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The electromagnetic (EM) bands used for radio astronomy could be thought of as an extension of the visible band. If you were looking for a white object, finding it on a black background would be visually easier than finding the same object on a white background. This is the same for radio astronomy, if the surroundings are flooded with RFI, detection of radio waves from celestial objects would be near impossible. The following section discusses 7 steps to select, protect and reduce RFI to aid the ability of radio telescopes to receive signals from beyond our galaxy.
Inhabitants per square kilometre map. Fewer inhabitants mean less RFI
Step 1: Select a RFI quiet site
The increase in use of the EM spectrum that causes RFI can be minimized by selecting a site that has very low population density.
For this reason, KAT-7, MeerKAT and SKA-MID (and HERA, HIRAX, etc) were built on the radio quite SARAO site in the Karoo where the inhabitants are less than 1 per square kilometre.
RFI is measured by means of a spectrum analyser and a suitable antenna. Using the required equipment, the measurements shown in the plots to the left were produced.
With frequency on the x-axis and Flux (amplitude) on the y-axis, a comparison of RFI spectrums as measured at HartRAO (Gauteng) and MeerKAT (Karoo) are shown.
The noisy spectrum seen at HartRAO makes radio astronomy difficult. The MeerKAT however, only shows a few frequency spikes caused by broadcasting TV, radio-links and satellites.
Step 2: Legal Protection
The Karoo Astronomy Advantage Area (KAAA) has been declared in the Astronomy Geographic Act (AGA) and is protected by regulations. This legal protection is managed by the Astronomical Management Authority (AMA)
All contractors working on site must comply to the site information and instructions document as laid out in the induction training section.
All SARAO staff must comply to the RFI policies and SARAO site instructions
Step 3: Special Designs for Infrastructure
Any Infrastructure such as electrical systems, buildings, vehicles etc that SARAO deploys to the site can cause RFI and special designs are needed to prevent this.
These photos (left and below) show the Karoo Array Processor Building (KABP) which was built underground and includes special faraday cage rooms to contain the EMI from the data centre (servers and computers).
Step 4: Special Design for Telescope
The Radio Telescopes (Kat-7, MeerKAT, SKA-MID) can cause self-generated interference, therefore, all receivers, drive systems and data communications need to be carefully designed and qualified in an EMC chamber to prevent and mitigate any EMI produced.
Step 5: RFI Controls
All equipment deployed permanently to site needs to be tested and if it complies a RFI Certificate of Compliance (CoC) is issued
All equipment deployed to site on a temporary basis (Construction, Commissioning, Maintenance, Visitors, Vehicles) must apply for a permit. The device will be tested and based on a risk assessment a RFI permit with restrictions of where and when it can be used will be issued.
See - RFI Permit Procedure - for more information
Step 6: Ongoing monitoring of RFI on site
Mobile (Bakkie), fixed RFI monitoring and RFI scanner systems have been deployed to site to detect and track down any RFI on site.
We are watching you - don't cause RFI!
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