Posted on January 5th, 2016

GTM – Finding the Collection Date and Time of Satellite Imagery, Part II

Continuing on last month’s Geospatial Tip of the Month (GTM), in the January edition we will look at finding the collection date and time for the Airbus satellites (i.e. Pléiades 1 and SPOT 6/7), EROS B and KOMPSAT-3.

Given the introduction offered last month, just a very quick recap that one of the key advantages of satellite imagery is a date and timestamp that is extremely accurate and can be verified with metadata. It is the point of this two-part GTM series to show our readers how to find the metadata files for the key high and medium-resolution satellites and then to uncover the date and timestamps. So without further ado, we jump into part two of this GTM series!

Airbus Satellites Metadata

The first metadata we will look at are from Airbus’ high-resolution satellites, the 50-centimeter (cm) panchromatic and 2-meter (m) multispectral Pléiades 1 (i.e. P1A and P1B), and medium resolution satellites, 1.5-m panchromatic and 6-m multispectral SPOT 6/7. Before we jump into looking at these two satellite’s metadata, a quick note that Airbus operates many satellites (and sells data from even more) so if your data is not SPOT 6/7 or Pléiades 1, then the folder names might be different and the metadata files in a slightly different format. But we expect the lesson you will learn here will help you find the date and timestamp for the other satellites, including SPOT 1 to 5, which Airbus either operates and/or sells. Finally, you might also find this to be the case for the other satellites the imaging companies covered here operate (such as ImageSat and SI Imaging Services) but we do not specifically discuss.

For Pléiades 1 (P1) imagery, the metadata is located in a subfolder of the main folder containing the TIFF (or JPEG2000) imagery files. In general, the imagery files for P1 data are located in a folder with a name similar to one of these:

IMG_PHR1A_MS
IMG_PHR1B_MS
IMG_PHR1A_P
IMG_PHR1B_P

There could be an underscore followed by some numbers (e.g. IMG_PHR1B_MS_003) in these folders names. From there, look for the folder called:

LINEAGE

The metadata file is in XML format so it may open with Internet Explorer as it will on my computer, look for a file with a name like this:

STRIP_DS_PHR1A_201509190307335_FR1_PX_E116N39_0818_03844_DIM.XML

Towards the top of this file, there is a category called:

<Time_Configuration>

In this category, you will find values like this:

<START>2015-09-19T03:07:33.5558849Z</START>
<END>2015-09-19T03:07:40.0797810Z</END>

As with other metadata on date and timestamp, these values are reported in UTC. So in our example, the strip that contains the area of interest we ordered started imaging on September 9, 2015 (e.g. 2015-09-19, Year-Month-Day) at 03:07:33.5558849 (e.g. Hours:Minutes:Seconds in UTC) and ended it imaging the strip about 6 seconds later at 03:07:40.0797810.

For SPOT 6 and 7 data, the process to find the metadata files is very similar as with P1. You will find the imagery files for SPOT 6 and 7 buried in a folder chain that looks something like this:

\PROD_SPOT6_001\VOL_SPOT6_001_A\IMG_SPOT6_MS_001_A

Your specific folder might have SPOT7 instead of SPOT6 in the name, you might also have numbers different than 001 or letters other than A or MS but in general it will be pretty similar. Now look for this folder again:

LINEAGE

And now the metadata file is XML format, named something like:

STRIP_DS_SPOT6_201508191322112_FR1_FR1_SE1_SE1_W052S04_04713_DIM.XML

And again, close to the top of the XML file in the same section called Time_Configuration, you will find the date and timestamp:

<START>2015-08-19T13:22:11.2493530Z</START>
<END>2015-08-19T13:22:35.6243530Z</END>

This date and timestamp is in the exact format as describe above for P1.

EROS B Metadata

EROS B is a 70-cm black and white only satellite operated by an Israel company, ImageSat. The folder structure for this data is very simple, there is only one folder option at each level (though it is a series of subfolders) so the metadata and imagery files are very easy to find. The metadata file is in PASS format, it will be named something like this:

MBT1-e2078406.PASS

I use the Microsoft program, WordPad, to open these files. Any text editor should do the same. Similar to P1 and SPOT 6/7, the date and timestamp will be for the entire strip of imagery; and so it is not specific to the area of interest as with DigitalGlobe metadata. Towards the very top of the metadata file, look for these lines:

sweep_start_utc     2015-07-23,19:56:35.05000
sweep_end_utc       2015-07-23,19:56:38.80000

And in similar fashion to the other satellites discussed already, the timestamp is in UTC so that the start imaging time would be on July 23, 2015 (e.g. 2015-07-23, Year-Month-Day) at 19:56:35.05000 (e.g. Hours:Minutes:Seconds in UTC) and stopped at 19:56:38.80000.

KOMPSAT-3 Metadata

KOMPSAT-3 is a 70-cm panchromatic and 2.8-m multispectral high-resolution satellite operated by the South Korean firm, SI Imaging Services. And as has been the trend for the satellites discussed here, the date and timestamp metadata is for the entre strip of imagery. But uniquely, this metadata is reported for each of the panchromatic and four multispectral bands individually as they are all collected independently but still at very (very, very…) close times.

The file and folder structure of KOMPSAT-3 is also simple as all of the imagery and metadata is located in the same single folder. The metadata file will be in XML format with a name like this:

K3_20130713205707_06169_16811282_L1G_Aux.XML

When you open this file, you will see that it is extremely long with thousands of line of code. And conveniently (ugh!) the date and timestamp are close to the bottom of the XML file. One trick you can use to find the date and timestamp is collapsing the </Metadata> section by clicking on this line of text – at least that is how Internet Explorer works (I will show you this step in the YouTube video that accompanies this GTM).

You will find the start and stop time for the collection of the strip in the first section of <PAN>. We have elected to use these start and stop times as this is the first set you come across (making it easier to find) and as these times will be very close for all of the bands. It is these lines of text that provide you with the date and timestamps:

-<ImagingStartTime>
<UTC>20130713205714.991963387</UTC>

-<ImagingEndTime>
<UTC>20130713205717.326652527</UTC>

It is pretty obvious these times are in UTC but the date and time are a bit harder to interpret. The date is at the start of the string, like this:

20130713 = 2013 (Year) – 07 (Month) – 13 (Day)

And then the time in UTC at:

205714.991963387 = 20 (Hours) : 57 (Minutes) : 14.991963387 (Seconds)

So then the start imaging time for this strip was July 13, 2013 at 20:57:14.991963387 and stopped at 20:57:17.326652527.

Now that you have seen the process for finding the date and timestamp for a number of medium and high-resolution satellites, you will likely be able to find similar information for a variety of satellites we have not covered here. See you next time!

A five plus-minute YouTube video showing you where you can find the collection date and time of the Airbus satellites (including Pléiades 1 and SPOT 6/7), EROS B and KOMPSAT-3.

Do you have an idea for a future GTM? If so, let me know by email at brock@apollomapping.com.

Brock Adam McCarty
Map Wizard
(720) 470-7988
brock@apollomapping.com

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