International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 6

Number 2

June 2012

165

1 INTRODUCTION

Inmarsat started operations 1981 with only Inmarsat-

A maritime service and for that reason devised an in-

itial synonym: INternational MARitime SATellite

(INMARSAT). The SES is electronic equipment

consisting in antenna and transceiver with peripheral

devices usually installed on board ships or sea-

platforms and rigs.

Later, Inmarsat developed other SES standards

with mandatory and obligatory equipment, such as

B, M, mini-M, C, mini-C, D, D+, Fleet F77, F55 and

F33, including latest FleetBroadband. In Figure 1.

are presented Above Deck Equipment (ADE) and

Below Deck Equipment (BDE) general block dia-

grams of electronic units for SES terminals. The

main elements of BDE are the following units [01]:

1 Cabin Interface Unit (CIU) – With build in PC or

system processor Cabin Interface Unit controls

and monitors the whole system operations of the

transceiver and direction of the antenna dish, and

also performs different task of baseband signal

conditions between all obligation and optional

BDE peripheral equipment (Tel, Fax, Tlx, Video

on one hand and Data) and Baseband Processor

on the another hand.

2 Baseband Processor – This processor simply per-

forms baseband processing of all transmitting and

receiving audio, video and PC or data signals. In

such a way, Baseband Processor comprises In-

termediate Frequency (IF) amplifier, modems and

timing circuits for multiplexing up and down sig-

nals. The synthesizer produces the highly stable

frequencies required for modulation and demodu-

lation and for signal switching.

3 Interface Terminal – Connects navigation Gyro

Compass and Motion Sensor Units with CIU de-

vice for satellite tracking and electronic control of

ADE.

4 Modulator and Demodulator – Both represents

the nucleus of any transceiver. First modulates

the baseband signal onto the IF carrier of trans-

mitting signal and second demodulates the base-

band signal from the IF carrier of the receiving

signal.

Shipborne Satellite Antenna Mount and

Tracking Systems

S. D. Ilcev

Durban University of Technology (DUT), Durban, South Africa

ABSTRACT: In this papers are introduced the very sensitive components of the ship’s antenna tracking sys-

tem as the weakest chain of the Maritime Mobile Satellite Service (MMSS). Also are presented the complete

components of Ship Earth Station (SES), such as antenna system and transceiver with peripheral and control

subsystems independent of ship motion. The communications Mobile Satellite Antennas (MSA) for Maritime

Satellite Communications (MSC) are relatively large and heavy, especially shipborne directional Inmarsat B

and Fleet-77 antenna systems. Over the past two decades the directional antenna system, which comprises the

mechanical assembly, the control electronic and gyroscope, the microwave electronic package and the anten-

nas assembly (dish, arrays and steering elements), is reduced considerably in both physical size and weight.

These reductions, brought about be greater EIRP from satellite transponders coupled with GaAs-FET tech-

nology at the front end the receiver leading to higher G/N RF amplifiers, has made the redesigning, adopting

and installing of shipborne antennas even on tracks and airplanes a reality.

166

Figure 1. Maritime ADE and BDE Configuration

Courtesy of Book: “Global Mobile Satellite Communications”

by St. D. Ilcev [01]

5 BDE Diplexer (DIP) – Enables direction of

transmitting signals from Modulator to the ADE

and receiving signals from ADE to the Demodu-

lator.

The ADE is mounted below the waterproof con-

tainer or radome on the Stabilized Platform. The fol-

lowing units compose the ADE assembly:

1 ADE Diplexers (DIP) – The first diplexer passes

all transmitting signals to the Up Converter and

from Down Converter to the Demodulator. The

second diplexer guides transmitting signals from

HPA to the SAU and from SAU to the LNA.

2 Up and Down Converters – This unit accepts the

modulated IF carrier from modulator and trans-

lates it to the uplink transmitted RF via HPA, by

mixing with Local Oscillator (LO) frequency,

while the Down Converter receives the modulat-

ed RF carrier from the LNA and translates its

downlink receiving RF to the IF.

3 High Power Amplifier (HPA) – This unit pro-

vides amplification of transmitting signals by the

Traveling Wave Tube Amplifier (TWTA) and

Klystron Amplifier. The second HPA enables

higher gain and better efficiency than TWTA but

in smaller bandwidth of 2%. The amplified uplink

signal goes via DIP to the SAU.

4 Low Nose Amplifier (LNA) – The LNA device

provides initial amplifier stage of downlink signal

coming from SAU via DIP without introducing

much additional temperature noise. In this sense,

the two most commonly used LNA products are

new GaAs (Gallium Arsenide) FET (Field Effect

Transistor) and old Parametric amplifiers. Thus,

the recent developed GaAs FET LNA enables

very low noise temperatures and takes advantages

of its stability, reliability and low cost.

5 Antenna Control Unit (ACU) – Antenna Control

Unit is providing control of the ship antenna Sta-

bilized Platform (Stabilizer) and Tracking Sys-

tem. Thus, it maintains the antenna direction to-

wards the focus of satellite against any motion of

the ship.

2 ANTENNA MOUNT SYSTEMS

The MSA system is generally mounted on a plat-

form, which has two horizontally, stabilized axes (X

and Y), achieved by using a gyrostabilizer or sensors

such as accelerometer or gyrocompass units. The

stabilized platform provides a horizontal plane inde-

pendent of mobile motion such as roll or pitch. For

example, all mobiles have some kind of motions, but

ship motion has seven components during naviga-

tion such as: roll, pitch, yaw, surge, sway, heavy,

and turn, shown in Figure 2. Turn means a change in

ship heading, which is intentional motion, not

caused by wave direction, and the other six compo-

nents are caused by wave motion. Surge, sway, and

heave are caused by acceleration.

2.1 Two-Axis Mount System (E/A and Y/X)

An antenna mount is mechanically moving system

that can maintain the antenna beam in a fixed direc-

tion. In MMSS, the mount must have a function to

point in any direction on the celestial hemisphere,

because ships have to sail across the heavy seas. It

is well known that the mount of the two-axis antenna

configuration is the simplest mount providing such

functions [02]

Figure 2. Components of Ship Motion

Courtesy of Book: “Mobile Antenna Systems Handbook” by

K. Fujimoto and J.R. James [02]

Figure 3. Two and Four-axis Mount Systems

Courtesy of Book: “Mobile Antenna Systems Handbook” by

K. Fujimoto and J.R. James [02]

167

There are 2 typical mounts of the-axis configura-

tion: one is E/A (elevation/azimuth) mount and the

other is the Y/X mount. Simplified stick diagrams of

both mounts are given Figure 3. (A) and (B) respec-

tively. Thus, in the E/A mount, a full steerable func-

tion can be obtained by choosing the rotation range

of the azimuth axis (A-axis) from 0-90

. In the Y/X

mount a full steerable function is achieved by per-

mitting the rotation angle from –90

to +90

to both

the X and Y-axis. In fact, this is the basic configura-

tion for the ships utility, so a special function re-

quired for its antenna mount system is to compen-

sate the ship motions due to sailing and ocean

waves, and to keep the antenna beam in nearly a

fixed direction in space. In the case of the pointing

and tracking under ship motions, the required rota-

tion angle range of each axis is from 0

to more than

360

for the A-axis, and from –25

to +120

for the

E-axis with respect to the deck level, assuming that

the operational elevation angle is de facto restricted

above 5

[01]. Otherwise, both mount types have

several disadvantages.

2.2 Three-Axis Mount System (E/A/X, E’/E/A and

X’/Y/X)

The three-axis mounting system is considered to be

modified two-axis mount, which has one additional

axis. The three-axis mount of an E/A/X type is

shown in Figure 4. (A), which is the E/A mount with

one additional X-axis. The function of the X-axis is

to eliminate the rapid motion of the two-axis mount

due to roll. However, in this system, the possibility

of the gimbal lock for pitch is still left near the zen-

ith, when the E-axis is parallel to the X-axis. The

three-axis mount of an E’/E/A type shown in Fig-

ure 4. (B) is the E/A mount with an additional cross-

elevation axis E. In the mount system, the charge of

the azimuth angle is tracked by rotating the A-axis,

and the change of the azimuth angle is tracked by a

combined action of the E and E’ axes. Hence, the E

and E’ axes allow movements in two directions at a

right angle. With an approximate axial control, this

mount is free from the gimbal lock problem near

both the zenith and the horizon. The three-axis

mount of an X’/Y/X type is the two-axis Y/X mount

system with the X’-axis on it to remove the gimbal

lock at the horizon, presented in Figure 4. (C). When

the satellite is near the horizon, the X-axis takes out

the rapid motion due to yaw and turn. In this sense,

the X’-axis rotate within ± 120o, so the X’-axis can

only eliminate the rapid motion within the angular

range. In general, this axis mount is rather more

complex than that of four-axis mount, because steer-

ing and stabilization interact with each other [02].

Figure 4. Three-axis Mount System

Courtesy of Book: “Mobile Antenna Systems Handbook” by

K. Fujimoto and J.R. James [02]

Figure 5. Functional Block Diagrams of Step and Program

Tracking

Courtesy of Book: “Mobile Antenna Systems Handbook” by

K. Fujimoto and J.R. James [02]

2.3 Four-Axis Mount (E/A/Y/X)

The stabilized platform made by X/Y-axis to take

out roll and pitch, and two-axis mount of the E/A

type is settled on the stabilized platform. This is

four-axis mount solution, illustrated in Figure 3. (C).

The tracking accuracy of this mount is the best solu-

tion because the stabilization function is separated

from the steering function, and at any rate, four ma-

jor components such as roll, pitch, and azimuth and

elevation angles are controlled by its own axis indi-

vidually as well. Accordingly, the four-axis mount

has been adopted in many SES antenna systems of

the current Inmarsat-A and B standards [01].

3 ANTENNA TRACKING AND POINTING

SYSTEMS

Tracking and pointing system is another important

function required of the antenna mount system. It

should be noted that the primary requirement for

SES tracking MSA systems are that they have to be

economical, simple and enough reliable. Tracking

performance is a secondary requirement when an an-

tenna beam width is broad [02].

1 Manual Tracking - This is the simplest method,

wherein an operator controls the antenna beam to

maximize the received signal level. At first, the

operator acquires the signal and moves the anten-

na around one axis of the mount. If the signal lev-

el increases, the operator continues to move the

antenna in the same direction. If the signal de-

creases, the operator reverses the direction and

continues to move the antenna until the signal

level is maximized. The same process is repeated

around the second axis and the antenna is held af-

168

ter both axes when the received signal level de-

creases. This method is suitable for Land Mobile

Satellite Communications (LMSC) and especially

for portable and fly-away terminals.

2 Step Tracking - Among various existing auto

track systems, the step track system has recently

been recognized as a suitable antenna-tracking

mode for SES terminals because of its simplicity

for moderate tracking accuracy. In such a way,

recent design and development of integrated cir-

cuits and microprocessors have brought a greatly

remarkable cost reduction to the step track sys-

tem, which principle is the same as that of the

manual track. The only difference is that an elec-

tric controller plays the role of an operator in the

manual track. The schematic block diagram of the

step tracking system is shown in Figure 5. (A).

Sample-hold circuits are used to hold the signal

level, which are compared before and after the

antenna has been moved by a present angular

step. If the level is increasing, the antenna is

moved in the same direction, and vice versa, if

the level is decreasing, the direction will be re-

versed. This process will be carried out alternate-

ly between two axes levels, which accuracy de-

pends on the sensitivity of comparators. As a

result, the beam center is maintained in the vicini-

ty of the satellite direction. Thus, wrong decisions

on the comparison of levels generally arise from

the S/N ratio and the level changes due to the

multipath fading and the stabilization error.

3 Program Tracking - The concept of the program

system is based on the open-loop control slaving

to the automatic navigation equipment, such as a

ship gyrocompass, GPS, the Omega and Loran-C

systems. In the program tracking, the antenna is

steered to the point of the calculated direction

based on the position data of the navigation

equipment. Since the satellite changes because of

roll, pitch, and turn direction, a function to re-

move the rapid motions is required in the pro-

gram track, which block diagram is shown in

Figure 5. (B). The error of navigation equipment

is negligibly small for the program track system,

while the error of it mainly depends on the accu-

racy of sensors for roll, pitch, and turn directions,

what is the stabilization error. In fact, an adequate

sensor for the program track system is a vertical

gyro, because it is hardly affected by the lateral

acceleration. When the stabilization requirement

is lenient, the conventional level sensor, such as

inclinometer, a pendulum, and a level, may be

used with careful choice of the sensor’s location.

Therefore, the controller calculates the direction

of the satellite orbit to compensate differences for

the ship’s motions affected by all components. In

any event, the simpler the axis configuration of the

mounts, the more complex the program calculation

procedure becomes.

Figure 6. Safe Distance of Inmarsat-C Antenna from Obstruc-

tions

Courtesy of Manual: “Sailor Maritime Inmarsat-C” by Thrane

& Thrane [03]

More exactly, since the program controller has to

execute different calculations of many trigonometric

functions, a microprocessor is a candidate for the

controller. However, the program tracking system is

also applicable to the four-axis mount. A combina-

tion with the step track system is more desirable be-

cause the error of the program track system can be

compensated by the step track system and its error

due to the rapid ship motions can be compensated

for by the program tracking system [01].

4 OMNIDIRECTIONAL SHIPBORNE MSA

MOUNTING

When installing MSA is necessary to find a location

on board of ship that is as free from any obstructions

as possible. On the other hand, also is important to

maintain a certain distance to other communication

antenna systems, especially radar installations. Fi-

nally, the best place for the MSA on board ship

would be above radar scanning antennas or far a way

from them. Otherwise, the minimum safe distance

should be maintained to HF antenna 5 m, to VHF

antenna 4 m, and to magnetic compass 3 m [04].

The omnidirectional antenna is designed to pro-

vide satellite coverage even when the vessel has

pitch and roll movement up to 15

. In this sense, to

maintain this coverage the ship antenna should be

free from any obstructions in the area down to 15

below the horizon, as is shown in Figure 6. (A).

Since this may not be possible in the fore and aft di-

rections of the vessel, the clear area can be reduced

to 5

below the horizon in the fore and aft directions

and 15

below the horizon in the port and starboard

directions. Otherwise, any compromise in this rec-

ommendation will degrade performance. If an ob-

struction such as a pole or a funnel is unavoidable,

the distance to these objects should large enough, so

that the obstruction only covers 3o. For instance, if

the diameter of ship obstruction object is 0,1 m, the

169

safe distance should be about 2 m, as is shown in

Figure 6. (B).

The safety levels for the Thrane & Thrane

Capsat-C Antenna Unit and similar Inmarsat-C aeri-

als are based on the ANSI standard C95.1-1982.

Namely, this standard recommends the maximum

power density at 1,6 GHz exposed to human beings

not to exceed 5 mW/cm

. Therefore, at the maxi-

mum radiation output power from Inmarsat-C an-

tenna of 16 dBW EIRP corresponds to a minimum

safety distance of about 30 cm. So, the future stand-

ard from the European Telecommunication Standard

Institute (ETSI) concerning 1,5/1,6 GHz Mobile

Earth Station (MES) the new recommendation will

be maximum 8 W/m

(0,8 mW/cm

) with minimum

safety distance on 62 cm at 16 dBW of EIRP [01].

5 DIRECTIONAL MSA MOUNTING AND

STEERING

The directional ship Above Deck Equipment (ADE)

consists of an antenna unit mounted on a pedestal,

an RF unit, power and control unit, all covered by a

radome. Ideally, the antenna should have free optical

sight in all directions above an elevation angle of 5o.

The antenna must be placed as high as possible on

the best position on board of ship to avoid blind

spots with degradation and/or loss of communication

link, caused by different deck obstacles [05].

5.1 Placing and Position of SES Antenna Unit

The directional antenna has a beamwidth of 10

and

ideally requires a free line of sight in all directions

above an elevation angle of 5

. Possible obstructions

will cause blind spots, with the result of degradation

or even loss of communication link with the satellite.

So, complete freedom from degradation of the signal

propagation is only accomplished by placing the

ship antenna above the level of possible obstruc-

tions.

Figure 7. Theoretical Antenna Installation

Courtesy of Book: “Global Mobile Satellite Communications”

by St. D. Ilcev [01]

This is normally not feasible and a compromise

must be made to reduce the amount of blind spots.

The degree of degradation of the communication de-

pends on the size of the ship obstructions as seen

from the antenna, hence the distances to them must

also be considered. However, it should be remem-

bered that the antenna RF beam of energy possesses

a width of 12

angle cone and consequently, objects

within 10 m of the radome, which cause a shadow-

ing sector greater than 6

, are not likely to degrade

the electronic equipment significantly. Preferably,

all obstructions within 3 m of the shipborne antenna

system should be avoided.

Obstructions less than 15 cm in diameter can be

ignored beyond this distance. Knowing the route that

the ship normally sails allows a preferable sector of

free sight to be established, thus facilitating the loca-

tion of the antenna unit.

In such a way, the antenna beam must be capable

of being steered in the direction of any GEO satellite

of the Inmarsat constellation, whose orbital inclina-

tion does not exceed 3

and whose longitudinal ex-

cursions do not exceed ±0.5

. Therefore, means must

be provided to point the antenna beam automatically

towards the satellite with sufficient accuracy to en-

sure that the G/T and EIRP requirements, namely re-

ceive and transmit signal levels, are satisfied contin-

uously under operational conditions.

Careful and important consideration should be

given to the placing of an Inmarsat-A or B standard

ADE with antenna radome. Essentially, the focal

point of the parabolic antenna must be pointing di-

rectly at the GEO satellite being tracked without any

interruption of the microwave beam, which may be

caused by any obstruction on the ship. Inmarsat

specify that there should be no obstacle that is likely

to downgrade the performance of the equipment in

any angle of azimuth down to an elevation of –5

which is not easy to achieve. Thus, the SES design

and installation guidelines of Inmarsat explains a

theoretical satellite antenna installation instructions

mode satisfying this advice but with the disad-

vantage that the antenna system is very high above

the vessel’s deck and would be impossible to install

in such a way, see Figure 7. Differently to say, this

type of installation is not practical because in reality

a ship’s satellite antenna would certainly be very ad-

versely affected by extremely strong and gusty wind,

will have higher inclination angles, vibration and it

would be difficult to gain access for maintenance

purposes [06].

If ship’s structures do interrupt the antenna beam,

blind sectors will be caused, leading to degraded

communications over some arc of azimuth travel.

Thus, if it is like that and as is often the case, it is

impossible to find a mounting position free from all

obstructions; the identified blind sectors should be

recorded. It may be possible for the operator, when

in an area served by two satellites, to select the one

whose azimuth and elevation angles with respect to

170

the ship’s position are outside the blind sector. Ob-

viously, this method is not enough practical because

the satellite overlapping sectors inside of Inmarsat’s

four ocean regions cover relatively small areas. The

best solution to avoid all blind sectors is to place the

antenna unit on top of the radar mast or on a special-

ly designed mast.

5.2 Antenna Mast and Stabilizing Platform

The mast has to be designed to carry the weight of

the antenna unit, maximum 300 kg, depending on

model design or manufacturer, presented in Figure 8.

(A). It must also be able to withstand the forces im-

posed by severe winds up to 120 knots on the ra-

dome and strong vibrations due to very rough seas

on the whole ADE construction. The top end of the

mast should be fitted with a flange with holes match-

ing the bolts extending from the bottom of the ra-

dome. In addition, the flange must not be so large as

to interfere with the hatch in the bottom of the an-

tenna unit. In this sense, the holes through the mast

flange must be positioned symmetrically around the

ship’s longitudinal axis.

If the height of the mast makes it necessary to

climb up to the antenna unit, a ladder must be pro-

vided on the mast column. A guardrail must be at-

tached to the upper section for safety purposes. Fi-

nally, if the height of the mast exceeds

approximately 4.5 m, an access platform should be

attached to the mast about 1.5 m below the radome

bottom.

Figure 8. ADE Mast and Stabilized Platform

Courtesy of Manuals: “Saturn 3” by EB Communications and

“Maritime Communications” by Inmarsat [04]

The radome completely encloses the periphery of

the base plate assembly to protect the electronic and

mechanical components from corrosion and weather.

It is usually fabricated from high-gloss fibreglass

and is electronically transparent to RF signals in the

assigned frequency band. The radome is secured to

the circular or square antenna stand (base plate) with

several screws and can be removed easily without

special tools.

The antenna-stabilized platform is housed inside

the radome and consists in the electrical and me-

chanical elements, presented in Figure 8. (B). At this

point, there are two antenna control stepper motors.

First, there is the azimuth step motor, which controls

the position of the antenna reflector in the horizontal

A plane (azimuth) and second, is the elevation step

motor that controls the vertical E plane (elevation).

Each motor has four phase-inputs coming from the

drive circuit in the BDE Control Board and a supply

voltage from the master power supply located on the

antenna stand. Accordingly, as a stepper motor turns

the antenna, it also adjusts the setting of the relevant

sensor potentiometer. The sensor voltage supply is

the reference voltage for the A/D converter on the

Control Board. In the other words, two stepper mo-

tors move the satellite antenna in both azimuth and

elevation angles and moves the relevant sensor po-

tentiometers, which provide feedback information

on the position of the antenna. In fact, the antenna

stabilization system, or gyroscope with two gyro

motors, stabilizes the platform for the antenna

against the roll and pitch of the ship. A two-turn so-

lenoid clamps the antenna platform to the gyroscope

assembly. Therefore, a diplexer passes the Rx signal

from the antenna to the LNA and the Tx signal from

the transceiver assembly to the antenna. The LNA

amplifies the Rx signal and the HPA amplifies the

Tx signal. The parabolic dish radiates EM energy to

and from the antenna feeder in Rx or Tx direction,

respectively. In such a mode, the antenna assembly

is mounted below the radome for protection purpos-

es [07].

5.3 Antenna Location Aboard Ship

The ship’s antenna unit should be located at a dis-

tance of at least 4.5 m from the magnetic steering

compass. At this point, it is not recommended to lo-

cate the antenna close to any interference sources or

in such a position that sources such as the radar an-

tenna, lie within the antenna’s beam width of 10o

when it points at the satellite. The ADE should also

be separated as far as possible from the HF antenna

and preferably by at least 5 m from the antenna sys-

tems of other communications or navigation equip-

ment, such as the antenna of the satellite navigator

or the VHF and NAVTEX antennas. In addition, it is

not practical to place the antenna behind the funnel,

as smoke deposits will eventually degrade antenna

performance. Regardless of the location chosen for

the antenna, it should be oriented to point forwards

in parallel with the ship’s longitudinal axis when in

the middle of its azimuth range, which will corre-

spond to zero degrees on the azimuth indicator [08].

171

The EM RF signals are known to be hazardous to

health at high radiation levels. In such a way, it is

inadvisable to permit human beings to stand very

close to the radome of an SES when the system is

communicating with a satellite at a low elevation

angle. In this case, Inmarsat recommends that the

radiation levels in the vicinity of the antenna should

be measured. The crew members and passengers

should not be admitted to areas closer than 10 m

away from the antenna unit at desk level above 2 m,

measured beneath the lowest point of the radome, as

shown in Figure 9. (A).

Figure 9. Antenna Radiation Precautions and Azimuth Limit

Courtesy of Manual: “Saturn 3” by EB Communications [04]

No restrictions, therefore, are required when the

antenna radome is installed at least 2 m above the

highest point accessible to crew and passengers. Au-

thorized personnel should not remain close to the an-

tenna system for periods exceeding 1 hour per day

without switching off the RF transmitter. However,

radiation plan diagrams may be produced and locat-

ed near the antenna as a warning for crew members,

passengers and ship’s visitors, or distances from the

antenna may be physically labeled at the relevant

place.

5.4 Satellite Determination and Antenna Azimuth

Limit

An Inmarsat-A, B and M MSA must be capable of

locating and continuously tracking the GEO satellite

available or selected for communication, namely if

the ship has in view only one satellite or if the ship

is in an overlapping position, respectively. Thus,

Inmarsat-C has an omnidirectional antenna and does

not need a tracking system. Locating and tracking

may be done automatically, as in the case of an SES,

or manually, as with a portable MES [01].

In fact, it is common practice to believe that the

GEO satellites are fixed and that once the link has

been established it will remain so as long as the mo-

bile does not move. However, ships or other mobiles

are always moving during operational management

of voyages and satellites are under the influence of a

number of variable astrophysical parameters, which

cause it to move around its station by up to several

degrees. At this point, an ADE tracking system must

counteract this by repositioning the SES antenna at

regular intervals and in case of need. The carrier

signal is monitored continuously and, if a reduction

in its amplitude is detected, a close-programmed

search is initiated until the carrier strength is again at

maximum. No loss of signal occurs during this pro-

cess, which is automatically initiated. Obviously, the

greatest tracking problem will arise when the SES is

moving at speed with respect to the satellite. In a

more general sense, an Inmarsat-A and B MSA may

be moved through any angle in azimuth and eleva-

tion as the vessel moves along its course. In this

case, it is essential that electronic control of the an-

tenna is provided. In practice, ship antenna control

system may be achieved by manual and/or even au-

tomatically by simple electronic feedback methods

[02].

1 Manual Commands – When the radio or naviga-

tion operator onboard ships has selected manual

control, elevation is commanded by up and down

keys, whereas azimuth positioning is controlled

clockwise and counter-clockwise keys. In such a

case, a command would be used when the relative

positions of both the vessel and the satellite are

known. Azimuth and elevation angles of antenna

can be derived and input to the equipment, by us-

ing the two A and E charts of Inmarsat satellite

network coverage. Once the antenna starts to de-

tect a satellite signal, the operator display indi-

cates signal strength. Fine positioning can now be

achieved by moving the ship antenna in A and E

in 1/6

degree increments until maximum field

strength is achieved.

Figure 10. Antenna Pointing

Courtesy of Manual: “Saturn 3” by EB Communications [04]

2 Automatic Control – Once geostationary satellite

lock has been achieved, the system will automati-

cally monitor signal strength and apply A/E cor-

rections as required in order maintaining this lock

as the vessel changes course.

3 Automatic Search – An automatic antenna search

routine commences 1.5 minutes after switching

on the equipment, or it may be initiated by the

operator. Therefore, the elevation motor is caused

to search between 5

and 85

limits, whereas the

azimuth motor is stepped through 10

segments.

In s such a manner, if the assigned common sig-

172

naling channel signal is identified during this

search the step tracking system takes over to

switch the antenna above/below and to each side

of the detecting signal location searching for

maximum signal strength.

4 Gyroscopic Control – Using this mode the lock is

maintained irrespective of changes to the vessel’s

course by sensing signal changes in the ship’s gy-

ro repeater circuitry. In the proper manner, satel-

lite signal strength is monitored and if necessary,

the A/E stepper motors are commanded to start

searching for the maximum signal strength.

5 Antenna Rewind – The antenna in the ADE is

provided on a central mast and is coupled by var-

ious control and signal cables to a stationary sta-

ble platform. Thus, if the antenna was permitted

to rotate continuously in the same direction, the

feeder cables would eventually become so tightly

wrapped around the central support that they

would either prevent the antenna from moving or

they would fracture. To prevent this happening, a

sequence known as antenna rewind is necessary,

as is shown in Figure 9. (B). In fact, an antenna

has three areas with rewind time of approximately

30 seconds plus stabilizing time, giving a total of

about 1.5 minutes:

− Operational Area is the antenna-rotating limit

in the azimuth plane. In fact, the antenna can

rotate a total of 540

, which is shown as a white

area in Figure 9. (B). Normally, the vessel an-

tenna will operate in the operational area,

which is between 60

and 480

− Rewind Area is necessary for the following

reasons: if the antenna moves into one of the

rewind areas, i.e., 10

to 60

or 480

to 530

(antenna azimuth lamp lights) and if no traffic

is in progress, the antenna will automatically

rewind 360

to get into the operational area and

still be pointed at the satellite, which is illus-

trated as a dotted area in Figure 9. (B). For ex-

ample, the antenna moves from position 1 to 2

and the rewind lamplights. If the SES is occu-

pied with a call and the ship turns so that the

antenna enters the rewind area, no rewind will

take place unless the antenna comes into the

azimuth limit area.

If this happens, rewind will take place and the

call will be lost. The azimuth-warning indicator

on the operator display will light to indicate

that antenna rewinding is in progress.

− Azimuth Limit Area is an important factor be-

cause when the antenna is in this area the azi-

muth limit lamp lights. If the antenna moves in-

to the outer part of the azimuth limit area, i.e.,

to 10

or 530

to 540

, rewind will start au-

tomatically, despite traffic in progress.

5.5 Antenna Pointing and Tracking

The directional reflector antenna is highly directive

and must be pointed accurately at the satellite to

achieve optimum receiving and transmitting condi-

tions. In normal operation the antenna is kept point-

ed at the satellite by the auto tracking system of, for

example, Saturn 3 SES. Before the auto tracking can

take over, the antenna must be brought within a cer-

tain angle in relation to the satellite. This can be ob-

tained using the command “find” or by manually

setting the antenna using the front push buttons on

the terminal or via teleprinter command. For manual

pointing it is necessary to provide the ship’s plotted

position, ship’s heading by gyro, azimuth angle and

elevation angle map of the satellite [01].

Figure 11. Azimuth and Elevation Angle

Courtesy of Manual: “Saturn 3” by EB Communications [04]

1 Ship’s Plotted Position – The plotted position is

needed to decide which satellite can be used,

namely which Inmarsat network area can be

tuned: Indian Ocean Region (IOR), Pacific Ocean

Region (POR), Atlantic Ocean Region-West

(AORW) or Atlantic Ocean Region-East

(AORE), depending on the ship’s actual position,

as presented in Figure 10. (A). Sometimes, the

ship can be in an overlapping area covered by two

or even three Inmarsat satellites. In this case it

will be important to choose convenient CES and

to point the antenna towards one of overlapping

ocean regions.

2 Ship’s Heading by Gyrocompass – The method of

the permanent heading of the ship course deter-

mined by gyrocompass is needed for the antenna

auto-tracking and focusing system during naviga-

tion, illustrated in Figure 10. (B). In this case, this

scenario is not needed for omnidirectional ship

antennas, such as for Standard-C, mini-C and D+

equipment. This connection is very important for

new Inmarsat Fleet and FleetBroadband stand-

ards.

3 Azimuth Angle – This is the angle between North

line and horizontal satellite direction as seen from

the ship, as is shown by example of 259o, in Fig-

ure 11. (A). The actual azimuth angle for the var-

ious satellites due to the ship’s plotted position

can be found on the azimuth angle map.

173

4 Elevation Angle – The elevation angle is the sat-

ellite height above the horizon as seen from the

ship, as is shown by the example of 38o, in Fig-

ure 11. (B). In this case, the actual elevation angle

for the various satellites due to the ship’s plotted

position can be found on the Elevation angle map.

6 CONCLUSION

It is obvious that shipborne MSA configuration

needs to be compact and lightweight. These re-

quirements will be difficult to achieve because

ship’s directional antenna has quite heavy compo-

nents for stabilization and tracking, and because the

compact antenna has two major electrical disad-

vantages such as low gain and wide beam coverage.

Therefore, a new generation of powerful satellite

transponders with high EIRP and G/T performances

should permit the effective design of more powerful,

compact and lightweight MSA for ship applications.

On the other hand, new physical shapes of radome

and less weight of components are very important

requirements in connection with compactness and

lightweight, what will permit easier installation and

regular maintenance of ship antennas. With ship-

borne antennas on oceangoing large ships, installa-

tion requirements are not as limited compared to

fishing vessels or very small boats and yachts, be-

cause even small ships have enough space to put

mast on compass deck and to install an antenna sys-

tem. However, in the case of small ships, especially

yachts, very low profile and lightweight equipment

is required, such as the Inmarsat Fleet F33, Mini-M

or C omnidirectional antenna installations.

REFERENCES

[01] Ilcev D. St. “Global Mobile Satellite Communications for

Maritime, Land and Aeronautical Applications”, Book,

Springer, Boston, 2005.

[02] Fujimoto K. & James J.R. “Mobile Antenna Systems

Handbook”, Artech House, Boston, 1994.

[03] Group of Authors, “Sailor Maritime Inmarsat-C Installa-

tion Manual”, Thrane & Thrane, Soeborg, 1997.

[04] Group of Authors, “Saturn 3 standard-A Installa-

tion/Operator’ Manuals”, EB, Nesbru, 1986.

[05] Evans B.G., “Satellite Communication Systems”, IEE,

London, 1991.

[06] Gallagher B. “Never Beyond Reach”, Book, Inmarsat,

London, 1989.

[07] Rudge A.W., “The Handbook of Antenna Design”, IEE,

London, 1986.

[08] Law E.P. “Shipboard Antennas”, Artech, 1983.