Currently, LOFAR consists geographically of a core area, remote stations and international stations. Sensor fields at the remote stations and in the core will be equipped with 48 High Band Antenna tiles, 96 Low Band Antennas, microbarometers (for infrasound detection), and several auxiliary systems e.g. for weather monitoring and GPS time/position measurements. The outermost stations will also be equipped with three-axis vibration sensors (geophones). There will also be locations that only will be equipped with microbarometers and geophones out to distances to the core of about 50 km. The international LOFAR stations placed outside the Netherlands will have 96 Low Band Antennas and 96 High Band Antenna Tiles.
Two types of geophysical sensors are used. Three-axis vibration sensor
(geophone) will be placed 10m below the surface to get below the
ground-water level. The geophones are passive elements connected
through 2x3 signal lines. Signals are digitised in a special
sample-unit using an 0.5 ms samplerate at 24 bit. Data are transported
as 32 bit words, so the geophones at a Remote Station generate a 2.4
Also, KNMI microbarometers are used. These are housed in 0.5 m diameter tubes, placed just below the surface, with six tubes connected to suppress local noise. The microbarometer is read-out by a standard PC-based data-acquisition card. The sampling rate is roughly 120 Hz at 24 bit. The datarate for a single microbarometer is 3 kb/s.
Agriculture sensor systems will be installed near selected fields, details for these sensors are yet to be defined.
The LOFAR radio telescope
The Low Band Antenna (LBA) element
LBAs sample the frequency range between 10 and 80 MHz and consist of simple dual polarization (X and Y) droop dipoles above a conducting ground plane with the wires at an angle of 45 degrees with the ground. The field of view of an LBA extends to the horizon.
The LBA has been designed to actively suppress signals above 80 MHz (the FM-band). The configuration resembles a band pass filter where the low frequency corner is far below 10 MHz. Below 30 MHz a best effort has been made. There the performance is mostly limited by the radiation characteristics of the dipole.
The High Band Antenna (HBA) Tiles
HBAs sample the frequency range between 120 to 240 MHz. They are assemblies (tiles) of 16 bowtie shaped dual dipole antennae arranged in a 4x4 grid with a spacing of 1.25 m between the dipoles. Each HBA tile is equiped with an analogue radio frequency (RF) beamformer, which limits the field of view of an individual tile to approximately 30 degrees full-width at half-maximum (FWHM) at a frequency of 150 MHz. The tiles itself are placed at a distance of 15cm from each other within a station. The FM band is suppressed in the antenna amplifier for both antennas to minimize intermodulation products from FM transmitters.
The antenna signals are handled by a broad-band integrated receiver and digital processing system. The receiver uses direct conversion of a 100 MHz band. Each receiver is connected to a Low Band and a High Band antenna; only one of these can be selected at a time. The 100 MHz signal will be buffered for ~1 sec for Cosmic Ray detection and Transient Processing.
In the first digital processing step 256 kHz subbands are formed. Only a subset of these bands is further processed. The maximum total bandwidth selected for further processing will be 32 MHz. Each Remote Station delivers a single dual polarization beam at 32 MHz, or 8 dual polarization beams at 4 MHz or any combination in between. The resulting output data rate is slighter higher than 2 Gb/s. The secondary filtering stage (to 1kHz channels) is done in the Central Processing system. Currently, it is investigated if the system can also fully support a bandwidth of 48 MHz.
In other words, in the stations the electromagnetic field received by the station antennas is converted into signals with spatial and spectral sensitivity in the digital domain. Two antenna systems are used to be able to observe in two distinct frequency bands. Both antennas include amplifiers to drive analog connections to the signal processing chains. The compound high band array also includes an analog beamformer. The signal processing chain starts with amplification and analog filtering of the antenna signals after which the analog signals are converted to the digital domain. The signal processing chains continue for each receiving element (antenna) with digital filters after which coherent spatially sensitive signals are created in the beamformers. The station output is a time series of voltage beams, although intermediate signals can also be made available for specific observations.