Emmet Fletcher

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, there are – in essence – three types of sensor: surveillance, 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 focussing (no pun intended) on the major characteristics of a tracking sensor.

Tracking sensors usually have a very small field of view – rather like when you use a domestic telescope and you can see a very small area of the sky. 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 maintainence 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 maintenence 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 counterintuitveilly 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 maintainence 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 maneouvres.

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

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