Tunnel Vision

Recently, Australia's regulator the ACMA, received notice of interference to the 7 kilometre CLEM7 tunnel in Brisbane.

The interfering signal only occurred between 9.00-11.00am.

The CLEM7 communicating systems operate immediately above 403 MHz in the land mobile band.

Field Operations officers discovered that every week the interference changed direction.

Eventually they triangulated the direction towards Moreton Island, which indicated the interference could be associated with shipping, dredgers, wave and channel buoys.

Another thought was surveyors could be moving a malfunctioning or spurious differential global positioning system (DGPS) from location to location.

Eventually one of the team remembered hearing the same type of noise when working in Fiji and successfully identified the interference as coming from a weather balloon’s radiosonde transmitter, which measures various atmospheric
parameters and transmits them to a fixed receiver.

The Brisbane office of the Bureau of Meteorology quickly confirmed that a weather balloon is released every morning at 9.00 am with a radiosonde attached transmitting on 403 MHz. This of course explained why the position of the interference kept changing. The wind direction would be consistent for a week, for example, giving the teams great direction readings, and then suddenly change with a new wind direction.

While the met bureau have a spectrum frequency allocation of 400–403 MHz, the transmitter was drifting above this range and causing interference to CLEM7’s land mobile radio systems.

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(from the ACMAsphere 57 Newsletter, www.acma.gov.au)