Data from a micro-vibration monitor on board TechDemoSat-1 aids design of future spacecraft
What Is Micro-vibration?
Imagine taking a photo of something in the distance with a camera. We all know that it is 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 is to “camera shake.” This is why professional photographers use tripods to stabilize long lenses.
The same principles apply to our imagers on our Earth observation satellites in orbit. The three SSTL-300 S1 satellites we launched last year
are flying one-meter resolution imagers in an orbit of 651 kilometers—the terrestrial equivalent of standing here in Denver and taking a photo of a road in Albuquerque using a telescopic long lens, with the aim of clearly identifying the road in the photo and the cars driving on it! This is a big task, 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 micro-vibration, and it is this micro-vibration that must be minimized to ensure that the images taken by our satellites are as clear and sharp as possible.
The effect of micro-vibration on some of our spacecraft imagers is enhanced because of the way the camera takes images. Spacecraft like our SSTL-300 S1 satellites use a push-broom detector rather than an area detector like you would find in a typical digital camera. A push-broom detector captures a one-dimensional array of pixels—similar to the way a document scanner or photocopier operates; that is, the image of the ground is built up one line at a time as the spacecraft flies overhead. This graphic illustrates this concept:
What Causes Micro-vibration?
We think of space as a quiet place, free of gravitational forces, where things float around peacefully. So what is it that generates this micro-vibration? Well, the spacecraft itself in fact. Almost all spacecraft contain some moving parts. On Earth observation satellites, typically 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 micro-vibration 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 dampen vibrations.
How Do We Manage Micro-vibration?
So it’s clear that we have to understand the levels of micro-vibration generated by moving parts on our spacecraft in order to mitigate the effects on our imagers. However, we don’t want to overcompensate by building a fortress around the imagers, as we also need to minimize the size and weight of our spacecraft to keep launch costs low.
Micro-vibration management generally centers on ground-based testing and analysis. We conduct tests on the spacecraft before launch while it is still in the clean room, and then the image chain is simulated to predict behaviors 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 micro-vibration 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).
Opportunities to measure micro-vibration in orbit and better understand how the many variables change between “down here” and “up there” are invaluable, but 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 it’s not hard to imagine how quickly we seized the opportunity to fly an experimental micro-vibration monitor on TechDemoSat-1, a technology demonstration mission that we launched in 2014.
We designed a micro-vibration monitor comprising of a 16-channel, high-frequency data acquisition system and onboard data storage. We used high-sensitivity accelerometers to measure the vibrations present around key parts of the spacecraft’s structure.
In order to make the most of the experiment, we wanted to measure as many different sources of micro-vibration as possible, and on TechDemoSat-1 we had the opportunity to fly three different types of reaction wheels, rather than just one type as is usual for a typical mission. We flew our 10SP, 10SP-0 and 100SP-OC wheels, five wheels in total, with each producing a different micro-vibration pattern.
We also measured micro-vibration produced by the antenna pointing mechanism, which is shown in operation in the following video.
With this in-orbit micro-vibration monitoring system, the noise of the main micro-vibration sources is measured and the effect of space on their behavior can be better understood.
We can operate TechDemoSat-1 from our own Spacecraft Control Centre in Guildford, U.K., and we are able to interrupt normal spacecraft operations and switch on each of the micro-vibration 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 modeling and test campaigns. The results are helping further enhance the stability of future imagers under development here at Surrey.
So next time you are positioning your tripod on a stable surface in order to take a photo with your long lens, think of the optics on board the satellites passing overhead in an orbit of 651 kilometers, traveling at a speed of 17,000 miles per hour, and being disrupted by micro-vibrations from their fellow passengers on board the spacecraft!