A COSPAR ID names an object by its launch — year, launch number in that year, then a letter for each piece — so Cartosat-3 is 2019-081A.
A NORAD ID is a plain serial number. The international designator, often called the COSPAR ID, tells you something more: which launch an object came from. It is named after the Committee on Space Research (COSPAR), which adopted the system, and it has been used since January 1963.
The pattern is year – launch number – piece. CelesTrak documents it as a four-digit year, a three-digit launch number for that year, and one to three letters for the piece. The letters I and O are never used, so they cannot be confused with 1 and 0.
When one rocket carries several satellites, they share the first part and get different letters. Debris and the spent rocket stage from that launch get later letters too. So if you see two objects with the same year and launch number, you know they rode the same rocket.
The oldest entry in the catalogue, the rocket body that launched Sputnik in October 1957, carries 1957-001A. That is a modern designator given to it afterwards: objects launched until the end of 1962 were first named in an older Harvard scheme using Greek letters.
Every satellite page shows an International Designator row next to the NORAD ID. Cartosat-3's says 2019-081A. The same code is hidden inside the orbital data: line 1 of its TLE contains `19081A`, which is the designator with the century and the dash removed.
The two IDs work together. The NORAD ID is what this site uses in web addresses and searches; the international designator is what you need to group all the objects from one launch, such as every satellite from a PSLV rideshare mission.
The International Designator row reads 2019-081A, and line 1 of the Orbital Elements (TLE) block contains 19081A. Open Cartosat-3
Inside a TLE, line 1 stores the designator in three fixed fields: columns 10–11 hold the last two digits of the launch year, 12–14 the launch number of the year, and 15–17 the piece. Cartosat-3's line 1 therefore begins `1 44804U 19081A`. Dropping the century makes the year ambiguous: software such as satellite.js reads 57–99 as 1957–1999 and 00–56 as 2000–2056; CelesTrak's newer OMM and SATCAT formats write the full year, as in 2019-081A.
Launch numbers are usually in chronological order within a year, but McDowell's catalogue notes it is not a strict rule. A few objects whose parent launch cannot be identified sit in the official catalogue under a year with launch number zero.
Piece letters run A to Z, then continue with two letters (AA, AB…) and three letters for launches that produce a lot of debris. With I and O skipped, there are 24 single letters, 24 × 24 = 576 two-letter codes and 24 × 24 × 24 = 13,824 three-letter codes — room for more than 14,000 pieces from one launch.
Worked example: a PSLV mission that carries one main satellite and several small co-passengers produces one designator per object, all beginning with the same YYYY-NNN, plus a letter for the PSLV upper stage if it stays in orbit. Querying CelesTrak by INTDES (yyyy-nnn) returns all of them at once.
It came from launch number 81 of 2019 in the catalogue's count, and it is piece A of that launch — the main payload.
The NORAD ID is a serial number for a single object. The COSPAR ID identifies the launch first and then the piece, so objects from the same rocket share most of their designator.
They are omitted to avoid confusion with the digits 1 and 0.