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SatIdentifier

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Identify satellite in you camera, telescope fov, or even overhead

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# SatIdentifier

The inverse of a satellite tracker. Instead of asking *where is this satellite*, it
asks **who is in my field of view** — so an unidentified trail on a frame can be
matched against the catalogue.

Give it a site, an epoch and a timespan, a pointing (RA/Dec J2000, Alt/Az, or LVLH
angles from orbit) and a field of view. It finds every catalogued object that
crosses that field and draws each trail on a gnomonic sky chart over a real star
background, directly comparable against your frame.

The site itself can be a **satellite** (space-based SSA): pick any object in the
loaded catalogue by NORAD number and the scan runs from *its* sensor — who crosses
my field, seen from orbit.

![SatIdentifier — sky chart with predicted trails, all-sky context view, crossings table and satellite info](https://raw.githubusercontent.com/exoplanet5/SatIdentifier/main/docs/screenshot.png)

Local Python backend (catalogue fetching, caching, persistence — standard library
only) + browser frontend (SGP4 in Web Workers, canvas chart). Companion to
[SatObserver-MX](../satobserver), sharing its architecture and its house style.

Module APIs are in [CONTRACT.md](CONTRACT.md); how it was built and what was
measured is in [DEVLOG.md](DEVLOG.md).

## Requirements

**Packaged app** (in `release/`) — no runtime dependencies; Python and the star
catalogue are bundled. Network access is needed for catalogue fetching (cached
data works offline). The full catalogue loads from CelesTrak with no account,
or — complete, including analyst and newly catalogued objects — from a free
[space-track.org](https://www.space-track.org) account (saved locally on
first use).

- **macOS** (`SatIdentifier-macOS-arm64.zip`): Apple Silicon. Unsigned — first
  launch on another machine needs right-click → Open once. User data lives in
  `~/Library/Application Support/SatIdentifier/`.
- **Windows** (`SatIdentifier-windows-x64.zip`, when present): produced by CI on
  a Windows runner, **untested on real hardware** — the same caveats as
  SatObserver-MX's Windows build apply.

**To run from source** (browser mode):
- Python ≥ 3.10 — **standard library only**, no packages needed
- Any modern browser (developed against Chrome; the packaged app uses WKWebView)

**To rebuild the .app**:
`python3 -m venv .venv-build && .venv-build/bin/pip install pywebview pyinstaller`
(icon regeneration additionally needs `pillow`), then:

```sh
.venv-build/bin/pyinstaller --noconfirm --clean --windowed \
  --name "SatIdentifier" --icon build_icon/SatIdentifier.icns \
  --add-data "app:app" --osx-bundle-identifier "local.satidentifier" desktop.py
```

## Run

**macOS app**: unzip `release/SatIdentifier-macOS-arm64.zip`, double-click
`SatIdentifier.app`. Native window; closing it quits.

**Dev / browser mode**:

```sh
python3 server.py
```

Starts a local server on http://127.0.0.1:8476 and opens your browser.
Options: `--port N`, `--no-browser`. Or double-click `SatIdentifier.command`.
In dev mode data lives in `./data/`.

## Workflow

1. **Catalogue** — load the full catalogue from either provider: *CelesTrak*
   (bulk catalogue file, no account; the legacy TLE file omits objects
   catalogued after 2026-07) or *Space-Track* (full GP; free account required,
   saved locally; the complete set — analyst and 6-digit objects included).
   Add CelesTrak single-object queries (NORAD / COSPAR / name-contains, no
   account needed — the same tab's *Fetch SATCAT metadata* button downloads
   the SATCAT table that feeds photometry, not elements), McCants classified
   elements, or paste TLEs — everything merges into one catalogue,
   deduplicated by NORAD with the newest elements winning, and **epoch-age
   statistics are shown per source**, because stale elements are the main
   cause of a failed identification.
2. **Sites** — add your observing site and mark it active. Two kinds: **Ground**
   (lat / lon / alt) or **Orbit** (an observing satellite, picked from the loaded
   catalogue by NORAD number or name — the TLE is resolved live from the
   catalogue, so the observer can never go stale against the targets).
3. **Pointing** — start time, timespan, pointing, field of view.
4. **Crossings** — press *Scan*.
5. **Sky Chart** — compare the trails against your frame.

## Observing from orbit (space-based SSA)

With an Orbit site active, the same pipeline answers "who crosses my sensor's
field" for a satellite-mounted instrument:

- **Az/El become LVLH angles** (labelled AzL/ElL): Az from the along-track
  (velocity) direction toward the orbit normal, El from the local horizontal
  toward zenith (radially out; −90° = nadir). The *Mount* tracking mode becomes
  **LVLH** — a body-fixed staring sensor, drifting through the stars at the
  orbital rate exactly as a parked ground mount drifts at the sidereal rate.
- **Earth-limb occlusion replaces the horizon**: a target is dropped only while
  the line of sight passes through the Earth itself, and entry/exit times resolve
  a target *rising from behind the limb* the same way they resolve a field edge.
  No refraction, ever, in orbit.
- The observer **never identifies itself**, and its own TLE error is added to the
  match tolerance — at the sensor it is indistinguishable from the target's.
- All-Sky is a ground-horizon projection and says so on an Orbit site; the Sky
  Chart works for both kinds.

The soundness proofs run over orbital geometry too: the geometric cull returns
the identical crossing set with the cull disabled, and every reported position
agrees with an independent two-body recompute to ~0.01″ (see `tools/test_scan.js`
section [o]).

## What it computes

- **Crossings**: entry / closest-approach / exit times, separation, RA/Dec (J2000),
  Az/El, range, range rate, angular rate and position angle, estimated magnitude,
  sunlit/penumbra/umbra, orbit class, TLE age.
- **Two angular rates**, because which one streaks your exposure depends on how you
  were tracking. Against the **stars** (`d(RA,Dec)/dt`) is the streak on a
  sidereally-guided frame — note a geostationary object is *not* stationary here, it
  drifts at ~15″/s, which is why GEO streaks in tracked images. Against the
  **horizon** (`d(alt,az)/dt`) is what a parked mount sees, where GEO is a fixed dot
  and the stars trail instead — on an Orbit site this second rate is measured
  against the LVLH frame, i.e. what a body-fixed staring sensor records. The chart
  draws whichever matches your tracking mode, including the field rotation a parked
  mount sees, and the trail drawn across the field between entry and exit is the
  thing to hold up against your frame.
- **Star background** (Gaia DR3): the depth adapts to the field — a local tiled
  database to **V = 13** serves views narrower than 3° (built once with
  `python3 tools/make_starcat.py --deep-tiles`, ~60 MB, never touches the
  network at runtime); wider views use the bundled catalogue (V ≤ 10.5,
  549 037 stars), shedding depth as the field grows. The deep build is
  **optional**: out of the box, without it, every field draws from the bundled
  catalogue and simply tops out at **V = 10.5**. Star dots follow
  Stellarium's rendering law, fading smoothly to invisibility at the limit.
  Sun/Moon (with lunar phase), constellation overlays, satellite labels and
  satellite tracks each have a toolbar toggle; the All-Sky panel uses the
  bright-star set (V ≤ 4.6) and carries the Milky-Way glow. No twilight or
  daylight tinting in either view.

## Accuracy — read this before trusting an identification

Frame handling is deliberately careful, because the tool compares satellite positions
against a star field:

- Coordinates are **J2000 mean equinox**. SGP4 returns TEME, which is *not* J2000 —
  precession since J2000 is **0.36° in 2026**, twenty times a typical telescope field.
  The full TEME → J2000 rotation (IAU-1976 precession, IAU-1980 nutation, equation of
  the equinoxes) is applied. Verified against satellite.js's own alt/az path over 1102
  samples spanning LEO/HEO/GEO and four sites: agreement **1.4e-5 arcsec**.
- Refraction is always applied to Alt/Az (1.7′ at El 30°, 5.4′ at El 10°), and the
  inverse is solved by iteration so mode switching is exactly reversible.

**But the frames are not the limiting error — the elements are:**

| Term | Magnitude | Applied? |
|---|---|---|
| TLE position error (fresh → a week old) | ~1–20 km ⇒ **7′–2°** at 500 km range | irreducible |
| UT1−UTC ignored by default | ≤ 0.9 s ⇒ **≤ 2.9′** | optional `DUT1` setting |
| Refraction | 1.7′ at El 30°, 5.4′ at El 10° | always applied |
| Annual aberration | ≤ 20.5″ | no — see below |
| Light time + orbital aberration (Orbit sites) | ≤ ~5″ each | no — two orders under TLE slop |
| Precession + nutation | < 1″ | **yes** |

Aberration is applied to neither satellites nor stars, so the chart and the objects
stay mutually consistent; correcting one side alone would make things worse. TLE age
is shown per row and in Satellite Info, because it is the number that decides whether
an identification is believable.

**Magnitudes are estimates, and mostly priors.** The method is shown per row:

| Method | Meaning |
|---|---|
| `qsmag` | McCants standard magnitude — a real observed value |
| `rcs` | Diffuse sphere from the SATCAT radar cross-section |
| `model` | Documented constellation brightness (Starlink, OneWeb) |
| `type` | Size class from SATCAT object type (R/B ≫ PAY ≫ DEB) |
| `default` | 1 m sphere. A guess, flagged as one |

Two data facts make the lower tiers necessary rather than optional: **CelesTrak
publishes no RCS above NORAD 50000** (measured: 41–97% coverage below 40000, 25.6% for
40000–49999, 0.0% above 50000), and **mmccants.org/programs/qsmag.zip currently
returns HTTP 404**. On a modern catalogue most objects therefore land on a prior with
a magnitude or more of real scatter. Do not read the magnitude column as photometry.

## Repository layout

```
server.py                  Python 3 stdlib-only backend: static files + JSON API
desktop.py                 pywebview shell for a native window
SatIdentifier.command      double-click launcher (dev mode)
app/
  index.html               loads CSS + scripts in a fixed order
  css/app.css              the whole design system, dark theme
  assets/stars_deep.bin    Gaia DR3 to V=10.5, 549 037 stars, 5.5 MB (preferred)
  assets/stars_m9.bin      Tycho-2 to V=9.0, 130 183 stars, 1.3 MB (fallback)
  js/frames.js             coordinate frames, precession/nutation, refraction, TAN
  js/propagate.js          SGP4 wrapper and the topocentric solution
  js/scan.js               worker pool, merge, budgeting
  js/worker/scan-worker.js the three-stage scan engine
  js/stars.js              deep star catalogue, cone queries
  js/photometry.js         five-tier magnitude model, Earth shadow
  js/chart.js              gnomonic sky chart (main view)
  js/crossings.js          crossings table (main list)
  js/pointing.js           the input window
  js/allsky.js             all-sky context view with the FOV footprint
  js/state.js  util.js  clock.js  windows.js  sources.js  locations.js  satinfo.js
tools/
  make_starcat.py          builds the star catalogue asset; --deep-tiles builds
                           the local V=13 tile set into data/deepstars/ (gitignored)
  test_*.js                verification harnesses — see DEVLOG
docs/                      screenshot
data/                      state, caches, credentials, deep star tiles (gitignored)
```

## Credits

Propagation: [satellite.js](https://github.com/shashwatak/satellite-js) (SGP4/SDP4).
Stars: Gaia DR3 (Gaia Collaboration 2022) and Tycho-2 (Hog+ 2000) via VizieR, plus
BSC5/HYG bright-star photometry by way of
[d3-celestial](https://github.com/ofrohn/d3-celestial). Catalogue data: CelesTrak,
Space-Track, Mike McCants. The scan's structure — hoisting SGP4 init out of the time
loop, rotating the pointing rather than the catalogue, and gating expensive work
behind a cheap scalar test — is taken from Bill Gray's
[sat_code](https://github.com/Bill-Gray/sat_code); the precession handling follows the
reasoning in his `lunar/precess.cpp`. Problem framing owes a debt to
[SatSkyMap](https://github.com/lnicastro/SatSkyMap).

SatIdentifier

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