Parkes RFI Studies - Airport DME Beacon
Reference Fault Report - 1433:RFI in pulsar multibeam system
The RFI reported in 1433, manifested itself as strong interference in the modulation spectrum, with significant spectral components at 675Hz, and 1350Hz, of which, a snapshot of the RFI is shown below.
Investigations conducted at the time, included viewing the IF spectrum (60-360MHz) in the control room, of which, a snapshot is shown below. It was noted that the only obvious items were the DRCS signal at ~80MHz, which, due to the down-conversion frequency, is the known signal at 1500MHz, and has never produced RFI, like the effect seen in the modulation spectrum. The only other noted signal, was the airport distance measuring equipment (DME) beacon, which, has been known since the receiver system was installed. It appears at the notched position in the IF spectrum. During the design phase of the Multibeam receiver system, the DME was known as a source of interference, given it's frequency of operation, 1168MHz, and with the Multibeam receiver Pulsar survey covering the range, 1200-1500MHz, adequate steps were taken, to minimise it's effects, by providing notch filters to remove most of the signal, at the edge of the band. Further investigations included the placement of the IF spectrum in a square law detector, and feeding the resultant signal into the DC amplifiers, and feeding a voltage to a spare loud speaker, in the control room. It was then noted that a signal, appeared evey 40 seconds, with a series of beeps and pauses, which coincided with the strong signal in the modulation spectrum at 1350Hz. The more astute members performing the investigation, namely Dr. Dick Manchester, and Dr. John Reynolds, noted that the beeps and pauses, were in fact a morse code signal, signifying PKS, ie, Parkes, implicating the airport DME. Further examination of the notch filters in the RF module of the Multibeam receiver, showed that a significant number of filters had changed from their designed notch centre frequency of 1168MHz, some by as much as 4MHz.
Consultations with the Pulsar's observations project leader, Dr. Dick Manchester, showed that the project team had been aware of the RFI sequence (a 135Hz comb, with strong peaks at 675Hz and 1350Hz), since early in the Multibeam Pulsar survey, and the survey analysis system had a software filter to remove it from the modulation spectrum, however, this filter did not completely eliminate the RFI when it is strong. When the RFI was first reported, the Pulsar survey had established a new set of observations, which required the radiotelescope, to search that part of the sky, which forced the underside of the focus cabin, to be in such a position, for there to be a clear line-of-site view from the underside of the cabin (and hence, access to the Multibeam receiver feed system), to the Parkes airport. This, coupled with the now out-of-tuned notch filters, allowed significant power into the receiver chain, when the beacon was at it's most active, during the 40second pulse phase, as well, during the normal phase of operation, during interrogation.
At the Parkes airport, the DME beacon, used by aircraft, for on-route navigation, provides slant range distance of an equipped aircraft from an equipped ground reference point. The beacon, located at a frequency of 1168MHz, from an allocated spectrum for such aids of 960-1215MHz, has pulse coding as the method of modulation. Subsequent discussions with technical personnel at Air Services Australia (ASA), showed that the DME has a tone identification signal transmitted at a repetition rate of 1350 pulse pairs per second (pps), with a signal characteristic associated in the form of dots and dashes (International Morse Code), with the beacon identity pulses at least once every 40seconds. The existance of a 135Hz comb, probably derives from the fact that the master transmitter operates at 1350Hz, and that there are subharmonics of that reference, 135Hz, 270Hz, 405Hz, 540Hz, 675Hz,....,1350Hz, as evidenced in the modulation plot shown below.
In order to mitigate the effects of the DME beacon, on future Pulsar observations, using the Multibeam receiver, and Pulsar Filterbank backend system, a number of options were investigated. From careful analysis, taking into consideration the effects of the Multibeam RF module bandpass filter shape, and the attenuative effects of the Multibeam feedhorns when operated below their cutoff frequency, it was decided to propose to the ACA, that the DME beacon frequency, be lowered to a much lower frequency, in the allocated licensed spectrum, for such services. ACA advised that the lowest possible frequency, for the DME beacon, in the existing frequency allocations, was around 1018MHz. Careful analysis from the ATNF Engineering Receiver Group, showed that such a frequency would provide better than 25dB of extra attenuation on the DME beacon, compared to the existing beacon levels for correctly tuned notch filters at 1168MHz, and 40dB of extra attenuation on the DME beacon, compared to the existing beacon levels for the out-of-tuned notch filters. The beacon has subsequently been changed, to the lower frequency of 1018MHz, and as such, the modulation RFI has been excised. Snapshots of the beacon at the new frequency of 1018MHz are shown below, measured from the RFI Monitoring Tower, using equipment particular to RFI monitoring.