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**Volume Title**
ASP Conference Series, Vol. **Volume Number**
**Author**
c **Copyright Year** Astronomical Society of the Pacific
Monitoring and Control of EMBRACE, a 4608 Elements Phased
Array for Radio Astronomy
Patrice Renaud1
, Christophe Taffoureau1
, Philippe Picard1
, Jean
Borsenberger2
, Steve Torchinsky1
, Henrik Olofsson3
and Franc ̧ois Viallefond2
1Observatoire de Paris, Station de Radioastronomie de Nan ̧cay, France
2Observatoire de Paris-Meudon, France
3Onsala Space Observatory, Sweden, France
Abstract. EMBRACE is a technology demonstrator for the decimetre wavelength
range of the Square Kilometre Array. As a demonstrator instrument, the primary goal is
to test and verify its merits as an SKA candidate design. For this purpose, we have de- velopped the control software for EMBRACE including the real-time control software,
the data acquisition and the observation setup systems. We have reused and adapted
the LOFAR C++ control software and implemented a similar architecture on the LCU
(Local Control Unit) computer. Station Control Unit (SCU) software provides a Python
interface to LCU for users to easily setup observation scripts for various types of obser- vation and to capture integrated data. Tests with satellites and strong radio sources are
in progress to validate the system and characterize the demonstrator.
1. Introduction
EMBRACE is a SKA Pathfinder for the mid frequencies. Two EMBRACE stations
were built,one at Westerbork in the Netherlands (G.W. Kant 2011) and the other at
Nanc ̧ay, largely financed by the European Commision Framework Program 6 project
SKADS. EMBRACE is the first large-scale demonstrator of the dense aperture ar- ray technology for radio astronomy. EMBRACE@Nanc ̧ay is a phased-array of 4608
densely packed antenna elements (64 tiles of 72 elements each). For mechanical, and
electromagnetic performance reasons, EMBRACE@Nanc ̧ay has, in fact, 9216 antenna
elements, but only one polarization (4608 elements) have fully populated signal chains.
EMBRACE is capable of real-time analog beamforming in two independant direc- tions using a LOFAR backend. Multiple digital beams can be formed inside each RF
beam. Key parameters for the Station control software are:
Nanc ̧ay Physical collecting area: 70 m2
Number of polarizations: single
Number of independant fields of view (RFbeams): 2
Max number of digital beams: 248
Tuning frequency range: 500 - 1500 MHz
Observable frequency range: 900 - 1500 MHz
Instantaneous bandwidth: 48 MHz
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The observable frequency range was reduced by a high pass filter which was installed
to eliminate high power transmissions from nearby digital television stations.
EMBRACE digital and RF processing is built from a LOFAR backend, which
provides both integrated data (statistics) and raw data (beamlets). The LOFAR LCU
(Local Control Unit) real-time control software was adapted to the specific needs of
EMBRACE. An interface layer, implemented on a Station Control Unit (SCU) com- puter, provides interaction with the LCU and to setup and run observations. Tests are
in progress to study functionalities of the demonstrator.
2. EMBRACE Processing
Figure 1. RF and Digital EMBRACE
Processing
EMBRACE@Nanc ̧ay uses a hierar- chy of four levels of analog beam- forming leading to 16 inputs to
the LOFAR backend system for
digital beamforming. The first
beamforming is of 4 Vivaldi ele- ments done on the integrated cir- cuit “beamformer chip” developed
at Nanc ̧ay (S. Bosse 2010). The
output of 3 beamformer chips is
summed together on a “hexboard”
and 6 hexboards make a tile. At
Nanc ̧ay, we have one further analog
summing stage with 4 tiles making
a tileset. The output of the tilesets is fed into a LOFAR-type RCU and RSP system for
digital beamforming. The LCU (Local Control Unit) computer calculates coefficients
for RF and Digital beamforming.
1 second integrated data (statistics data) are calculated by backend firmware for
calibration. There are 3 kind of statistics: subbands statistics (power distribution for
the total observation bandwidth -100 MHz-), beamlet statistics (power for each subband
selected and for 2 directions), and crosslet statistics (cross-correlation data between all
receiver inputs). Raw data (beamlets) are output to 3 × 1 Gbps ethernet ports.
Observation setup and integrated data acquisition is provided by Station Control
Unit (SCU) software.
3. Local Control Unit (LCU) software architecture
The EMBRACE Monitoring and Control (MAC) system is based on the LOFAR Local
Control Unit (LCU), with significant modifications and extensions added in order to
operate on the EMBRACE array. EMBRACE has a 4-level hierarchy of analog beam- forming making it more complex than a LOFAR station. In particular, the MAC must
configure the phase-shift parameters applied by beamformer chips for each of the 4096
antenna elements. The number of parameters is double the number of antenna elements
in order to configure the two independent RF beams. The LCU can also configure a
time delay parameter available on each tile (group of 72 antenna elements).
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The EMBRACE LCU permits the creation of multiple digital beams pointing to- wards astronomical sources on the sky. Beams are formed by complex weighted ad- dition of the sub-band separated signals of up-to n × 72 antenna elements, where n is
the number of tiles. The LCU can also configure the station into multiple sub-arrays.
For example, it is possible to create an array of half of the antennas and another array
consisting of the remainder.
Figure 2. Control architecture
RFBeams can be calibrated to correct com- plex electronic phase differences between
the different signal paths. The phase cor- rections computed are applied by combin- ing them with the directional station beam- former weights. The same principle is ap- plied for digital beamforming. All control
commands are synchronized to the SYNC
signal from the Station Control Operation
(SCO) subsystem which includes time sig- nals from GPS, a rubidium clock, and local
oscillators.
There are two independent control sys- tems, one for each RF beam. Each control
system is composed of independent processes, except for the the antenna tile driver
which is common to both RF beam chains, as there is only one hardware address per
tile.
The LCU is accessible by a server inside the application layer.
4. Station Control Unit (SCU) Software
Figure 3. Observation
script software components
Station Control Unit (SCU) is the interface
between the EMBRACE user and the control
software (LCU). An extensive Python pack- age library was developed for EMBRACE and
is installed on the SCU (Station Control Unit)
computer. This software gives scripting func- tionnality for users to easily setup observation
scripts for various targets and types of obser- vation. With access to the powerful function- ality of the Python scripting language, the EM- BRACE user can build complex observation
setups by using the numerous standard pack- ages provided by Python.
Integrated statistics data calculated con- tinuously by the firmware backend at 1 sec- ond intervals are acquired by the SCU from
the LCU and saved into FITS files. Raw data
(beamlets) are captured from LCU Ethernet
1 Gbps outputs by a dedicated acquisition pro- gram (for performance reasons) and saved into binary files.