keep delta_time instead of renaming to offset_time

[zxdawn]

Describe the bug

In the official TROPOMI products, it's delta_time but renamed to offset_time.

satpy/satpy/etc/readers/tropomi_l2.yaml

Lines 45 to 49 in f901396

	offset_time:
	name: 'offset_time'
	file_type: tropomi_l2
	file_key: 'PRODUCT/delta_time'
	standard_name: offset_time

int delta_time(time=1, scanline=2906);
  :long_name = "offset of start time of measurement relative to time_reference";
  :units = "milliseconds since 2019-07-02 00:00:00";
  :_FillValue = -2147483647; // int
  :_ChunkSizes = 1U, 2906U; // uint

Is it better to keep it as the original name? I'm producing the data for some KNMI researchers. They usually use delta_time.

[djhoese]

It looks like you (@zxdawn) and @tommyjasmin are the main people who use/used this reader. @tommyjasmin originally wrote it and my guess is that this name change was either arbitrary or was to match existing software related to McIDAS/ADDE as part of is work at the SSEC. @tommyjasmin any memory of this?

[tommyjasmin]

Hi @zxdawn and @djhoese - I do not recall making any changes for this dataset, but I barely recall what I did a month ago :-)

I will look into it soon as I can (but today the meetings are thick). I would not have changed it for McIDAS, since McIDAS-V has it's own Java reader (I wrote that too), and while I see an ADDE server in our future, the Python ADDE servers are still in their relative infancy.

[djhoese]

@tommyjasmin Thanks. The main question is why something called delta_time in the file was changed to be called offset_time in Satpy.

[zxdawn]

@tommyjasmin Thanks. And I'm not sure that the renaming of time is necessary:

satpy/satpy/etc/readers/tropomi_l2.yaml

Lines 50 to 54 in bd784ca

	ref_time:
	name: 'ref_time'
	file_type: tropomi_l2
	file_key: 'PRODUCT/time'
	standard_name: ref_time

[tommyjasmin]

Attn: @djhoese and @zxdawn

Hello Xin - I am not sure why I renamed the delta_time variable in Satpy - the only reason I can think of is, possibly, because that's what the long_name attribute mentions (see below). I think you can go ahead and change the .yaml for both of these variables as suggested if you have permission. Otherwise I can do it.

group: PRODUCT {
variables:
float delta_time(line=358, ele=450);
:coordinates = "latitude longitude";
:_FillValue = -2147483647; // int
:long_name = "offset from reference start time of measurement";
:units = "milliseconds";
:_ChunkSizes = 1, 358, 450; // int

[zxdawn]

@tommyjasmin Got it! I will create a PR soon. Thanks.