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Jan David Mol authoredJan David Mol authored
Beam
The beam == DeviceProxy("STAT/Beam/1") device sets up the beamforming on the station. It configures the HBA tile beam former, which adds the signals of its 16 elements within the tile. These element signals can be delayed a configurable amount of time, allowing their sum to become more sensitive in a certain direction. Each tile can have their own pointing direction configured.
Beam Tracking
Beam tracking automatically recomputes and reapplies pointings periodically, and immediately when new pointings are configured. It exposes the following interface:
| HBAT_tracking_enabled_R: |
Whether beam tracking is running.
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| HBAT_pointing_direction_RW: |
The direction in which the beam should be tracked for each antenna. The beam tracker will steer the beam periodically, and explicitly whenever the pointings change.
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| HBAT_pointing_direction_R: |
The last applied pointing of each antenna.
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| HBAT_pointing_timestamp_R: |
The timestamp for which the last set pointing for each antenna was applied and set (in seconds since 1970).
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Beam Steering
The beam steering is responsible for pointing the beams at a target, by converting pointings to recv.HBAT_bf_delay_steps. The beam steering is typically controlled by the beam tracker. To point the antennas in any direction manually, you should disable beam tracking first:
| HBAT_tracking_enabled_RW: |
Enable or disable beam tracking (default:
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| HBAT_set_pointing(pointings): |
Point the beams towards the specified
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Computation Parameters
To form a beam from a set of antennas in a certain direction, the signals coming from each antenna needs to be aligned, such that signals coming from that direction become coherent. In tile beam forming, this is accomplished by proportionally delaying the signals from the antennas that are closer to the target.
To compute the required delays, both the direction of the source and the position of the antennas are required. Furthermore, a reference position is used, a virtual position towards which the antenna signals are aligned. The reference position is typically in the center of the set of antennas.
HBA positions
For the tile beam, these positions are stored as follows in the recv device:
| recv.HBAT_antenna_itrf_offsets_R: |
The relative offsets of the elements within each HBA tile with respect to its reference position, in ITRF.
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| recv.HBAT_reference_itrf_R: |
The absolute reference position for an HBA tile, in ITRF.
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NB: A LOFAR antenna field is constructed such that all tiles lie within the same plane, instead of following the curvature of the Earth. This means that the relative offsets of the elements with an HBA tile in ITRF is the same for every tile in the antenna field.
HBA delays
Each HBA tile is steered by configuring the number of delay steps to introduce for each element.
Direction information
We use python-casacore to compute the direction of a given pointing with respect to our antennas and reference positions. Casacore in turn uses measures tables for the precise measurements of celestial positions, geodetical information, and time calibrations (f.e. leap seconds). These tables need to be installed and periodically updated to maintain the pointing accuracy:
| measures_directory_R: |
Directory of the active set of measures tables. The directory name includes the timestamp denoting their age.
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| measures_directories_available_R: |
List of installed sets of measures tables.
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| download_measures(): |
Download (but do not activate) the latest measures tables from ftp://ftp.astron.nl. Returns the directory name in which the measures were installed.
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| use_measures(dir): |
Activate the measures tables in the provided directory. This necessitates turning off and restarting the Beam device, so the command will always appear to fail. Turn the device back and the selected measures tables will be active.
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