LOPES — A LOFAR Prototype Station

Dynamic spectra

Example of a dynamic spectrumThe dynamic spectrum plots on this website display the received power by LOPES, in the frequency range from 40−80 MHz as a function of time. Every vertical line represents less than 1 ms of data, as this is the amount of data recorded for every cosmic ray trigger.

The ordinate (vertical axis) shows 4096 frequency channels with a bandwidth of 10 kHz. The analog voltage measured by the dipole antennas is digitized at a rate of 80 megasamples. That means 216 samples per millisecond with a dynamic range (bitrate) of 212 (i.e. –2048 to 2047). Fourier transforming the 216 samples in one event yields 215 frequency channels with 1 kHz bandwidth, after which blocks of eight channels are averaged to obtain the 10 kHz bandwidth mentioned above.

The intensity in the plots represents the averaged logarithmic power (abs FFT) from all antennas. No beamforming is performed here. Averaging means that all signals from the entire sky received by each antenna are averaged in the plots. The color map is logarithmic, has fixed limits for all plots and the values are in arbitrary units, as we do not have an amplitude calibration yet.

ds_031028_0_evx_a1_100000.jpg Horizontal lines at fixed frequencies indicate man-made interference or Radio Frequency Interference (RFI). The strongest interference is around 62.194 MHz, and arrives from a tv broadcasting station in Rainberg, 99 km south (155°) of LOPES. An advantage of the amplitude modulated tv video signal carrier is that it has a constant phase; it can be used to phase calibrate our antennas to improve the accuracy when pointing the telescope.

LOPES is optimized for the detection of Cosmic Ray Air Showers. Nethertheless, it is possible to observe other phenomena with it. Solar bursts, for expample, appear as bright, vertical strokes dying out towards lower frequencies as can be seen around the date of the strongest solar burst ever on October 28, 2003 (see picture on the right).

Strong lightning events also appear in our data, for example on July 23, 2004. Other narrow vertical lines exceeding the bandpass 43.6−76.1 MHz, running into the lower and upper edges in plots before December 3, 2003 were produced by asynchronities in the data aquisition process, which were eliminated afterwards.