Hovercraft navigation - The problem and a solution

ÀRCH1E1

Hovercraft

Problem and a Solution

by PAUL HORROX

HOVERCRAFF NAVIGATION SYSTEM EQUIPMENTS, A COMPROMISE

"S,'

is a highly flexible integrated navigation

sys-tem designed by EASAMS (E-A Space and Advanced

Military Systems Ltd) to meet the problems of safe guidance

of high-speed hovercraft. Like most engineering projects, it is a development based upon previous experience. In the

case of hovercraft navigation, this experience must be derived, primarily, from marine and aeronautical

back-grounds which must be blended to provide solutions to the problems which arise when both régimes are combined.

Since the only equipment available for hovercraft navi-gation stems from either marine or aeronautical

develop-ment, it

is inevitable that,

in many cases, a product

developed for either is not optimum for a hovercraft. The

hovercraft requirements for a navigation radar illustrate

this point. They can be summarised as:

High definition, je short pulse and narrow beam,

for collision avoidance.

High data rate, dictating high scan rate and short

persistence displays.

Short range scale capability, for close in-shore

pilotage.

Medium power (low scanner height makes high-powered radar wasteful).

Low weight.

Compatibility with aircraft-type power supplies.

Aircraft radars do not normally include (a), (c) or (d), whereas marine radars do not conform to (b), (e) or (f).

This situation, to a greater or lesser extent, confronts

the system designer of a hovercraft navigation system in the selection of nearly all the equipment required.

A hovercraft navigation system must, therefore, at this

stage, be a compromise of both marine and aeronautical

equipments, selected to meet the primary requirements and adapted by either modification or special interface units to

make them compatible with each other and with other

hovercraft systems, eg power supplies.

Although this tends to be wasteful in terms of weight

and interface complexity, it is the only practical solution short of developing equipment specifically for hovercraft application. Such development cannot yet be commercially justified. It is ori this basis of compromise, therefore, that the next generation hovercraft navigation system is being

developed.

Lab.

y.

Scheepsbouwkunde

Technische Hogeschool

avigation_TheIft

Head of Hovercraft Navigation System Projects

E-A Space and Advanced Military Systems Ltd

Camberley, Surrey

ThE PROBLEMS OF HOVERCRAFT

NAVIGATION

The problems of hovercraft navigation have been dis-cussed at some length in several journals. It is pertinent,

however, to reiterate them here. In brief they are:

Collision avoidance. The craft moves at relatfvely high speed in a congested environment with only

one plane in which to provide separation and in

which it can manoeuvre.

The craft, being airborne, is subject to wind drift.

The surface over which the craft travels moves with surface wind, tides, currents and with the

passage of the craft itself.

The imprecise manoeuvring characteristics of the

craft, including pronounced slip in turns, make

appreciation of actual direction of travel difficult to assess.

The rough ride, even in moderate sea states,

pre-cludes conventional navigational observations and manual plotting.

The environment is harsh from the noise, vibration and corrosive viewpoints.

Optimum sites for magnetic heading reference sensors are difficult to find on a hovercraft and

calibration and correction of such devices difficult

to achieve.

Short turn-round time at base (compared with

ships) and, in some roles, long-duration sorties

(compared with aircraft) raise problems when

inertial navigation is considered.

In short, the hovercraft inherits many of the problems of both marine and aeronautical navigation and also produces some of its own.

THE "SEALANE" NAVIGATION SYSTEM

"Sealane" is the name given to the navigation system

being developed for the Wellington Class hovercraft

cur-rently under construction by che British Hovercraft

Cor-poration.

The system concept was proposed by EASAMS in 1966 in answer to a broad requirement formulated by the Naval School of Direction and Navigation, HMS Dryad, in con-junction with the Interservice Hovercraft Unit at

Lee-on-L

y

SUB- SYSTEM DECCA NAVIGATOR L

BLOCK DIAGRAM

A L

£ DOPPLER SUB- SYSTEM COMPUTING SUB-SYSTEM X z

GYRO- COMPASS SUB-SYSTEM

SYSTEM TEST COMMANDS COMMAND HE AU INC TO DESTINATION ACTUAL TRACA AND X- TRACK E NEON TURN WAONING _ACTUAL H0ADING SYSTEM TEST PANEL

- iL

r--

HELICOPTER i RADAR TRANSPONDER TRANSCEIVER RECEIVER (oPrEoL) i s

y

1RAN5PØNDE SIGNALS

4---PILOT S

COMMAND STEERING INDICATOR

DEC CA NAVIGATOR RECEIVE R MOVING NAVIGAI IO N 0.0. DRIFT TRIM COMPASS CONTROL CONTROL UNIT

LMAP

DISPLAY CONTROL CONTROL PANEL UNIT

* /'

*

'u, DE CAM (1G R D(COM(T(R DICAMETEN PANEL RID I GREGN I \'uriPLtj NAVIGATION RADAR 00F PI. Il

-I2 DISPLAY DO IF T AND GIS M U TE R) HE LOINS REPEATER 4-HEADING NA VIGAT O R s) X NAVIGATION RADAR IG DISPLAY

HIS AND 01V TRUE MOTION SHIFT (RADAR OPERAToR'S) OPTICAL PLOTTING

MANUAL

AUTO N

CONTROL

ATTACHMENT

8

Drift and A/S Meter

Decca 71 H Doppler

Soient. The latter had been evaluating an experimental

computer-based navigation system in their SR.N3

hover-craft and had, therefore, recognised the combination of

navigational problems peculiar to the hovercraft.

In 1967 a System Specification was issued by EASAMS, nominating the equipment required and the methods to be

employed for integrating the various sub-systems into a composite system. Off-the-shelf marine or aeronautical equipment, most amenable to adaptation to make them

compatible with the system requirements, were used

wherever possible.

Design and development of the system commenced in August 1968 and ground rig trials of the system are now

taking place.

Decca Transar Group 8 Navigation Radar

To Computing

sub system

OPTI CAL

Plotting

attachment

The system navigates in one of three modes. The mode may be selected or may be entered as an automatic

rever-sion if a sensor fails. In the primary mode, navigation is accomplished on the hybrid dead reckoning/automatic

fixing principle. The DR component is provided by a suit-ably modified Decca 71 helicopter doppler measuring along

and across heading velocities, together with an

Arma-Brown land vehicle gyro-compass providing the heading reference. Automatic fixes are derived from a Decca Navi-gator Receiver Mk 19.

A Decca Omnitrac 70 general-purpose digital computer forms the central digital processor for the system.

In the hybrid "Decca/Doppler Mix" mode, mixing is accomplished by a predictor-corrector technique which

filters the noise in the Decca fixes and, at the same time,

corrects the bias error inherent in any integrating dead reckoning system. Monitoring circuits in the Navigator

Receiver and Doppler output "fail" signals to the computer when either sensor is suspect. The computer then reverts to using only the remaining input for deriving all required navigational parameters. Combined selector/indicator push buttons on a Navigation Control Panel allow mode to be selected and indicate in which mode the system is operating

by either selection or reversion.

Calculated present position is presented to the navigator

as digital decimal read-outs on the Navigation Control

Panel and as a trace on a Decca Mk 6 digital Flight Log.

Initial acquisition of the correct Flight Log chart and

subsequent chart changes are automatic when a chart roll covering the area containing the present position has been inserted into the Flight Log.

Control of the Mk 19 Navigator Receiver may be

accomplished either automatically or manually. In"AUTO", chain and slave pair selections are carried out by the com-puter using data derived from data tracks along the margin

of the Flight Log charts. In "MANUA.1", chain and slave

pairs must be selected manually but the computer reads

these selections so that automatic processing of Decca fixes can still take place. Thus automatic fixing is independent

of Flight Log operation. In addition to the

computer-derived automatic fixes, the Mk 19 Receiver also provides conventional Deconieter read-outs for cross-checks.

Transponder

receiver

Radar Transceiver

Gyro compass sub-system

Compass

gyro unit

For collision avoidance and in-shore navigation a Decca

Transar Group 8 marine navigation radar is used. This

incorporates a 25 kW peak power transmitter operating

with a 9 ft scanner having 0.8° horizontal beam width and a scan rate of 32 rpm. Radar returns are displayed on two

PN display units. The main 16 in dia display is manned

by a radar operator. The secondary 12 in dia display is

navigator operated. Both may be switched independently

to any of eight range scales between 0.5 _{and 48 nrns.}

Both displays are console mounted to provide _a

corn-fortable viewing angle from seated operators. The displays are North stabilised by inputs of true scanner orientation

derived from the scanner via the heading reference

sub-system. Both include heading markers, track markers and :ontrollable range and bearing markers.

The 16 in display may be selected to relative motion,

ff-set relative motion or true motion. With _{the last two}

elections, the computer provides the voltages _{to off-set}

he time base origin. These voltages may be set in response o demands from a "joystick" control in the radar operator's lesk, which allows him to set the time base origin where te wishes. They are also controlled, however, with a True

4otion selection, by Incremental changes in Easting and

1orthing of craft position so as to ground stabilise the P!. Compass remote controller Comss Re-transmission Unit jHeading JOutputs

The 16 in display may be fitted with an Optical Plotting

Attachment which allows a roller map or a selection of

graticules, plotting surfaces

_{or cursors to be optically}

superimposed ori the PPI display. The superimposed

dis-play required is selected by controlling the brilliance of

integral lighting in the selected graticule

_{or in the chart}

container so that its image is reflected in a suitably angled semi-reflecting mirror across the PPI face.

Using this facility, the radar picture can be matched to superimposed charts for identification of_{landmarks, buoys,} etc, during in-shore pilotage. Thus the navigator with his

Flight Log display, and the radar operator with a

chart-matched PPI, both have easily assimilated, _but

indepen-dently derived, pictorial displays of craft position in in-shore waters,

Chart matching also permits error accumulation rate to be observed when the system is operating in the "Doppler" mode. This is shown by "drift" between the radar picture and superimposed chart. A correction vector to cancel this

can be set into the computer by either the _{navigator or}

-n paling sub-system

10

Drift trim control panel

radar operator and doppler inputs are then compounded with the correction vector in calculating position. This

facility, together with the capability of injecting manually

set drift and ground speed into the computer, permits all position computations and displays to be slaved to the radar presentation in the event of failure of both Decca

and Doppler.

The system provides a Destination Steering facility. This allows the navigator to specify on the Navigation Control

Panel two destinations to which he wishes to proceed in

sequence. These may be specified as latitude/longitude or

as a bearing and distance. In either case, the

comple-mentary parameters to those set are calculated once the starting position is defined by initiating a "Steer"

com-mand. Thereafter, the position of the craft is related to the designated track, and distance-to-go and cross-track error are read out.

Steering signals and horizontal situation are displayed

to the hoverpilot on a Command Steering Indicator. This

includes a conventional rotating compass card reading

heading against a fixed lubber and a computer or manually

set command heading index which indicates on the card the heading to be steered to converge upon and hold the

required track. Actual track-made-good and linear cross-track eri or displays are also included on the instrument.

Command steering indicator

Zone indent Lane indent

meter meter

Deco meters

Red Green Purple

To Computer

sub-system

Decca Navigator Receiver Mk 19 sub-system

A feature of this instrument, peculiar to the hovercraft, is the provision of warning indicators showing the direction of an impending turn on nearing termination of a leg, and

the provision of a command button allowing the pilot to

demand that the command heading index shall indicate the

approximate heading for the next leg in place of that for

the leg nearing termination. This facility is available from within half a mile of the turning point and allows the pilot

to make the necessary trim alterations and anticipate the

turn as is sometimes necessary in cross-winds.

To allow the system to be checked for correct operation prior to a sortie, the whole system can be made to operate dynamically with the craft static. This allows most of the system functions to be carried out and the normal displays

to be monitored for correct presentation. To assist in the check-out of displays, the correct functioning of which

would be time-consuming, a number of computer-initiated checks are also included, allowing the whole system to be checked with a high degree of confidence in 15 minutes by

one operator. Fault diagnosis can be accomplished via a

System Junction Box and Test Panel which provides some

700 test points on the signal lines between the various units within the system, in addition to those available to

special test equipment through monitor sockets on indi-vidual units.

Although not by any means "the ultimate", the "Sealane" Navigation System provides many features which are new in the realm of hovercraft navigation.

R.F.Amplifier

Decca Flight Log Display Head Mk 6

Auto-manual changeover relay Mk. 19 RX control unit Ìu

4-.L1