Jon F. Bell, Peter J. Hall, Warwick E. Wilson, Robert
J. Sault,
\\ Rick J. Smegal, Malcolm R. Smith, Willem van Straten, \\
Michael J. Kesteven, Richard H. Ferris, Frank H. Briggs, \\
Graham J. Carrad, Malcom W. Sinclair, Russell G. Gough, \\
John M. Sarkissian, John D. Bunton \& Matthew Bailes, PASA, 18 (1), in press.
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Properties of Interfering Signals
The kinds of interfering signals we chose included terrestrial communications (television and microwave links), space-based communication and navigation systems (GLONASS, GPS, LEO satellite) and astronomical sources such as the sun. These were chosen because they either already cause problems or because they are expected to cause more problems in the future. Details of the transmissions discussed below were obtained from the Australian Communications Authority (ACA) databases (Australian Communications Authority 1998, Sarkissian 1999) , except where otherwise noted. - Sun: While the sun is an interesting source of radio waves to some, to others it is one of the most dominant sources of interference. Unlike the communications signals discussed below, it is not band limited in any sense and affects observations at all frequencies. It is particularly troublesome to spectral line and continuum observations, but pulsar observations have some immunity due to their periodic nature. In principle it can be treated as just another source of interference.
- MW links: A number of microwave (MW) links were recorded. These are digital point-to-point or fixed-to-mobile services. They do not necessarily impinge on vital spectral lines, but they are the most numerous of the unwanted signals. As an example, the 1499 MHz NSW government point-to-point MW link is a persistent source of interference due south of the Parkes telescope, causing 3-4% of the data to be discarded from the Parkes multibeam pulsar survey (Manchester et al. 2000) . The 1503 MHz microwave link from Mt Dowe (east of the CSIRO ATNF Australia Telescope Compact Array at Narrabri, NSW) is another persistent source of interference.
- MDS TV: The Multipoint Distribution Service (MDS) TV transmitter on Mt Dowe due east of the ATCA at Narrabri has strong transmissions around 2350 MHz. While these do not affect known spectral lines, they do disrupt 13cm continuum observations.
- GLONASS: Many papers in the astronomical literature cite problems with interference from the Russian Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) system of navigational satellites when trying to observe 1612 MHz OH spectral line emission. GLONASS satellites transmit at frequencies between 1602-1616 MHz and have shared primary user status with radio astronomy for the 1610.6-1613.8 MHz band (Combrinck, West, & Gaylard 1994). There are 24 carrier frequencies spread over the 14 MHz band at intervals of 0.5625 MHz. The carrier is modulated by a pair of noise-like, equal power, pseudo-noise (PN) codes of 0.511 and 5.11 MHz. The unfiltered sinc2 side lobes of these signals have relative power levels as high as -25 dB, extending out to 20 MHz either side of the main carrier in some cases (Galt 1991). GLONASS satellites launched more recently do have some band-limiting filters. Galt (1991) & Combrinck et al. (1994) both present data demonstrating the damaging effect of GLONASS signals on astronomical data. Some reports have stated that up to 50% of observations have had to be discarded (Galt 1991).
- GPS: GPS is the rather better known US equivalent to GLONASS. GPS also has a constellation of 24 satellites. In this case all the satellites transmit at frequencies of 1575, 1380 MHz and other military frequencies. Each satellite uses a different pseudo-noise code but they all contain 1023 chips (
or 
phase shifts) and run at a chip rate of 1.023 MHz. There is also an equal power 10 MHz wide military signal. The signals in the 1380 MHz band cause the most trouble for radio astronomy. This is because redshifted 1420 MHz HI emission or absorption at particular velocities may occur in the 1380 MHz region. In practice, it severely affects about 5% of the data from the Parkes multibeam HI survey (Barnes et al. 2000). - LEO Satellites: Low Earth orbit and hence fast moving satellites are likely to present significant challenges in the future. For example the parametric signal modelling technique used to suppress GLONASS signals (Ellingson, Bunton, & Bell 2000) required the carrier phase to be adjusted about once every millisecond. If such a technique was to be applied to signals from a LEO satellite, the update rate would be about 10 times faster. The LEO satellite chosen for one recording session was UO-11, designed for amateur radio experimentation at 2401.5 MHz. UO-11 orbits at a height of 674 km.
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