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W S T SKA
SKADS C 2009
S.A. Torchinsky, A. van Ardenne, T. van den Brink-Havinga, A.J.J. van Es, A.J. Faulkner (eds.)
4-6 November 2009, Chateau ˆ de Limelette, Belgium
Profiling the EMBRACE tile beam using GPS satellite carriers
A.O.H. Olofsson1,2
, S.A. Torchinsky
3
, L. Chemin1
, S. Barth3
, S. Bosse3
, J.-M. Martin1
, W. Paule3
, P. Picard3
,
S. Pomarede `
3
, P. Renaud3
, C. Taffoureau3
, G.W. Kant4
, J.E. Noordam4
, S.J. Wijnholds4
, R. Keller5
, and
S. Montebugnoli6
1 GEPI, Observatoire de Paris, 5 place Jules Janssen, Meudon, France
2 Onsala Space Observatory, Chalmers University of Technology, SE-439 92 Onsala, Sweden
3 Station de radioastronomie de Nanc ̧ay, 18330 Nanc ̧ay, France
4 ASTRON, Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, The Netherlands
5 Max-Planck-Institut fuer Radioastronomie (MPIfR), Auf dem Huegel 69, 53121 Bonn, Germany
6
I.N.A.F.-I.R.A., Via Fiorentina 3508/B, Medicina (BO), Italy
Abstract. The L2C carriers of multiple GPS satellites have been used to trace out a nearly complete beam pattern out to 45◦
away from the main lobe centre for a horizontally mounted single EMBRACE tile. The beam was formed along its bore-sight
direction, i.e., staring at the local sky zenith. The result is very close to design specifications although there is evidence for at least
one side lobe rising above the achieved noise level. We have also used the older L2 carrier to estimate the system temperature,
although an exact figure in addition requires knowledge of the aperture efficiency.
1. The EMBRACE instrument
The dense aperture plane phased array system called
EMBRACE is the Electronic MultiBeam Radio Astronomy
ConcEpt. Densely packed Vivaldi antenna elements are organ- ised into tiles each consisting of 72, single-linear polarised
elements. There are two arrays being constructed. One in
Westerbork consisting of 144 tiles and one in Nanc ̧ay with 80
tiles. A prototype tile is undergoing characterisation testing in
Nanc ̧ay.
EMBRACE is a prototype being designed for developing
technology needed for the mid to low frequency portion of the
Square Kilometre Array (SKA) project. For more details about
EMBRACE see in these Proceedings Picard et al. (2010);
Wijnolds et al. (2010); Berenz et al. (2010); Monari et al.
(2010); Bianchi et al. (2010)
2. GPS – a brief introduction
2.1. Frequencies
The GPS satellites transmit at a number of frequencies and
while the primary carrier is at 1575 MHz, it is the L2 signal
at 1227.6 MHz which is the most useful for EMBRACE test
observations. At this frequency (λ=24.421cm), the largest di- mension of the tile RF beam is about 15◦
if the instrument is
approximated with a traditional dish with a diameter equivalent
to the tile side of 1068 mm. The signal is right-hand circularly
polarised which is advantageous since the projection onto a lin- early polarised receiver does not change over time.
2.2. Orbit
The GPS satellites are in what one might call “semi- synchronous” Earth orbits (SSEO), i.e., they perform two or- bits in exactly one sidereal day. Thus their groundtrack will be
repeated every 24h and this puts constraints on which satellites
will ever be available for direct overhead passes. All GPS satel- lites have inclinations of about 55◦ which is fortunate since it
allows zenith passages at the Nanc ̧ay latitude (about 47.4◦
).
The SSEO is at a large distance (∼20,000 km) which means
that they are visible in a large area on the ground and that a pass
takes a long time (up to 7h).
2.3. Tracking
The information about identity and positions of the satellites
can be found from NORAD, e.g. via the webpage http://
celestrak.com/NORAD/elements/. Positions are calculated
and extrapolated from so called Two Line Keplerian Elements
(TLEs) which contain, among other things, orbital parame- ters and drag coefficients. The latter is more important for low
earth orbit (LEO) satellites for which orbital decay occurs more
swiftly. The TLEs are updated regularly when new telemetry is
available.
At any given time, about 10 of the satellites are above the
horizon in Nanc ̧ay. An illustrating snapshot is shown in Fig. 1.
Furthermore, two of them pass almost directly overhead,
GPS BIIR-11 and GPS BIIA-25 (maximum elevation>89◦
),
and another handful comes well within the HPBW. It is worth
reiterating that each satellite continuously provides observation
opportunities once every 24 hours.
3. Method
The “EMBRACE Nanc ̧ay Tile”a was put in a recently fabri- cated cradle in the fully horizontal setting, see Fig. 2. The fine- pointing of this setup was not known a priori. The reference ro- a A tile retroactively modified to mimick the ultimate design chosen
and shipped to Nanc ̧ay in advance of standard array tiles.
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254 A.O.H. Olofsson et al.: Profiling the EMBRACE tile beam using GPS
Fig. 1: Global map in Plate Carree ́ projection (also called Equirectangular projection) showing the ground track for two consecutive orbits of
a satellite passing overhead at Nanc ̧ay (light-blue line) and the instantaneous sub-satellite-position of all other operational satellites. Also of
interest is the red line showing the area within which a satellite (at this orbital height) must be, such that it is above the horizon from the chosen
station (EMBRACE-Nanc ̧ay, coordinates lat. 47.3819◦
, long. -2.1993◦
)
Fig. 2: The single EMBRACE tile mounted on its trolley in the hori- zontal position.
tation of the tile was such that the sides were along the Cardinal
Directions. Due to the diagonal placement of the Vivaldi ele- ments, that corresponds to a linear polarisation of 45◦with re- spect to an east-west line. A 30 MHz bandpass was sampled
every minute with 500 channels using a spectrum analyser that
included the GPS L2 frequency at 1227.8 MHz. At the time
of writing, 335 hours of observation had been performed using
two different spectrum analysers as backends. To ensure that
the pattern was sufficiently sampled in all directions, the tile
was rotated in steps of 15◦ between zero and 60◦
inbetween
some of the measurements.
The intensity was integrated over the central 2 MHz as the
L2C (not the weaker L2) carrier was strongly concentrated here
and we attempted to avoid a pair of nearby parasitic signals.
In order to assign satellites to observed signals and to pro- vide sky trajectories as a function of time (which was later con- verted to tile-centric coordinates), we employed the older, free
version of “SatTrack” which is simple yet flexible since it may
be customised by the user (as long as it is not spread under
the copyright agreement). The positions were calculated every
30 seconds and later resampled to the points in time of the sig- nal samples.
MatLab was later used for number crunching and visuali- sations once the identification phase was over.
4. Results
4.1. Detections
We first list the satellites that were unequivocally detected (but
with some caveats, see below) in order of succession. The ob- served time signal from one measurent is shown in Fig. 3. to- gether with satellite markers at their transit times.
Comments to table:
– The satellites with an M (for “Modernized”) in their desig- nation was found to have much stronger signals offering a
very large SNR in each spectrum (and hence in each pass).
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A.O.H. Olofsson et al.: Profiling the EMBRACE tile beam using GPS 255
Fig. 3: A time sequence of the integrated intensity from the first multi-day measurement performed. The signal has been marginally smoothed.
Designation Peak Carrier Signal overlap
elevation type (or comment)
GPS BIIA-24 86.2 L2 w. A-25
GPS BIIA-25 89.4 L2 w. A-24,
dominant
GPS BIIR-11 89.0 L2
GPS BIIR-03 88.4 L2 w. A-10,
dominant
GPS BIIA-10 89.8 L2 w. R-03
GPS BIIR-04 83.2 L2 marginally
w. R-03/A-10
GPS BIIR-02 89.3 L2
GPS BIIRM-4 87.3 L2C (strong)
GPS BIIRM-3 83.9 L2C (strong)
GPS BIIRM-7 79.6 L2C (strong) far from
beam centre
GPS BIIRM-5 86.9 L2C (strong)
This is consistent with the stated addition of the C (Civil)
signal also at the L2 frequency in this generation of GPS
satellites (previously only used at L1 at 1575 MHz).
– Overlap means that one satellite had not yet completed its
pass before another approached transit and the correspond- ing signal is a superposition of the two carriers.
– Two additional satellites, GPS BIIRM-6 and
GPS BIIRM-8, were also likely detected but only
through their passage through suspected side lobes, see
further below.
4.2. 2D beam pattern
In order to produce a two-dimensional beam pattern, we com- bined the signals from the satellites with the L2C carrier (which
has a much higher signal-to-noise ratio than the older L2) taken
with five different tile rotations. Figure 4 shows the trajecto- ries for the portions of the signal sequences we used. We could
make sure that these satellites had comparable intrinsic line
strengths by looking at points in which their trajectories over- lapped. The variations at the intersections we checked were
within 10% of the beam-centre strength. Blending – i.e., when
the next satellite approaches the beam pattern before the pre- vious one has left it – was sometimes a problem and we trun- Fig. 4: Trajectories of 5 different satellites observed using 5 different
tile rotations. The coordinates are fixed with respect to the tile and
shown as looking onto the tile where φ=0 is along the local x-direction
(along a tile side incresing to the right). With the tile in the reference
postion, the x-axis is parallel to an W-E line, increasing to the east.
cated the signal in these cases. It is important to note that while
blending effects can produce artifacts, it cannot falsely produce
a flatter – or better – pattern. Thus our results are to be consid- ered a worst-case scenario.
We have neglected to take into account any variation of
transmitter antenna gain when the satellite are farther from
zenith. Even if we assume a “worst-case” scenario, in which the
transmitter has Gaussian beam which is pointing to the nadir
(directly down-looking) as seen from the satellite, we estimate
that the HPBW would be &50◦b while the ground-station would
only be 10◦
away from nadir when the satellite is at 45◦
eleva- tion in the local sky. Nor would the increased path length away
from zenith impose a relevant change; the received power has
a square dependence on the distance but the high orbit ensures
that the difference is only 1% one full beam width (13◦
) away
from zenith and about 15% at 45◦
elevation.
b Based on an unconfirmed bore-sight GPS antenna gain of 11.5 dB.