The Murchison Wide-field Array (MWA) is a radio telescope located in a remote part of western Australia and it's made up of 4,096 antennas designed to detect radio waves between 70 and 300 MHz. One of its primary goals is to detect very weak radio signals coming from the reionization when the first stars and galaxies were forming.
The MWA radio telescope is located inside a 300-kilometre-wide radio quiet zone which is supposed to eliminate all signals from terrestrial sources. This is very important because any signals from a source on Earth would likely be much stronger than any signals from the cosmos.
It turns out that the 300km wide quiet zone may not be wide enough as the array was still picking up some interference as shown below. The image shows a signal between the frequencies of 181.5–187.5 MHz, and with times between 35 and 50 s.
By analysing the signals, they were able to determine that the interference was from Australian digital TV channel 7 and the signals was being reflected off an airplane traveling at 38,400 feet (11.7 kilometres) in altitude and at a velocity of 792 kilometres per hour.
A recent study has found that aircraft are present above the horizon line at the site at least 13% of the time, establishing a lower limit on the data potentially lost to reflected interference from aircraft.
It is now hoped that this type of new interference can now be modelled and be subtracted from the final data from the observations.
A highly detailed account of how they found the interference can be found HERE
Comment... I found the above news item of interest for a number of reasons. There are similar but smaller VHF radio astronomy telescopes scattered across Europe as part of the LOFAR network and these must surely suffer from similar interference considering the higher density of aircraft over Europe and the extensive DAB radio network.
It's also a reminder that the VHF bands are certainly not 'line of sight'. There are multiple propagation modes that allow signals to be heard even briefly over many hundreds of kms.
1 comment:
Passive radar uses this exact technique, using other people's high power transmissions reflected off aircraft. It can be used to track aircraft using only a receiver. At lower frequencies it could detect stealth aircraft and potentially it can be used at embassies and covert sites to track aircraft without transmitting anything. Digital terrestrial TV may be a good source of signal as it uses high power digital pulses similar to a radar transmitter. Using Doppler you can filter out any non moving reflections so only display moving targets.
Post a Comment