8 RFI (Radio Frequency Interference)

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At radio frequencies there are a few narrow radio astronomy bands that are exclusively reserved for doing radio astronomy. It means that radio emissions in these bands are forbidden and RFI should not occur. Outside these bands frequencies can be allocated for many different purposes and observations can be affected by interference. Here is a summary of the RFI situation at the WSRT:

- 3.6 cm and 6 cm bands. These bands are practically free of RFI.

- 13 cm band. Some RFI, most notably in the band from 2375-2410 MHz, with a strong peak at 2386 MHz.

- L-band. Quite good, but not completely free of RFI. Harmful interference has been measured at the following frequencies: 

1227.6 continuous GPS L2
1240-1300 intermittent radio amateurs
1300 continuous relay station
1320.7 continuous radar
1381.05 intermittent GPS L3 
1575.42 continuous GPS L2
1602-1615.5 continuous Glonass satellites
1616-1627 continuous Iridium satellites
1710-1800 intermittent GSM handsets

- 49 cm band. Due to nearby TV transmissions spectral observations are difficult. The range 590-608 MHz is not usable. This band should be avoided.

- 92 cm band. During day-time lots of RFI at different frequencies, but quite reasonable during nights. Avoid the edges of the band, because of the presence of a very strong RFI source just out of the band.

- UHF-high and UHF-low bands. (UHF-low can no longer be used due to mechanical issues.) The UHF-bands are heavily used for broadcasting TV-signals and for telecommunication. The following frequencies are completely blocked for observations: 

390-395 C2000
420-430 data communication
930-960 GSM

Apart from that, between 700 and 790 MHz the occupation is so high that observations with a bandwidth of more than 2.5 MHz are hardly possible. The RFI outside these frequencies is often intermittent and usually allows observations. But be aware that the observations can contain RFI.

We are continuously investigating the RFI situation at the WSRT and trying to improve the situation. Recently modifications have been applied to the DZB backend to increase its dynamic range, thus allowing better observations close to strong RFI sources. Currently we are investigating the addition of filters to the 92 cm receiver to suppress the very strong RFI just outside that band.

In order to minimize the effect of interference, standard frequency range are available for continuum observations  (see § 5). 

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