ACMA's Field Operations Section has many
responsibilities, including the important role of
identifying and resolving interference to
radiocommunications services. Recently, the team
received notice of interference to the new 6.8
kilometre CLEM7 tunnel in Brisbane. An ACMA team
was assembled to investigate the issue.
The CLEM7 tunnel is one of the largest
infrastructure projects ever to be completed in
Queensland. Construction of the $3 billion tollway
commenced in September 2006 and was completed seven
months ahead of schedule in March 2010. The CLEM7
opened to traffic on Monday 15 March 2010, with
more than one million trips recorded in the first
three weeks of toll-free operation.
In May this year, the ACMA's Field Operations
Section received a report about interference to
CLEM7's land mobile radio systems. The interfering
signal only seemed to occur between 9.00 am and
11.00 am for a period of exactly two hours. The
CLEM7 communicating systems operate immediately
above 403 MHz in the land mobile band.
In an effort to track down the interference,
Field Operations officers used a range of direction
finding equipment, including the new Rhode and
Schwarz PR100-a receiver used to ascertain where
the interference is coming from. The issue, they
discovered, was that every week the interference
seemed to change direction.
In June, the team attempted to triangulate the
direction of the interference to get a clear
indication of where it was coming from. The result:
a firm direction towards Morton Island, which
indicated the interference could be associated with
shipping, dredgers, wave buoys (used to measure
wave height) or channel buoys (used to help guide
shipping into busy ports).
'After researching possible licenses on RADCOM
and contacting the Brisbane Port Authority, we
received confirmation that they were not aware of
any devices that were operating in the 403 MHz
range. The direction of the interference then
changed once again,' said ACMA Field Operations
Officer, Ian Barker.
Many theories were put forward by the team,
including the possibility that a ship was moving up
and down the river creating a moving interference
vector. Another thought was that surveyors could be
moving a malfunctioning or spurious differential
global positioning system (DGPS) from location to
location. A DGPS is used by surveyors as an
extremely accurate measuring tool.
One morning as the team was monitoring the CLEM7
interference signal from the ACMA office in
Brisbane, Dhammika DeSilva, from the Interference
Management and Monitoring Section, said he had
heard a similar signal while working in Fiji.
Dhammika suggested that this interference may be
caused by a weather balloon's radiosonde
transmitter, which measures various atmospheric
parameters and transmits them to a fixed
'This information seemed to fit with the type of
interference we were experiencing,' said ACMA Field
Operations Officer Michael Cooper. 'Some quick
phone calls to the Brisbane office of the Bureau of
Meteorology confirmed that they released a weather
balloon every morning at 9.00 am with a radiosonde
attached that transmitted on a frequency of
approximately 403 MHz.'
The next morning, members from the Field
Operations team met at the Brisbane Meteorological
Weather Station and proved that the weather
balloon's transmitter was causing the interference.
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.
The Brisbane Meteorological Weather Station
advised the Field Operations team that, while they
have a spectrum frequency allocation of 400-403
MHz, the frequency of the transmitter on the
weather balloon was drifting above this range and
causing interference to CLEM7's land mobile radio
Radiosonde's weather balloons are classified as
meteorological aids. The Australian Radiofrequency
Spectrum Plan allows their operation between 400.15
and 403 MHz. However, the radiosondes being used in
Brisbane were causing interference because they
were operating above 403 MHz in spectrum allocated
for land mobile services.
This is potentially a national issue and the
Field Operations Branch is currently working with
the Australian Bureau of Meteorology to prevent any
further interference to land mobile radio