Radio Frequency Interference

RFI at the VLBA

As the VLBA pushes the recording media capabilities to higher and higher bit rates, astronomers will have access to more and more bandwidth for their observations. While that increase in bandwidth improves continuum sensitivity and allows access to more spectral lines in one observation, it comes at the cost of making the telescope more susceptible to radio frequency interference (RFI).

In addition, an ever increasing number of potential interfering sources are being created every day. New satellite-based internet providers such as SpaceX’s Starlink plan to place tens of thousands of radio-emitting satellites in orbit in the next decade. The radar necessary for emergency braking and self-driving cars, the proliferation of cellular communications and wireless data access, and the general expansion of human habitats have the potential to dramatically increase sources of RFI on the ground, as well.

Considerable effort has gone into making the VLBA's electronics as linear as possible so that the effects of any RFI will remain limited to the actual frequencies at which the RFI exists. Non-linear effects, such as receiver saturation, should occur only for those very unlikely, and usually very brief, times when the emitter is within the antenna primary beam.

RFI is primarily a problem within the low frequency (C, S, L, and the low-band system) bands, and is most serious at less remote stations. With increasing frequency and increasing resolution comes an increasing fringe rate, which is often very effective in reducing interference to tolerable levels.

The bands within the tuning range of the VLBA which are specifically protected for radio astronomy are: 322.0-328.6 MHz, 1400–1427 MHz, 1660.6–1670.0 MHz, 4990–5000 MHz, 15.35–15.4 GHz, 22.21–22.5 GHz, 23.6–24.0 GHz, and 42.5–43.5 GHz. No external interference should occur within these bands.

RFI seen in VLBA data can be internal or external. Great effort has been expended to eliminate all internally-generated RFI. Nevertheless, some internal RFI remains, which we are working hard to eliminate. Nearly all such internally-generated signals are at multiples of 128 MHz. So far as we know, all such internal signals are unresolved in frequency, and therefore will affect only a single channel.

The VLBA has installed filters to reduce RFI from known sources at some stations (e.g., the US Border Patrol radio communications channels at Kitt Peak and Fort Davis). If observers identify any persistent RFI in their data, they are encouraged to contact the VLBA staff either directly or via the helpdesk.

Examples of known strong RFI and given in the table below.

The most current RFI measurements at the VLBA stations can be found on the VLBA RFI webpage.

Satellite Transmissions and the Clarke Belt

The last two entries in the table deserve extra discussion. These are satellite transmissions, whose severity is a strong function of the angular offset between the particular satellite and the antenna. It appears that significant degradation can occur if the antennas are within ~10° of the satellite. The great majority of the satellites are along the Clarke Belt—the zone of geosynchronous satellites. As seen from the Pie Town station, this belt is at a declination of about −5.5°. There are dozens—probably hundreds—of satellites parked along this belt, transmitting in many bands: at a minimum, S, C, Ku, K, and Ka-bands. Observations of sources in the declination rate of +5° to −15° can expect to be significantly degraded due to satellite transmission.

C-band (4–8 GHz), X-band (8–12 GHz), and Ku band (12–18 GHz) are subject to strong RFI from satellites in the Clarke Belt.

The Sirius digital radio system, and probably the satellites in the 2178–2195 MHz band, comprises three satellites in a 24-hour, high eccentricity orbit with the apogee above the central U.S. For the Sirius system, the orbit is arranged such that each of the three satellites spends about eight hours near an azimuth of 25° and an elevation of 65°. The corresponding region in astronomical coordinates is between declinations 50° to 65°, and hour angles between −1 and −2 hours. Observations at S-band within that area may—or may not—be seriously affected. Due to the very long baselines of the VLBA, only one station is typically affected by RFI from these satellites at a time (although the PT, LA, and Y27/Y1 baselines may be impacted simultaneously).

Although most of the stronger sources of RFI are always present, it is very difficult to reliably predict their effect on observations. Besides the already noted dependence on frequency and baseline, there is another significant dependency on sky location for those satellites in geostationary orbit. For these transmitters (e.g., the frequency range from 3.8–4.2 GHz), the effect on observing varies dramatically on the declination of the target source. Sources near zero declination will be very strongly affected, while observations north of the zenith may well be nearly unaffected, especially at the highest resolutions.

Observing and Post Processing Considerations

The VLBA electronics, including the DiFX correlator, have been designed to minimize gain compression due to very strong RFI signals, so that in general it is possible to observe in spectral windows containing RFI, provided the spectra are well sampled to constrain Gibbs ringing and spectral smoothing (such as Hanning) is applied. Both AIPS and CASA provide useful tasks which automatically detect and flag spectral channels/times which contain strong RFI.

Extracting astronomy data from frequency channels in which the RFI is present is much more difficult. Testing of algorithms which can distinguish and subtract RFI signals from interferometer data is ongoing.

Calibration of VLBA data when strong RFI is present within a subband can be difficult. Careful editing of the data, using newly available programs within CASA and AIPS, will be necessary before sensible calibration can be done. The use of spectral smoothing, typically Hanning, prior to editing and calibration is strongly recommended when RFI is present within a subband.

Identification and removal of RFI is always more effective when the spectral and temporal resolutions are high. However, the cost of higher spectral and temporal resolution is in database size and, especially, in computing time. A good strategy is to observe with high resolution then average down in time and frequency once the editing is completed.