Conformal Array Antenna for Observation Platforms in Low Earth Orbit
Author
affilliation
G. Caille, E. Vourch
Alcatel Espace (F)
M.J. Martín
CASA (E)
J.R. Mosig
LEMA-EPFL (CH)
A. Martín Polegre‡
at Electromagnetics Division, ESTEC
Résumé
Une antenne réseau à éléments non
placés dans le même plan a été
conçue, fabriquée et testée.
Elle doit permettre aux plates-formes d'observation
en orbite basse de télécharger à grande
vitesse des données vers plusieurs stations sol
simultanément. Le principe de l'antenne fait
appel à des techniques de pilotage en phase du
réseau permettant de pointer le faisceau dans
telle ou telle direction.
L'une des priorités était de réduire
l'incidence des sauts d'amplitude et de phase lors
du déplacement du faisceau, pour écarter
les risques de perte du verrouillage par le
récepteur de la station sol.
Contractors:
Alcatel Espace (F), CASA (E), LEMA-EPFL (CH),
Funding:
Basic Technology Research Programme.
Introduction
Increased performance requirements placed on
Earth observation satellites in low Earth orbit
(LEO) require larger amounts of data to be downloaded
to the ground station. The antennas currently used
for this task on satellites such as ERS-1 and -2
and Envisat-1 have single fixed beams which
completely cover all of the visible surface of
the Earth. The gain pattern of this fixed beam is
shaped to compensate for the distance from the
satellite to a ground station. The wide beamwidth
required by this approach can only be met with a
low-gain antenna and a correspondingly high
transmitter power would be needed to support
the high data rates expected in the future.
Figure 1. The spot beam concept.
To achieve an increase in data rate while maintaining
a low transmitter primary power consumption (i.e.
low transmitted power) and to enable simultaneous
links with several ground stations, a multi-beam
scanning antenna using phased array techniques has
been designed. A demonstration model with up to three
beams in three adjacent sub-bands has been manufactured
for an observation mission having an altitude of
about 800 km. A conformal array was found to be the
best option for this mission because it had higher
gain compared to a passive fixed beam. It was also
capable of electronic scanning, which gave no
mechanical disturbances to the platform, and
provided fast change-over between ground stations.
It also offered high reliability with graceful
degradation. Table 1 shows the main features of
the antenna.
Table 1. The principal properties of the
conformal array antenna.
| Frequency |
X-band (8.1, 8.2 and 8.3 GHz) |
| Polarisation |
Right-hand circular (axial ratio < 3 dB) |
| Operation mode |
transmission only (three simultaneous beams) |
| Platform altitude | 800 Km |
| Coverage |
360 degrees in azimuth,
0-62.3 degrees in elevation
isoflux condition on Earth's surface |
| Gain at edge of coverage |
greater than 20 dBi |
| Pattern jumps while scanning |
0.15 dB in amplitude
2.67º in phase |
| Radio frequency power | 1.23 dBW |
| Array arrangement | conical configuration,
24 sub-arrays of 6 patches |
| Mass | 5.7 Kg (excluding active modules) |
| Dimensions | 37.7 cm diameter, 17.3 cm in height |
Antenna design and implementation
The antenna concept, chosen jointly by ESA and
the prime contractor, Alcatel Espace, was a
semi-active [1] conformal array. The antenna
needed a large field of view which covered 360
degrees in azimuth and 0 to 62.3 degrees in
elevation, matched to a template in which gain
was higher at high elevation angles.
To meet the requirement above, the chosen array
configuration has 24 identical sub-arrays arranged
in a conical geometry. Each sub-array provides
maximum directivity towards the maximum elevation
angle and has a radiation pattern in elevation
similar to the elevation template imposed on the
whole array. Twenty four power amplifiers drive
the sub arrays through a set of eight three-by-three
Butler microwave couplers, which were designed
and manufactured by CASA.
The phase of the inputs to the Butler matrices
is controlled by a set of phase shifters.
The Butler matrices translate phase distribution
created by the phase shifters into a set of amplitude
and phase excitations for the sub-arrays. This method
of controlling the distribution of the excitation of
the elements of the array is known as the 'semi-active
concept', and it has some advantages over the conventional
active concept in which both amplitude and phase at
the sub-array inputs are controlled. These include:
all solid-state power amplifiers operate at equal
power, which improves their efficiency;
the amplitude and phase jumps on the scanned
beam are kept very low while the beam is continuously
steered, preventing loss of receiver lock on ground
station, and therefore reducing the bit error rate.
In our case, 5-bit phase shifters were sufficient to
keep the phase jumps in the scanned beam below 3
degrees, which is a very stringent requirement;
two-dimensional beam scanning is achieved with
a one dimensional set of 24 phase shifters per beam;
the use of Butler matrices routes all of the input
power to the set of sub-arrays that radiates most
efficiently in the desired direction, with resulting
higher gain.
Each sub-array consists of a set of six microstrip
double-stacked patches in cavities. The cavity
reduces the mutual coupling between radiating elements.
Each patch radiates with circular polarisation from a
single feed point. Accurate electromagnetic modelling
software developed by LEMA, was used to develop an
efficient radiating element without building costly
prototypes.
Figure 2. A conformal array antenna
(courtesy of Alcatel Espace)
A demonstration model antenna, for one beam only,
was manufactured and tested. No amplifiers were included,
so the antenna was tested in receive mode, and phase
control was performed by means of a set of vector
modulators (with 5-bit phase control).
Conclusion
A conformal array antenna can provide the flexibility
that platforms in low Earth-orbit need to download their
data to ground stations. It does this by producing multiple
beams for various ground stations. Even if only one ground
station is used at a time, the higher gain of this antenna
compared to a fixed beam antenna allows the ground station
antennas to be down-sized or, equivalently, the data rate
to be increased.
Moreover, missions on which disturbances on the platform
attitude are critical can take advantage of a phased array
with no moving parts. Although the demonstration model was
targeted at a low Earth-orbit mission, the concept is
equally applicable to future interplanetary science missions.
Reference
[1] Roederer A.G., French Patent 89-12584 / 2652452.
‡ Mr. Martín Polegre is with Aurora BV (NL)
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Preparing for the Future Vol. 8 No. 2
Published June 1998
