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: http://www.naic.edu/alfa/ealfa/meeting1/minutes/rfi.html
Дата изменения: Mon May 8 23:01:36 2006 Дата индексирования: Sun Dec 23 01:19:13 2007 Кодировка: Поисковые слова: martian surface |
Note that the full ALFA band covers 1225 to 1525 MHz. Current allocations in the frequency range around that of ALFA include:
960-1215 MHz | Aeronautical radio navigation |
1300-1350 | GPS, Gallileo |
1350-1390 | Try to avoid, but can be used |
1395-1400 | Satellite uplink desired |
1400-1427 | Radio Astronomy |
1427- | Mobile, aeronautical stuff |
The Global Positioning System dubbed GPS is a constellation of satellites of which some are always above the horizon (5-8 satellites always available) so it's always visible. The three principle transmitter bands are denoted: L1, L2 and L5
General coverage: -158 dB. Note that:
In the surroundings of the currently protected 1400-1427 band. US has proposed a satellite system with an uplink around 1400 MHz, and a downlink around 1427 MHz. Note that the timing explorer on Space Lab was at 1427 MHz.
Note that the web site provides graphical records of the fractional occupation of the spectrum by rfi in 0.1 MHz bins, also on AO website. These show the total hours covering particular frequency range, and the fraction of that time occupied by RFI.
Karen O. | What is lowest frequency of the radar pair at Puntas Salinas? |
Tapasi G. | 1223 MHz. |
Wim vD. | There are no unique RFI mitigation algorithms; it is telescope dependent. One approach certainly is to employ radar blankers. But also, we need simple RFI removal software. GPS, for example, is not on all the time, but you need software to excise it from scans. You also need to consider: "How short should your sampling rate be, so that you can excise RFI?". The latter is critical for correlator concept. In order to assess how far out in z (low in freq) we can go, we have to know how clean is the spectrum. This also relates to the issue of maximum BW: 100 or 200 MHz bandwidth? From my experience, the spectrum seems to be pretty clean out to 1225 MHz, and so there is a case for 200 MHz bandwidth. |
Phil P. | You also need to be sure not to saturate the analog signal. For example, Aerostat 126 can blow out a digitizer. Sometimes you may want to use multiple pieces of narrow bandwidth each so you don't saturate the whole band. |
Riccardo | Wide field surveys are not going to have extremely long integration times, so they probably will not focus on detection of signals over wide bandwidth. We have to look at the tradeoffs between bandwidth coverage versus potential problems. |
Wim | The point is that it is not entirely clear. |
Wolfram F. | We also need software to excise to cross-correlate beams to remove RFI. |
Lister | Clearly cross-correlation can help. |
Riccardo | There are a bunch of questions that we need to investigate. Do you clean first in time freq domain? (Yes). Is it critical to make multiple passes? |
Lister | Median filtering for given pixel on sky is the only nonparametric method used on HIPASS. It is quite robust. It doesn't get rid of extra rms around GPS L3, though it does get rid of the L3 signal itself. |
Noah B. | Can you coordinate movement of frequency of radars? |
Riccardo | Yes, at least for the San Juan airport radars. For example, Wolfram did it all the time in order to make his Hercules supercluster observations. |