How does RFI affect Radioastronomy Data?
The nature of unwanted signals, whether generated locally, or from off-site, can manifest themselves, as varying forms of RFI, dependent upon the type of observations at the time. Spectral line and continuum observing, are mostly affected by signals with either bandwidth, as in either an analogue or digitally modulated signal, with significant spectral bandwidth and energy, like an analogue modulated TV signal, or a digitally modulated communications signal(an example being Digital Radio Concentrator Service - DRCS), or, by a continuous wave (CW) type signal, and it's harmonics, usually generated by a personal computer (PC), or by signal processing equipment, with high speed digital clocking circuits, such as the samplers used in radioastronomy backend systems. Either way, these signals, have energy, may be either narrow-band, as in a CW signal, or with bandwidth, as in a modulated signal, and present themselves as unwanted emissions in the spectrum of interest to the radioastronomer, for spectral line, or continuum observations. The other type of observing, which occurs quite regularly, is Pulsar observing. This type of observation, can be affected in a completely different way by RFI, compared to spectral line and continuum observations, as described above. In Pulsar observing, minute variations in the way the RF spectrum changes with time, is of great importance, hence, small amplitude fluctuations in the RF spectrum can cause RFI. The most common form of RFI affecting Pulsar observing, is to have an amplitude modulated signal within the RF spectrum pass-band, like an analogue TV signal, or to have a broad-band noise like signal, like that produced from a DC motor with commutator ring and carbon brushes, which produces a spark everytime the brushes pass over the commutator rings. The electrical sparks produce broad-band noise, even up to L-Band (1500MHz), giving an RF spectrum, which is modulated with 50Hz, and higher harmonics, end hence, for the Pulsar observations, a modulated spectrum dominated by the RFI.
Some examples of the varying forms of RFI can be found at:
Analogue TV Signal WIN CH39 analogue TV signal from Mt Canobolas
Digital TV Signal ABC CH36 digital TV signal from Mt Canobolas
DME Beacon at L-Band Parkes Airport Distance Measuring Equipment Beacon
DRCS Signal at L-Band Digital Radio Concentrator Service
Modulation Spectrum with 50Hz Broad-Band Noise Pulsar modulation spectrum polluted with broad-band noise
