SPHERICAL NEAR FIELD RADOME TEST FACILITY FOR NOSE-MOUNTED RADOMES OF AIRCRAFT

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SPHERICAL NEAR FIELD RADOME TEST FACILITY FOR NOSE-MOUNTED
RADOMES OF COMMERCIAL TRAFFIC AIRCRAFT
Marcel Boumans marcelb@orbitfr.de
Ulrike Wagner ulrikew@orbitfr.de
ORBIT/FR-Europe GmbH
Johann-Sebastian-Bach-Str. 11, 85591 Vaterstetten, Germany
ABSTRACT
Typically radome tests are performed on outdoor far field
ranges or compact ranges. ORBIT/FR has designed, build
and qualified a unique spherical near-field radome test
facility for the nose-mounted radomes of commercial
traffic aircraft for the so-called “after repair” tests
according to the international standard RTCA/DO-213, as
well as the aircraft manufacturers Component
Maintenance Manuals. The facility is extremely compact
(chamber size 5.7 m x 5.2 m x 3.2 + 0.7 m, L x W x H),
can handle radomes as small as used on the Canadair and
as large as used on the Airbus-380 and can be installed
directly in the repair workshop for such radomes.
The tests performed are transmission efficiency and side
lobe level increase. The system is completely automated,
so that a workshop technician can operate the facility.
Utmost attention has been paid to operational aspects and
both operator and equipment safety.
After the measurements are done, a test report is fully
automatically generated according to RTCA requirements
and classifications. The facility is equipped to test all
standard Airbus, Boeing, Canadair and Dash nose
radomes.
Keywords: Radome Measurements, Transmission
Efficiency, Sidelobe Increase, Spherical Near Field
1. Introduction
RTCA Inc. is a not-for-profit corporation formed to
advance the art and science of aviation and aviation
electronic systems for the benefit of the public. The
organization functions as a Federal Advisory Committee
and develops consensus based recommendations on
contemporary aviation issues.
The RTCA/DO-213 standard is defined in the documents
“Minimum Operational Performance Standards for Nose-
Mounted Radomes” dated Jan 14, 1993, with “Change
No. 1 to RTCA/DO-213 Minimum Operational
Performance Standards for Nose-Mounted Radomes”,
dated Jun 23, 1995.
The presented test facility allows RF testing compliant to
the RTCA/DO-213 standard, in particular for
Transmission Efficiency and Side Lobe Level Increase
(the so-called “after repair” test requirements). Software
modifications to also include Incident Reflection, Beam
Deflection and Beam Width are under development. This
facility is not intended for any of the environmental tests
also defined in document RTCA/DO-213.
In addition the aircraft manufacturers requirements,
which sometimes define deviations from the standard
RTCA/DO-213, are taken into account. In particular the
Airbus Component Maintenance Manual (CMM) requires
sidelobe measurements throughout the entire predictive
windshear (PWS) electromagnetic window, where
RTCA/DO-213 requires this only in the forward
direction.
The facility design is unique in that it is based on antenna
near field technology. This allows to reduce the size of
the facility significantly, so that it can be build indoor,
and can be environmentally controlled. This reduces
infrastructure cost and increases availability and
accuracy. The acquired data is transformed to the far
field, so that the data analysis is based on highly accurate
far-field data.
It can be used for Qualification/Preproduction Testing,
Quality Assurance/Production Testing and so called After
Repair Testing. The facility is designed for use with a
multitude of radomes, for easy and safe handling of these
radomes, and for fast and efficient testing. In particular
for production and after repair testing, the facility is
highly automated, so that workshop personnel not
familiar with RF testing can easily do RTCA/DO-213
compliant testing. Test reports are generated fully
automatic.
Since the design is based on standard antenna / radome
test technologies, the advanced user can perform a
multitude of tests beyond what is defined in the
RTCA/DO-213. Thus this facility also presents a sound
investment for future standards and applications.
2. Facility Overview
Figure 1. shows the ORBIT/FR near field radome test
facility for nose-mounted radomes. The near field probing
surface is spherical. The positioning is realized with a fast
rotating azimuth positioner (30 rpm), which carries the
Device Under Test (radome, radar antenna on gimbal
positioner, aircraft bulkhead, where applicable), and an
elevation arm. Note that the orientation of the facility is
such that the aircraft “flies” to the zenith of the facility.
Figure 1. General overview of test facility
The test facility chamber outside dimensions are approx.
5.7 m x 5.2 m x 3.9 m (W x L x H), with a pit for the
azimuth positioner of 0.7 m deep. Thus it can be easily
installed right in the radome workshop. Access inside the
facility is through a large double leaved door of 4.0 m x
3.3 m (W x H). Typically the chamber is build as a
shielded, self supporting structure, so that there is no
interaction / disturbance from the radar signals inside the
facility with the outside world and vice versa. The inside
of the chamber is installed with 200 mm pyramidal
absorbers everywhere. In those areas of the floor where
people have to be for installation purposes, so-called
walk-on absorbers are installed.
Figure 2. The facility “as built”
All the instrumentation is mounted in a rack inside the
test chamber, right next to the elevation positioner. The
system is controlled through Fiber Optic LAN by either a
computer directly next to the facility, or optionally any
other computer connected to the same network.
The RF system is based on a network analyzer. For
Production testing and After Repair Testing, typically a
single frequency is used (e.g. 9.35 GHz), for
Qualification Testing multiple frequencies between 9.3
and 9.5 GHz are used. The RF probe on the elevation arm
is a dual linear polarized single choke circular probe.
Figure 3. Radar antenna under the radome
Facility temperature control is through one inlet into and
one outlet out of the test chamber, and can either be
connected to the building air conditioning supply or a
separate unit just for this facility. Note that the absolute
temperature does not need to be as strictly controlled as
the temperature variation during one measurement (+17
to +28 deg C versus ± 2 deg).
The radar antenna is mounted on the gimbal positioner,
which has a travel of ± 88 deg in azimuth and ± 33 deg in
elevation (relative to the aircraft). An automated linear
translation of approx. 8 mm (representing ¼ λ in X-band)
is installed between the radar antenna and the gimbal.
Note that RTCA/DO-213 requires the transmission
measurements to be done in two antenna positions
separated by ¼ λ in the boresight direction.
The entire gimbal unit is installed on a vertical slide
which allows to position the unit at the correct position
inside the radome. In addition this allows to lower the
radar antenna below the radome mounting surface, so that
it is protected while a new radome is being installed.
Figure 4. Radar antenna in low position for easy radome
mounting
The facility is designed to be able to handle a multitude of
different radomes with a minimum of operator
adjustments. For this purpose, each different size of
radome is mounted on its own support plate, made out of
a light weight fibre reinforced plastic honeycomb
sandwich structure. Thus it can easily be adapted to
include the aircraft bulkhead or to include the real
aircrafts interface (hinges, latches, screws), or any other
means to position and hold the radome. Since the
radomes are supported by these plates and point upright,
the loads on the interfaces are relatively small.
The plates are coded such that it is not possible to move
the antenna inside the radome to a position which would
allow the antenna to hit the radome. Also the interface
between the support plate and the azimuth positioner is
designed such that the radome is automatically mounted
correctly in radial and axial directions.
In this way most radomes can be mounted and
dismounted by just two or three people who carry the
support plate with radome into and out of the facility.
This action only takes a few minutes.
3. Facility Control
The operation of the facility is highly automated for the
measurement types described in the RTCA/DO-213, and
minimum antenna and RF knowledge is necessary to
perform these tests according to the RTCA/DO-213
requirements. The measurements can typically be
executed from fully automated batches, provided by
ORBIT/FR.
On the other hand, there is still a lot of flexibility in
regard to variation from the pre-defined measurements.
Such variation can consist of selecting a sub-set of the
gimbal angles (e.g. to concentrate on an area of the
radome where improvement or repair is necessary) and of
selecting reduced RF sampling ranges (e.g. to speed up
measurement time for indicative measurements).
3.1 Test facility and test preparation
Presuming the facility is powered off, first the test facility
needs to be powered on, temperature stabilize and the
applicable radar antenna needs to be mounted (if there is
a large variation in the sizes of the radomes to be tested,
also the radar antennas need to be available in different
diameters). Then data is entered in regard to the test
radome (e.g. Type and Serial Number).
Detailed instructions for the operator appear on the
computer monitor.
3.2 Measurement selection and execution
First the test standard is selected (RTCA/DO-213 resp.
CMM or customer defined “Fast Check”), the
measurement types are selected (Transmission Efficiency,
Side Lobe Levels and Beamwidth, Incident Reflection
and / or Beam Deflection). Note that for “After Repair
Testing” only Transmission Efficiency and, in some
cases, Side Lobe Level Increase are required.
Then the gimbal angles are selected (full set, as defined in
RTCA/DO-213 resp. CMM, or partial set).
Before measurements can be started, the temperature of
the facility needs to be stabilized to within ± 2 deg of the
reference temperature (see also Chapter 2), each time the
large doors have been opened. Note that if the reference
temperature is close to the temperature outside of the test
chamber, no waiting times would be required.
Now a calibration (reference) measurement is made of the
radar antenna without the radome. Then the radome is
mounted and, after the temperature is stabilized again, the
measurements can be performed fully automated. At the
end of all measurements the radome is removed again,
and a second reference of the radar antenna without
radome is performed to verify that the facility was stable
during the measurements.
3.3 Data Analysis and Reporting
After the measurements are finished, test reports are
generated automatically. For “Fast Check” or “Partial
Check” the statement at the lower end of the report will
then state that these measurements were NOT executed in
accordance to RTCA/DO-213 and / or CMM.
4. Safety
Many safety aspects have been included in the design and
implementation of the Radome Test Facility. These
aspects relate to both equipment safety (test facility
including radar antenna and radomes) as well as
personnel safety.
4.1 Equipment Safety
The equipment safety relates to the risk that any
component of the test facility, or the radome under test,
can get damaged in operation. In particular, if many
different sizes of radomes are to be tested, then such a
risk is real: if a large radome was measured, and after that
a much smaller radome is to be measured, then the radar
antenna could hit the radome wall if the facility would not
correctly be set up to handle the much smaller radome.
If the system is only used for regular weather radar tests,
then it is only operated at frequencies from 9.3 to 9.5
GHz. Thus nothing needs to be changed to the RF
instrumentation, including the RF probe, at any time. The
RF probe on the elevation arm is normally parked at the
ceiling of the facility near the “zenith” position when any
person has to work inside the facility. This reduces the
risk that the arm and the RF probe are damaged during
installation work.
Before the radomes are installed in the facility, they are
fitted to support plates unique to the radomes, with
handles or other devices to carry the plate / radome
assembly. The radomes are mounted upright on the
azimuth positioner (the aircraft is “flying” to the zenith of
the facility). This makes the assemblies easy to carry and
install. It will be difficult to install a radome on the wrong
support plate by virtue of clear indications and
instructions on the plates, the size of the plates relative to
the radomes, or the interfaces between the plates and the
radomes. The support plates are fitted with hardware
coding which allows to install the plate to the azimuth
positioner in only the correct way, and which give
information to the test facility about the position of the
radar antenna inside the radome.
The installation platform is 0.8 m above the chamber
inside floor level, so that installation personnel has best
handling control of both the size and the weight of the
radomes. The radar antenna is lowered below the support
plate interface frame before a new radome is installed.
Thus it is virtually impossible to damage the radar
antenna with the radome during installation.
Lifting and lowering of the gimbal positioner is done
automatically by the software. The radar antenna can only
be lowered below the interface frame when the gimbal is
set close to zero azimuth and zero elevation because of a
limit switch interlock mechanism included in the gimbal
positioner / vertical lift, thus preventing that the radar
antenna hits the interface frame. The radar antenna cannot
be lifted to a height which would allow the antenna to hit
the radome wall, because the support plate includes limit
switch information for the vertical lift.
4.2 Personnel Safety
Personnel safety relates to the risk that an operator of the
facility, or any other person, gets hurt by this facility.
This relates to mechanical, electrical and RF aspects.
In a machine with moving parts, there is always the risk
that somebody gets hit or squeezed by any of these parts.
In this facility, with the 30 rpm azimuth positioner which
can carry very large radomes, the gimbal positioner on
which radar antennas need to be mounted, etc., such risks
need to be carefully considered. Through design,
instructions and training this risk needs to be minimized.
Several hardware measures have been included in this
facility to increase the personnel safety.
Emergency stop buttons are included in three locations:
one near the elevation positioner, one near the door and
one outside the chamber, typically near the control
computer.
Also a switch is connected to the door, which disables
any positioner movement as soon as the door is opened.
This will prevent that anybody not familiar with the
facility, who could enter the facility unintended, cannot
get hurt by any moving parts.
Further a shielded window is installed in one of the
facility walls, as well as a video network camera inside
the facility. Thus the operator can easily make a final
check before starting the measurement. The camera can
be accessed through any computer on the network, so that
one can also have a regular look inside the facility for
long unattended measurements.
From the operational and health point of view carrying
and handling heavy and large objects, the facility is
optimized by having a very large door of 4 m wide, a
single step of only 5 cm from the outside to the inside of
the facility (walk-on absorbers) and by having the
installation platform at 80 cm height.
In regard to the electrical safety, the standard national and
international electrical wiring and installation standards
are followed.
In regard to RF safety, the transmitted power is very low
(less than 50 mW). In addition, the facility is typically
build inside a shielded chamber. This is primarily to
prevent interference from weather radar signals from real
aircraft in the vicinity of the facility, or to prevent the
signals from the facility to interfere with weather radar
signals from real aircraft, but also lowers the radiation
outside the facility to undetectable levels.
5. Test Results
First the facility has been tested on stability and
reproducibility. This means that all 45 antenna / gimbal
combinations have been measured repeatedly, and
variation in the angles of the beam peak as well as in the
maximum power level have been compared.
Repeatability in beam peak angle has been shown to be
better than ± 0.1 deg. Note that the power variation of the
beam peak within this angular range is only ± 0.02 dB, or
less than ± 0.5 %.
Repeatability in power level at the beam peak has been
shown to be better than ± 0.05 dB, or ± 1 % between the
reference measurements with which a radome test is
started and the final reference measurement after all
radome measurements are finished. Note that up to 14
hours can be between these two measurements for a large
radome.
These stability and repeatability results are well within
the requirements stated in RTCA/DO-213.
The following tables and plots show some typical test
results for both a typical Kevlar (Class C) and a high
quality quartz glass (Class A) radome. Note that the
shown tables and graphs are generated fully automated.
Customer specific logos, test object information and
signature fields can be added according to the customer
requirements.
Figure 5. Transmission Loss of typical Kevlar radome
Figure 6. Sidelobe Increase of typical Kevlar radome
Figure 7. Transmission Loss of high quality quartz glass
radome
Figure 8. Sidelobe Increase of high quality quartz glass
radome
6. Measurement times
In the spherical NF measurement system, the phi-axis
rotates at 30 rpm, thus makes one revolution in 2 seconds.
The theta axis, which is the step axis, needs approx. 2
seconds to move to the next theta position.
A large radome, like on a Boeing 747, requires an
acquisition step angle of 0.72 deg. This presumes that any
part of the radome can reflect some radiation (eg.
reflection lobes), which is the worst case assumption.
For a full characterisation of the radome / antenna system
the theta axis needs to be scanned from 0 (zenith) to
slightly beyond 90 degrees (horizon), resulting in approx.
9 minutes for a single measurement. A fully compliant
RTCA “after repair” test requires 45 gimbal positions at 2
“quarter wavelength” positions of the antenna, plus two
reference measurements, or a total of 92 measurements.
Thus the total measurement time for a B-747 radome is in
the order of 14 hours.
A smaller radome, like used on a DASH, requires 1.44
deg increments, and results in total measurements times
in the order of 8 hours.
These test times have been confirmed in practice.
Although the test times for the largest types of radomes
are quite long, it must be realized that the facility operates
fully automated, and no operator actions are required
from after the radome is mounted until all 2 x 45 gimbal
angles are measured and the radome can be removed
again. Thus, in practice, one to two radomes can be
measured per day, which is typically more than sufficient
for a radome repair workshop.
If higher measurement speeds are required, then more
probes can be mounted on the elevation arm.
7. Conclusions
A very compact RTCA/DO-213 compliant “after repair”
RF test facility has been build and qualified, but also
Qualification / Preproduction and Quality Assurance /
Production Testing defined in RTCA/DO-213 can be
tested with this facility.
It can be installed in the repair workshop and can be
operated by workshop personnel. Tests are made and
evaluated fully automated.
Since this avoids packing for and transportation to an
external RF test facility, significantly reduced repair times
can be realized.
8. REFERENCES
[1] “Minimum Operational Performance Standards for
Nose-Mounted Radomes” dated Jan 14, 1993, with
“Change No. 1 to RTCA/DO-213 Minimum Operational
Performance Standards for Nose-Mounted Radomes”,
dated Jun 23, 1995,
[2] “CMM Component Maintenance Manual”, various
Aircraft models, Airbus Industrie,

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