The Space Reviewin association with SpaceNews

orbital debris illustration
Tracking all the active satellites and orbital debris around the Earth is a challenging task, even for the US Defense Department. (credit: NASA)

The numbers game

What’s in Earth orbit and how do we know?

<< page 1: observations, tracks, and catalogs

Mirror, mirror on the wall, who has the fairest catalog of them all?

While there is no doubt that the US military has the most data on objects in orbit, it is not the only entity keeping track and collecting data. Perhaps the second best network of sensors and independent catalog of orbital positions is kept by the Russian military. In similar fashion to the US, Russia maintains a network of both dedicated space surveillance and additional duty missile warning radars, as well as optical telescopes. Unlike the US, all of the Russian assets are located on the Asian continent. This means that while Russia has excellent coverage over its own territory, particularly for low Earth orbit, it does not have adequate deep space coverage on the other side of the world and very limited coverage of most of the GEO belt.

The US military undoubtedly has the best catalog in the world, but there are three caveats to this assessment.

Recently, the European Council of Ministers approved a plan to study how best to develop a European SSA capability. Initially starting by combining the data from existing European radars and telescopes, the plan might eventually entail building new sensors. China is looking into this problem as well, with their existing optical telescopes and research at the Purple Mountain Observatory located near Nanjing in Jiangsu Province. They have stated that they currently have a catalog of about 100 objects, but have plans to develop a Chinese Space Surveillance System, including building a network of optical sensors. Although unknown exactly to what extent, it is also believed that certain phased array radars managed by the Chinese military are also used for space surveillance. Many other countries have singular telescopes and radar dishes which could be used to track satellites.

But space surveillance and SSA is not limited to governments and militaries. The largest optical space surveillance telescope network in the world is the International Scientific Optical Network (ISON) and consists of 25 scientific and academic instruments at 18 different sites managed by the Russian Academy of Sciences. Additionally, there are hundreds of amateur observers around the world that do an excellent job of tracking the larger, and often classified, objects that governments don’t like to talk about. Armed with binoculars, stopwatches, and backyard telescopes, they can be surprisingly accurate and well informed.

When it comes to judging the “best” catalog, there are two main criteria to consider:

  1. The quality of the orbital data, both how accurate the orbital elements are and how often they are updated.
  2. The accuracy of the descriptive information, such as what the object is and the launching State.

With regard to the first criterion, the US military undoubtedly has the best catalog in the world, mainly because of the size of the sensor network they can pull data from, the accuracy and quality standards for that data, and the care that is put into maintaining the catalog. There are three caveats to this assessment. First, the satellite catalog released publicly by the US military is not the best it has. The TLEs on Space Track are of a relatively poor accuracy compared to what the US military uses for its own internal analyses and there are at least 6,000 objects being tracked by the US military that do not appear on Space Track because a launching State cannot be identified, as explained earlier in this article.

Second, the US military does not have a monopoly on highly accurate positional data. The mathematical theories and algorithms behind this process are not classified and have been actively discussed in international forums by experts from the US and other countries for decades. The optical telescopes in the ISON network can generate highly accurate element sets that are on par with the very accurate SP vectors the US military does not release publicly (see “Billiards in space”, The Space Review, February 23, 2009). In a few cases, the US military is actually behind the curve and still using decades-old astrodynamic techniques to create these element sets, mainly because it costs too much to redesign the hardware and software and recertify the system.

There is also the issue of the satellite owner-operator positional data. Anyone who operates a satellite can get very accurate information on its location, either as reported from the satellite itself by the onboard guidance system or through the process of communicating with the satellite. Almost always this positional data collected by the satellite operator is more accurate than anything collected by any third-party sensor and for the most part not being utilized as part of the US satellite catalog.

While there are some technical issues relating to data formats and models that would need to be worked out for this to happen, the main sticking points have traditionally been policy-related. Some commercial operators are uncomfortable dealing with the US military; many would like the data exchange to flow both ways. However, in the past the US military has generally decided not to share data with commercial operators and only provide information about potential issues on the “trust us” model. Ironically, many of these same commercial providers are now cooperating with the SOCRATES-GEO program run by the Center for Space Standards and Innovation because CSSI has found ways to deal with these issues. There are indications the US military may be changing these policies in the near future.

Third, just because the US military has the best current catalog does not mean that it doesn’t have significant room for improvement. In particular, the lack of any sensors on the Eurasian and African continents, in the Southern Hemisphere, and deep space tracking capacity in general are significant handicaps for the US military catalog. For example, the annual European Space Agency report The Classification of Geosynchronous Objects lists many objects in or near GEO that are not included in the public US military catalog.3 This list includes 103 dead payloads or rocket bodies listed in the US military catalog without orbital elements and another 67 pieces of debris not listed at all but which can be clearly identified with a particular launch. And as mentioned earlier in this article, the US military catalog currently has hardly any of the estimated hundreds of thousands of objects smaller than ten centimeters in size. The US military is attempting to address both of these issues, mainly through unilateral measures, but is running into financial and bureaucratic difficulties.

When it comes to the second criterion, the quality of the descriptive information, the US catalog is also notably lacking. The main issue stems from trying to assess the correct name or mission for an object based on public information, which may be inaccurate, hard to track down, or in a foreign language. Having worked this job in the past and trained these analysts myself, I can tell you that it is extremely challenging. In addition to the high turnover and training issues identified in a previous article, there is the additional issue of naming conventions. The US military has a whole set of naming conventions for various Russian, Chinese, and other foreign boosters and satellites that stem from military intelligence practices. Unfortunately, these names are often quite different from the official public names used by the launching State and sometimes classified.

The operators of Celestrak, who have maintained a list of operational satellites for decades, send a weekly list of discrepancies found in the official Space Track catalog to the JSpOC, a list that is very long indeed.

There are also special cases that are particularly challenging, with perhaps the most serious being new launches. Just because a radar or optical telescope is tracking an object doesn’t mean they know which object they are tracking, especially if it is newly launched and in a very similar orbit to other pieces from that same launch. Additional information such as size and shape is needed to distinguish between two similar objects and sometimes that information isn’t always available when it comes time to catalog objects. The absolute worst-case scenario is when a booster releases multiple microsatellites. Telling one ten-centimeter cube apart from a dozen others nearby and getting the naming right is very difficult indeed. This can lead to issues where an object gets cataloged as the satellite from a launch when it is actually the rocket body and vice versa. Sometimes these mistakes are not discovered until the piece of debris suddenly maneuvers. The Russian Dnepr-1 booster in particular has had past launch attempts with as many as 18 payloads.

For deep space, often a satellite being launched into the geostationary belt needs to make multiple maneuvers over days or weeks to get to its final orbit. Keeping tabs on all these maneuvers can be difficult, especially if the owner-operator has not made public the sequence of events. Other times satellites in GEO will move to new locations in the belt. Most often this is because they are new commercial satellites that are put into one orbital slot for technical checkout and then weeks or months later moved into an operational slot while the old satellite is maneuvered into the graveyard region.

These are not theoretical problems—they happen every day and the end result is an inaccurate catalog. The operators of Celestrak, who have maintained a list of operational satellites for decades, send a weekly list of discrepancies found in the official Space Track catalog to the JSpOC, a list that is very long indeed.

Perhaps the most controversial issue is the number of currently active satellites. This has come up a few times recently in news articles, briefings, and Congressional testimony. Lieutenant General Larry James, Commander of 14th Air Force and the JSpOC, said in testimony before the House Subcommittee on Science and Aeronautics:

Today we are tracking approximately 19,000 objects; 1,300 active payloads and 7,500 pieces of debris… [and] we conservatively project the number of active satellites to grow from 1,300 to 1,500 over the next 10 years.

Currently, the best open source analysis of the number of active satellites comes from a database maintained by the Union of Concerned Scientists. This database is pulled from the public US military satellite catalog, Celestrak, satellite catalog information from other nations including Europe, and amateur observers. As of April 1, 2009, it lists just under 900 active satellites in orbit, including many not officially acknowledged by the US military, giving a disparity of 400 compared to the numbers cited by the US military.

It is unclear where the US military’s number of active satellites comes from. Perhaps there is a difference in naming convention and definition of what an active satellite is. General James cited 19,000 total objects being tracked, of which 1,300 are active payloads but only 7,500 pieces of debris and doesn’t elaborate on what category the other 10,200 objects fall under if they are not active satellites or debris. It could also be that the 1,300 number cited by the US military was derived from unclassified sources so as to not compromise the true number gleaned by military intelligence. One basis for this derivation could be that oft-cited historical “fact” that payloads have made up between five percent and ten percent of the catalog; 1,300 is about seven percent of 19,000.

It takes a village to build a (good) catalog

Clearly, the issue of maintaining a comprehensive and accurate catalog of objects in Earth orbit is extremely important. Safe and secure operations of satellites and human activities in Earth orbit depend on such information. A good satellite catalog (as measured by the two main criteria outlined earlier) is also essential for monitoring the long-term health of the space environment and devising strategies to protect the long-term sustainability of the space environment.

Despite the issues raised earlier, the JSpOC should be commended for tackling such an extraordinarily difficult mission and doing as well as they have given their existing personnel and resource limitations. The important lesson is that maintaining an orbital catalog is difficult, and asking any organization to do so without also giving them the proper tools and resources is setting that organization up for failure.

The important lesson is that maintaining an orbital catalog is difficult, and asking any organization to do so without also giving them the proper tools and resources is setting that organization up for failure.

One immediate area of concern is that the US military, and government in general, needs to improve how they take constructive criticism on this issue. The flaws and shortcomings of the SATCAT are well known to the analysts that work with it on a daily basis, yet in the past the immediate reaction anytime an outsider pointed out these some issues was one of defensive aggression that immediately shuts down any further cooperation. In the broader context, many of those in the US government who are not working day to day with the catalog unfortunately believe the mission boilerplate—that the JSpOC really does track everything in Earth orbit from cradle to grave. Thus they tend to see any criticism of the catalog or the SSA efforts of the military as unfair, untrue, biased, or based on ulterior motives.

These reactions are unfortunate, and they are compounded by the usual swarm of contractors and private companies that have been drawn to the problem like vultures scenting a fresh kill. Some of these entities actually do have the best interests of the United States and the US military in mind and are honestly looking to help. But others just see SSA as another potentially lucrative funding stream to add to their defense portfolio. Sorting between these two groups is extraordinarily difficult but necessary.

With the recent high profile events of the 2007 Chinese ASAT test, the 2008 destruction of USA 193, and the 2009 Iridium-Cosmos collision, the US is joining other governments around the world in finally realizing the importance of SSA and maintaining an accurate catalog. However, they have yet to find a way of providing the resources to do so without breaking the bank, mainly because they have been primarily focusing on unilateral solutions – each building and operating their own sensor networks.

The United States needs to realize that it is not alone in this effort and there are resources available to help succeed in this mission. In many cases, these resources already exist and bringing them into the process does not necessarily mean huge economic costs. However, it does mean a rethinking of the philosophy behind the approach of SSA as well as policy towards cooperation. Part of this re-thinking is to realize that SSA is not solely a military mission: while it does indeed have a strong military component, it also plays a big role in commercial and civil space affairs. Just as the military does not have a monopoly on space remote sensing imagery, weather data, navigation, or communications, commercial, civil and international players need to be brought into the discussion on SSA and mixed with unilateral military solutions.

One of the great lessons of the Internet Age is that more eyeballs on a problem almost always leads to more accurate information and more timely updates.4 Gone are the days when the prevailing assumption is that a small group of “experts” maintaining a set of data is the best way to go. By opening it up to more people, you greatly increase the chances of finding mistakes and reduce the mind share needed to keep a handle on the data.

Additionally, having large numbers of people looking a data set almost always results in some surprising new ways of analyzing, sorting and creating value from that data. Of course, there are some elements of a satellite catalog that should not be publicly crowd-sourced - in particular intelligence on capabilities and limitations of foreign military satellites. But aside from that small section, the other pieces of data that are already made public and shared (element sets, object names, object type) would benefit.

One of the great lessons of the Internet Age is that more eyeballs on a problem almost always leads to more accurate information and more timely updates.

This means that methods of bringing commercial, civil, and international sources of data into the SSA process to improve the existing satellite catalog need to be explored while simultaneously increasing access to that catalog for international civil and commercial use. At the same time, efforts must be made to separate out and protect information that is truly of a critical, national security nature for the United States and its allies.

Part of this international cooperation and dialog should be on better defining the terms used in SSA, including what information should be in the satellite catalog and what an “active payload” is. Commercial operations and governments should be encouraged to share more data on initial orbits for newly launched objects and especially in-orbit maneuvers of existing objects. Doing so would reduce the workload on SSA systems, improve the accuracy of the satellite catalog, and contribute to trust and transparency.

As is the case with many regimes and problems, knowledge is the key to safe and secure operations in space. A comprehensive and accurate satellite catalog is the essential foundation of this knowledge and enables many other services and functions. While the US military does a yeoman’s job with this mission currently, there are many initiatives that the US government can undertake to improve the situation. The incentive is assured access to and continued use of space for the US, part of which depends on the actions of all the other actors in space. Only when everyone is on the same page of music and understands the space environment can we truly make progress towards space sustainability.


  1. Schumacher, Paul, “Prospects for Improving the Space Catalog”, AIAA Meeting Papers on Disc, September 1996, A9641248, AIAA Paper 96-4290
  2. Ibid.
  3. R. Choc and R. Jehn, “Classification of Geosynchronous Objects”, Issue 11 (February 2009), European Space Agency, European Space Operations Centre, Space Debris Office.
  4. D. Brabham,. “Crowdsourcing as a Model for Problem Solving: An Introduction and Cases”, Convergence: The International Journal of Research into New Media Technologies (2008)