The Boeing company conducted a flight test of a SATCOM system similar to the ARINC 741 configuration on a production model 747-400. A flight plan was specifically designed to test the system over a wide variety of satellite elevations and aircraft attitudes as well as over land and sea. Interface bit errors, signal quality and aircraft position and navigational inputs were all recorded as a function of time. Special aircraft maneuvers were performed to demonstrate the potential for shadowing by aircraft structures. Both a compass rose test and the flight test indicated that shadowing from the tail is insignificant for the 747-400. However, satellite elevation angles below the aircraft horizon during banking maneuvers were shown to have a significant deleterious effect on SATCOM communications

Murphy, Timothy A.

Stapleton, Brian P.

English

NASA Technical Reports Server

N92_2 -,
Flight Test of ARINC 741 Configuration
Low Gain SATCOM System on
Boeing 747-400 Aircraft
Timothy A. Murphy and Brian P. Stapleton
Boeing Commercial Airplane Group
PO Box 3707 - MS 47-31
Seattle WA. 98124-2207
(206) 655-6908
FAX (206) 655-9252
ABSTRACT
On May 23, 1989 The Boeing Company
conducted a flight test of a SATCOM system
similar to the ARINC 741 configuration on a
production model 747-400. A flight plan was
specifically designed to test the system over a
variety of satellite elevations and aircraft attitudes
as well as over land and sea. Interface bit errors,
signal quality and aircraft position and
navigational inputs were all recorded as a func-
tion of time. Special aircraft maneuvers were
performed to demonstrate the potential for
shadowing by the aircraft structures. During the
flight test, messages were routed via a Ground
Earth Station in Santa Paula, California to Collins
Radio in Cedar Rapids, Iowa. These messages
were also monitored by ARINC in Annapolis,
Maryland through the ACARS ARINC network.
On May 20, 1989, a compass rose test was
performed in preparation for a flight test. Both
the compass rose test and the flight test indicated
that shadowing from the tail is insignificant for
the 747-400. However, satellite elevation angles
below the aircraft horizon during banking
maneuvers were shown to have a significantly
deleterious effect on SATCOM communications.
BACKGROUND
Satellite communications will undoubtedly
play an important role in future civil aeronautical
operations. Presently HF communications (2-30
MHz) provides the communications link when
the aircraft is beyond line of sight with VHF.
HF provides SSB analog voice communications
using ionospheric propagation. The FAA as well
as other aviation regulatory bodies recognizes the
superior reliability of SATCOM over HF with its
propagation vagaries. Used wisely, satellite
communications technology will enable more
efficient utilization of the higher traffic density
airline routes in both the Pacific and Atlantic
Ocean regions.
Cognizant of the potential benefits of this
technology, Boeing Commercial Airplane Group
has been involved in the development of
aeronautical satellite communications systems for
over two decades. In more recent times Boeing
has been an active participant in the industry
regulatory activities associated with satellite
communications. It is truly exciting that the
introduction of a standard, full time,
commercially available satellite communications
service is imminent. As an airframe
manufacturer, Boeing will be a system integrator,
responsible for integrating and testing all the
avionics components and finally certifying and
delivering complete systems.
On May 20, 1989, in preparation for a
flight test, Boeing conducted a compass rose test
of the SATCOM system. This test verified that
the system wiring was complete and functional as
well as verifying that the installed equipment was
operational.
On May 23, 1989, Boeing Avionics
Engineering conducted a flight test of a
SATCOM system similar to the ARINC 741
configuration I on a production model 747-400.
This flight test was conducted using
developmental engineering units as a step in
design validation. Due to the nature of the
aircraft and the avionics equipment, the test
instrumentation was minimal. Hence the
recorded performance parameters are not easily
related to standard performance measures such as
signal to noise ratios or physical quantities such
as power. However, some qualitative
observations can be made from this flight test
data.
International Mobile Satellite Conference. Ottawa, 1990
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TEST DESCRIPTION
A communications link was established
between the 747-400 at Paine Field, Washington
and the COMSAT earth station at Santa Paula,
California. The modulation used was ABPSK
(Aviation BPSK) at a rate of 600 bits per second
with rate one half Forward Error Correction
coding (as described in the ARINC 741
characteristic). The Ground Earth Station (GES)
equipment was controlled from a Collins site via
a conventional telephone modem link to the Santa
Paula station. Collins personnel on the ground in
Cedar Rapids were able to view text messages
received from the aircraft as well as compose text
messages and uplink them to the aircraft. Hence,
two way link operation was verified.
The aircraft was instrumented to record a
number of forward link performance parameters
over time throughout the flight. The data
recorded were: aircraft latitude, longitude,
heading and attitude (pitch and roll). In addition
to these parameters, data provided by the Collins
engineering Satellite Data Unit (SDU) were
recorded: signal quality and bit errors. The
signal quality indication is a relative measure of
signal strength. The value is an indication of I
squared plus Q squared as measured by the
demodulator. The bit errors indication received
from the SDU reported the number of (channel)
bit errors detected in the 6 most recent Signal
Units (or 1152 channel bits). The data collected
were post processed to evaluate the effects of
aircraft attitude and shadowing by aircraft
structure on system performance. The aircraft
position and attitude data were used to transform
the satellite azimuth and elevation into the aircraft
coordinate reference frame.
No return link performance parameters
were recorded. In fact, very little information as
to the performance of the return link was
available to the flight test crew. On the forward
link there is a p-channel which was monitored
continuously. On the return link, the aircraft
transmitted only when a text message was
composed and sent. There was an acknowl-
edgment on the control panel in the aircraft when
a message had been received at the GES.
The satellite used in this test was the
INMARSAT Pacific Ocean Region satellite,
positioned at approximately E 180 °. The GES
function was performed by experimental
equipment installed by Collins at the COMSAT
station in Santa Paula, California.
EQUIPMENT CONFIGURATION
Figure 1 shows a block diagram of the
SATCOM avionics as configured on the Boeing
747-400 aircraft. This configuration represents
the low gain portion of the standard installation
offered by Boeing for the 747-400 at the time of
this flight test. The low gain SATCOM system
design parameters are (from the ARINC 741
characteristic) a receive G/T > -26 dB/K and a
minimum transmit EIRP of 13.5 dBW. The low
gain antenna is a quadrifilar helix intended to
provide omnidirectional azimuth coverage from
zenith down to 5 ° elevation angles. The antenna
is housed under an aerodynamically shaped
radome which is less than 6 inches high. The
design goal is to achieve a minimum of 0 dBic
gain for 85% of the coverage volume defined
from zenith to 5° elevation angles. The low gain
SATCOM system installed on the 747-400 is
believed to have had a receive G/T of -24.3 dB/K
(assuming a 0 dBi aircraft antenna). The transmit
EIRP is believed to have just met the minimum
required, 13.5 dBW.
Figure 2 shows the SATCOM provisions
as offered by Boeing at the time of this
engineering test. The low gain antenna is located
on top centerline at body station 650 (see figure
2). Since the low gain antenna is mounted on the
top of the upper deck, only the tail fin extends
above the horizon as seen by the antenna. Hence
the tail fin may block the direct line of sight to the
satellite for some aircraft attitudes, and bearings.
COMPASS ROSE TEST
The 747-400 aircraft was positioned on a
compass rose at Paine Field Washington (latitude
N 47'54.1", longitude W 122'17.1"). The
nominal elevation to the satellite at this location is
12.5 °. The aircraft was towed in a circle with
stops being made at approximately 10 degree
increments in order to record the bit error rate and
signal quality at each azimuth position. The
signal quality and bit error indications were
recorded manually from the DLC-800 controller
for several seconds at each azimuth position.
Figure 3 shows the data collected during
the compass rose test. The radial axis is in units
of average signal quality. The angle reference is
the beating to the satellite in the aircraft reference
frame. Care was taken to position the aircraft
such that the tail was directly in line with the
satellite. The nominal elevation was such that
line of sight to the satellite was not actually
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International Mobile Satellite Conference, Ottawa, 1990
blocked and no airframe effects were evident in
the data recorded.
FLIGHT TEST
Figure 4 shows the flight path chosen for
the test. The flight path was designed
specifically to test the system over sea and a
variety of land terrain. The Pacific Northwest is
at the edge of coverage for the Pacific Ocean
Region INMARSAT and therefore offers an
excellent opportunity to evaluate the system at its
performance limits and beyond. Special
maneuvers were performed at three locations,
Sedar Waypoint, Moses Lake, Washington and
Glasgow, Montana in order to test the satellite
link at a variety of satellite bearings and
elevations in three different basic multipath
environments.
Near Glasgow, the aircraft adopted a
heading specifically chosen to move the tail into
the line of sight to the satellite. The test crew
were unable to detect any harmful effects on the
link performance due to tail shadowing. Also,
the recorded flight data does not reflect any
degradation in the link due to tail shadowing.
Figure 5 shows the elevation of the
satellite, the signal quality and the bit errors
recorded over a period of time spanning about
half an hour. This data represents the flight
maneuvers conducted over Glasgow Montana
(latitude N 48'13", longitude W 106'37").
Glasgow represents the most extreme nominal
operating environment included in the test with a
nominal elevation to the satellite of =2.3 ° .
During the orbiting maneuver, from the aircraft
reference frame the satellite dipped to more than
22 ° below the horizon. During this extreme
condition, the receiver indicated a high number of
channel bit errors, messages were not received
by the aircraft and messages transmitted from the
aircraft were not received at the GES. However,
during level flight in the vicinity, with a nominal
elevation to the satellite of =4 ° (in the aircraft
reference frame) messages were transmitted and
received successfully.
Figure 6 shows the elevation of the
satellite, the signal quality and the bit errors
recorded over a period of time spanning the
flight maneuvers conducted at the Sedar
Waypoint (latitude N 45°30.5 ', longitude W
126043.0 ') which is off the west coast of the state
of Oregon. This geometry results in multipath
from the sea for all azimuth directions. The
nominal elevation to the satellite in an 'earth
normal' reference frame at this location is 16.4 ° .
It can be seen from figure 6 that the system
continued to operate with a low number of bit
errors when the satellite was below the aircraft
reference frame horizon (as much as 10%
However, as would be expected, at some very
low elevation angles, (greater than 12 ° below the
horizon), the receiver lost lock and a high
number of bit errors is indicated.
Figure 7 shows the average signal quality
level and bit errors as a function of the elevation
of the satellite relative to the aircraft antenna
reference frame. This function was produced by
averaging the data taken over the time period
illustrated in figure 6.
Figure 8 shows data taken during the
maneuvers conducted over Moses Lake (latitude
N 47°12.7 ', longitude W 119018.9 ' )
approximately in the center of the state of
Washington.. Again it can be seen that the
system continued to operate when the satellite
was several degrees below 0 ° elevation relative to
the aircraft antenna reference frame.
Figure 9 shows the average signal quality
level and bit errors as a function of the elevation
of the satellite relative to the aircraft antenna
reference frame for the time period illustrated in
figure 8.
CONCLUSIONS
This was a successful engineering flight
test of an ARINC 741 type low data rate
SATCOM system aboard a 747-400. Two way
link operation was demonstrated under a variety
of operating conditions. In general the SATCOM
system performed far better than expected at low
satellite elevation angles. There were instances
where the communication link failed during
banking maneuvers resulting in very low
elevation angles and shadowing by aircraft
structures. Furthermore, it was found that tail
shadowing does not appear to be significant for
the 747-400 SATCOM system.
REFERENCE
t ARINC Characteristic 741 "Aviation Satellite
Communication System", Aeronautical Radio,
Inc.
International Mobile Satellite Conference, Ottawa, 1990
159
ASSUMED PERFORMANCE:
SKY TEMP.=100 K
EIRP = 13.5 dBW
SYSTEM TEMP.
ATANT= 272 K
GfF = -2,4.3¢_/K
V BALL 511611 LOWGAIN ANTENNA
A ] BALL 511610 DIPLEXER/LNA
NF= 1.SdB
X R G IN = 55 dB
] 1 ANT TO XMIT ATI'EN.=0.8 dB
I HPA-900Po=40 WATTS MIN.
COLLINS 277-0519-040 _ _ COLLINS 277-0519-040
SPLITTER/COMB INER SPLITTFaR]COM BINER
ARINC_ 429 BUS ,i] SDU-900TO BOEING DATA [
RECORDERS
Transmit Frequency - 1642.875 MHz
Receive Frequency - 1541.375 MHz
DLC-800
NOT STANDARD
FOR BOEING
INSTALLATION
I RFU-900 I INTERFACE FORN =I0dB FLIGHT TEST ONLY
I I
Fig. 1. Block Diagram of Low Gain Aeronautical Earth Station
STA 1650
(2) SIDE 12 dBi HGAs*
STA 660 (2) LNA/DIPs •
LGA ItPA • (2) BSUs • STA 1650
.... 1 :o.i°
• SDU (6MCU) •
SDU - SATET.LITE DATA UNIT
RFU - RADIO FREQUENCY UNIT
HGA - HIGH GAIN ANTENNA
LGA - LOW GAIN ANTENNA
HPA - tIIGIt POWER AMPLIFIER
LNA - LOW NOISE AMPLIFIER
HPR - HIGH POWER _Y
DIP- DIPLEXER
STA - BODY STATION
• RFU (4MCU) •
• HGA HPA (8MCU)•
• TOP HGA BSU/ACU (2 TO 6MCU) A
• SIDE HGA tLPR •
• COMBINER •
• SPLITTER •
* PARTIAL PROVISIONS - STRUCTURAL DOUBLER INSTALLATION
• SPACE PROVISIONS - NO TRAY OR BRACKET
i$ MOUNTING PROVISIONS - TRAY OR BRACKET INSTALLED
• EQUIPMENT INSTALLED
Fig. 2. Standard Boeing SATCOM Installation Provisions as of 9/89.
160
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SIGNAL QUALITY
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Flight test of ARINC 741 configuration low gain SATCOM system on Boeing 747-400 aircraft

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