History: transition from 4-line elements to 5-line elements

The number of objects tracked by the USAF has exceeded the capacity of the 5-digit catalogue numbers of the 2-line elements format, which will result in its obsolescence. This is not the first time that something like this has happenned.

By the summer of 1964, the USAF had catalogued more than 800 objects.

At the time, orbital elements were transmitted in a 4-line (aka 4-card) format, that was limited to 3-digit catalogue numbers.

Here is an example:

0  005  312 58 BETA      002 US11 62 04 27 312 04 29  98
1  005  312 37781.00000000 01.35993597 -.168030-06 -.110000-10 120
2  005  312 034.2490 275.4970 297.2870 .1862006 0133.99 000681 167
3  005  312 156.2020 004.4050 -03.0140 -.612-04 -.140-3 016243 089

The definition is here:

http://satobs.org/seesat_ref/misc/4-line_elements_format.pdf

My source for the above is the 1963 document The SPADATS Mathematical Model:

https://apps.dtic.mil/dtic/tr/fulltext/u2/415001.pdf

SPADATS stood for Space Detection and Tracking System.

In February 2005, the late Pierre Neirinck mailed me copies of some historical documents from the early days of satellite tracking, including the announcement by NORAD of a proposed 5-line elements format, which I have scanned into a pdf:

http://satobs.org/seesat_ref/misc/Announcement_of_5-line_elements.pdf

It was sent on behalf of AVM Maurice Lipton, RCAF, to the RAE (Royal Aircraft Establishment). This copy appears to be from the files of the late Dr. Desmond King-Hele, whose name was hand-written in the upper right corner of page 1. It was stamped with the date September 17, 1964.

The introduction explains the motivation behind the new format:

"The projected growth of the satellite inventory catalogued in the
SPADATS/SPACETRACK System requires that allowance be made in the data records
basic to the cataloguing activity for unique, numeric satellite identification
to accommodate that growth. Specifically, satellite observation and orbital
elements records and reporting formats will be revised for five-digit satellite
numbering."

Also noteworthy was the introduction of a classification code: "S" if secret, "C" if confidential, "U" if unclassified.

Other documents sent to me by Pierre indicate that the 5-line format had come into effect by January 1, 1965, by which time 967 objects had been publicly catalogued. 

I say *publicly* catalogued, because Cosmos 50 (1964-070A / 919) had exploded on November 5, 1964, but most of its tracked debris was not catalogued until February 1965. Ninety-three (93) fragments were catalogued in that month, all with decay dates in November 1964.

Not relevant to this post, but perhaps of interest: I have previously reported evidence that some of the Cosmos 50 debris was seen to re-enter from Hong-Kong, less than 15 minutes after the explosion:

http://satobs.org/seesat/May-2013/0022.html

The 2-line format eventually replaced the 5-line format, but retained the 5-digit catalogue numbers. Several new formats will soon replace the 2-line format, with support for 9-digit formats. Since most, if not all, appear to support variable field widths, there probably is no practical limit to the size of the catalogue numbers.

Ted Molczan



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