SKA Aperture Arrays
A key feature of the SKA, its enormous collecting area, will be realised by complementing traditional dish radio telescope designs, which will be used in the high frequency component of the SKA with aperture arrays.
But what is an aperture array?
An aperture array (an artist impression can be seen above) is a large number of small, fixed antenna elements coupled to appropriate receiver systems which can be arranged in a regular or random pattern on the ground. A signal “beam” is formed and steered by combining all the received signals after appropriate time delays have been introduced to align the phases of the signals coming from a particular direction.
Innovative, efficient and low cost, aperture array antennas provide a large field of view and are capable of observing more than one part of the sky at once.
By simultaneously using different sets of timing delays, this “beam forming” can be repeated many times to create multiple independent beams, yielding an enormous total field of view. The ability to configure numerous beams will permit the system to look at multiple regions of the sky simultaneously, massively increasing the telescope survey speed.
The number of useful beams produced, or total field of view, is essentially limited by signal processing, data communications and computing capacity. The SKA from the outset is challenging industry and technology to the limits, with concepts and designs that, as with the Apollo program in the 1960s and 70s simply did not exist at the time of inception.
More on aperture synthesis and beam forming
Aperture arrays using substantial digital processing systems are inherently very flexible since the system can ‘trade off’ between the field of view and bandwidth and hence provide an instrument that can be matched to that required by the experiment it is set to work on.
This approach which uses essentially no moving parts is key to the design concept for the SKA, allowing rapid deployment and, hopefully, lower maintenance requirements.
What will they look like?
Used in the low and medium frequency ranges, aperture arrays comprise a large number of small radio wave receptors arranged on the ground. The design concept for the Low Frequency Aperture Array in Australia is shown above. The Mid Frequency Aperture Array is one of the SKA’s Advanced Instrumentation Programmes (AIPs), a research and development arm looking at future technologies. An artist’s impression of what it may look like when deployed in South Africa can be seen on this page.
Whereas with a traditional radio telescope radio signals bounce off the surface of a dish and are then captured at the focus, with aperture arrays the radio signals are captured when they first hit the receptor on the ground.
The signals from all the elements are then added together electronically, in phase, to synthesise reception beams, and the result is a fast and flexible system.
- For the SKA’s phased arrays the relative phases of the respective signals coming in from space arriving at the the telescopes will be varied in such a way that the effective signal from each location in space hitting the array is increased in a desired direction and suppressed in others
- The Messenger spacecraft was the first to ever use a phased array communications system.