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Reducing "camera shake" by measuring microvibration in orbit

Data from a microvibration monitor on board TechDemoSat-1 aids design of next-gen spacecraft.
What is microvibration?

Imagine taking a photo of something in the distance with a camera.  We all know that it is really important to hold the camera still to prevent “camera shake” blurring the photo, and the bigger the lens on the camera the more sensitive it will be to “camera shake” which is why professional photographers use tripods to stabilise long lenses.

The same principles apply to our imagers on our Earth Observation satellites in orbit.  The three SSTL-300S1 satellites we launched last year are flying one metre resolution imagers in an orbit of 651km – in terrestrial terms that’s equivalent to standing here in Guildford and taking a photo of a road in Frankfurt using a telescopic long lens, with the aim of clearly identifying the road in the photo, and indeed that there are cars driving on it!  It’s a big ask, and one that requires serious attention to reducing “camera shake” in orbit.  The tiny disturbances on a spacecraft that could potentially lead to “camera shake” are generically referred to as microvibration and it is this microvibration that must be minimised to ensure that the images taken by our satellites are clear and sharp.    

The effect of microvibration on some of our spacecraft is enhanced because of the way they take images.  Spacecraft like our SSTL-300S1 satellites use a Push Broom detector rather than an area detector like you would find in a typical digital camera.  A push broom detector is a 1 dimensional array of pixels that works in a similar way to a document scanner or photocopier, i.e. the image of the ground is built up one line at a time as the spacecraft flies overhead.  If this is a little difficult to understand, here is a graphic to help explain what we mean.  

Push broom detectors



Click to enlarge. Simulated images demonstrating effects of microvibration on different types of detector


What causes microvibration?

We think of Space as a quiet place, free of gravitational forces, where things float around peacefully – so what is there on a spacecraft to generate this microvibration?  Well, the spacecraft themselves in actual fact.  Almost all spacecraft contain some moving parts – typically on Earth Observation satellites, these will be reaction wheels that spin up to control attitude, and Antenna Pointing Mechanisms that target transmitting and receiving downlinks to ground stations on Earth.  No matter how perfectly these mechanisms are built, they will always produce a level of mechanical noise, creating microvibration on the spacecraft and potentially affecting the imager.  But it gets even worse because in space, once things start vibrating they can carry on for a very long time due to the lack of any atmosphere to damp out vibrations. 

How do we manage microvibration?

So it’s clear that we have to understand the levels of microvibration generated by moving parts on our spacecraft in order to mitigate the effects on our imagers.  However, we don’t want to over-compensate by building a fortress around the imagers as we also need to minimise the size and weight of our spacecraft to keep launch costs low. 

Microvibration management generally centres around ground-based testing and analysis, where tests are conducted on the spacecraft before launch while it is still in the cleanroom, and then the image chain is simulated to predict behaviours in orbit.  Using this approach has allowed us to design very stable imaging systems in the past, but there is always an uncertainty around how things will change in orbit.  The further we push the performance of our imagers the more concerned we have to be about the potential changes between the microvibration effects measured on the ground (in the 1g environment, at atmospheric pressure, and room temperature) and the real effects in orbit (in microgravity, with no atmosphere, at vastly varying temperatures). 

However, an opportunity to measure microvibration in orbit and better understand how the many variables change between “down here” and “up there” is invaluable, and surprisingly difficult to orchestrate.  This is because on most of the satellites we design and build for customers, there are limited opportunities to fly experimental bits of kit which will add weight to the spacecraft, and also require in-orbit operations time – a valuable commodity to a customer with a space mission to run. 

A perfect opportunity 

So then, it’s not hard to imagine how quickly we seized the opportunity to fly an experimental microvibration monitor on TechDemoSat-1, a technology demonstration mission which we launched in 2014.  

TechDemoSat-1, flight ready in our cleanroom before shipment to the launch site


We designed a microvibration monitor comprising of a 16 channel high frequency data acquisition system and on-board data storage.  High sensitivity accelerometers are used to measure the vibrations present around key parts of the spacecraft’s structure.  

The left hand side of this module tray contains the microvibration monitor


In order to make the most of the experiment, we wanted to measure as many different sources of microvibration as possible, and on TechDemoSat-1 we had the opportunity to fly three different types of reaction wheel, rather than just one type as would be usual for a typical mission.  SSTL’s 10SP, 10SP-0 and 100SP-OC wheels were flown on the satellite, five wheels in total, with each producing a different microvibration pattern.  

Click to enlarge. Positioning of the reaction wheels on TechDemoSat-1


We also measured microvibration produced by the Antenna Pointing Mechanism which is shown in operation in this video.


With this in-orbit microvibration monitoring system, the noise of the main microvibration sources is measured and the effect of space on their behaviour can be better understood.

TechDemoSat-1 can be operated from our own Spacecraft Control Centre at our HQ in Guildford, and so we are able to interrupt normal spacecraft operations and switch on each of the microvibration sources in turn to take measurements in isolation, repeating measurements taken during the test campaigns on the ground, before the spacecraft was launched.  This gives us data from the ground and from orbit, and we can examine both for differences to make more accurate predictions of how firing up moving mechanisms on a spacecraft may affect the imager, and thus plan refinements to our modelling and test campaigns.   The results are helping further enhance the stability of the next generation of imagers under development here at SSTL.  

The Spacecraft Operations Centre at SSTL


So next time you are bedding in your tripod to a stable surface in order to take a photo with your long lens, spare a thought for the optics on board the satellites passing overhead in an orbit of 651 kilometres, travelling at a speed of  17,000 miles per hour (27,400km/hr), and being disrupted by microvibrations from their fellow passengers on board the spacecraft!  

Click to enlarge. One  metre image of Athens Olympic Stadium, taken by one of the DMC3/TripleSat Constellation satellites, August 2015 as it orbited at 651km above the Earth

With thanks to:
Trevor Seabrook, Senior Mechanical Analyst
Betty Cabon, Mechanisms Engineer
Guy Richardson, Mechanical Expert
 

 

 
 

 

 

21 March 20160 Comments1 Comment

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