Air-borne broadband interference detection by LOFAR

Transcription

1 Air-borne broadband interference detection by LOFAR Auteur(s) / Author(s): Hans van der Marel Olaf Wucknitz Organisatie / Organization ASTN Argelander-Institut für Astronomie Datum / Date 2 May 2011 Controle / Checked: Hans van der Marel ASTN Goedkeuring / Approval: Hans van der Marel ASTN Autorisatie / Authorisation: Handtekening / Signature ASTN ASTN 2011 All rights are reserved. Reproduction in whole or in part is prohibited without written consent of the copyright owner. ASTN-FO / 10 ASTN-RP-484

2 Distribution list: Group: Others: Document history: Revision Date Chapter / Page Modification / Change April Creation May 2011 Entire document Minor textual updates ASTN-RP / 10

3 Table of contents: Summary Introduction Measurement set-up LOFAR Observation L2011_ Measured signal Analysis Discussion and conclusion Bibliography List of figures: Figure 1 Map with the operational LOFAR stations in green and the stations in construction in yellow... 5 Figure 2 Autocorrelation values in frequency vs time of station RS106. The colour indicates the autocorrelation value... 6 Figure 3 Altitude and X and Y position as a function of time (UTC)... 7 Figure 4 Fitted X and Y positions of the location of the interference as a function of time. The altitude has been fixed to m Figure 5 The reconstructed track of the interference emission (red pins) and the location of the LOFAR stations (green pins) used in the analysis... 8 Figure 6 Detail of the reconstructed track using LOFAR and the data at Casperflight. The pin with the same time stamp as the Casperflight plot is indicated (note the 1 hour difference for UTC and CET) List of tables: ASTN-RP / 10

4 Summary During a LOFAR observation on February 13-14, 2011 broadband interference was detected for part of the observation. The interfering signal was detected in the frequency range MHz between 21:23:06 and 21:51:46 UTC on February 13. Analysis shows that the interference is air-borne and based on data from open sources it has been identified to come from Aeroflot flight AFL231 from Moscow to Brussels. 1 Introduction After the inauguration of the LOFAR telescope in June 2010, scientists and engineers reveal more and more the capabilities of this unique low-frequency radio telescope. Apart from being a powerful telescope for static and variable astronomical sources, LOFAR can also act as a passive radar for near-earth objects and airplanes. During a recent observation on Taurus A (also known as the Crab nebula), emission with a peculiar signature was detected from an airplane. Olaf Wucknitz from the Argelander-Institut für Astronomie of the University of Bonn performed the analysis of the data and published the results on the website of the University of Bonn [1] and presented the results at the LOFAR Status Meeting at ASTN, Dwingeloo, The Netherlands on March 23, This report summarizes the results. 2 Measurement set-up 2.1 LOFAR LOFAR consists of a large number of antenna fields of which most are situated in the southeast of the province of Drenthe in the Netherlands, but also some abroad, in Germany, France, the UK and Sweden. Currently 27 stations in the Netherlands and 5 stations abroad are operational, while 6 more stations in the Netherlands and 3 abroad will soon become operational as well. The map of Figure 1 shows the locations of the stations. Each station has two distinct antenna types: the Low Band Antenna (LBA) operates between 10 and 90 MHz and the High Band Antenna (HBA) between 110 and 250 MHz. The HBAs are placed in tiles of 16 antennas. The Dutch stations consist of 96 LBAs, 48 HBA tiles and a cabinet with electronics, while the foreign stations have 96 LBAs, 96 HBA tiles and a cabinet with electronics. The signals of the antennas are processed by the electronics at the stations and send over a glass fibre network to the central correlator in Groningen. ASTN-RP / 10

5 Figure 1 Map with the operational LOFAR stations in green and the stations in construction in yellow. 2.2 Observation L2011_23410 Observation L2011_23410 was scheduled for 13/14 February 2011 from 14:00 until 02:00 UTC. The aim of the observation was to measure the polarisation of the pulsar in the centre of Taurus A (also know as the Crab nebula). The observation was also a long-baseline test to investigate the calibration of all stations and short baselines using the long-baseline data. The observation has been done with the HBAs; 244 contiguous subbands covering the frequency range MHz were used. There were 64 frequency channels per subband and the integration time was 1 s. In total 11 stations were used: 8 Dutch stations and 3 international stations. The station codes are: CS032, RS106, RS205, RS208, RS306, RS307, RS406, RS503, DE601, DE603 and FR606. The main beam of LOFAR was tracking Taurus A. Due to technical problems station DE601 failed and 50% of the subbands were bad. The subbands that gave good data were 0 29, 61 90, and Rev.: Date: 5 / 10 ASTN-RP May 2011

6 all bl0014 RS106HBA-RS106HBA RR freq [MHz] log10 (short-term mean) time [h] Figure 2 Autocorrelation values in frequency vs time of station RS106. The colour indicates the autocorrelation value. ASTN-RP / 10

7 3 Measured signal On all baselines between the Dutch stations strong spikes with a duration of less than 1 s were visible. They covered a frequency range of MHz in the observation and were detectable for a period of about 30 minutes. From the spectra it seems likely that the signal extends below 115 MHz. In Figure 2 the autocorrelation values of station RS106 are shown for the frequency vs time. Note that the figure shows a relative signal strength, there is no information about the absolute signal strength available. There are no known frequency allocations in the band with such a large bandwidth. 4 Analysis The analysis of the data has been performed by Olaf Wucknitz. Only data from the Dutch stations were used. For the analysis the origin for X and Y is 6.86 E longitude and 52.9 N latitude, a position close to the centre of the LOFAR core. The data from all Dutch baselines were converted to delay-spectra (i.e. signal strength as function of delay). Because of projection effects, the altitude of the emission could only be determined accurately at small distances. A limited time range when the emission was closest to the stations was scanned in X, Y and Z with 12.5 m steps) using a 6 MHz wide band. The altitude is plotted in Figure 3 as a function of time and the X and Y positions are plotted in Figure 3 as a function of time as well. Figure 3 Altitude and X and Y position as a function of time (UTC). Having determined the altitude of the emission to be approximately m, the X and Y positions are fitted within a range of ± 300 km from the centre of the LOFAR core with a resolution of 100 m. Each integration within the time range 21:15 22:00 UTC that had interference signal levels well above the average noise level has been fitted, which results in the X and Y positions as indicated in Figure 4. Outside this time window no signal has been detected. ASTN-RP / 10

8 Figure 4 Fitted X and Y positions of the location of the interference as a function of time. The altitude has been fixed to m. Figure 5 The reconstructed track of the interference emission (red pins) and the location of the LOFAR stations (green pins) used in the analysis. ASTN-RP / 10

9 Figure 5 shows the reconstructed track with all positions marked with red pins on a Google map [2]. The green pins indicate the LOFAR stations that have been used in the analysis. Figure 6 Detail of the reconstructed track using LOFAR and the data at Casperflight. The pin with the same time stamp as the Casperflight plot is indicated (note the 1 hour difference for UTC and CET). ASTN-RP / 10

10 Comparing the reconstructed track with data that is available at Casperflights [3] and taking into account that the LOFAR time is in UTC and the time in Casperflights is in CET (1 hour difference during winter time) it was found that the reconstructed track coincides perfectly with that of Aeroflot flight AFL231 from Moscow to Brussels. In Figure 6 one can see that the plane is at the same position as the source of the interference. The altitude obtained from the LOFAR measurement is in agreement with the altitude according to Casperflight. The plane has the registration VP-BUO and is of type Airbus A More information about the plane and the flight can be found in [4]. 5 Discussion and conclusion During an astronomical observation in which LOFAR was tracking Taurus A, air-borne broadband emission from other directions was detected. The track of the emission could be reconstructed and an identification of an airplane could be made. The plane has been detected from the moment that it just came above the horizon at more then 300 km to the northeast of LOFAR until about 40 km to the southwest of LOFAR. During a short period when the plane was above the LOFAR area no interference was detected. The fact that the plane could be detected better when it was approaching LOFAR indicates that the plane was transmitting primarily in forward direction. For these frequencies it is not possible to have an antenna with limited dimensions to fit into a plane that is very directive. This explains that the interference could also be detected when the plane had passed LOFAR. From communication that Axel Jessner had with Felix Butsch from DSF (German air traffic control) it appears that Aeroflot is aware of the emission. The cause seems to be a strange malfunction of air communication equipment built by Honeywell. It is under investigation by EADS and DFS. The case reported in this document shows that LOFAR can act as a passive radar, even if the beam(s) of LOFAR are pointing at astronomical objects. This property makes LOFAR a versatile tool for many types of observations, and at the same its sensitivity in many directions are a threat for astronomical observations of weak objects. Bibliography [1] [2] [3] [4] ASTN-RP / 10