Radio Frequency Interference (RFI)

 

SARAO conducts regular RFI monitoring, both by analysing telescope data as well as independent on-site monitoring. However, some RFI may only emerge after deep integrations and may not be detected by standard RFI flaggers. Users who find any anomalies not noted below are encouraged to inform the observatory by raising a helpdesk ticket under ‘Data quality’.

Frequency occupation

Frequency ranges of the most persistent sources are described in Table 1 and subsequent figures.

Table 1: Summary of major RFI contaminated regions of the L-band range.

RFI source

Frequency range (MHz)

Digital TV (UHF)

8 MHz “rectangular” broadband:

(306 + Channel# x 8) +/- 4 MHz

GSM (Mobile phones) (UHF + L-band)

880 - 915 uplink

925 - 960 downlink

Aircraft transponders

Multiple <1 MHz bandwidth intermittent signals between 962 and 1213 MHz

GPS

L1: 1565 - 1585
L2: 1217 - 1237
L3: 1375 - 1387
L5: 1166 - 1186

GLONASS

L1: 1592 - 1610
L2: 1242 - 1249

L3: 1202.025

Galileo

E1: 1575.42

E5a: 1176.45

E5b: 1207.14

E5 AltBOC: 1191.795

E6: 1278.75

Iridium

1616 - 1626

Inmarsat

1526 - 1554

L-band statistics

There are three major contributors to RFI in the MeerKAT L-band frequency range: Global System for Mobile Communication (GSM), Distance Measurement Equipment (DME) on aircraft and Global Navigation Satellite System (GNSS) satellites.

We show an example of the typical RFI environment, as observed during a 30 hour run on primary calibrators over Christmas Day 2019, in Figures 1 through 4. The science data processor (SDP) pipeline flagger output is also shown.

Note that, in the interests of speed, the online flagger uses a very aggressive static mask (based on long term observation statistics), which masks all known transmission bands on baselines out to 1 km. This enables reasonable calibration solutions to be produced. Users may wish to disable these flags on their target fields and adopt their own flagging strategies. A second pass at flagging after applying calibration solutions also improves the detection of fainter RFI signals.

Figure 1: RFI frequency occupancy for a 30 hour continuous observation. The blue line shows flagging on core baselines (<1 km), which includes a static mask as well as outlier detection, while the orange line shows flagging of the longer baselines. Only a single polarisation (XX) is shown. The YY polarisation shows similar behaviour.

 

Figure 2: As for Figure 1, but zoomed in on the quietest portion of L-band. Galactic HI can be seen (and is often flagged by the calibration pipeline) at 1420 MHz. The GPS L3 signal is sporadic but seen fairly often.

Time dependence

Activity from aircraft transponders is at a minimum from ~23:00 to 06:00 SAST under typical operating conditions. Figure 3 is of an observation that includes this window.

 

Figure 3: Fraction of baselines flagged per scan. The time range shown here runs from 2019-12-24 22:59 UTC (bottom) to 2019-12-26 04:29 UTC (top).

Baseline dependence

The frequency ranges in Table 1 are included in the SDP pipeline static mask but are only applied on baselines shorter than 1 km (see Figure 4) since most of the RFI decorrelates on longer baselines. Note that this is an aggregate mask built up over time, and it is possible to recover more channels on an individual observation if manual or outlier-based flagging is used instead.

Figure 4: Fraction of time that baselines of a particular length are flagged. Note that the antenna separation axis is not linear. The time range of the observation is 2019-12-24 22:59 to 2019-12-26 04:29 UTC. The SDP calibration flag report also shows flags from calibration solutions. Horizontal lines of flags would correspond to baselines to an antenna that has been flagged either for a fraction of time or for the entire observation.

 

UHF statistics

There are no satellite-based transmitters in the UHF band. The primary sources of RFI are GSM, and Digital TV transmissions (DTV). All DTV transmissions are beamed away from site, with the exception of distant transmitters which are occasionally detected - they are generally transmitted over the horizon by anomalous tropospheric ducting. It is expected that the situation will improve in future as further sites discontinue terrestrial transmission.

Figures 5 and 6 below show the aggregated results of UHF (4K mode) observations carried out between 2021-02-27 to 2021-09-11, totaling about 70 hours of integration, in random pointing directions. Flags generated by the SDP ingest flagger (prior to calibration) are used for this analysis.

The very faint RFI detections are under investigation. The following frequency ranges are affected by faint but persistent RFI:

759.1 - 759.2, 798.6 - 798.9, 925.1 - 925.3, 965.5 - 965.6, 1028.8 - 1030.9, 1040.3 - 1041.8, 1062.9 - 1065.0 MHz

Additionally sporadic spikes are found at the following frequencies:

647.1, 653.1, 735.3, 741.4, 765.3, 896.6, 899.3 - 899.4 and 928.0 - 935.0 MHz.

These often result in positive / negative localized spikes in the target source spectra.

Figure 5: Fraction of the time flagged for all baselines. The grey shaded areas indicate expected RFI bands. Red solid line is the weighted median, the blue solid line is the weighted mean and the blue shaded area is the weighted standard deviation.

 

Figure 6: Fraction of the time flagged for long baselines. The grey shaded areas indicate expected RFI bands. Red solid line is the weighted median, and the red shaded area is the weighted standard deviation.

 

Figure 7: Fraction of the time flagged for short baselines. The grey shaded areas indicate expected RFI bands. Blue solid line is the weighted median, blue shaded area is the weighted standard deviation.