Diss Pipeline

DISS Pipeline

Expert guide for the eLTS DISS binary-file processing pipeline (C-band uplink, G1 Galileo).

Architecture Overview

DISS binary file
    └─ DissFilter (src/filter/diss_filter.py)
           ├─ _process_normal_band_block()   ─► MsgSubfrParser  ─► FNavParser / INavParser / CNavParser / GNavParser
           │                                  ► IntSubfrParser
           └─ _process_s_band_block()        ─► SBandParser    ─► FNavParser / INavParser / CNavParser / GNavParser

Each parser delegates per-word extraction to parser/nav_utils/{f_nav,i_nav,c_nav,g_nav}/ where the bitfield descriptors live.

Key Classes

Config Files

Service Descriptors (Normal-Band)

Defined in config/diss_config.json → SERVICE_DESCRIPTORS:

Normal-Band Packet Layout (§4.2.4.2, msg_subframe.json)

Each of the 12 packets in the Message sub-frame is 264 bits (33 bytes):

• Bits 0-2: 3-bit service descriptor
• Bits 3-263: 261 bits of payload data

Field 1 (packets 0-5): FNAV, INAV, SAR, CNAV pages — each has a lookupValue in msg_subframe.json, so collect_lookup_info() can build page_info.

Field 2 (packets 6-11): G/NAV PRS pairs — no lookupValue → page_info["GNAV"] never exists → msg_subfr_parser.extract_nav_page() must branch for GNAV before the page_info lookup block.

S-Band Packet Layout (§4.2.4.2, Figure 18)

Fixed packet size: 26080 bytes (S_BAND_PACKET_LEN in s_band_config.json). All bit offsets below are data-relative (i.e. after the 184-bit HEADER).

Access pattern (same for all nav types):

data_bits = bytes_to_bits(packet)
section = data_bits[HEADER:][slot_start : slot_start + batch_size]

BATCH_START Structure (48 bits)

Every nav/almanac/PRS batch begins with a 48-bit header:

bits 0-3:   TYPE_NAV_SV_DATA  (4 bits)
bits 4-9:   LAST_VALID_WORD_IDX (6 bits)
bits 10-15: MSG_NUM            (6 bits)
bits 16-47: TOA                (32 bits)

Within TOA (bits 16-47 of BATCH_START):

• TOW: bits 16-35 (20 bits)
• WN: bits 36-47 (12 bits)

subframe_id and Slot Selection

# src/parser/diss/common_utils.py
def generate_subframe_id(time_ms: int) -> int:
    return ((time_ms // 1000) + 1) % 30   # → 0..29

Used by GNavParser.extract_gnav_page() to select the G/NAV word slot in Field 2:

slot_idx = subframe_id % 3   # → 0, 1, or 2

Symmetry Pattern for Nav Parsers

All nav parsers follow the same pattern:

class XNavParser(NavParser):
    def extract_xnav_page(self, subframe, req_page_type, subframe_id, subframe_data, msg_filt):
        """Normal-band extraction."""
        ...

    def extract_xnav_page_S_band(self, packet, batch_number, req_page_type, subframe_data, msg_filt):
        """S-band nav batch extraction."""
        ...

    def extract_xnav_almanac_S_band(self, packet, batch_number, req_page_type, subframe_data, msg_filt):
        """S-band almanac batch extraction."""
        ...

When adding a new nav type or method, wire it in both:

1. msg_subfr_parser.MsgSubfrParser.extract_nav_page() — normal-band routing
2. s_band_parser.SBandParser.extract_nav_page() — S-band routing

And add headers in __init__ of both parsers.

_parse_packet() Return Type

SBandParser._parse_packet() returns a tuple (fieldIndex, number_of_mismatch) — never a bare int. Callers unpack with:

processed_count, _ = self.s_band_parser._parse_packet(...)

A bare return self.fieldIndex on early-exit paths is a bug; it must be return self.fieldIndex, self.number_of_mismatch.

Byte Alignment Rule

utils.bits_to_bytes(bit_string) requires len(bit_string) % 8 == 0. Always pad before converting:

pad = (-n_bits) % 8
word_bytes = bits_to_bytes(word_bits + "0" * pad)

G_NAV_WORD = 283 bits → pad 5 zeros → 288 bits = 36 bytes
Pre-encryption G/NAV = 164 bits → pad 4 zeros → 168 bits = 21 bytes