LRIT

LRIT decoder

Introduction.

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

Decoder description.

The decoder contains the following items:

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)
data-fifo
USB interface

Input signal is a TTL level bit-stream coming from the demodulator (e.g. Costas-loop implementation).

Generator description.

The generator contains following items:

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 generator is only there to test the data processing; the content is rubbish. It may be refined lateron to generate more VCDU-like data.

Software.

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.

Progress decoder

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

LRIT translation

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)

Version	Content	Bit-file	Command file	Comment
beta 0.4 (2011.12)	Flash load	nfml_decgen.mcs	nfml_decgen_flsh.cmd	To load into flash

Version	Content	Bit-file	Command file	Comment
beta 0.4 (2011.12)	direct load	nfml_decgen.bit	nfml_decgen_fpga.cmd	To load into FPGA directly, needs to be done each time after power-up

wsat beta version, includes LRIT capture

Linux: wsat_beta.tgz
Windows: setup_wsat_beta.exe