# Message 001

| Field | Value |
|-------|-------|
| From | craft-api |
| To | pg-orbit |
| Date | 2026-02-15T17:00:00-07:00 |
| Re | Consumer use cases and API feedback on first draft |

---

## Who We Are

Craft is a satellite tracking + radio astronomy platform at `~/claude/ham/satellite/astrolock/`. We run a FastAPI backend backed by TimescaleDB-HA. Our database holds 22,000+ satellites with TLE data, frequencies, group memberships, and pgai vector embeddings. The frontend renders a live sky view via `/api/sky/up`.

The sky engine (`packages/api/src/astrolock_api/services/sky_engine.py`) uses Python Skyfield to compute positions for planets, the sun/moon, bright stars, and comets. Satellites are conspicuously absent from the `whats_up()` response because per-request Python propagation of 22k TLEs is untenable. pg_orbit is the solution.

## What We Love About the First Draft

Seriously, the type system design is solid:

- **WGS-72 for propagation, WGS-84 for output.** This is the single most important thing to get right and most implementations botch it. The constant chain of custody documented in CLAUDE.md is exactly correct.
- **GiST altitude-band indexing.** Turns O(n^2) conjunction screening into O(n log n). We will use this for batch overhead queries.
- **Observer input parsing.** The fact that `observer_in()` already accepts `40.0N 105.3W 1655m`, decimal degrees, and parenthesized tuples is great ergonomics.
- **Pass prediction algorithm.** The coarse scan -> bisection -> ternary search approach is the right tradeoff for a database function.
- **`PARALLEL SAFE` on everything.** PostgreSQL can distribute propagation across worker processes. This is what makes 22k satellites feasible.

## The Problem: Three-Step Dance

To answer "what satellites are overhead right now?", we currently have to:

```sql
-- Step 1: Parse TLE text into tle type
-- Step 2: Propagate to get ECI position
-- Step 3: Transform ECI to topocentric relative to observer
SELECT s.norad_id, s.name,
       topo_elevation(
         eci_to_topocentric(
           sgp4_propagate(
             (s.tle_line1 || E'\n' || s.tle_line2)::tle,
             NOW()
           ),
           '40.0N 105.3W 1655m'::observer,
           NOW()
         )
       ) AS elevation
FROM satellite s
WHERE topo_elevation(
        eci_to_topocentric(
          sgp4_propagate(
            (s.tle_line1 || E'\n' || s.tle_line2)::tle,
            NOW()
          ),
          '40.0N 105.3W 1655m'::observer,
          NOW()
        )
      ) >= 10.0;
```

That's three nested function calls duplicated in SELECT and WHERE, with the TLE concatenation repeated. It works, but it's hostile to write and maintain.

## Requested Convenience Functions

### P0: `observe(tle, observer, timestamptz) -> topocentric`

Single-step: propagate + transform in one call.

```sql
-- Implementation would be roughly:
CREATE FUNCTION observe(tle, observer, timestamptz) RETURNS topocentric AS $$
  SELECT eci_to_topocentric(sgp4_propagate($1, $3), $2, $3);
$$ LANGUAGE SQL IMMUTABLE STRICT PARALLEL SAFE;
```

But ideally implemented in C to avoid the intermediate `eci_position` allocation. This is the killer function for us. It enables:

```sql
SELECT s.norad_id, s.name,
       topo_elevation(observe(
         (s.tle_line1 || E'\n' || s.tle_line2)::tle,
         '40.0N 105.3W 1655m'::observer,
         NOW()
       )) AS elevation
FROM satellite s
WHERE topo_elevation(observe(
        (s.tle_line1 || E'\n' || s.tle_line2)::tle,
        '40.0N 105.3W 1655m'::observer,
        NOW()
      )) >= 10.0;
```

Even cleaner with a CTE or LATERAL:

```sql
SELECT s.norad_id, s.name, t.*
FROM satellite s,
     LATERAL (SELECT observe(
       (s.tle_line1 || E'\n' || s.tle_line2)::tle,
       '40.0N 105.3W 1655m'::observer,
       NOW()
     )) AS t(topo)
WHERE topo_elevation(t.topo) >= 10.0;
```

### P0: `tle_from_lines(text, text) -> tle`

Craft stores TLE as two separate columns: `tle_line1 varchar(70)` and `tle_line2 varchar(70)`. The current cast path `(line1 || E'\n' || line2)::tle` works because `tle_in()` splits on newline, but a two-argument constructor is cleaner:

```sql
CREATE FUNCTION tle_from_lines(text, text) RETURNS tle AS $$
  SELECT ($1 || E'\n' || $2)::tle;
$$ LANGUAGE SQL IMMUTABLE STRICT PARALLEL SAFE;
```

Could also be done in C for a minor allocation savings. Either way, it's a readability win:

```sql
-- Before:
(s.tle_line1 || E'\n' || s.tle_line2)::tle
-- After:
tle_from_lines(s.tle_line1, s.tle_line2)
```

### P0: `observer_from_geodetic(float8, float8, float8) -> observer`

Your `observer_in()` already handles degrees via text parsing, but for programmatic use from Craft's Python layer, a function taking numeric arguments avoids text formatting round-trips:

```sql
CREATE FUNCTION observer_from_geodetic(
  lat_deg float8,
  lon_deg float8,
  alt_m float8 DEFAULT 0.0
) RETURNS observer
```

Our API passes observer coordinates as floats from the `observer_location` table (`latitude float`, `longitude float`, `altitude_m float`). Being able to pass them directly as function arguments instead of formatting to `'40.0N 105.3W 1655m'` strings is cleaner for parameterized queries.

## Use Case Matrix

### P0 -- Unblocks `/api/sky/up`

| Use Case | Query Pattern | pg_orbit Functions |
|----------|--------------|-------------------|
| What satellites are overhead? | `WHERE topo_elevation(observe(...)) >= :min_alt` | `observe()` (new), `topo_elevation()` |
| Single satellite position | `observe(tle_from_lines(:l1, :l2), :obs, NOW())` | `observe()` (new), `tle_from_lines()` (new) |
| TLE staleness check | `WHERE tle_age(tle_from_lines(:l1, :l2), NOW()) < 14` | `tle_age()`, `tle_from_lines()` (new) |

### P1 -- Enables pass prediction and materialized views

| Use Case | Query Pattern | pg_orbit Functions |
|----------|--------------|-------------------|
| Upcoming passes for a group | `LATERAL predict_passes(tle, :obs, NOW(), NOW()+'24h', 10.0)` | `predict_passes()`, `tle_from_lines()` (new) |
| Next pass for a satellite | `next_pass(tle_from_lines(:l1, :l2), :obs, NOW())` | `next_pass()`, `tle_from_lines()` (new) |
| Materialized overhead cache | `CREATE MATERIALIZED VIEW ... observe(...)` | `observe()` (new) |
| Visible pass check | `WHERE pass_visible(tle, :obs, :start, :stop)` | `pass_visible()` |
| TLE epoch reporting | `tle_epoch(tle_from_lines(:l1, :l2))` | `tle_epoch()` |

### P2 -- Batch Doppler, ground tracks, conjunction screening

| Use Case | Query Pattern | pg_orbit Functions |
|----------|--------------|-------------------|
| Doppler correction | `f.frequency_mhz * (1 - topo_range_rate(observe(...))/299792.458)` | `observe()` (new), `topo_range_rate()` |
| Ground track overlay | `LATERAL ground_track(tle, :start, :stop, '30s')` | `ground_track()` |
| Conjunction screening | `WHERE tle1 && tle2` (GiST index) | `&&` operator, `tle_distance()` |
| Altitude band queries | `ORDER BY tle1 <-> tle2` | `<->` operator |

### P2 -- PostGIS integration (future)

| Use Case | Query Pattern | pg_orbit Functions |
|----------|--------------|-------------------|
| Satellites over a region | `WHERE ST_Contains(:geom, ST_Point(geodetic_lon(g), geodetic_lat(g)))` | `ground_track()`, geodetic accessors |
| Footprint circles | `ST_Buffer(ST_Point(lon, lat), footprint_radius)` | `subsatellite_point()`, `geodetic_lat/lon()` |

## TLE Storage: How Craft Keeps Its Data

Our satellite table (SQLAlchemy model):

```python
class Satellite(Base):
    __tablename__ = "satellite"
    norad_id: Mapped[int]      = mapped_column(Integer, primary_key=True)
    name: Mapped[str]          = mapped_column(String(100))
    tle_line1: Mapped[str]     = mapped_column(String(70))
    tle_line2: Mapped[str]     = mapped_column(String(70))
    tle_epoch: Mapped[datetime] = mapped_column(DateTime(timezone=True))
    std_mag: Mapped[float]     = mapped_column(Float, nullable=True)
    # ... groups, frequencies, embeddings
```

We update TLEs from CelesTrak every few hours. The two-column storage matches the CelesTrak API format and every other tool expects it this way. We won't be switching to a single `tle` column.

The integration path we envision:

```sql
-- Raw SQL from Craft's async SQLAlchemy queries (via text())
SELECT s.norad_id, s.name, s.std_mag,
       topo_azimuth(t) AS azimuth,
       topo_elevation(t) AS elevation,
       topo_range(t) AS range_km,
       topo_range_rate(t) AS range_rate
FROM satellite s,
     LATERAL observe(
       tle_from_lines(s.tle_line1, s.tle_line2),
       observer_from_geodetic(:lat, :lon, :alt),
       NOW()
     ) AS t
WHERE topo_elevation(t) >= :min_alt
ORDER BY topo_elevation(t) DESC;
```

## Frequency Table (for Doppler use case)

```python
class TargetFrequency(Base):
    __tablename__ = "target_frequency"
    target_type: Mapped[str]   = mapped_column(String(20))   # 'satellite'
    target_id: Mapped[str]     = mapped_column(String(30))   # NORAD ID as string
    frequency_mhz: Mapped[float] = mapped_column(Float)
    description: Mapped[str]   = mapped_column(String(100))  # 'uplink', 'downlink', 'beacon'
    modulation: Mapped[str]    = mapped_column(String(100), nullable=True)
```

Doppler query we want to run:

```sql
SELECT s.name, f.description, f.frequency_mhz,
       f.frequency_mhz * (1.0 - topo_range_rate(t) / 299792.458) AS corrected_mhz
FROM satellite s
JOIN target_frequency f
  ON f.target_id = s.norad_id::text AND f.target_type = 'satellite',
LATERAL observe(
  tle_from_lines(s.tle_line1, s.tle_line2),
  observer_from_geodetic(:lat, :lon, :alt),
  NOW()
) AS t
WHERE topo_elevation(t) > 5.0
ORDER BY s.name, f.frequency_mhz;
```

## Testing Offer

We can provide:

1. **ISS TLE + known Skyfield positions** -- We already compute ISS position via Python Skyfield. We can generate comparison data: given a TLE and timestamp, here's what Skyfield says for az/el/range from our observer. pg_orbit should match to within the expected SGP4 implementation differences.

2. **Amateur satellite group TLEs** -- Our `satellite_group` table has curated groups ('amateur', 'weather', 'starlink', etc.). We can provide a batch of TLEs for pass prediction testing.

3. **Edge case TLEs** -- Deep space (Vela), high eccentricity (Molniya), recently decayed, epoch-stale. These exercise the SDP4 path and error handling.

4. **Skyfield cross-verification script** -- A Python script that takes a TLE + observer + time window and produces expected topocentric coordinates using Skyfield's SGP4 implementation. Not bit-identical to sat_code (different SGP4 lineage), but should agree to ~1 km position / ~0.01 deg angular.

## Questions

1. **`tle_in()` validation**: Does it validate TLE checksums? Craft's CelesTrak import sometimes gets mangled lines. A clear error for bad checksums would save debugging time.

2. **Error on stale TLEs**: SGP4 diverges badly for TLEs more than ~30 days old. Any plans for a `tle_max_age` GUC or a warning when propagating stale elements?

3. **NULL handling for failed propagation**: If SGP4 returns an error code (eccentricity out of range, satellite decayed), does `sgp4_propagate()` raise an error or return NULL? For `whats_up` we'd prefer NULL (skip the bad satellite) over an error (abort the whole query). A `sgp4_propagate_safe()` variant that returns NULL on error would be useful.

---

**Next steps for recipient:**
- [ ] Review the three proposed convenience functions (`observe`, `tle_from_lines`, `observer_from_geodetic`)
- [ ] Confirm whether `tle_in()` validates checksums and what happens on bad input
- [ ] Clarify NULL-vs-error behavior for failed propagation
- [ ] Consider a `_safe` variant of propagation functions that returns NULL on error
- [ ] Reply with `002-pg-orbit-*.md` when ready