TOI processing LFI

Overview

The LFI Level2 Pipeline analyzes each horn of the instrument separately, one pointing period at time and stores results in objects the length of one OD. Each diode of the horn is corrected for diode specific systematics, differenced and then combined with its complementary diode in the same radiometer. The horn is then calibrated and the photometric calibration is applied.

LFI Pre-processing

Before the run of the Level2 pipeline and to improve the analysis the Mission information and data sampling divisions are stored in the database.

The Mission information is a set of objects, one for each Operational Day (OD, as defined in MISSION), in which are stored Pointing Period data: DPC pointing ID (where 1 is the first pointing of the nominal mission), PSO pointing ID, start OBT of the pointing maneuver, start OBT of the stable pointing, end OBT of the pointing, spin axis ecliptic longitude and latitude.

The sampling information is a set of objects, one for each LFI frequency, in which are stored for each pointing ID: start OBT of the pointing maneuver, start OBT of the stable pointing, end OBT of the pointing, number of samples of the pointing, number of stable samples of the pointing, start sample of the stable pointing and sample number from the start of the nominal mission. Valid samples and OBTs are defined where any of the radiometers from that frequency cohort contain valid data.

ADC Correction

More on P02 and P02a.

Evaluation

Bob sent me the email. Copy the page: http://belzebu.lambrate.inaf.it/lfiWiki/bin/view/LFI/AdcCharacterisation

Application

For each of the 44 LFI diodes there is the corresponding object in the Database. Each object contains 4 columns: the input voltages coming from the sky channel and the corresponding linearized output, the input voltages coming from the reference channel and the corresponding linearized output.

Data loaded by the module are used to initialize two different interpolators using CSPLINE and the functions from gsl (GNU Scientific Libraries) libraries. The interpolators are then used to correct each sample.

Spike Removal

Some of the LFI receivers exhibit a small artifact with exactly 1 second repetition, which visible in the power spectra. The effect is a set of spikes at 1 Hz and harmonics. The spurious signal is very well modeled and is removed from the timelines. More information can be found in P02 and P02a.

Modeling

The cause of the spikes at 1 Hz and harmonics is a tiny 1 second square wave embedded in affected channels. The method to estimate the 1 Hz signal is to build a template in time domain synchronized with the spurious signal. The first step is dividing each second of data into time bins using OBT. The number of bins is computed using:

[math] nbins = fsamp * template\_resolution[/math]

where fsamp is the sampling frequency, while template resolution is 136 at 70 GHz, 80 at 44 GHz and 56 at 30 GHz. Then the bins vector is initialized with time intervals.

We can write the process adding an index to the time sample: lower index denotes the particular time sample, while the upper index labels the bin into which the sample falls. The linear filter can be written as:

[math] s(t_{i}^{j}) = a_j \left(1- \Delta x (t_{i}^{j}) \right) + a_{j+1} \Delta x (t_{i}^{j})[/math]

Here [math] \Delta x (t_{i}^{j})[/math] is the filter weight which is determined by where within the bin sample lies. If we use [math] t^j [/math] with only an upper index to denote the start of each bin, then we can write the filter wright as follows:

[math] \Delta x (t_{i}^{j}) = {{{t_i^j - t^j} \over {t^{j+1} - t^j}}} [/math]

In other words, the filter weight is the time sample value minus the start of the bin divided by the width of the bin.

We must estimate the parameters [math] a_j [/math] from the data. With the assumption that the instrument has stable noise properties, we can use a least square algorithm to estimate the bin values:

[math] {\partial \over \partial a_k} \sum_{i,j} \left( s(t_i^j) – d_i^j \right)^2 = 0 [/math]

This can be represented in matrix equation:

[math] M_{jk}a_k = b_j [/math]

with the following definitions:

[math] M_{k,k-1} = \sum_i (1 - \Delta x (t_i^{k-1})) \Delta x (t_i^{k-1}) [/math]

[math] M_{k,k} = \sum_i (1 - \Delta x (t_i^k))^2 \Delta x (t_i^{k-1})^2 [/math]

[math] M_{k,k+1} = \sum_i (1 - \Delta x (t_i^k)) \Delta x (t_i^k) [/math]

[math] M_{k,k+n} (|n| \gt 1) = 0 [/math]

[math] b_j = \sum_i d_i^k (1- \Delta x (t_k^i)) + d_i^{k-1}\Delta x (t_i^{k-1}) [/math]

With these definitions we have to make use of periodic boundary conditions to obtain the correct results, such that if [math] k = 0 [/math], [math] k-1 = n-1 [/math] and [math] k = n-1 [/math], [math] k+1 = 0 [/math]. Once this is done, we have a symmetric tridiagonal matrix with additional values at the upper right and lower left corners of the matrix. The matrix is solved with LU decomposition. In order to be certain of the numerical accuracy of the result, we can perform a simple iteration. The solving of the linear system and the iterative improvement of the solution are implemented as suggested in Numerical Recipes.

Application

For each of the 44 LFI diodes there is the corresponding object in the Database. Because of the amplitude of the spikes we choose to apply the correction only on the 44 GHz radiometers. Each object contains 3 columns: the bins start time vector, the sky amplitudes and the reference amplitudes.

For each sample the value to be subtracted is computed using:

[math] V = skyAmp_k (1 - \Delta x (t_k)) + skyAmp_{k+1} \Delta x (t_k) [/math]

where k is the index of the bins at a given time.

Gap Filling

During the mission some of the data packets were lost (see P02). Moreover in two different and very peculiar situations LFI was shutdown and restarted, giving inconsistencies in data sampling. All of those data aren't used for scientific purpose but to avoid discrepancies in data analysis all of the radiometers at the same frequency must have the same samples.

To accomplish this the length of the data stream to be reduced in a specific pointing period is compared with the data stored in the sample information object. If the length is not the same the OBT vector is filled with missing sample times, the data vector is filled with zeros and in the flag column the bit for gap is raised.

Gain Modulation Factor

The pseudo-correlation design of the LFI radiometers dramatically reduces the [math] 1/f [/math] noise when the [math] V_load [/math] outputs are subtracted from the [math] V_sky [/math] outputs. The two time streams are slightly unbalanced, as one looks at the 2.7 K sky and the other looks at the ~4.5 K reference load. To force the mean of the difference to zero, the load signal is multiplied by the Gain Modulation Factor (R). For each pointing period the factor is computed using:

[math] R = \lt V_{sky}\gt \over \lt V_{load}\gt [/math]

Then the data are differenced using:

[math] DD = V_{sky} – R V_{load} [/math]

This value for R minimizes the 1/f and the white noise in the difference timestream.

At this point the maneuver flag bit is set to identify which samples have missing data, using the information stored in the sampling information object. This identifies which data to ignore in the next step of the Pipeline.

The R values are stored in the database. At the same time the mean values of [math] V_load [/math] and [math] V_sky [/math] are stored in order to be used in other steps of the analysis.

Diode Combination

The two complementary diodes of each radiometer are combined. The relative weights of the diodes in the combination are chosen for optimal noise. We assign relative weights to the uncalibrated diode streams based on their first order calibrated noise.

Evaluation

From first order calibration we compute an absolute gain [math] G_0 [/math] and [math] G_1 [/math], subtract an estimated sky and calculate the calibrated white noise [math] \sigma_0 [/math] and [math] \sigma_1 [/math], for the pair of diodes. The weights for the two diodes ([math] i [/math] = 0 or 1) are:

[math] W_i = {\sigma_i^2 \over G_{01}} {1 \over {\sigma_0^2 + \sigma_1^2}} [/math]

where the weighted calibration constant is given by:

[math] G_{01} = {1 \over {\sigma_0^2 + \sigma_1^2}} [G_0 \sigma_1^2 + G_1 \sigma_0^2] [/math]

The weights are fixed to a single value per diode for the entire dataset. Small variations in the relative noise of the diodes would in principle suggest recalculating the weights on shorter timescales, however, we decided a time varying weight could possibly induce more significant subtle systematics, so chose a single best estimate for the weights for each diode pair.

Detector ID	Weight
LFI18M-00	0.567304963
LFI18M-01	0.432695037
LFI18S-10	0.387168785
LFI18S-11	0.612831215
LFI19M-00	0.502457723
LFI19M-01	0.497542277
LFI19S-10	0.55143474
LFI19S-11	0.44856526
LFI20M-00	0.523020094
LFI20M-01	0.476979906
LFI20S-10	0.476730576
LFI20S-11	0.523269424
LFI21M-00	0.500324722
LFI21M-01	0.499675278
LFI21S-10	0.563712153
LFI21S-11	0.436287847
LFI22M-00	0.536283158
LFI22M-01	0.463716842
LFI22S-10	0.553913461
LFI22S-11	0.446086539
LFI23M-00	0.508036034
LFI23M-01	0.491963966
LFI23S-10	0.36160661
LFI23S-11	0.63839339
LFI24M-00	0.602269189
LFI24M-01	0.397730811
LFI24S-10	0.456037835
LFI24S-11	0.543962165
LFI25M-00	0.482050606
LFI25M-01	0.517949394
LFI25S-10	0.369618239
LFI25S-11	0.630381761
LFI26M-00	0.593126369
LFI26M-01	0.406873631
LFI26S-10	0.424268188
LFI26S-11	0.575731812
LFI27M-00	0.519877701
LFI27M-01	0.480122299
LFI27S-10	0.484831449
LFI27S-11	0.515168551
LFI28M-00	0.553227696
LFI28M-01	0.446772304
LFI28S-10	0.467677355
LFI28S-11	0.532322645

Planet Flagging

Why we flag planets.

Looking for Planets in data

Michele's code. Under review.

Application

The OBT vector found by the search are saved in a set of object, one for each horn. In Level2 Pipeline those OBTs are compared with the OBT vector of the data to raise planet bit where needed.