The LOFAR Phase 1 Baseline consists geographically of a Compact Core area and 45 Remote Stations. Sensor fields at the Remote Stations and in the Compact Core will be equipped with 100 High Band Antennas, 100 Low Band Antennas, 13 three-axis vibration sensors (geophones), three microbarometers (for infrasound detection) and several auxiliary systems e.g. for weather monitoring and GPS time/position measurements.

The Low Band Antenna element The Low Band Antenna element is optimised for the 30-80 MHz range, with a sharp cut-off filter above 80MHz. The suppression below 30MHz is matched to the environment. The antenna LNA will only suppress these frequencies to the extent needed to maintain linearity over the full band. The receiver will have a separate filter for observations below 30MHz that matches the signals to the available dynamic range in the ADC to allow night-time observing for 80% of the night-time. An alternative design would be not to suppress the band further in the receiver and accept a further reduced duty cycle (possibly down to 10%). The Low Band Antenna elements can be used down to 30 degrees elevation.

The High Band Antenna
The High Band Antenna can be used between 120-240 MHz. The FM band is suppressed in the antenna amplifier for both antennas to minimize intermodulation products from FM transmitters. This antenna is a composite system consisting of 4x4 dual-polarization dipoles and a dual-polarization RF beamformer.

Antenna signals
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 2 Gb/s. The secondary filtering stage (to 1kHz channels) is done in the Central Processing system.

Geophysical sensors
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 13 geophones at a Remote Station generate a 2.4 Mb/s datastream.

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.

In the Compact Core, with ~2km diameter, a total of 3200 High Band and 3200 Low Band antennas are placed in 32 (sub)stations; the number and configuration of geophones and microbarometers is yet to be defined.

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