# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 1.3 (EUS)

***[Responsible Working Group Member:]{.underline}*** Richard Harrison

***[Date of Completion:]{.underline}*** 23 July 2002, updated 25
November 2002

***[Action:]{.underline}*** To demonstrate that the restricted telemetry
rate is sufficient to achieve the scientific goals of EUS.

***[Report:]{.underline}***

The nominal telemetry rate for the EUS instrument is 17 kbit/s. The full
EUS detector image is 4kx4k pixels. At 12 bits per pixel, it will take
197 min to read one exposure. Since each exposure will form part of a
raster, the raster cadence will be significantly longer.

Studies from instruments such as CDS/SOHO (ref. 1) have shown that
careful line selection is far more important than data compression in
managing the data return. Much of the spectrum is not required. Indeed,
for specific studies, specific emission lines are required. A good rule
of thumb from SOHO is that a selection of between 6 and 15 lines is good
for most scientific purposes.

The EUS nominal resolving element is 0.5 arcsec along a 34 arcmin slit
(4k pixels). The nominal spectral resolution is of order 0.01 Å/pixel.
To obtain full line widths for million K lines, plus sufficient nearby
background, one would want to return about 0.3 Å, i.e. 30 pixels.

The table shows a selection of potential cases. In each case a number of
required lines is defined as is a length along the slit (spatial
direction). The time to return such an exposure is given with a stated
compression factor. The rastered image cadence is then given for four
cases. The spatial length is given in pixels because of the varying
distance to the Sun. We assume a return of 30 pixels across each line
and 12 bit words.

  ------- --------- ------------- ---------- ----------- ------- ------- -------
  No. of  Spatial   Compression   Time to    Cadences                    
  lines   Length    Factor        return     for 200                     
                                  exposure   (100                        
                                             arcsec),                    
                                             500 (250                    
                                             arcsec),                    
                                             1000 (500                   
                                             arcsec) and                 
                                             2000 (1000                  
                                             arcsec)                     
                                             steps                       
                                             (minutes)                   

  10      100       10            2.1 sec    7           18      35      70

  6       500       3             21.2 sec   71          177     353     707

  6       500       10            6.35 sec   21          53      106     212

  10      500       10            10.6 sec   35          88      177     353

  6       1000      3             42.4 sec   141         353     707     1413

  6       1000      10            12.7 sec   42          106     212     423

  10      1000      10            21.2 sec   71          177     353     707

  6       2000      3             84.7 sec   282         706     1411    2823

  6       2000      10            25.4 sec   85          212     423     847

  6       4000      10            50.8 sec   170         414     846     1694
  ------- --------- ------------- ---------- ----------- ------- ------- -------

This table shows that the telemetry rate is very limiting for the
majority of rastered images. Given the dynamic/transient nature of the
Sun's atmosphere, we should be looking for cadences of order minutes.

For some, specific solar applications, the figures of the above table
are fine. This would include

-   Small area (under 50x50 arcsec) rasters to investigate
    fine-structure dynamic events in the solar atmosphere (e.g. 3.5 min
    raster with 10 lines in 50x50 arcsec FOV);

-   Single-slit location sequences (i.e. no raster -- the slit stays at
    one place monitoring intensities with one spatial dimension) to
    study transient intensity events such as blinkers and explosive
    events (e.g. 10.6 sec resolution observation over 250 arcsec with 10
    lines).

-   Spectral atlas studies, which are seeking evidence for line
    identifications and discoveries using long-duration runs.

However, for many other studies, we do need to identify methods for
improving the performance to avoid compromising the scientific return.
Several options are possible:

1.  Increase the telemetry rate: This option should be sought from the
    Project in any case;

2.  Return line profile parameters rather than the full profile
    information;

3.  Return image differences rather than full images.

These are just three options, one of which is beyond the control of the
EUS study team! Returning the line profile parameters would reduce the
data return by a factor of up to 0.1 (at best, 3 parameters instead of
the 30 bins). Alternatively, we may wish to return 15 bins (2:1 summing)
across the line rather than the full 30, if we are wary of the profile
parameter method. The savings are illustrated below for the 1000 pixel
length studies of the table above.

  ---------------------- --------- ------------- ---------- ----------- ------- ------- -------
  No. of lines           Spatial   Compression   Time to    Cadences                    
                         Length    Factor        return     for 200                     
                                                 exposure   (100                        
                                                            arcsec),                    
                                                            500 (250                    
                                                            arcsec),                    
                                                            1000 (500                   
                                                            arcsec) and                 
                                                            2000 (1000                  
                                                            arcsec)                     
                                                            steps                       
                                                            (minutes)                   

  *[1. Basic                                                                            
  Method]{.underline}*                                                                  

  6                      1000      3             42.4 sec   141         353     707     1413

  6                      1000      10            12.7 sec   42          106     212     423

  10                     1000      10            21.2 sec   71          177     353     707

  *[2. 2:1 Line Profile                                                                 
  Summing                                                                               
  Method]{.underline}*                                                                  

  6                      1000      3             21.2 sec   70          177     353     707

  6                      1000      10            6.3 sec    21          53      106     212

  10                     1000      10            10.6 sec   36          89      177     353

  *[3. Line Parameter                                                                   
  Method]{.underline}*                                                                  

  6                      1000      3             4.2 sec    14          35      71      141

  6                      1000      10            1.3 sec    4           11      21      42

  10                     1000      10            2.1 sec    7           18      35      71
  ---------------------- --------- ------------- ---------- ----------- ------- ------- -------

In addition to this, the feasibility of the image differencing method
should be examined by any proposing EUS team. However, the figures in
the second table do now show cadences of order under 10 minutes and this
is encouraging.

This report has assumed the nominal telemetry rate of 17 kbit/s. This
figure is based on the 240 Gbit onboard memory for the payload and the
single ground-station data dump scenario. In anticipation of
improvements in ground-station coverage and on-board memory capacity, we
briefly examine improvements in the telemetry rate of factors of 3 and
10, i.e. 51 kbit/s and 170 kbit/s.

  ------------ --------- ------------- ---------- ----------- ------- ------- -------
  No. of lines Spatial   Compression   Time to    Cadences                    
               Length    Factor        return     for 200                     
                                       exposure   (100                        
                                                  arcsec),                    
                                                  500 (250                    
                                                  arcsec),                    
                                                  1000 (500                   
                                                  arcsec) and                 
                                                  2000 (1000                  
                                                  arcsec)                     
                                                  steps                       
                                                  (minutes)                   

  Telemetry                                                                   
  Rate of 51                                                                  
  kbit/s:                                                                     

  6            4000      10            16.9 sec   56          140     280     560

  6            2000      10            8.45 sec   28          70      140     280

  6            1000      10            4.23 sec   14          35      70      140

  Telemetry                                                                   
  Rate of 170                                                                 
  kbit/s:                                                                     

  6            4000      10            5.08 sec   17          70      140     280

  6            2000      10            2.54 sec   8           35      70      140

  6            1000      10            1.27 sec   4           17      35      70

  Telemetry                                                                   
  Rate of 51                                                                  
  kbit/s with                                                                 
  Line                                                                        
  Parameter                                                                   
  Compresion                                                                  
  Method:                                                                     

  6            4000      10            1.6 sec    5.2         16      28      56

  6            2000      10            0.8 sec    2.6         8       14      28

  6            1000      10            0.4 sec    1.3         4       7       14

                                                                              
  ------------ --------- ------------- ---------- ----------- ------- ------- -------

It is clear that there are very significant improvements in the
performance with increased telemetry rates, providing reasonable
rastered observations with cadences of under 10 minutes.

***[Conclusion:]{.underline}***

*The figures demonstrate that the operation of an EUS instrument is
feasible with the 17 kbit/s telemetry allocation given careful data
selection and compression. The basic scientific goals of the instrument
are not compromised but it is very restricting. It is clear that a
greater telemetry allocation is highly desirable and would provide a
MUCH improved scientific return..*

*The nominal telemetry allocation is extremely low and we would suggest
that the following actions be considered by the Solar Orbiter Project
and by any proposing EUS team to improve significantly the scientific
return:*

-   *The Project to consider possibilities for increasing the EUS
    telemetry rate allocation from 17 kbit/s; factors of 3 or 10 show
    significantly better scientific return, and it is felt that with
    increased onboard memory and improved ground coverage, this will be
    feasible. [This must be taken as an urgent action.]{.underline}*

-   *In addition, proposing EUS teams should study methods for novel
    data selection and compression, such as returning image differences
    rather than raw images, and line profile parameters, to aid certain
    scientific study requirements.*

***[References:]{.underline}***

1\. Harrison et al., 1995, Solar Phys. 162, 233.