Survillance and Tracking for SSA

Why is there so much interest in the theme of Space Surveillance? It can be tempting to thing that this is just an overblown group of self-interested parties trying to drum up funding for obscure research efforts. The reality couldn’t be any more different than this. Over the last five years, prostate real concern about the way in which the near space environment is becoming polluted by our past activities has been growing. The collision between Iridium 33 and Cosmos 2251 on the 10th Februrary 2009 highlighted these concerns. How could an operational spacecraft be destroyed by hitting a piece of junk? The millions of dollars of investment destroyed in a millisecond was one thing, mind but the secondary effect was the increased concentration of debris in those orbital regimes – and the effect this had on neighbouring satellites’ operations.

Of course, this wasn’t the first time that alarm had propagated through the military, governmental and industrial sectors regarding the hidden threat from space debris. On the 24th of July 1996, the French Cerise satellite had a sudden attitude failure. Further investigation concluded that it had been hit by a piece of debris (later suspected to be part of a Ariane launch) and this had reduced the ability of the satellite to point itself in the right direction. This was the first confirmed conjunction between two orbiting objects.

So what are we doing about this? It’s not that we haven’t had any space surveillance activities before. The first space surveillance system (Minitrack) was in place before the launch of Sputnik in 1959. During the cold war, the capabilities to detect and track orbiting satellites increased enormously in both the USA and Russia. Of course, the idea for these systems wasn’t to detect debris, but to observe what the other side was doing and ensure compliance with the various non-proliferation treaties in place (notably the 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies – also known as the 1967 Outer Space Treaty).

As a result, both the US and Russia have highly sophisticated surveillance and tracking systems. The US system (USSTRATCOM JFCC-SPACE) has a declared detection resolution of around 10cm diameter in Low Earth Orbit (LEO). The ability to accurately track objects at higher orbits varies. The Russian
Why is there so much interest in the theme of Space Surveillance? It can be tempting to thing that this is just an overblown group of self-interested parties trying to drum up funding for obscure research efforts. The reality couldn’t be any more different than this. Over the last five years, prostate real concern about the way in which the near space environment is becoming polluted by our past activities has been growing. The collision between Iridium 33 and Cosmos 2251 on the 10th Februrary 2009 highlighted these concerns. How could an operational spacecraft be destroyed by hitting a piece of junk? The millions of dollars of investment destroyed in a millisecond was one thing, mind but the secondary effect was the increased concentration of debris in those orbital regimes – and the effect this had on neighbouring satellites’ operations.

Of course, this wasn’t the first time that alarm had propagated through the military, governmental and industrial sectors regarding the hidden threat from space debris. On the 24th of July 1996, the French Cerise satellite had a sudden attitude failure. Further investigation concluded that it had been hit by a piece of debris (later suspected to be part of a Ariane launch) and this had reduced the ability of the satellite to point itself in the right direction. This was the first confirmed conjunction between two orbiting objects.

So what are we doing about this? It’s not that we haven’t had any space surveillance activities before. The first space surveillance system (Minitrack) was in place before the launch of Sputnik in 1959. During the cold war, the capabilities to detect and track orbiting satellites increased enormously in both the USA and Russia. Of course, the idea for these systems wasn’t to detect debris, but to observe what the other side was doing and ensure compliance with the various non-proliferation treaties in place (notably the 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies – also known as the 1967 Outer Space Treaty).

As a result, both the US and Russia have highly sophisticated surveillance and tracking systems. The US system (USSTRATCOM JFCC-SPACE) has a declared detection resolution of around 10cm diameter in Low Earth Orbit (LEO). The ability to accurately track objects at higher orbits varies. The Russian

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The ESA OGS (image credit: Emmet Fletcher)

There are sometimes confusing statements made regarding the different sensors used within a Space Situational Awareness (SSA) system. Even if we are discussing radar or optical, health there are – in essence – three types of sensor: surveillance, viagra tracking and imaging. Since imaging sensors are a very special type – and do not figure in the development of the ESA SSA system, we will ignore them for this specific discussion. What we will discuss here is the difference between surveillance and tracking sensors – a difference that is often overlooked or ignored or just not understood, but is of prime concern when discussing the development of an effective space surveillance system.

Tracking Sensors

It is probably easier to start any discussion of the differences between tracking and surveillance sensors by focusing (no pun intended) on the major characteristics of a tracking sensor.

Tracking sensors usually have a very small field of view. Given a fixed detector performance, the smaller the field of view, the more precise the locations of the objects detected within this field of view (when comparing like-for-like). This is fantastic when you want to increase the precision of an object for when you already have some orbital data of, such as a piece of debris for which you have a rough orbit and may collide with an operational spacecraft. You just take this rough orbit and set your tracking sensor to point along this orbit at the position you think the debris should be. When you see the debris, you can then create a more precise orbit – since your detector is looking at a very small region of space and so has a high precision.

The problem is – of course – that since you only see a small area of the sky, if the error on your rough orbit is too high, you might not see the debris at all (it might slip by outside your field of view). It also makes these sensors very inefficient (read: almost useless) for the build-up a catalogue of objects. Since the view is small, it is difficult to trap new objects, unless you are very lucky. Even then, given the small view, you only have a very short reading as the debris passes across the sensor. This results in an initial orbit guess (orbit determination) which can have very high errors. For the development and maintenance of a catalogue, we need a surveillance sensor.

Examples of European tracking sensors: TIRA (Germany), BEM Monge (France), OGS (ESA), CAMRA (UK)

Surveillance Sensors

The TFRM (image credits: Emmet Fletcher)

A surveillance sensor is the workhorse of a surveillance system. It provides the data for both the initial catalogue development (the so-called ‘cold start’) as well as the day-to-day maintenance of the catalogue.

The main difference between the tracking and surveillance sensor is that the surveillance sensor sees a very large area of the sky at the same time. It is also not actively looking for objects, but rather passively (which counter-intuitively can be active) waiting for debris – any debris – to pass over it. Once it detects something passing over it, the data related to this pass is processed and passed to the catalogue maintenance system.

In this way, the surveillance sensor creates a ‘fence’ which is triggered by any object passing through it. No prior information is needed by the sensor to generate new data regarding a specific debris object and the system therefore does not need to be ‘tasked’ to look out for an object. In reality, the fence can also be generated using an active sensor scanning the sky with a frequency that ensures nothing will be missed. This is the case for radar systems which quickly scan across a path. It doesn’t look in all directions at all times, but still forms an effective fence.

Through the use of surveillance sensors, a catalogue can be built up. The precision of this catalogue will not be very high initially, although the design of the surveillance network should be such that the eventual precision using just the surveillance assets will be enough to give a reliable warning of potential collisions with operational satellites. When the warning is triggered, then comes the turn of the tracking sensors to refine the orbit of this debris and provide the precise information that satellite operators need to plan their manoeuvres.

Examples of European surveillance sensors: GRAVES (France), RAF Fylingdales (UK/US), TFRM (Spain)

If you have any comments, clarifications, corrections or suggestions – please comment!

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