NexradWx

An Android app for weather enthusiasts who want raw radar data, not a rendered summary: NEXRAD Level II moments (reflectivity, velocity, correlation coefficient) decoded from NOAA's public archive, plus raw surface station observations from the National Weather Service.

Modules

• nexrad-core — pure Kotlin/JVM library, no Android dependency. This is where all the hard, easy-to-get-wrong logic lives, and it has a real unit test suite (28 tests):
  • decode/Level2Decoder.kt — a from-scratch NEXRAD Level II Archive decoder (ICD 2620002): volume header, bzip2-framed LDM records, Message Type 31 (Digital Radar Data Generic Format), generic data moment blocks for REF/VEL/SW/ZDR/PHI/RHO. Verified against a byte-accurate synthetic fixture (src/test/resources/build_fixture.py + Level2DecoderTest.kt) that exercises bzip2 framing, moment scaling, below-threshold (NaN) and range-folded (+Inf) sentinels, and radial status flags.
  • color/ColorTables.kt — dBZ / velocity / correlation-coefficient color ramps.
  • render/PpiRasterizer.kt — polar-to-pixel PPI rasterizer (radar-centered, north-up), independent of any UI toolkit so it's unit-testable and reusable.
  • site/RadarSite.kt — catalog of 162 WSR-88D sites (CONUS, Alaska, Hawaii, Guam, Puerto Rico, and the overseas military units), sourced from NOAA's Historical Observing Metadata Repository via the py-ART project's nexrad_common.py (BSD-3).
• app — the Android application (Kotlin + Jetpack Compose): site picker, a 3-tab PPI view (Reflectivity / Velocity / Correlation Coefficient), and a station browser with raw METAR-derived observations.

Data sources (both free, no API key)

• Radar: s3://unidata-nexrad-level2 (Unidata's public mirror of the NOAA archive, anonymous/no-sign-request). NOAA's own noaa-nexrad-level2 bucket -- long documented as the canonical public archive, and what this app originally used -- now returns AccessDenied for all anonymous requests, confirmed directly against S3 (not a proxy/sandbox artifact: verified from two independent networks, including a real device). Unidata's mirror has the same key layout and is still genuinely public, but syncs with a multi-hour lag rather than the ~4-10 minutes NOAA's bucket used to offer, so this is raw data but noticeably delayed. Cutting that lag means reading Unidata's separate real-time chunk feed instead (s3://unidata-nexrad-level2-chunks, keys like <SITE>/<volume>/<yyyyMMdd>-<HHmmss>-<seq>-S), reassembling the S/I/E sequence per volume -- confirmed reachable and public, but not implemented here.
• Stations: api.weather.gov (National Weather Service). Before shipping, replace the placeholder USER_AGENT in NwsStationRepository.kt with your own app name/contact — NWS asks every client to self-identify.

Scope decisions worth knowing about

• Only REF/VEL/RHO are surfaced in the UI (per the ask); SW/ZDR/PHI are already decoded by Level2Decoder and available on Radial.moments if you want to add more tabs.
• No on-device GPS integration yet — the station browser takes a typed lat/lon instead of requesting ACCESS_FINE_LOCATION at runtime. The permissions are declared in the manifest for when that's wired up.
• Velocity/spectrum-width units are left in the ICD-native m/s rather than converted to knots.

Building

Requires a JDK 17+ and the Android SDK (compileSdk 34). This repo's Gradle wrapper (./gradlew) is fully set up:

./gradlew :nexrad-core:test   # runs the decoder/color-table/rasterizer/catalog unit tests
./gradlew :app:assembleDebug  # requires the Android SDK

A note on how this was built

This project was developed in a sandboxed environment with no Android SDK and no network access to dl.google.com (where the Android Gradle Plugin and SDK components are hosted), so :app could not be compiled or run there. :nexrad-core has no Android dependency, so it was fully buildable and testable in that sandbox — all 28 tests pass. The :app module's Compose/ViewModel/networking code was written and carefully reviewed by hand but is unverified by a real compiler; run ./gradlew :app:assembleDebug (or open in Android Studio) as the first step on a real machine, and treat that as part of the initial review.