Note: Descriptions are shown in the official language in which they were submitted.
1~Z47~Z3~
SATELLITE TRACKING ANTENNA SYSTEM
BACXGROUND OF THE INVENTION
This invention relates to an antenna apparatus
and in particular to a system for satellite tracking
from an unstable platform.
This invention finds specific utilization in a
ship borne satellite communication system having an
antenna operated to track a satellite despite the
motions of a ship.
Maritime communications are designed to provide
ship-to-shoxe, and in some cases ship-to-ship communi-
cations, utilizing a communication satellite as a
transmitting link. Given the environment of use,
off-shore, the satellite antenna tracking system must
be capable of prolonged, sustained operation, must be
easily maintained and highly reliable.
Such systems first acquire through some form of
external inputs as to position the desired communi-
cations satellite which is customarily placed in a
stationary geosynchronous earth orbit. This requires,
at a minimum, satellite elevation and azimuth data.
Once the satellite has been ac~uired, the pointing
attitude of the antenna is then continuously updated
during the duration of the ship's voyage to maintain
lock-on with the satellite irrespective of changes in
the ships heading and position. Changes in the
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heading of the ship are generally automa~ically com-
pensated in the azimuth axis, typically b~ direct link
to the ship's compass. Position changes are rela-
tively insignificant over short periods of time, ~or
example, in stationkeeping operations. For a yeosyn-
chronous satellite, a one hundred mile change in the
ship's position represents less than a 2 tracking
error, so changes occur gradually relative to changes
in the ships position. To maintain lock when dis-
tances are traversed a tracking algorithm is used
which constantly monitors signal strength and seeks
the position at which it is maximized. Due to the
nature of the algorithm it can only be used to correct
long term errors and not rapid excursions.
A primary difficulty in maintaining lock-on with
the satellite is the ship's motion, primarily pitch
and roll disturbances. In a heavy sea state, rolling
and pitching motion by wave action can be severe and
sudden as well as the turning of the ship to a dif~
ferent, often inadvertent change of heading. Each of
these motions re~uire a change in the orientation of
the antenna to maintain lock-on with the satellite.
Additionally, motions of the ship are often ~uite
sudden and therefore can be applied to the antenna
with considerable force given its orientation dis-
placed from the axis upon which motions of the ship
occur.
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It is customary to mollnt the ant~nna str~cture at
the highest point on the ship to minimize reflections
from the ship's superstructure and from the sea
surface. These reflections tend to cause distortions
or pertubations in the signals received from the
satellite. While mounting at the highest point on the
ship minimizes these disturbances and additionally
tends to reduce interruption of received or trans-
mitted signals which occur by having the signal path
blocked by parts of the ship, the forces applied to
the antenna are exacerbated at this location. That
is, since the antenna is mounted at a position signi-
ficantly removed from the origin of the axis of
rolling, pitching and yawing of the ship, the actual
translation of the antenna is amplified. Moreover, in
the context of large vessels having engines, winches
and the like, vibration is a factor. Consequently,
the antenna structure must be configured to maintain a
lock-on with the satellite irrespective of all of
these external forces tending to move the antenna
suddenly randomly and with great force.
Accepted techniques of maintaining antenna sta-
bility have been promised on first establishing a
stabilized platform and then mounting the antenna on
the stabilized platform. This technique generally
uses gravity sensors, gyros, and accelerometers to
determine on a real time basis motions of the ship.
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Gears and the lilce are then wti.lized to maintain a
platform in a 1.ocal. horizontal pl.ane irrespoctive ol~
motions oE the ship beneath it. The antenna mounted on
the platEorm can then track the satellite irrespective oE
5 movement of the ship. This technique ls shown with
variations in terms of active sensors in United States
Patents 3,893,123, 3,999,184~ 4,020,491, 4,035,805,
4,118,707. In these systems with a stabilized pedestcll,
-the antenna is then configured :Eor motion by elevation
10 over azimuth. In such a system, as the ship rotates under
the an-tenna, the azimuth axis compensates for ship's
heading changes. Azimuth axis correction is therefore
360. Motion in the elevation axis is generally 90. It
can be appreciated that the elevation over azimuth
15 techniques is derived from ground based antenna systems
where the mount can be leveled and fixed.
A recognized problem with this system is that as -the
ship rotates and azimuth corrections are made the
connecting feed cables tend to wrap around the antenna
20 mast. In order -to con-tinue operations, these cables must
be periodically unwrapped by antenna rotation before it
can con-tinue tracking the satellite. Thus, a loss in
communications results when this unwrapping process
occurs.
A more serious problem is the complexity and weight
associated with this system. Gravity sensors,
~.~h~e7?J3~
accelerometers and gyroscopes us~,d to provide sensor
inputs for the stabilized platform are expensive and
not considered to be highly reliable elements in the
harsh off-shore environment. Moreover, each of the
elements must be counterweighted such that the pede-
stal itself is balanced and then the antenna system on
top of the pedestal is also balanced. Such systems
tend to be relatively heavy, about 300-400 pounds for
the stabilized platform and about 700-800 lbs for the
complete system including a 4 ft. antenna including
the radome. Given the fact that this entire apparatus
is mounted on a pedestal above the ship superstruc-
ture, it is then also necessary to, in some cases,
reballast the ship to avoid excessive rolling. Given
the complexity and weight of such systems, there usage
has generally been confined to large ships capable of
carrying and supporting such systems.
In an attempt to eliminate complexity, but not
neccessarily weight, a second technique has been to
define a passive stabilized platform upon which the
antenna is mounted. Thus, rather than utilizing
active sensors on the platform, the pedestal is
mounted on a universal joint which is heavily
ballasted to establish a pendulum type structure. In
order to provide stability along one axis and therby
decouple motion in one direction, it is customary to
mount a pair of counterrotating momentum wheels on the
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pedestal~ A pair of momentum wheels is necessary to
cancel out the torque which would be generated by the
single unit. Such a passive system eliminates the
complexity and aspects of unreliability in prior art
active systems, however, the weight penalty remains.
Moreover, passive stable platform utilizes the same
elevation over azlmuth motion having the wire wrapping
problem as defined relatively to an active stabilized
platform.
SUMMARY OF THE INVENTION
Given these deficiencies in the prior art, it is
an object of this invention to define a, ship borne
antenna system which will continuously and accurately
attract a satellite irrespective of movement of the
ship.
Another object of this invention is to provide a
ship borne satellite antenna system does that not
require a rewrap cycle to unwind a feed cable wrapped
about the pedestal mast.
A further object of this invention is to provide
a ship borne antenna system of reduced weight and
complexity which finds utilization on a wide range of
ocean going vessels.
A still further object of this invention is to
provide a ship borne antenna system'which is highly
reliable and capable of prolonged operation in the
open sea.
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These and other objects of this invention are
attained by utilizing a gimbaled mount for a light
weight tracking antenna. The present invention pro-
ceeds from a recognition that a stabilized platform is
a precursor to provide known reference points for
antenna movement. Additionally, by not utilizing an
elevation over azimuth tracking system a continuous
overhead operation is possible irrespective of whether
the antenna is pointing along any principle axis.
In accordance with the present invention, the
same two axes of motion used for antenna tracking,
that is roll and pitch, are also used to compensate
for the lack of a stable platform. Signals indicative
of the ships displacement from a horizontal position
are fed to a microprocessor which then computes cor-
rections necessary to modify antenna pointing angles.
Bearing and elevation data is first received from a
serial data link for purposes of antenna-satellite
lock-on. Then, roll and pitch data obtained from
onboard sensors is provided to the microprocessor to
provide the necessary corrections for delay and linear
acceleration as a function of ship movement. With
these corrected pitch and roll inputs and the required
bearing and elevation, the microprocessor then calcu-
lates the position of the antenna along two axes
utilizing spherical coordinate translation algorithms.
The microprocessor then calculates the rate of
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rotati.on Eor each clXi9 needed to move to the ~e~luired
position Erom the last known posit;.on an that axis.
Stepper motors are used to drive the system.
This inven-t.ion will be described in greater detail by
referring to the attached drawing and the description of
the preEerred embo-.liment which Eollows:
BRIE.F DESCUIPrION OF T~ ~ DR~WINGS
Figure 1 is a schematic elevation view illustrating
an antenna ship mounting utilizing a standard elevation
over azimu-th mount as referred to above;
Figure 2 is a schematic e].evation drawing
illustra-ting the gimbled mount in accordance with the
present invention;
Figure 3 is a perspective elevation end view of a
ship illustrating the definition of the ring axis of
rotation of -the antenna system;
Figure 4 is a schematic elevation side view
illustrating the dish axis of rotation of the an-tenna
system of this invention;
Figure 5 is a schematic plan view illustrating
limited third axis motion Eor azimuth correction to
accoun-t for a special condition where the sa-tellite is low
on the horizon and directly aligned with the longitudinal
axis of the ship and the ring axis of this system;
Figure 6 is a schematic elevation view illustrating
the components of the antenna system of this invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figures 2, 3 and 4, the antenna
system of this invention will first be described
relative to the respective axis of movement.
A ship 10 having a superstructure 12 carries a
radome 14 to position an antenna 16 at the highest
possible point to minimize interEerence by reflection
and the like. The radome 14 is generally fixed to the
ship by means of an antenna mast 18. It lS understood
that in Figures 2, 3 and 4 the radome is shown grossly
out of scale to illustrate the .antenna disposed
therein. In practice, in accordance with the present
invention, the radome 14 would be only so large as
necessary to house and allow for movement of a
dish 14, typically 48 inches in diameter.
A feed cable 20 provides the electronic link
between the ship onboard electronics and the antenna
electronics. The cable merely hangs from the diplexer
fixed to the back of the dish 14.
In accordance with the present invention, the
antenna 16 is mounted in a gimbal mount configuration
allowing free movement along two axis. That is, the
antenna design in accordance with the present
invention has two primary axis. These are designated
as the ring axis of Figure 3 and the dish axis of
Figure 4. With the ship steady and level, these two
axis can point to any position in the sky with only
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180 motion re~uired of ei-ther axis. This primary
mode of movement is distinguishable from the elevation
over azimuth mount which utilizes 90 of elevation
motion and 360 of azimuth. This difference allows
the gimbal mount of the present invention to eliminate
the problem of rewrapping since both axes are used for
compensation. Because the antenna must always point
upward, the cable merely hangs down. The antenna does
not rotate relative to the ship and therefore no
rewrap cycle is required.
A ring 22 is mounted onto the radome 14 and is
aligned parallel to the lubber line of the ship.
Thus, as shown in Figure 3, the ring axis is disposed
orthogonal to the plane of the drawing running the
length of the ship. Rotation about this axis shown by
the the arrows 24 allows not only for 180 rotation,
but also a full 45 depression in either direction to
compensate for severe rolling of the ship.
As shown in Figure 3, arrow 26 dipicts the
position of the antenna feed 28 with the ring 22
aligned horizontally. Assuming the antenna 16
remaining stationary relative to the ring 22, if the
ring then rotates counterclockwise to an orientation
shown in dotted lines 30, the orientations of feed
will be in the direction of the arrow 32. If the
ring 22 rotates clockwise, again with the dish re-
maining stationary relative to the ring, it assumes a
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position shown via dotted lines 34. The direction of
the feed would then be represented by the arrows 36.
It can thus be appreciated that by movement of the
ring 22 along the arcuate path shown by arrows 24, the
positionin~ of the dish 16 relative to any change in
the ship's roll axis can be effectuated to maintain a
stable pointing angles vls-a-vis a fixed position in
the sky, i.e. a synchronous satellite.
Referring now to Figure 4, the dish axis, that is
the axis of movement of the parabolic reflector 16 is
illustrated. As will be described herein, the para-
bolic reflector dish 16 is mounted on the ring 22 and
carries with it a motor for motion relative to the
ring 22. Consequently, a dish axis of rotation is
established orthogonal to the ring axis. The dish
axis runs athrawtship.
With the dish axis parallel to the ship, the
feed 28 would point directly vertical as shown by
arrow 38. If, however, the dish 16 were to be rotated
clockwlse, as shown by the dotted lines 40, the feed
would point in the direction represented by arrow 42.
If the dish 16 is rotated along the dish axis in a
counterclockwise direction, as shown by the dotted
line position 44, the fee 28 would point in the
direction of arrow 46. It can therefore be appre-
ciated that the dish axis defines 180 over the top
motion shown by the arrow 48. Motion along the dish
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axis thereEore compensales for E).itch mol-.;on oi the shi.p.
Referring now to Figure 6, the essential. comporlerllF,
oE the antenna systern of this inve.ntion are illustrated.
The radome comprises two sections, a lower section 14 and
a compatible mating upper section 15. The radome is
generally made of plastic or fiberglass and provides an
environmental protective shell for the antenna components
therein. The radome is moun~.ed on mast 18 generally
positioned on the shipls superstructure. A fiberglass
ring 22 is journaled for rotation relative to the radome
upper section 15. That is~ as shown in Figure 6, the ring
22 has at each end a pair of bearings 50, 52 which are
respectively journaled for rotation in bearings 54, 56.
The bearings are nonmetallic PTFE on plasticO The ring 22
may be a solid or a hollow fiberglass ring configured to
handle the load of the parabolic dish 16 and its
associated equipment in a stable manner. That is, the
ring 14 is configured to withstand the necessary vibrat.ion
and shock loadings imposed by motion of the ship as well
as movement oE the dish 16 in a stable manner without
flexing. A hollow hexagonal shape is shown in Figure 3;
it being understood that any high strength-to-weight ratio
structure can be configured for the ring 22. Shaft 52
carries with it a gear 58. A stepper motor 60 is
.
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13
mounted to the radome shell poxtion 15 for rotating a
second gear 62. A belt 64 is used as a transmission
mechanism to convert motion of the motor 60 into
rotation of the ring 22 via gears 58 and 62.
While not illustrated, it is appreciated that the
stepper motor 60 will be housed in an environmentally
secure structure. While a bel-t drive is shown other
known drive techni~ues may be employed. Also, the
stepper motors may be replaced with other known types
of motors, for example, servo motors may bs employed.
Also, the stepper motors may be replaced with other
known types of motors, for example, servo motors may
be employed.
The parabolic dish 16 is preferably made of
graphite fiberglass fibers to provide a temperature
stable conductive surface requiring no additional
coatings. Typically, such clishes are approximately 48
inches in diameter. Dish 16 has a flanged circum-
ferential backing structure 66 to provide the neces-
sary structural stength for mounting the dish 16 onto
the ring 22. A pair of mounting brackets 68 couple
the dish 16 and its intergal flange 66 to the ring 22
via shafts 70. It will therefore be appreciated that
the axis of the shafts 70 define the dish axis. The
dish then mounts inside the ring and is journaled for
movement relative to the ring. As the ring is rotated
the dish is similarly rotated. A two degree of
freedom gimbal mount is therefore defined.
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In accordance with known communicati~n antennas,
dish 16 also carries a feed 28 disposed at its geo-
metric center. The feed 28 comprises a helical head.
The geometric configuration of the dish 16 and the
feed 28 are well known and established in this tech-
nology. Mounted on shaft 17 is a gear 72. A second
stepper motor 74 carrying with it a gear 76 drives the
dish 16 about the dish axis utilizing belt 78 as a
transmission mechanism. Stepper motor 74 mounted on
the dish 16 therefore "pulls" the dish about the dish
axis during rotation. It will be.appreciated that
other techniques of driving the dish relative to the
ring may be employed. Also shown ,in Figure 6 are the
associated diplexer electronics 73 providing the
electronic coupling between the antenna and its feed
to ship onboard electronics. The diplexer 73 is
mounted to the back of the dish 16 in a position to
allow static balancing of the motor 74 and its asso-
ciated gear 76. Consequently, the diplexer also acts
as a counterweight for the motor allowing static
balancing without the need of additional weights. It
is apparent that while offering advantages mounted as
described, the diplexer can be mounted elsewh,ere.
Wiring for the stepper motor 74 is carried on the
ring to an appropriate position and then routed exter-
nally with the associated leads for the stepper
motor 60. The antenna wiring from the diplexer merely
-
~ 72~4~
.~ .
hang from the antenna. Given the fact that there is
no continuous rotational movement about pedestal 18,
it will be appreciated that no wrapping of the wire 20
occurs relative to either the pedestal, the ring or
the dish. Consequently, the problem of wiring wrap
and the attendant rewrap cycles are eliminated by this
invention.
The all up weight of the structure shown in
Figure 6 exclusive of the mast 18 is in the order of
50 lbs. It is appreciated that this is approximately
a ten-fold decrease relative to the weight of prior
art systems. This significant decrease in weight is
important since it minimizes if not eliminates the
requirement for additional balIasting to trim out a
ship. Importantly, given the antennas reduced weight,
it can be carried by smaller vessels. Fishing vessels
and larger yachts can therefore realize offshore
satellite communication capability heretofore unknown.
Another advantage of the present inventlon is
that given its weight, installation is materially
simplified. The prior art systems require cranes, or
in some case helicopters, to lift the antenna system
and position lt on the mast 18. The present invention
in contrast, can easily be positioned by two people.
Moreover, given the elimination of the reguirement o
the stable platform, the attendant accelerometers,
gravity sensors and gyros located on the platform, the
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entire device is materially simpliEied. Reliability
i6 therefore enhanced.
In operation, the dome is positioned on the
mast 18 with the ring axis aligned parallel with the
ship`s lubber line. The device is leveled relative to
the static trim of the ship. The motors 60 and 74 are
stepper motors of conventional design utilizing
optical encoders to provide positive feedback of
stepper motor rotation. Such stepper motors are known
per se and have been proposed for driving the stabi-
lized platform of prior art devices to compensate
pitch and roll movements (see, U.S. Patent 4,035,805).
The stepper motors 66 and 74 are initially driven
under computer control to zero position end points so
that an initial position of both the ring and dish are
established, though actual antenna position can be
determined in other ways, such as feedback devices.
The position of the ship in terms of latitude and
longitude coordinates are then fed into a first
microprocessor. This data comes from external inputs
such as LORAN or NAVSAT receivers. This processor
typically a Zilog Z-80 microprocessor also receives as
a second input satellite position data and converts
these inputs into bearing and elevation signals to
initially orient the dish. The initial bearing and
elevation data signals are also used as input to a
second microprocessor, also a Z-~0 which receives real
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time pitch an~ roll data from sensors located onboard
the ship. Pitch and roll sensors, not shown, are, in
accordance with the present invention, not positioned
on the antenna system but rather housed o~-board to
provide data directly responsible to ship motion.
The seconcl microprocessor recei.ving bearing and
elevation data together with pitch and roll data then
performs a coordinate translation utilizing spherical
coordinates. The translation routines may be written
in Z-80 machine language employing high speed algo-
rithms for determining trigonometric functions when
needed. The thus assembled machine language routines
are stored in read only memories (ROM) connected to
the Z-80 microprocessor. The outputs are signals to
the stepper motors 60 and 74 to drive the ring 22 and
the dish 16 to lock-on for satellite acquisition.
Thereafter, the second microprocessor receiving pitch
and roll data on a real time basis continually updates
disk position by providing continuous signals to the
stepper motors for positive tracking.
Consequently, as can be appreciated, a two-axis
stabilized system about pitch and roll is defined
utilizing the present invention without the necessity
of stabilized platform to mount the antenna. The use
of the microprocessors eliminates the prior art re-
quirements for first defining and maintaining a stable
platform and then, providing elevation over azimuth
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data fox driving the dish mounted on the platform.
Rather, a dynamic system is defined herein for con~
tinuously driving the dish utilizing a gimbaled mount.
An advantage of utilizing a motor for driving the
system in continuous operation is that greater relia-
bility is achieved. It has been found that motors
used to provide stable platforms tend to develop flat
spots given the fact that motion, especially pitch
correction, occurs over a very limited bandwidth and
that all motors are not continuously in operation.
However, that reliability is enhanced by continuously
driving stepper motors to avoid flat spots and seizing
as a function of bearing failure and lubricant dis-
sipation.
Thus, a ship is generally continuously undergoing
incremental motion. When the ship is pitching,
rolling and yawing, stabilization is provided by
read-outs from on board sensors and corrections are
continuously made given the finite movement possible
with st~pper motor actuation. Roll motion up to a
re~uired 30 may be compensated directly by the ring
axis which is normally aligned with the roll axis of
the ship. However, in rare situations the satellite
may be on the horizon requiring that the ring axis be
depressed at least 30. This depr~ssion is shown in
Figure 3. Compensation for pitch motion, up to 15 is
more complicated because the dish axis will not
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directly compensate for pitch as the ring axis does
for roll. Rather, a co~ination of ring and dish axis
motion is required for pitch motion compensation.
While the dish axis motion is small, the roll action
motion is dependent on the bearing of the satellite
from the ship. For example, when the satellite is low
on the horizon, and directly aligned with the ring
axis, a full 360 of motion could be required of the
ring axis for minor changes in azimuth and yaw.
In order to prevent this motion, the present
invention allows for a limited ~otion about the
azimuth axis. As shown in Figure 5, a condition may
exist where the satellite lies directly on the ring
axis. In order to prevent this situation, potentially
requiring a full 360 of motion of the ring axis to
continuously align itself, a technique is used to
allow for incremental changes. Specifically, rotation
of the entire dome 14 occurs to a limited extent. As
shown in Figure 6, a third motor 80 is provided and
coupled via gear mechanism 82, mounted on the pedestal
18, for driving the entire dome 14 to move the ring
axis away from the satellite direction. Other
mechanical arrangements to produce this rotational
motion are possible. Also the motor need not be in
the azimuthal direction; elevating the axis by
tilting the dome is also possible. The main point is
that the axis is moved from the line to the satellite.
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This limited movement in the azimuth clirection is
shown in Figure 5. The axis is limited to approxi-
mately 40. Given this limitation, it is appreciated
that the ring axis must sometimes pass through the
direction of the satellite in order to affirmatively
move away from it. When such motion is required, the
action is timed so that the satellite is above the
plane of the ship when third axis motion occurs.
Given the availability of motion in the third
axis, the total required motion of the ring axis can
therefore be limited to approximately 270, that is
with 45 of depression on either side of the hori-
zontal.
It is appreciated that other modifications of
this invention may be practiced without departing from
the essential scope of this invention. While this
invention has been describecl in use relative to a ship
it is apparent that it may be used in other vehicles
or environments of use where motion influences
tracking ability. Also, while a dlsh antenna is
illustrated, this invention can be used with other
types of antenna structure, for example a helical
antenna or the like.
