Radio Frequency Interference (RFI)
SARAO conducts regular Radio Frequency Interference (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 MeerKAT Service Desk ticket under ‘RFI’. With the MeerKAT extension work that commenced in 2023, RFI has continued to pose a challenge for MeerKAT telescope operations. To describe the changing RFI environment the RFI statistics reported are generally for the period August 2024 to April 2025. The RFI statistics below are a representation of both environmental factors and instrumental.
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 in the frequency range covered by the MeerKAT receivers.
RFI source | Frequency range (MHz) |
Digital TV (UHF) | 8 MHz “rectangular” broadband: (306 +Digital TV Channel# x 8) +/- 4 MHz 768 - 778 Vodacom downlink 801 - 811 MTN downlink 811 - 821 Telkom downlink |
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 |
GLONASS | L1: 1592 - 1610 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 |
Globalstar | 2483.5 - 2495.0 |
Wi-Fi* | 2400 - 2495 |
Bluetooth* | 2400.0 - 2483.5 |
The GSM uplink, Wi-Fi and Bluetooth bands should be clear of RFI on the MeerKAT site and are rarely seen. We request that users who do note RFI in these bands report the frequency range and date and time by raising a MeerKAT Service Desk under ‘RFI’.
RFI statistics for MeerKAT
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 1–3 below show the aggregated results of UHF (4K mode) observations carried out between 2024-12-24 10:01:39.381 to 2025-04-12 12:26:17.633, totaling about 51 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: 647.1, 653.1, 875 MHz.
These often result in positive / negative localized spikes in the target source spectra.
Figure 1: 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 2: 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 3: 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.
SARAO has a couple of in-house tools to monitor and analyse RFI, one particular tool, which will be used here for environmental RFI patterns is KATHPRFI framework. This is a framework that uses offline Data Science Processer (DSP) flagger on MeerKAT Visibility Format (MVF) datasets. In general, the framework works by constructing MASTER and COUNTER arrays, these are multi-dimensional arrays of time, frequency, baseline, elevation and Azimuth for a chosen MeerKAT science observation. For a given MeerKAT science observation, a descriptive statistics about the RFI is contained in the mentioned arrays. The MASTER array for a given dimension counts the number of RFI samples per voxel and COUNTER counts the total number of observations per voxel. For compactness and relevance, in our case we focus only on time and frequency dimensions.
We start by consolidating the RFI statistics for the period of August 2024 to December 2025 (this amounts to approximately 760 total observation hours) for time of the day dependence and frequency band dependence.
First, we consider time-binned fractional RFI flagged for multiple science observations over a given month: Let us provide a precise mathematical description of the computation: Let be a matrix representation of hourly fractional RFI flagged values for a given observation:
where is fractional RFI flagged value of an observation x at hour h for month m.
Now for each monthly , an hourly median is computed to get the robust RFI representation for a given month m. This can be formulated as follow:
Now, we then determine the final profile by smoothing variability across the month by averaging the medians across all the months M as follow:
Figure 4: The above plot presents the fractional RFI flagging (using cal_rfi flags) for the period August 2024 to April 2025, focusing on UHF horizontal cross-pol observations. The flagged RFI IS shown as a function of time of the day (UTC). We observe that RFI levels are relatively high in the early hours then remain relatively low in the late mornings to early afternoon, then rise again irregularly in the afternoon towards evening across all the months.
Figure 5: The above plot presents the fractional RFI flagging for the period August 2024 to April 2025 UHF horizontal cross pol observations, shown as a function of Frequency in MHz. The frequency spectra above appears consistent with the MeerKAT frequency occupation ranges table.