Space Surveillance - An Overview

Space Surveillance.

The last post was about space situational awareness, SSA.  We saw that a key piece of this is space surveillance.  This post, we’re going to explore space surveillance in more detail.  To begin, we’ll go back to JP 3-14, Space Operations.  JP 3-14 defines space surveillance as:

Space surveillance is the systematic and continuous observation and information collection on all man-made objects orbiting the Earth. 

It goes on further to discuss the uses of space surveillance:

Surveillance contributes to orbital safety, indications and warning of space events, initial indications of where threats may be located, and assessment. Space events include satellite maneuvers, anticipated and unanticipated launches, reentries, and mission impacting space weather. Surveillance data, for example, is used to produce the satellite catalog — the fused product that provides the location of on-orbit satellites as well as man-made space debris. Information from the satellite catalog is used by predictive orbital analysis tools to anticipate satellite threats and mission opportunities for friendly, adversary, and third party-assets.

Recently, General William Shelton, commander of Air Force Space Command, said that space surveillance is the single most important aspect of space operations, that it underlies all other space activities.  If you consider that you must know the position of every trackable object in the sky to ensure that you don’t bump into it (shades of Iridium and Cosmos 2251), that you avoid it when launching or moving your satellite, and that you can find and hold an enemy’s satellites at risk, you begin to understand where his statement comes from.  Every operation, every event in space depends in some way in knowing where you are and where everything else is.

We’re going to begin our exploration of space surveillance with sensors, those radars and telescopes that find and track satellites and debris.  Right now, we won’t discuss other ways to do this – using the satellite’s own telemetry, for example.  We’ll stick to the common tools and methods.  And we’ll start with radar.

Radar stands for radio detecting and Ranging.  It has a long history reaching back to World War II.  Invented by British scientists and engineers, it was originally used to find and track German bombers enroute to Britain.  (As an aside, one of my favorite science fiction writers, Arthur C. Clarke, was a member of the team that invented and perfected wartime radar.  He also invented the concept of geosynchronous communications satellites.)  Radar was soon adapted to tracking ballistic missiles being tested and deployed.  From this grew the original space tracking capability.  MIT Lincoln Laboratories performed the very first radar space track in 1957 when the Millstone radar tracked Sputnik 1.  Tracking since that time has become standardized – although still as much a science as an art – and the resulting products, such as the ubiquitous NORAD two-line element sets (elsets) are used worldwide.

Radar is a complex technology and I won’t attempt to summarize it here.  Suffice it to say that it depends fundamentally on transmitting a radio signal to a target at some frequency, receiving same and comparing the time difference of arrival (TDOA).  There’s a lot more to it, of course – for example, the Millstone radar use a frequency of 445 MHz.  That’s 445 million radar pulses per second that are transmitted and received.  Different radars operate at different frequencies, from 10 MHz to 300 GHz.  Depending on the frequency, you can actually image an object and determine its motion (stable, tumbling, etc). 

The US operates a space surveillance network (SSN) consisting of radars and telescopes.  The figure below shows the sites.  One correction (since this is an older image):  replace SBV with Space-based Surveillance Sensor (SBSS).  The optical sites, which I’ll discuss in the next post, are Maui (MSSS), MOSS, Diego Garcia, and Socorro.  The rest are radar sites, primarily for low-Earth orbit (LEO).   The only dedicated deep space radar is the Globus II radar in Norway.

An interesting fact is the majority of the radars used for space track are also missile warning radars.  That is, in fact, their primary mission.  It also explains why they are all in the northern hemisphere – we have virtually no space track capability south of the equator. 

All these radars have undergone upgrades and replacements over the decades.  The last new radar is Eglin and it is over 20 years old.  All the sites have been modified several times, both the radar itself and the computing systems behind it that interpret and transmit the data.

Currently, space surveillance data goes several places:  the Joint Space Operations Center (JSpOC) at Vandenberg AFB in California, the Lincoln Laboratory Space Surveillance Analysis Center (LSSAC) just outside Boston, and the Dahlgren Naval Station’s Alternate Space Control Center (ASCC) in Maryland.  Each location processes the observations coming from the radar sites and develops a space catalog and, in some cases, performs more detailed analysis including some radar imaging at the LSSAC.  The penultimate destination for all this information is the JSpOC where the master Space Catalog is maintained.  We’ll discuss the JSpOC in a later post.

So there’s the beginning of space surveillance.  We’ll look at the optical sites next.