ACP-WGF29/IP13
International Civil Aviation Organization
2013-09-09
INFORMATION PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
TWENTY NINETH MEETING OF WORKING GROUP F
Nairobi, Kenya 5 – 12 September 2013
Agenda Item 7: Any Other Business
Interference Path Loss Measurements in the Radio Altimeter Band
onboard an Airbus A321
(Presented by Uwe Schwark)
SUMMARY
This Information Paper is to inform Working Group-F about a measurement campaign
recently launched by Airbus aiming at determining Interference Path Loss (IPL) between
exemplary measurement points inside the aircraft cabin and the radio altimeter receive
antenna. The minimum interference path loss is an important aspect to know in particular for
the integration of WAIC and radio altimeters onboard the same aircraft to allow for an
assessment of potential mutual effects between these systems. A first set of measurements
carried out suggests a minimum IPL of around 75 dB for measurement locations in the aft
cabin area and the aircraft being in a typical parking location in front of an airport terminal
building. IPL can reach up to105 dB for measurement locations in the forward cabin area and
the aircraft being situated in a location without any large electrically reflective objects in its
vicinity.
(9 pages)
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ACP WG-F/29 IP 13
1.
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BACKGROUND
At the 28th meeting of Working Group-F, March 11-22, 2013 in Lima, Peru, Working Paper 20 (ACPWGF28/WP20) was presented which discusses regulatory, standardization and certification activities seen
to be necessary for the safe introduction of WAIC systems. It was particularly discussed the roles of
ITU-R and ICAO in carrying out the required compatibility studies, when it comes to situations were two
aeronautical systems under the purview of ICAO share the same frequency band. This is for instance the
case for the frequency band 4 200 – 4 400 MHz used primarily for radio altimeters onboard civil and
governmental aircraft.
The case of an aircraft equipped with WAIC systems potentially interfering with the radio altimeter
onboard another nearby aircraft (off-board interference case) is comprehensively dealt within
compatibility studies brought into the WRC-15 preparatory process through Working Group-F and ITU-R
Working Party 5B.
The case of potential interference between WAIC systems and the radio altimeter onboard the same
aircraft is subject to aircraft certification rules and procedures. This view was supported by Working
Group-F at its 28th meeting. Key for being able to assess any potential mutual impact between WAIC
systems and the radio altimeter is knowledge of the Interference Path Loss (IPL) between transmit and
receive antennas of both systems. Airbus is currently carrying out a number of measurement campaigns to
derive IPL-figures for different assumed installation locations of WAIC antennas and different aircraft
surroundings. Although, the material contained within this Information Paper is not directly related to the
WRC-15 preparatory process on Agenda Item 1.17, since it addresses aircraft certification rather than
spectrum regulation, it was felt that Working Group-F should be made aware of this additional aspect
addressing the feasibility of WAIC systems and radio altimeters sharing the same band.
1.
INTRODUCTION
In August 2013 Airbus has carried out a first set of measurements onboard an A321 aircraft aiming at
determining Interference Path Loss (IPL) between certain locations within the aircraft cabin in which
WAIC transceivers likely will be installed and the radio altimeter receive antenna. IPL is an important
feature to know for being able to determine the degree of coupling between the two systems and any
mutual effects this coupling might potentially causes. This Information Paper is intended to Inform
Working Group-F about the initial findings derived from these measurements.
2.
SCENARIO
Measurement aircraft
The aircraft used for the measurement campaign is an A321 serial production aircraft specifically set
aside for the duration of the measurement campaign (see Figure 1). The used aircraft coming directly<
from the Final Assembly Line was fully equipped with all cabin installations, such as linings, seats and
cabin monuments.
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Figure 1: Measurement Aircraft (A321)
Aircraft locations
Measured IPL will depend on the surroundings in the vicinity of the aircraft under consideration. In
particular the presence of large electrically reflective structures in proximity of the aircraft is expected to
influence the measured IPL. For this reason, two different aircraft locations were selected. The first
location is a typical parking location in front of a terminal building, with the aircraft being
perpendicularly oriented to that building (see Figure 2). The second aircraft location was selected such
that there are no large electrically reflecting structures in the immediate vicinity of the measurement
aircraft (see Figure 3). Both locations are on the Airbus premises in Hamburg, Germany.
measurement
aircraft
Figure 2: Aircraft under consideration placed in front of a typical terminal building at Airbus
premises in Hamburg (Germany) – Gate Scenario
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ACP WG-F/29 IP 13
measurement
aircraft
Figure 3: Aircraft under consideration placed away from large electrically reflective structures at
Airbus premises in Hamburg (Germany) – Open Space Scenario
3.
MEASUREMENT SETUP, LOCATIONS & PROCEDURE
Measurement setup
The measurement setup used to determine the IPL uses a microwave signal generator (Rohde&Schwarz
SMR20) which was connected through a signal amplifier (30dB gain) to the radio altimeters
transmit/receive antenna at the installation tray foreseen to hold the radio altimeter in place (see Figure 5).
From here on the feeding cable and radio altimeter antenna installations as provided by the aircraft were
used.
On the cabin side two identical wideband omni-directional antennas were used as receiving antennas.
Each of these antennas is connected to separate spectrum analyzers, a Rohde&Schwarz FSVR7 real-time
spectrum analyzer and a Rohde&Schwarz FSIQ26 spectrum analyzer, respectively. For the actual
measurements these antennas were placed in various locations within the aircraft using three different
polarizations (see Figures 4, 5 and 6).
Transmitter
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Aft Cargo
Serial Link
Signal
generator
(R&S SMR20)
Control PC
Ethernet Link
Network
Switch
Interface
cable
coaxial
coaxial
Signal
Amplifier
(gain 30dB)
ACP WG-F/29 IP 13
RA-Tray
radio altimeter coaxial
antenna feeding cable
RA RX/TX
Antenna
Ethernet Link
Receiver
Aircraft Installation
Cabin
Signal Analyzer
(R&S FSVR7)
RX-Antenna
Signal Analyzer
(R&S FSIQ26)
RX-Antenna
Figure 4: Measurement setup used (gray elements describe components which belong to the radio
altimeter system installations)
Figure 5: Installation tray of the two radio altimeters installed onboard an A321in the aft cargo
compartment next to cargo door
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Figure 4: Example measurement point of the
measurement antenna on a window seat
(horizontally polarized)
Figure 5: Example measurement point of the
measurement antenna on a window seat
(vertically polarized in parallel to the plane
spanned by the aircraft’s roll and yaw axes)
Figure 6: Example measurement point of the measurement antenna below seat
(vertically polarized in parallel to the plane spanned by the aircraft’s pitch and yaw axes)
Measurement point within the aircraft cabin
Three sets of measurement points were defined (see Figure 7). These sets of measurement points are
located in the forward cabin section half way between forward doors and fwd lip of the belly fairing, in
the center of the cabin and the aft section directly above the radio altimeter antenna installation location.
Figure 7: Areas of measurements
Since exact information on installation locations for WAIC systems is not yet available and will likely
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vary from aircraft type to aircraft type, a set of measurement points has been defined, which is deemed
representative to cover the variety of WAIC system layouts within the aircraft cabin (see Figure 7). These
measurement points are located:
• Near the windows
• At the height of the seat back rests
• At the ceiling in the aisle
• On the floor in the aisle
• Below the seats
Measurement points on floor and seat backrest levels are spaced approximately 1 m apart from each
other.
Figure 7: Aircraft cross section - measurement points on floor, seat back rest and ceiling level
Measurement procedure
In order to get representative IPL values across the entire 4 200 – 4 400 MHz band, 21 measurements at
10 MHz sampling distance were taken per each of the measurement points and polarizations. The
resolution bandwidth settings of the spectrum analyzers were selected such that a minimum SNR of 10dB
was maintained for all measurement points within the cabin.
4.
MEASUREMENT RESULTS
Figure 8 provides an overview on the power levels at various stages of the transmission and reception
chain. A continuous sine wave test signal with constant signal power level of 25.7 dBm is fed into the
aircraft’s radio altimeter feeding cable. After the test signal has passed through the radio altimeter feeding
cable and antenna, the coupling path between radio altimeter and measurement antenna and coax cable
connecting the measurement antenna with the signal analyzer its power level X is measured by that signal
analyzer. The coax cable has a constant attenuation across the frequency range of interest of 11.3dB
allowing to determine the signal power level at the measurement antenna’s connector. Knowledge of the
test signal’s input power (25.7 dBm) and Y allows to determine Z. Z subsumes all attenuations and gains
caused by the radio altimeter feeding cable, the radio altimeter antenna, the coupling path between the
antennas and the measurement antenna (see Figure 8). Z is the IPL between the two reference points
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(radio altimeter feeding cable input and measurement antenna output). Table 1 summarizes the maximum
power levels which were measured per each set of measurement points and aircraft location and the
resulting coupling between radio altimeter feeding cable input and measurement antenna output. Worstcase coupling Zmin lies between 74.4 and 104.4 dB depending on the cabin area in which measurements
were taken and the aircraft environment. The minimum coupling occurs for the gate scenario and
measurement points in the aft cabin section. This can be explained by the fact that the radio altimeter
antennas on an Airbus A321 are located on the bottom of the aft fuselage directly below the aft locations
of the measurement points. Hence, the separation between the antennas in this case is minimized.
Furthermore, in the gate scenario one can expect additional coupling paths caused by reflections of the
test signal at electrically reflective structures in the aircraft’s vicinity. In case of the gate scenario it can be
assumed that the nearby terminal building or other aircraft on stands adjacent to the one of the
measurement aircraft act as such signal reflectors increasing the amount of energy coupled into the
measurement antenna.
4.200 + k*10 MHz
Signal
Generator
0 dBm
+30 dB
Amplifier
-4.3 dB
30 dBm
k = 0...21
coax
cable
25.7 dBm
2 m cable
-Z dB
RAfeeding
cable
RAAntenna
Measurement
Antenna
Coupling Path
-11.3 dB
Y dBm
coax
cable
X dBm
Signal Analyzer
15 m cable
Figure 8: Level plan for measurement setup
Aircraft environment
Gate
Gate
Gate
Open Space
Open Space
Open Space
Cabin area
Aft Cabin
Center Cabin
Fwd Cabin
Aft Cabin
Center Cabin
Fwd Cabin
Xmax / dBm
Ymax / dBm
Zmin / dB
-60
-70
-80
-70
-80
-90
-48.7
-58.7
-68.7
-58.7
-68.7
-78.7
74.4
84.4
94.4
84.4
94.4
104.4
Table 1: Measured max power levels (Ymax), max power levels at the receive antenna connector
(Xmax) and resulting minimum coupling (Zmin) for all combinations of aircraft environment and
cabin area in which measurements were taken.
5.
CONCLUSIONS
A measurement campaign has been carried out by Airbus seeking to determine Interference Path Loss
between various measurement points within the aircraft cabin and the radio altimeter antenna onboard the
same aircraft. The test aircraft used for the measurement campaign is an Airbus A321. Measured IPL lies
between around 75 dB and 105 dB depending on the locations of the measurement point within the
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aircraft cabin and the surroundings of the aircraft.
Acknowledging the fact that an aircraft approaching an airport for landing (the most critical phase of
flight in which the radio altimeter has to provide reliable altitude measurements) is far away enough from
reasonably large electrically reflective structures, measured IPL applicable for the “Open Space” scenario
can be assumed relevant for this flight phase. IPL values for the “Open Space” scenario are between
around 85 dB and 105 dB depending on the measurement point location within the cabin.