1、 Rep. ITU-R RA.2126 1 REPORT ITU-R RA.2126 Techniques for mitigation of radio frequency interference in radio astronomy (Question ITU-R 237/7) (2007) 1 Introduction This Report aims to provide a concise technical summary of the current state-of-the-art in techniques for the mitigation of radio frequ
2、ency interference (RFI) in radio astronomy. Specifically, this report considers techniques for the mitigation of man-made interference that originates from outside the instrument and is therefore beyond the control of the instrument operator. For the purposes of this report, the criterion for classi
3、fication of a signal as RFI is simply that it is an unwanted but detectable portion of a desired observation that has the potential to either degrade or inhibit the successful conduct of the observation. Some interference is not easily detectable but can still degrade the observations. Its mitigatio
4、n is much more difficult. The aim of mitigation techniques is to permit observation at the levels of sensitivity specified in Recommendation ITU-R RA.769, with percentage of data loss within the limits specified in Recommendation ITU-R RA.1513. These Recommendations provide the conditions for effici
5、ent observing in radio astronomy, and provide the numerical basis for calculation of tolerable RFI conditions in sharing and compatibility studies. Mitigation methods other than simple excision of RFI-contaminated data are not widely used in radio astronomy, mainly because they are not easy to devis
6、e or perform and may require the development of extensive special software. Until recently, the standard observing modes and signal processing techniques used in the course of making observations provided an inherent degree of interference mitigation that proved adequate to provide useful astronomic
7、al data in the presence of some interference. For example, “fringe stopping” in aperture synthesis imaging has the tendency to decorrelate the RFI received at widely-separated antennas, which tends to suppress the RFI in the associated correlation products Thompson, 1982. In the case of some synthes
8、is radio telescopes, such interference may result in a spurious bright source appearing in the maps at the celestial pole, making high declination observations difficult or impossible. Pulsars produce pulses of broadband noise, so a significant receiver bandwidth is needed to achieve a useful signal
9、-to-noise ratio. The noise making up the pulses is subject to frequency-dependent dispersion as it propagates through the rarefied plasmas in the interstellar medium. When observing a pulsar with a radio telescope, the pulse is deliberately de-dispersed using a combination of hardware and software,
10、to recover an accurate (non-dispersed) representation of the intrinsic pulse profile. This process tends to reduce RFI, because the process of de-dispersing the pulsar signal consequently disperses the RFI. Only limited mitigation is provided by these processes. Data are always degraded when interfe
11、rence is present. Increasingly astronomers find that the strength and temporal/spectral density of RFI is such that observations are “saturated” by RFI and made useless. Perhaps the most vulnerable observations are those made with single-dish radio telescopes (continuum or spectroscopy), because the
12、 improvement in sensitivity to astronomical signals afforded by increasing integration time leads to a proportional increase in sensitivity to RFI signals. While certain observing modes offer some intrinsic robustness to low levels of RFI, the low 2 Rep. ITU-R RA.2126 received signal strengths of co
13、smic radio emissions make radio astronomy highly vulnerable to interference. The impact of RFI extends beyond simply preventing or degrading certain observations or types of observation. It also limits the overall productivity of the radio astronomy station, making desirable observations prohibitive
14、ly difficult or expensive in terms of observing time requirements, processing complexity and operational overheads. An example is the increasing need for manual post-observation editing of data to remove RFI, as is sometimes practiced in aperture synthesis imaging Lane et al., 2005. While quite effe
15、ctive, it is difficult to automate and therefore becomes extraordinarily tedious as the observation length and observed bandwidth increase. The presence of RFI sometimes translates into dramatically increased requirements for both labour and telescope time, which is as limiting to science as is RFI
16、that irretrievably obliterates the emission being observed. These issues have motivated research into techniques for mitigation of RFI that might be considered “automatic” or “real time” in the sense that any given technique is nominally an integral part of the instrument, and operates without human
17、 intervention. This is the context in which the techniques described in the following section are presented. 2 Techniques for mitigating RFI The study of techniques for mitigating RFI contaminating the analog output of radio telescope receivers has been a topic of heightened interest in recent years
18、, spurred on by technological advances that enable real-time signal processing approaches to RFI mitigation. A helpful introduction to this area is provided in summaries of recent conferences addressing the issue; see for example Bell et al., 2000 and Ellingson, 2005. For the purposes of this Report
19、, a concise taxonomy of mitigation techniques might be organized as follows: 1. Excision, in the sense of “cutting out” RFI. For example, RFI consisting of brief pulses might be mitigated by blanking the data when the pulse is present; this is temporal excision. Alternately, persistent RFI might be
20、mitigated using array beam-forming techniques to orient pattern nulls in the directions from which the RFI is incident; this is spatial excision. A common property of all excision techniques is some loss of astronomy data, the possible distortion of the remaining data due to artefacts introduced by
21、the excision process. Since blanking is essentially a loss in observing time, there is a concomitant increase in the observing time required to reach the required sensitivity or measurement accuracy. 2. Cancellation, in the sense of “subtracting” RFI from the telescope output. Cancellation is potent
22、ially superior to excision in the sense that the RFI is removed with no impact on the astronomy, nominally providing a “look through” capability that is nominally free of the artefacts associated with the simple “cutting out” of data. However, as discussed below, the tradeoff with respect to excisio
23、n is usually that suppression is limited by the estimate of the interference received by the radio telescope. 3. Anti-coincidence, broadly meaning discrimination of RFI by exploiting the fact that widely-separated antennas should perceive astronomical signals identically, but RFI differently. In suc
24、h instances the RFI makes a contribution to the background noise level at each antenna rather than to the correlated signals. This degrades the correlated signal received, which may require an increase in the observing time to achieve the signal to noise ratio needed. Mitigation methods that are fre
25、quently or routinely used at observatories are mostly based on temporal excision, i.e. deletion of data that is believed to be contaminated by RFI. These are described in 2.1. Spatial excision ( 2.2) and methods involving cancellation ( 2.3 and 2.4) have been demonstrated using real or simulated ast
26、ronomical data, but are in most cases are under further development or used only in special circumstances. The various forms of spatial excision Rep. ITU-R RA.2126 3 generally require considerable special software and increased computer power. Anti-coincidence techniques ( 2.5) provide a very effect
27、ive means of identifying data that are contaminated by RFI, but cannot strictly be classified as mitigation since they do not provide a means of removing interference other than temporal excision. 2.1 Temporal excision (blanking) This is perhaps the oldest and best-known strategy for real-time mitig
28、ation of pulsed RFI. Interest in blanking seems to have emerged first in response to the problems encountered in observing in the 1 215-1 400 MHz band due to ground-based aviation radars. These radars typically transmit pulsed fixed-frequency or chirped sinusoidal waveforms with pulse lengths of 2-4
29、00 ms, 1-27 ms between transmitted pulses, and bandwidths on the order of 1 MHz. These pulses are often detectable through the sidelobe of radio telescopes hundreds of kilometres away. Although the transmission duty cycle is relatively low (typically less than 0.1%), accurate blanking is made diffic
30、ult by the short period between pulses. A second factor which makes blanking of radar pulses difficult is that reflections from terrain features and aircraft generate additional copies of the pulse which arrive long after the “direct path” pulse (see, e.g. appendix of Ellingson and Hampson, 2003. It
31、 is common for these multi-path pulses to be strong enough to corrupt the astronomical observation and yet too weak to be detected reliably. Thus, a blanking interval triggered by a detected pulse must typically be many times longer than the detected pulse, in order to ensure that all of the multi-p
32、ath copies of a detected pulse are also blanked. Blanking intervals having lengths up to 100s of microseconds (i.e. 10-100 times the pulse duration) are typically required Ellingson and Hampson, 2003. A number of real-time techniques for temporal excision have been proposed and developed to various
33、degrees. Friedman 1996, Weber et al. 1997, and Leshem et al. 2000, each describe methods for detecting impulsive interference and blanking the output accordingly. The National Astronomy and Ionosphere Center (NAIC) has developed a device for real-time mitigation of strong local radar pulses at the A
34、recibo Observatory (Puerto Rico). This device works by tracking the known pattern of the timing between pulses for this particular radar, and then blanking the output of the receiver in a time window around the expected pulse arrival times. More recent work in this area, including experimental resul
35、ts, is described in Ellingson and Hampson, 2003; Fisher et al., 2005 and Zheng et al., 2005, with the latter two references addressing the similar problem of pulsed interference from aviation distance measuring equipment (DME). The primary limitation of blanking is detection performance. This is bec
36、ause once an RFI pulse is detected, it can be completely removed by blanking. However, it is inevitable that some fraction of weak but potentially damaging pulses will not be detected. Over the time-scale of a single pulse, however, astronomical signals routinely have a signal-to-noise ratio (SNR) 1
37、 in order to achieve significant benefit. To achieve an output INR 1 using this method, it is usually necessary to implement some means to receive the RFI with INR greater than the INR perceived by the primary instrument. One way to achieve this (and in fact the approach advocated by Barnbaum and Br
38、adley is to use a separate directional antenna to receive the RFI. Since most large dishes have sidelobe gain that is approximately isotropic in the far sidelobe, the INR can be improved approximately in proportion to the gain of the auxiliary antenna used to receive the RFI. Thus, for example, a ya
39、gi with 20 dB gain could improve the INR available to the cancellation algorithm by about 20 dB, which could then reduce INR at the telescope output by a comparable factor. Subsequent work Jeffs et al., 2005 describes the extension of this “reference signal” approach to achieve better performance ag
40、ainst RFI from satellites by using multiple auxiliary signals from dishes with gains on the order of 30 dB. Another perspective on this problem from a more theoretical viewpoint is provided by Ellingson, 2002 in which it is found that the suppression achieved by a cancellation algorithm is approxima
41、tely upper bounded by the product of the input INR and L, where L is the number of samples used to estimate the waveform parameters, assuming a noise bandwidth equal to the Nyquist bandwidth, and is otherwise scaled by the ratio of the noise bandwidth to the Nyquist bandwidth. So, for example, to su
42、ppress a signal with INR equal to 20 dB by an additional 20 dB requires analysis of at least 10,000 Nyquist-rate samples, and proportionally more if the noise bandwidth is less than the Nyquist rate. Of course, the signal characteristics must also be stationary over this timeframe, thus this can eas
43、ily become the limiting factor. Another limitation of cancellation techniques that employ auxiliary antennas to obtain a reference signal with high INR is that such techniques can easily degrade into excision. For example, a single-dish radio telescope combined with a high gain auxiliary antenna can
44、 behave as a two-element array, with the result that the cancellation algorithm may synthesize a pattern null in the direction of the RFI, with the same consequences as those described above that are associated with null-forming 6 Rep. ITU-R RA.2126 Yet another consideration is that it is a potentia
45、lly onerous task to localize and point reference antennas for every source of RFI that affects an observation. An alternative temporal cancellation approach that avoids these difficulties is to synthesize distinct reference signals directly from the telescope output itself, by exploiting a priori kn
46、owledge of the modulation characteristics. For example Ellingson et al., 2001 demonstrated a technique for mitigation of RFI from a GLONASS satellite by partially demodulating the signal and then re-modulating the result to obtain a noise-free estimate of the RFI. They demonstrated reduction of INR
47、by more than 20 dB despite the fact that the RFI was received with INR on the order of -20 dB. In this case, the INR “deficit” was overcome by the effective increase in INR associated with the process of demodulation. It should be noted that this same technique could also be used to further improve
48、the INR by using auxiliary antennas. Unfortunately, signal modulations of the type used by GLONASS (i.e. direct sequence spread spectrum) represent only the “low hanging fruit” with respect to ones ability to obtain large INR improvements through partial demodulation. Most other signals do not exhib
49、it such large improvements with similar processing, and less can be done if the modulation is analog or has unknown structure. For example, work by Roshi, 2002 on a similar strategy for analog TV signals achieved only about 12 dB suppression despite beginning with an initially large INR, and work by Ellingson and Hampson, 2002 demonstrated suppression on the order of 16 dB against radar pulses using the estimate-synthesize-subtract strategy. In summary, while nominally more desirable than excision, temporal cancellation involves a significant risk that
