PPF “C-shape” Method B implementation results

Introduction PPF fixes an issue with initialisation of the “C-shape” area. The previous version did not initialise u with zero by default within the “C- shape”, as has been done by the previous approach (u/q)_ss*q=0 (PPF 4.0.0). The problem resulted in large scatter in areas where Signal values are lower than threshold, ie readouts are set to undefined and consequently Method B is not applied. The following results show results from PPF (with the above issue fixed) evaluating the following cases 1.PPF PMD signal threshold as is, ie. 35 BU 2.PPF PMD signal threshold set to 5 BU We investigate results from 26 th of March 2008 end of dump-orbit 7439 approx. the last 900 seconds before end of dump (at Svalbard), i.e. latitudes between 90 and about 60 degrees north (see next two slides). For this cases we saw the large scatter (however as it turned out pre-dominantly in POL_M_P data q_ij, ie. the high spatial resolution q-fractions (256 readouts per scan) around the C-shape area, which has been fixed by (4.1.0 results are not shown here since it was a simple bug!) The above two cases are also evaluated using POL_M data (multiple of 8 q- fractions q_imn averaged and put on a 187 ms grid as per PGS 6.1 Eq. 230, and Annex C Figure 13, lower part.

Note, that SRON (Gijs) analysis has (pre-dominantly) been based on POL_M_P data [SRON: please correct if this is wrong] whereas EUM offline analysis frequently uses POL_M data. The reason for us analysing POL_M data was that q_imn is used as input for the polarisation correction, whereas POL_M_P is “only” used by users making use of PMD signals directly. It is due to this that the large scatter by using Method B in PPF has not been identified at EUM immediately, since any kind of scatter seems to be significantly reduced when looking at averaged q_imn Stokes fractions. The last issue is important to keep in mind for issues concerning (potential) forthcoming PMD signal threshold changes and will be illustrated by the results presented here.

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to PMD Signal threshold 35 BU (nominal)

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to PMD Signal threshold 5 BU

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to POL_M_P q_ij PMD Signal threshold 35 BU

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to POL_M_P q_ij PMD Signal threshold 5 BU

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to POL_M q_imn PMD Signal threshold 35 BU

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to POL_M q_imn PMD Signal threshold 5 BU

Conclusion – part 1 Lowering the PMD signal threshold from 35 BU to 5 BU largely reduces the amount of Stokes fractions missing (especially in the UV, as shown here) and therefore increases the area where u-Stokes fractions can be calculated using Method B. The scatter in Stokes fractions (out side the limits) is, as expected, also increasing somewhat for lower limits of PMD signal threshold. However, this is pre-dominantly the case for POL_M_P (q_ij) high-resolution q-fractions and not for the averaged q_imn (POL_M) case as used for polarisation correction (this is also illustrated in the following 3D plots). The plot shows the PMD noise averaged over detector pixel covered by PMD band 1. The average is stable around 1.6 BU over the whole mission period for both PMDs.

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 312 nm PMD Signal threshold 35 BU POL_M_P q_ij POL_M q_imn

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 312 nm PMD Signal threshold 5 BU POL_M_P q_ij POL_M q_imn

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 338 nm PMD Signal threshold 35 BU POL_M_P q_ij POL_M q_imn

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 338 nm PMD Signal threshold 5 BU POL_M_P q_ij POL_M q_imn

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 757 nm PMD Signal threshold 35 BU POL_M_P q_ij POL_M q_imn

Basic PMD/Stokes fraction results from PPF th March 2008, orbit 7439, to Stokes fractions in 3D space same area as before 757 nm PMD Signal threshold 5 BU POL_M_P q_ij POL_M q_imn

Conclusions – part 2 The “C-shape” area using Method B is hardly visible in the q-fractions (using PPF 4.1.3). This is the expected behaviour since from previous analysis we know that the impact of u on q is small. However, this is different for calculation of the c- correction factor which we have not yet looked at. The 3-D plots confirm as before that reducing the PMD threshold to 5 BU increases (as expected) the scatter in q_ij, however, this increase is largely reduced for the averaged Stokes fractions q_imn used for the polarisation correction of main channels. Preliminary conclusion/suggestion: From a “stability” point of view it looks at current that a significantly lower PMD signal threshold will help to smoothen the transition area at high latitudes and significantly extend the area where a polarisation correction is applied. The drawback might be that users of full resolution q-fractions might be required to monitor the amount of scatter (error) in the Stokes fractions and to decide themselves where they want to cut-off.

Open issues SRON to double-check the conclusions presented here using two test orbits processed by PPF and with two different PMD threshold signal values 35 BU (nominal) and 5 BU). To be provided with this presentation. EUM to evaluate the polarisation correction carried out by PPF 4.1.3