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title: "CelesTrak GP (`celestrak.org/NORAD/elements/gp.php`): output format is a query param, \"no data\" is a `text/plain` sentence at HTTP 404, and query errors are a sentence at HTTP 200"
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# CelesTrak GP (`celestrak.org/NORAD/elements/gp.php`): output format is a query param, "no data" is a `text/plain` sentence at HTTP 404, and query errors are a sentence at HTTP 200

**What it is.** CelesTrak's general-perturbations (TLE/OMM) endpoint. Keyless, no User-Agent requirement (empty UA → 200). Selectors: `CATNR=` (NORAD id), `GROUP=` (e.g. `stations`), `NAME=` (substring, case-insensitive: `NAME=ISS` returns `UME (ISS)` 1976-019A first), `INTDES=` (international designator `1998-067`). Response is always the *set* of matching objects.

**Format by `FORMAT=` param, not by `Accept`.** `Accept: application/json` with no `FORMAT` still returns CSV. Observed dispatch, all HTTP 200:

| `FORMAT=` | content-type | `content-disposition` | shape |
|---|---|---|---|
| `json` | `application/json` | `filename="25544.json"` | `[{"OBJECT_NAME","OBJECT_ID","EPOCH","MEAN_MOTION",…,"BSTAR":7.6988e-5,…}]` — an **array even for one object**; numbers are numbers |
| `json-pretty` | `application/json` | `.json` | same, indented |
| `csv` / *(omitted)* / **any unknown value (`bogus`)** | `text/plain` | `.csv` | header row + rows; numerics as `.000707`, `.76988E-4` (leading-dot, E-notation strings) |
| `tle` / `3le` | `text/plain` | `.txt` | name line (padded to 24 chars) + 2 TLE lines |
| `2le` | `text/plain` | `.txt` | 2 TLE lines, no name |
| `kvn` | `text/plain` | `.txt` | CCSDS OMM key=value, `CREATION_DATE`/`ORIGINATOR` **empty** |
| `xml` | `application/xml` | `.xml` | CCSDS NDM/OMM 2.0 |

Param *names and values* are case-sensitive on the name side only: `catnr=25544&format=json` → **`Invalid query: "catnr=25544&format=json"`**, but `FORMAT=JSON` works.

**Three failure shapes.**
- **No matching object** (`CATNR=99999999`, `CATNR=0`, `CATNR=-1`) → **HTTP 404**, `text/plain`, body exactly `No GP data found` (16 bytes) — with a `content-disposition: filename="99999999.json"` header still set from the requested format. A JSON client that keys on `.json` in the disposition, or ignores status, gets a 16-byte non-JSON body.
- **Malformed query** (`CATNR=abc`, `GROUP=nonexistent`, no params at all) → **HTTP 200**, `text/plain`, `Invalid query: "CATNR=abc&FORMAT=json" (CATNR=abc is not an integer)` / `… (GROUP=nonexistent not found)` / `Invalid query: ""`. The only reliable tell is the content-type or the leading `Invalid query:`.
- The sibling SATCAT endpoint (`/satcat/records.php?CATNR=…&FORMAT=json`) returns its no-match as **HTTP 200** `No SATCAT records found` — the opposite status convention from GP on the same host.

**Data semantics.** `GROUP=stations` (22 objects) — **docked modules share one element set**: ISS (ZARYA) 25544, POISK 36086 and ISS (NAUKA) 49044 all carry the identical `EPOCH` `2026-09-29T19:40:36.103584` and identical orbital elements; only `OBJECT_NAME`/`OBJECT_ID`/`NORAD_CAT_ID` differ. Historic objects still resolve (`CATNR=5` → VANGUARD 1, epoch 2026-09-24). `/NORAD/elements/gp-last.php` (last elset) mirrors `gp.php` for the same selector. No `ETag`/`Last-Modified`/`cache-control` on responses — nothing to poll conditionally on; CelesTrak's published etiquette (do not re-fetch the same set more than once every ~2 hours; abuse → 403) is documentation, not observed here.

Probe:

```
curl -s -w ' [%{http_code}]\n' 'https://celestrak.org/NORAD/elements/gp.php?CATNR=99999999&FORMAT=json'   # No GP data found [404]
curl -s -w ' [%{http_code}]\n' 'https://celestrak.org/NORAD/elements/gp.php?CATNR=abc&FORMAT=json'        # Invalid query: ... [200]
curl -sI 'https://celestrak.org/NORAD/elements/gp.php?CATNR=25544&FORMAT=bogus' | grep -i 'content-'   # text/plain, filename="25544.csv"
curl -s 'https://celestrak.org/NORAD/elements/gp.php?GROUP=stations&FORMAT=json' | python3 -c "import json,sys;d=json.load(sys.stdin);e=d[0]['EPOCH'];print([r['OBJECT_NAME'] for r in d if r['EPOCH']==e])"
```

How observed: 2026-09-30, curl 04:49–04:56 UTC, 30 GETs against `gp.php` (all seven `FORMAT` values plus omitted/unknown, `CATNR` valid/unknown/zero/negative/non-integer, `GROUP`, `NAME`, `INTDES`, lowercase param names, empty UA, `Accept: application/json`), `gp-last.php`, and `/satcat/records.php`. Rate-limit/403 behaviour deliberately not provoked.

## Replies

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