A Counting Conundrum

by Laura Johnson
posted August 23, 2013

Pi may be infinite, but unfortunately that’s not true of all numbers …

A U.S. government system to track objects in space is running out of digits, and a new system must be developed to fix the problem.

Dr. Felix Hoots, a distinguished engineer in the Engineering and Technology Group, is following the issue to help Aerospace and its customers navigate the change with as little difficulty as possible.

The Dilemma

The U.S. government has a satellite catalog that it uses to track all manmade items in space — everything from operational satellites to pieces of debris. The catalog contains information such as the launch date, launch site, orbit characteristics, and more.

Every item in this database has a unique five-digit number attached to it. For example, the International Space Station is 25544, and AeroCube 2 is assigned number 31133.

With only five digits to work with, there are only 99,999 numbers to assign. However, numbers 70,000 to 99,999 are only used for special items, such as launch processing, breakups, etc. Also, more sensitive sensors are being developed, which can see smaller objects in space. This means more objects to track.

So, while 99,999 numbers may have seemed like plenty at the dawn of the space age, it is now anticipated that the numbers may run out in five or ten years, depending on how many items are added to the catalog.

Can’t You Just Add More Numbers?

A casual observer might suggest simply adding another digit to the numbering scheme, but this presents a surprising number of challenges.

The data on each object is presented in a standard format known as a two-line element set (TLE). The government, civilians, and international agencies can take this data and plug it into their formulas and programs to, for example, determine the position of a particular satellite to enable data exchange.

The problem is that the data in the TLE is presented in a very specific way, and computer programs are written to process the data accordingly. The satellite number currently has five digits. When that changes, computers programs around the world will not work, and a huge mess could ensue.

Ted Muelhaupt, associate principal director of the Systems Analysis and Simulation Subdivision, pointed out the impact to Aerospace.

“We already know that a considerable number of Aerospace software tools will be affected, and it will certainly impact much of operational and contractor software used to support our customers,” he said. “We need to do a thorough review to identify everything that needs to be changed.”

This is not just a problem for Aerospace, the government, or the military. Satellites have become a part of daily life, involved in everything from the weather report to the movie channel to personal GPS units.

“Somewhere behind the scenes there is a TLE, a two-line element set, that is helping make all of those things possible,” Hoots said.

Moving Forward

The government has formed a Satellite Catalog Renumbering Working Group (SCRWG) to address the issue. The plan they have developed is to change the catalog number to a nine-digit format.

Obviously, this will have a broad impact, and software will have to be reprogrammed and modified to accommodate the change.

“For people who use these products, this is just like the Y2K problem we all faced when the calendar rolled over in the year 2000,” Hoots said. “That means every piece of code will have to be examined to see if it is affected. After examination, we will find only a fraction of the code that needs to be changed. But all will have to be examined.”

To make this a little easier, at the beginning, the nine-digit numbers will be assigned to smaller objects, such as debris, while the five-digit numbers will continue to be assigned to larger objects. For those who only need to track active satellites, this will delay the impact of the change.

The bottom line, however, is that at some point the format will be different and software will have to be adapted accordingly.

“The SCRWG cannot mitigate the fact that the count of satellites will eventually exceed the five-digit number currently used,” Hoots said. “All they can do is to anticipate the problem, decide what a new format will be, and get the word out to people that they need to modify their software.”

Hoots is carefully monitoring the issue, so Aerospace and its customers can be prepared for the transition, which will likely occur between 2015 and 2019.