The compression rate of on board EIS data may vary depending on a few factors such as the observing target (QS, AR, CH, etc), slit/slot selection, exposures, etc.
The purpose of this study is trying to investigate how different compression schemes effect EIS data volume on board and work out a better estimation of compression rate for compression scheme (eg. DPCM, JPEG98, JPEG95, etc.)
The approach:#
1. to get actual data volume from MDP status information: the inclined curve means data packets from EIS on MDP, the vertical curve means data packets dumped to ground station. So in general, known a raster's start and end time can calculate actual data volume, and then compare it with the designed data volume of this raster to get data compression rate.
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2. to get related information from planning database/eis catalogue/fits header, for example: raster ID, compression scheme, designed data volume, SCI_OBJ, TARGET, slit/slot, exposures, etc.
3. prepare plots based on various factor combinations: compression rate vs. slit/slot, rate vs. target, rate vs. exposures
Some preliminary results (plots):
#
(The investigation here is for dates between 2007-Sep-15 and 2007-Dec-15, as EIS is operationally stable over this duration. The other data sets is in 2008-Mar (for JPEG85), and 2006-Dec (for JPEG75))
The average values of compression rate for each scheme are listed in Table here (how ever, only a single number cannot describe the varation of the rate.)
| Scheme | Total | QS | AR | CH | 1" | 2" | 40" | 266" | 10s | 15s | 20s | 30s | 60S | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| DPCM | 2.757 | 2.826 | 2.657 | 2.676 | 2.900 | 2.947 | 2.538 | 2.620 | 3.105 | 2.950 | 2.629 | 2.586 | 2.578 | |
| JPEG98 | 2.553 | 2.672 | 2.449 | 3.436 | 2.803 | 2.365 | 2.553 | 3.205 | 2.528 | 2.401 | ||||
| JPEG95 | 5.809 | 5.990 | 5.451 | 5.342 | 6.077 | 4.852 | 4.131 | |||||||
| JPEG90 | 8.273 | 7.387 | 3.533 | 7.790 | 2.753 | 3.678 | 8.329 | 3.678 | 2.293 | |||||
| JPEG85 | 5.528 | 5.578 | 4.922 | 6.148 | 5.539 | |||||||||
| JPEG75 | 11.294 | 11.294 | 11.294 |
There is difference between the number shown here and other previous results, for example, Hara'san work:
> 40" SLOT > DPCM 2.36 > JPEG98 2.70 > JPEG95 3.47 > JPEG92 4.22 > JPEG90 4.63 > JPEG85 5.74 > JPEG75 7.63 > JPEG65 9.43 > JPEG50 12.0 > For 10s exposure time.
As I mentioned above, this is a preliminary study on EIS compression factor. The method and results shown here need to check, confirm and compare with other number. There are still mystery hide behind these numbers. It is really just a start, so more contributions are very welcome!#
However, there is not always having value for SCI_OBJ in EIS fits header, also the range of value of SCI_OBJ keyword is varying, (SCI_OBJ_Example), sometimes we have to throw away some EIS data as it's hard to judge the SCI_OBJ belong to QS,AR or CH, eg. LMB, or FIL.
compFactor={compression_factor, $
study_ACR :'', $ ;string
study_id :'', $ ;string
rast_ACR :'', $ ;string
rast_id :'', $ ;string
ll_ACR :'',$ ;string
ll_id :'',$ ;string
start_time :'', $ ;string
end_time :'', $ ;string
fitsname :'',$ ;string
target :'',$ ;string
sci_obj :'',$ ;string
slit :'',$ ;string
def_volume :0LL,$ ;long64 int, unit: bits
mdp_volume :0.0,$ ;float, unit: kbits
comp_scheme :0,$ ;int
nexp :0,$ ;int
rast_req :0,$ ;int
exposures :fltarr(8) $ ;float, unit: sec
}
I attached the IDL sav file here
. Please download it and play it with your self, for example, I use:
if (str1[i].SCI_OBJ eq 'QS') && (str1[i].COMP_SCHEME eq 1) && (str1[i].MDP_VOLUME gt 0.) then ind[i]=1
to find those records having 'QS' SCI_OBJ and using DPCM compression scheme.