alpha5

NORAD Alpha-5 designator codec — encode and decode satellite catalog IDs between integers and the 5-character format used by Space-Track and modern TLE/3LE files (A0123 ↔ 100123). Zero dependencies. Pure ESM. ~70 lines of source. The spec is frozen by the US Space Force.

Install

npm install alpha5

Requires Node.js 20 or newer. Works in browsers as well — no runtime dependencies.

Usage

import { decode, encode } from 'alpha5';
// or, for a namespace import:
//   import * as alpha5 from 'alpha5';
//   alpha5.decode('A0123');

// Plain numeric designators round-trip unchanged.
decode('25544'); // 25544
encode(25544); // '25544'

// Alpha-5 designators decode to their canonical integer.
decode('A0123'); // 100123
encode(100123); // 'A0123'

// The I/O letters are reserved.
decode('I0000'); // throws Error
decode('O0000'); // throws Error

// Boundaries: A0000 = 100,000, Z9999 = 339,999.
encode(339999); // 'Z9999'
encode(340000); // throws Error: exceeds Alpha-5 range

API

decode(s: string): number

Decode a NORAD designator string into its integer value in the range 0..339_999.

Accepts both plain numeric strings ("25544", "00007") and Alpha-5 designators ("A0123", "Z9999"). Numeric inputs of any length are accepted, so JSON sources that omit leading zeros (e.g. "7" instead of "00007") still decode correctly.

Throws if the input:

• is not a string, or is empty
• has whitespace, a sign prefix, or a decimal/scientific/hex/octal/binary marker
• uses a reserved letter (I or O)
• uses a lowercase letter
• has a non-digit tail after the letter prefix
• decodes to a value greater than 339_999

encode(n: number): string

Encode an integer NORAD ID into its 5-character designator string.

• Values 0..99_999 are zero-padded to 5 digits (encode(7) === "00007").
• Values 100_000..339_999 use the Alpha-5 letter prefix (encode(100123) === "A0123").
• Output is always exactly 5 characters and never contains the reserved letters I or O.

Throws if n is not a non-negative finite integer, or exceeds 339_999. BigInt, booleans, strings, null, undefined, NaN, ±Infinity, and non-integer floats are all rejected — only plain finite integers are accepted.

The full letter table

Verbatim from the Space-Track Alpha-5 documentation:

Letter	Value		Letter	Value		Letter	Value
A	10		J	18		S	26
B	11		K	19		T	27
C	12		L	20		U	28
D	13		M	21		V	29
E	14		N	22		W	30
F	15		P	23		X	31
G	16		Q	24		Y	32
H	17		R	25		Z	33

I and O are omitted.

Why

When the satellite catalog approached the 100,000-object limit of the legacy 5-digit NORAD field, the US Space Force introduced Alpha-5 as a stopgap: the first character of the 5-char field becomes a letter (A=10 through Z=33, skipping I and O to avoid confusion with 1 and 0), extending the addressable range to 339,999 objects.

If you ingest TLE, 3LE, or GP/GP_HISTORY data from Space-Track, you need this codec. The official Space-Track API only accepts integer NORAD IDs for filtering, so any 5-char designator coming in must be decoded before you can use it as a database key or query parameter.

Spec: https://www.space-track.org/documentation#tle-alpha5

Stability

The Alpha-5 spec is frozen by the US Space Force. This library follows it exactly and is tested against every example in the official documentation, the full letter table, both I/O-skip boundaries in both directions, and a round-trip property check across the entire 0–339,999 range.

If the spec ever changes (Space-Track recommends migrating to XML/JSON/KVN for new work — Alpha-5 is itself a stopgap), this library will follow.

What's not provided

• No unpadded output option. The library produces canonical 5-character Alpha-5; if you want unpadded output for IDs below 100,000, that's just String(n). Variable-width output is not part of the Alpha-5 spec and would dilute the library's purpose.
• No bulk parsing of TLE/3LE lines. This is a codec for the catalog-ID field only. Use a TLE parser like tle.js for full TLE/3LE handling and pass the catalog field through decode after extraction.
• No error subclasses. All failures throw a plain Error with a descriptive message. If you need to distinguish failure modes, match on the message.

License

MIT © Andrei Lavrenov