Few *times* ago I started to check on GOES 16 transmissions to see if I can get any data from it and make OpenSatelliteProject work with it. Me and @usa_satcom noticed that the HRIT signal was transmitting using differential encoding that was not predicted on NOAA’s HRIT Specification (You can check it here http://www.goes-r.gov/users/hrit-links.html ). So I decided to send an email to NOAA asking what was the current HRIT specs for GOES-16. Of course I expected no answer from them (they would probably be really busy with GOES-16 Testing), but surprisingly they answered sending the specs and saying that any feedbacks would be helpful and appreciated. So the HRIT indeed uses Differential Encoding (NRZ-M to be more specific). Knowing that I could start changing OpenSatelliteProject to be compatible with HRIT.
In the last chapter of my GOES Satellite Hunt, I explained how to obtain the packets. In this part I will explain how to aggregate and decompress the packets to generate the LRIT files. This part will be somwhat quick, because most of the hard stuff was already done in the last part. Sadly the decompression algorithm is a modified RICE algorithm, and the Linux version of the library provided by NOAA cannot be used anymore because of incompatibilities between GCC ABIs ( The NOAA library has been compiled with GCC 2). Until I reverse engineer and create a open version of the decompression algorithm, I will use the workaround I will explain here.
In the last chapter I showed how to get the frames from the demodulated bit stream. In this chapter I will show you how to parse these frames and get the packets that will on next chapter generate the files that GOES send. I will first add C code to the code I did in the last chapter to separated all the virtual channels by ID. But mainly this chapter will be done in python (just because its easier, I will eventually make a C code as well to do the stuff).
In the last chapter of GOES Satellite Hunt, I explained how I did the BPSK Demodulator for the LRIT Signal. Now I will explain how to decode the output of the data we got in the last chapter.
One thing that is worth mentioning is that most (if not all) weather satellites that transmit digital signals use the CCSDS standard packet format, or at least something based on it. For example this frame decoder can be used (with some modifications due QPSK instead BPSK) for LRPT Signals from Meteor Satellites (I plan to do a LRPT decoder as well in the future, and I will post about it). I will not describe my entire code here, just the pieces for decoding the data. I will also not write the entire code here, since it can be checked in github. So before start see the picture below (again). We will some info from it as well.
In the last episode of my GOES Satellite Hunt I explained how I manage to build a reception system to get the GOES LRIT Signal. Now I will explain how to get the packets out of the LRIT signal. I choose the LRIT signal basically because of two reasons:
- It contains basically all EMWIN data + Full Disks from GOES 13 and 15.
- Less complexity on the demodulator side (Simple BPSK Demodulator)
So few people know that I started a crusade against GOES 13 Satellite. My idea was to capture the GOES 13 signal (that’s reachable in São Paulo) with a good SNR (enough to decode) and them make all the toolkit to demodulate, decode and output the images and other data they send. I wanted a high-res image, and the L-Band transmissions usually provide that (GOES for example is 1km/px with whole earth sphere in frame. A 10000 x 10000 pixels image)
So I choose GOES over other Weather Satellites mainly because GOES is a Geostationary Satellite. That means its position never change. That was needed for me, because L Band usually needs a relatively big dish to capture the signal, and if the satellite is moving, the antenna needs to track it. That means: Alt-Az tracker (or something else) that will be most likely more expensive than the whole capture system (at least in Brazil). Since GOES does not move, I could just point my dish and forget about it. It would always capture the signal.