Reasons to Conduct SSA from Orbit


Historically, the majority of the tracking of objects in Earth orbit has been conducted by “remote” sensors, based on the ground. The catalogues on which we currently rely for space traffic control are, in the main, based on data from telescopes and radars on the Earth’s surface. Space tracking is, therefore, something that is primarily “done to” space objects; rather than being “done by” space objects.

And the fact that space tracking is considered a hard problem, with a number of well-known capability gaps, is thus hardly surprising…’s really hard to detect and track centimetre-sized objects travelling rapidly across the sky from several hundred kilometres away.

One of the solutions to this issue may be to move a proportion of our space surveillance capability into orbit. This transition would have a number of potential benefits:-

  1. For a start, it’s much closer – small debris is easier to detect for optical, radar, and bi-static sensor systems as a result of the significantly reduced range to the target objects.
  2. It is also reasonable to assume that, in some instances, it might be possible to acquire resolved images of target satellites. This would provide configuration information that would help to confirm the identity of the objects, and also data on their rotational motion, both of which are rarely available from ground-based sensors. This is important information for the planning and safe completion of future rendezvous and proximity operations, servicing missions and debris removal operations.
  3. The frequency of tracking opportunities should improve, as there are no clouds to obstruct the line of sight to the targets, and the day-night cycle is less of an issue for optical sensors. In some cases, it may even be possible to “shadow” a target object in a similar orbit for a period of time, and certainly there should be opportunities to collect data on satellites when they are over the extensive (ocean and southern hemisphere) regions where there are very few terrestrial sensors
  4. Space-based sensors can use wavelengths for passive surveillance which don’t penetrate the Earth’s atmosphere. UV sensors would offer higher resolution imagery for a given size of aperture, and IR sensors could be used for the detection of thermal signatures. Both could also exploit spectral techniques to provide information on the material composition of an orbiting object.
  5. Propagation issues would also cease to be a problem for very high frequency, very wide bandwidth radars for active surveillance. SAR systems operating at frequencies above 100 GHz would not propagate far through the Earth’s atmosphere, but would work in space. The wide signal bandwidths available at these frequencies would potentially allow very high-resolution images of the target objects.
  6. The operational status of satellites is traditionally hard to determine remotely, but IR sensors could indicate whether an object is still being thermally controlled, and RF sensors can potentially get close enough to pick up the sidelobes of a directional antenna’s transmissions.
  7. In situ measurements of space weather are obviously a key element of the SSA data that can be collected in space, and there is now an ESA mission proposal to explicitly investigate the very small, (sub-centimetre), debris population using a space-based detector.
  8. And in a more cooperative future catalogue maintenance scenario, operator data collected by on-board GNSS receivers and then fed into the tracking network would, quite naturally, be collected in space too.

And a final benefit of moving to a space-based collection system is that the assets in orbit will need to determine their own positions and orientations accurately in order that they can make precise measurements. This would entail using technologies such as laser ranging systems for orbit determination, and the space-based surveillance satellites themselves could then function additionally as calibration targets for the ground-based sensor network.

[Image credit: Spaceflight Now]

Comments (2)

This is an interesting set of advantages for in-orbit SSA, so I have some questions for thought,
(1) What about different orbital planes? Does proximity also have the negative consequence of higher relative angular velocities so less time spent in the field of view of a sensor?
(2) Resolved images are of course interesting but what really prevents knowledge of rotation from ground-based sensors? Is it the lack of suitable measurements and trying to fit models to inadequate data? (Doppler broadening?)
(3) This is an excellent point and I’ve always wondered if quantum ghost imaging is an option but have never come up with a feasible solution for the high O.D. through clouds. (There is some excellent work on reciprocal coherent scattering theory however.)
(4) IR certainly would benefit from not requiring dry, clear sites to prevent the CO2 bands, for example, being swamped by scattering, and UV is an interesting option…although doesn’t optical have sufficient resolution already…is the gain so large moving to this difficult wavelength where array detectors aren’t so easily available?
(5–7) I can’t comment on and
(8) I don’t quite understand the benefit from in-space vs on-ground GNSS.

I have wondered about the use of High-Altitude Platforms in the stratosphere for near-the-ground-to-space applications.

Hi Nazim

1. Well, it depends in which direction you look. By analogy, cars on the same lane of a motorway have low relative velocities, whereas cars on the opposite carriageway pass very quickly….
2. Nothing “prevents” it, but there are potential ambiguities because some rotations take longer than an average pass to complete
3. Thanks!
4. The answer depends on what resolution is required – to answer some questions, that resolution can be very high indeed….
8. A satellite in space can measure its own position if it has an on-board GNSS receiver – SSA does not have to be exclusively conducted by “remote sensing” methods like radars and teelscopes

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