This LRIT decoder is build using the GODIL module with XCS500 FPGA
The load also contains HRPT, CHRPT and MHRPT decoders and generators.
Software wsat supports now also capturing of this LRIT data.
Switching between decoders is possible in 2 ways:
- By software (wsat)
- By hardware; code 1100 for jumpers J1, J2, J3, J4
The decoder contains the following items:
Input signal is a TTL level bit-stream coming from the demodulator (e.g. Costas-loop implementation).
- bit synchronizer; PLL with external oscillator (see HRPT decoder page)
VCO frequency: about the same as for the other decoders (10.646 MHz) (a small adjustment may be necessary;
exact needed frequency is LRIT symbol speed of 293kHz x 36 = 10.548 MHz)
- conversion bit-stream to 2-bits symbols, including symbol synchronizer
- Viterbi decoder: rate 1/2, K=7
- de-randomizer (x^8+x^7+x^5+x^3+x^0)
- USB interface
The generator contains following items:
The generator is only there to test the data processing; the content is rubbish.
It may be refined lateron to generate more VCDU-like data.
- A pattern generator:
- 4 bytes sync word (0x1acffc1d)
- 892 bytes "VCDU" data (0a 0b 0c......7f, so that is not 'real' valid data!)
- 128 bytes Reed Solomon Check symbols (just filled with 0xaa)
- randomizer, starts after each sync word (x^8+x^7+x^5+x^3+x^0)
- Viterbi encoder
- ser2par of the 2-bits symbols
The wsat software now just captures the incoming data and writes it to disk. It is stored "packet-aligned", i.e., the first 4 bytes in a file are the sync words and next packets are at multiple of 1024 bytes.
Things like packet identification, sequence control etc. are not implemented yet.
Also, Reed Solomon processing is not done yet.
- LRIT decoder is tested on GOES data and seems to work fine.
- Received data includes ASCII text of weather mesages. This proves that the LRIT decoder functionality is OK:
- lock on data OK
- Viterbi decoding OK
- de-randomizing OK
A program to translate received data into input data for existing software, like xrit2pic, is made.
This program works fine on MSG LRIT data, received using a different decoder (SDR) by Martin Blaho.
I am still working for GOES data; although in principle the same format is used the resulting files are different.
- LRIT MSG (received with SDR decoder)
- Most data translated correctly into Eumetcast-like files
- CRC check (60MByte file): OK, except for a few M_SDU's
- All errored files have errors in the SDU header;
if SDU header is OK then user data (CRC) is also OK. Strange...
- LRIT GOES (received with the LRIT decoder)
- CRC check (700k file) OK, except for a few M_SDU's
First LRIT picture: From SDR LRIT-decoder of Martin Blaho
Bitfiles for XC500 on GODIL 50 board
New in beta 0.4: Works also on inverted data (auto-correction)
|beta 0.4 (2011.12)
||To load into FPGA directly, needs to be done each time after power-up
wsat beta version, includes LRIT capture