NEXRAD level2: get_ldm drops trailing Message 31 radials from LDM records containing Message 2

[aladinor]

Summary

NEXRADLevel2File.get_ldm locates messages with fixed // 120 / % 120 arithmetic, assuming every LDM compressed record holds exactly 120 messages. Records that carry Message 2 (RDA status) slots still contain 120 Message 31 radials, but at shifted positions — the fixed arithmetic drops the trailing radials of those records, so affected sweeps come back short.

MRE (public file)

import fsspec
from xradar.io.backends.nexrad_level2 import open_nexradlevel2_datatree

url = "s3://unidata-nexrad-level2/2026/03/10/KLOT/KLOT20260310_231412_V06"
with fsspec.open(f"simplecache::{url}", s3={"anon": True}) as f:
    local = f.name

tree = open_nexradlevel2_datatree(local)
print(tree["sweep_4"].ds.sizes["azimuth"])  # 717

Expected

720 rays. An independent byte-level census (walking each decompressed LDM record message-by-message via the size_halfwords/message-type headers, skipping the fixed 2432-byte slots of non-31 messages) finds 720 Message 31 radials in this cut; the 3 missing rays are the trailing radials of the Message-2-carrying record.

Suggested fix

Walk messages by their headers instead of assuming 120 messages/record — non-31 messages occupy fixed 2432-byte slots and can appear at arbitrary positions.

xradar 0.12.1.dev9+gd723857f2 (main), Python 3.12.