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Receiving GK-2A LRIT Images

July 30, 2019 - sam210723

GEO-KOMPSAT-2A (GK-2A) is a geostationary meteorological satellite positioned at 128.2˚E and operated by the National Meteorological Satelite Center of the Korea Meteorological Administration. On July 22nd 2019 it replaced the aging COMS-1 satellite after a successful week of in-orbit testing.

Near real-time meteorological data from GK-2A is broadcast to end-users in most of Asia and Oceania via two L-band (1.7 GHz) downlinks called LRIT and HRIT (Low/High Rate Information Transmission).

This guide walks through setting up hardware and software to receive these images, as well as the types of images that can be received, image post-processing (overlays, false colour, animation) and data decryption.


Images

A full disk (FD) image is downlinked every 10 minutes for a total of 144 images per day, which is enough to create smooth animations. Currently only the infrared channel is present, however the Water Vapour and Visible Light channels were also seen during testing.

Full disk infrared animation (22/07/19)

Images not directly derived from GK-2A sensor data are also downlinked between full disk images. These include synoptic charts (surface pressure), sea temperature charts, swell forecasts, and sea ice maps. Additional image types (synoptic/isobar, sea surface temperature and sea ice charts)

Spectral Bands

The Advanced Meteorological Imager (AMI) instrument on-board GK-2A is capable of imaging Earth in 16 spectral bands. The 5 primary bands are visible light, near-infrared, short-wavelength infrared, mid-wavelength infrared (water vapour), and thermal infrared.

Visible Light Infrared Water Vapour
False Colour

Visible Light and Infrared channels can be combined using a colour look-up table to create a false colour image such as the Full Disk below. This image was created using GeoSatSignal by David Taylor (GM8ARV). Full Disk False Colour (0248 UTC 02/07/19)


Getting started

Hardware

  • Raspberry Pi 3B+ or 4B
  • RTL-SDR or Airspy Mini/R2
  • 70cm dish (or larger)
  • 1.7 GHz linear feed (cantenna)

Software

  • goesrecv
  • xrit-rx
  • osp
  • gnuradio
Raspberry Pi Setup
Windows Setup
Decryption

GK-2A LRIT/HRIT downlinks are encrypted using the Data Encryption Standard (DES). Users normally have to apply for decryption keys through the Korea Meteorological Administration website. Applications from government departments, research institutes, and large organisations seem to be prefered rather than from individual users.

During the development of software for COMS-1 LRIT, valid decryption keys were found in code samples provided publicly on the KMA website. These keys have continued to work with all GK-2A data to date.

To obtain these keys:

  1. Open the COMS Operation page
  2. Click the "HRIT/LRIT Data Decryption samples C code" link to download COMS_Decryption_Sample_Cpp.zip
  3. Extract the key file called EncryptionKeyMessage_001F2904C905.bin
  4. ZIP file can now be deleted
  5. Copy the key file to your xrit-rx folder

Decryption sample code ZIP

This Encryption Key Message file itself is encrypted and needs to be decrypted before use. The second part of the file name is the key: EncryptionKeyMessage_001F2904C905.bin. The keymsg-decrypt.py script in the xrit-rx "tools" folder .

python3 tools\keymsg-decrypt.py EncryptionKeyMessage_001F2904C905.bin 001F2904C905

For those interested, a detailed explanation of the key decryption process is available in this older blog post.