematic uncertainties, Planck Collaboration, 2014, A&A, 571, A3
  25. Planck 2015 results. III. LFI systematics, Planck Collaboration, 2016, A&A, 594, A3.
  26. Cryogenic characterization of the Planck sorption cooler system flight model, G. Morgante, D. Pearson, F. Melot, P. Stassi, L. Terenzi, P. Wilson, B. Hernandez, L. Wade, A. Gregorio, M. Bersanelli, C. Butler, N. Mandolesi, Journal of Instrumentation, 4, 2016-+, (2009).