Discussion

In Section 4 we analysed the expected sensitivity of the telescope to interference as a function of frequency by assigning a nominal value of 1 to the remote sidelobe telescope gain. The experimental tests have shown that this gain ranges typically from 0.01 to 1 and may be higher in directions where grating beams are formed. Thus Figure 2 may be used to define the interference tolerance of the MOST.

The site for the telescope was originally chosen hoping that the surrounding hills would provide a measure of protection from radio interference. The interference tests with the SMA have incidentally provided an opportunity to check the level of protection afforded. By comparing the measured path losses for the four test sites with those calculated for the same distance over a smooth Earth, we find that the hills provide about 20dB of additional protection.

The SMA has independently analysed the data from the tests and come to similar conclusions about the sensitivity of the MOST to local in-band transmitters: for the worst case the maximum tolerable irradiance is -184dBWm (-174dBm in an isotropic antenna). The SMA have used the data from the collaborative tests and path loss calculations to establish criteria for restricting the future use of the 825-845MHz band. Their report (SMA 1997) includes a table of permitted radiated powers, which would not cause detectable interference, as a function of distance and azimuth from the MOST, antenna height and frequency. As an indication of the power limits implied by the table we quote three examples. For a transmitter located 38km north of the telescope (antenna height of 10m) the maximum radiated power (EIRP) is 5W. The corresponding maximum allowable power at 66km is 50W and at 99km is 1mW. The SMA recommendations, if adopted, would therefore rule out the use of mobile phone transmitters in the MOST passband throughout the Canberra and Queanbeyan regions.

The Australian Communications Authority (ACA), which has subsumed the Spectrum Management Agency, has used the SMA report as the technical basis for Attachment 9 of the Draft Marketing Plan for the PCS Spectrum Auction (www.aca.gov.au/spectrum/auction/pcs). Attachment 9 entitled Radiocommunications Advisory Guidelines (Protection of Molonglo Observatory Synthesis Telescope) 1997 sets out the compatibility requirement that would provide the MOST with a reasonable level of interference protection from transmitters. A suggested approach to assessing the compatibility is also provided. Spectrum licences in the relevant bands will require that operation of transmitters under the licence must not interfere with the MOST. This requirement to protect the MOST will cease at the end of 2008.

We believe that the ACA guidelines would adequately protect the MOST provided they are strictly followed. After the sale of the spectrum it will be the responsibility of the University to monitor interference at the observatory site and, in the event of any problems, to negotiate directly with the users of the bands. In such negotiations the ACA would be prepared to act as a paid consultant.

At the same time the spectrum sale is proceeding, the DCA is setting up a Review of Spectrum Management Legislation to be completed by 30 June 1998 (www.dca.gov.au/whatwedo/govtrev.html). The terms of reference of this review make no mention of scientific uses of the spectrum.

© Copyright Astronomical Society of Australia 1997