Future satellite communications to

military aircraft by S. E. Nicol, G. Walton, L. D. Westbrook and D. A. Wynn

This paper outlines some of the techniques being developed to provide affordable, reliable satellite communications suitable for a wide range of military aircraft, from agile platforms such as fast jets and helicopters to

surveillance, tanker and transport aircraft.

1 Introduction

'I'he beginning of a new millennium also sccs an importanl milestone iii military wialioii cornimunications in the Uiiileil Kingdom (UQ, with the inlmductioti of tlic first supcr-hi~~-E1-fi.equency (SHF) airborne satcllite communication (satcoin) tcrrninals, which are diic t:o enler service on Nimrod mariliine recontiaissaticc aiicrak (MRA4). '1'Ms is expectcd lo be lollawed by similar SIIF satcom terminals on the UR's new airhorne standhff radar (ASTOR) plat.k~rm~.

Satcorn ierinirrals usinE thc Iiltr.~-high-.rrequcncy (UHF) hand have been fitled to larger UK aircrafl fora iiurnbcr of years and there are currently around 30 Itoyal Air Forcc (RA19 aircraft fitted with (or for) UHF sat:ellite communicslion. Alrhough relatively sirnplc to install iInd

comparatively incxpciisive, UHF satcoms (24&270 and 290-320 MI Iz bands) suffer iron1 very liinited capacity (lypically a few 25 kHz channels per satcllitc) and are prone lo midtipath and uninleiitioiial intcrfcrciice h e to their poor antenna sclcctivily2. SIlF satconis (7257.75 and 7.918.4 GHz Iiancls) o € h significantly increased k ~ h v i d t h (Iiutidreds of megahertz) fnr high daiai rates or increased iise of sprcatl-spcctrum techniques, togcther with lncdisctl (spot) cuveragtt and adaptive aiilenna tediniqucs - lor ~iiillitig unwanted signals or inLerkrence.

For airliorne plai.Corms, the aclvaiihgcs 01 SIIF satcoins come at the expensc of a significant additional Iiurileii in terms of antcniia siting and pointing, particular-ly Cor smaller, highly a.& aircraft. Salellitc antcnnas un aircraR must. be small cnough not to causc significant :ierorlynamic clrap; while at ihe saiiie timc bc large enough to support the desired data rate, arid tn prnvidc enough direclivity to minimkc inlerkrence with adjacent satcllitcs and avoid rlctcctioii by hostile forccs. Thc smaller arid xnnrc agilc thc air plallorm is, thc more difficult these trade-offs bccome. Aiiothcr fcaturcof satcoms, unique to a i r c d , is the effect of titiwatitcd modulat.ion from movinE parts such as

lielicopler rotor blades, propollcrs antl jet engines. This papcr gives an overview of the ticvclopment of

ail-hornc SAF and also extr~mely-hi~li-frc-frcclucncy (EIIF) satcom techiiqucs ancl tcrininal clernonsiralars by DEW (Delence Rvaluation atid Research Agency) and its partners lhrough the Miiiislry of Defence's applictl rcscarch Iwogt'aInnie. 'lliis rcscarch is aimed at providing alfordahle, scciirc and robusl salcorns lo a range of UK inililary aircraft, supporting ground-atlack and reconnaissancc rnlcs to surveillance, transport and tanker aircrall.

2 The military perspective

'I'he IJK Ministry olneleme (Moll) currently opcrates a conslellation ol h rcc gcostationary militai+y communi- cation satcllitcs collcctively known as Skynctq. The currcnl generatitinn of Slcynct 4 satclliks provides satellitc coinnitmicatioris for all three armed services at both UHF and SHF bands. In addition, as a niernber of thc Norlli Atlantic Treaty Organisation (NA'I'O) the 1IK has access to NKI'O satellites, antl those uC its allies, via ineaioranda olunderstantling,

k inar id for all types of military satcoilis is rising rapidly, duc principally to the need lor ever inorc t i d y information (mareness) to prosccutc operaliotis efreclively. Another f a h r f~~elling the increase in dcmaiid for satcoins is the grnwing cxpcctatioiis of servicc personnel familiar with a world of instant ~ loha l comtnimications and rapid availability of infnrination via Internet-typc scrviccs.

UK forces arc rcquircd l o operate in a wide variety of roles and in an incrcasing range o[ gengraphicd arras. Uniquely, satcoins can span dishrice, terrain ancl hostile forccs, to provide a global rcach for dispersed riinbilc platforms such as aircrah, submeritics, surface ships, vehicles and innnpaclts.

Rcccnt conflicts in Bosnia-Herzegovina and Kasnvo Iiave also demonstrated thc need to kiditale increascrl rcrnotc cletision-making Iiy scnior commanders out of

ISIRLTRONICS bi COMMUNlCATION ENGINIXRING JOURNAL 1;W?UARY 2000 15

Fig. 1 Potential use of satrom by military aircraft

\ deep strike/ jamminu reconnaissance,'-,

thcatrc (tactical decisions would normally be talreti locally). This clrarly rcquircs thc ~~rovisiori of ncar-rcal- time sihiational awarcncss via rcliablc, sccurc, glnbal conitniinications. Witti an increased einphasis nn joint operations, ttiere is also a growing need for reliable atid iritcropcrablc cornmunications with the IJK's allies atid coalition forces, through the adoption a( coiiiinon s~.aiirlartls.

Fig 1 illuslrales, schematically, the hreadth of rnilitar y aerial i d r : s Illat salcoins inay ht! requirctl tn siippcirt i n htiirc, providing glohnl beyond-linc-of-sifilit cornmunicatioris belween aircraft and commanders in thcakc ancl thc UK. Iiiforniatinn carriccl Iiy satcllitc could include:

ncar-rcal-time command ancl control - taslting, position reporting, ctc. data k o w recontiaissance and surveillanw aircrafl targeting data for stand-ofiweapons situ at ion awiirpii c ss trarisfor data from dissimilar ancl/nr gcnjirapliically scparaled linc-okighl (LOS) nctworlts.

In some cases, clircct satcllik conitnunications with an aircraft will not be ncccssary. For cxaniplc, air superiority fighters typically opcrak in ltic vicinity of an E3-D Sentry air-bornc early-warning (AEW) aircraft with which Uicy are in lincxLsiEhl coiiimuiiiution4. For thcsc fighlcr aircvaCt il would bc iiiore cfiicicnt to communicattc via their lincol sight liiik lo the E3 D and then via satcom Irnin Ilie E3-n, with the trnnsler belwwn the [WO

Table 1: Typical data rates for various air missions

liiilrs operating scanilcssly. Typical dah rales for soinc of thcsc military rolcs arc

givcn in Tiiblc 1. For somc applications - such as tasking pilots - short tcxtual nicssaEcs at vcry low data rates (tons of bits per scumcl) can be acleqiiate. lhese messages may bc critical, howcvcr, and cornmutiications at thcsc rates must lie highly robust. At the other etid oCtlie scale, unprocessed reconnaissance rlaia demmd large amounts of bandwidth - the amount increasingyea~~on~year as Ihe resolutiori 01 iinaging nnrl ratiar sensol' tcchtiolrlgic:s improves.

3 Use of civilian satellite systems

Thc usc of nonmilitary satcllitc systems, principally Inniarsat, by tho military to supplcmcrit capacity for tion- mission-critical cominunicatioris is already cnmimn. Civilian tnobilc sntcorn is a $2-5 billion glhd industry atid can provitlc significant aclclitional capacity using the latest st.ale-o Mhe-art tecl inology.

Thc usc ol coinmcrcial satcllitc systems is bound by itiIei+nalioaal l.reaties which l.ypicdly placc scvcrc condriiints on tlic usc of hcsc systcnis by the military. Onc such constraint is ttic right. of countries to prohibit, liceiisc (lax) and .(lc&imatcly) intcrccpl traffic to and from lcrminals opct-aling within thcir tcri-itorial h i t s . Coiutncrcial satclliks also have no specific built-iti resilience lo jainiiiiiig and liiiiitccl protcctinn against inhrmation warfare. Undcrstandably, too, thc location and iileril.ily 01 military satccrm uscrs is scnsitivc inhrinalinn. Neverlheless, modern commercial satcom

sysleins can provide a fdst-t.rack upKradc pAh for acronaiitical niilitary

Scenario Data type ~ ~ ~ i ~ ~ l data rates commimicalions b r pcacc-lwcping ailcl opcralions other than war, provided thpir

Tasking and position reporting short messages 50-300 bith limitations and vulnerabilities are Mlv untlerslood ancl appropriate prccaulions

Unlil rcccntly, Intnarsat was the sole

Verbal communication voice 2.4-4-8 kbith Reconnaissance imageshrideo 16kbiUs-20 MbiUs takcrl, Receive-only direct broadcast videolmultjmedia 2-20 Mbitk

l < I , K C ~ l ~ O N l C S & COMMIJNTCKI'ION TSNC;!NEFRING JOURNAL FEBRUARY 2000

provider of coinmcrcial acruiiaulical satellite services, using 10 MRz n l spcctriitn iti L-band (-14 GIIx)~ . In Octobcr 1998 the Tiirlium system, dcvclopcd by Molornla, began liinitcd operations. Iridiuin pravidcs truly glol>al coverage using ti6 satcllitcs hi polar orbits with 18 bca~ns pcr satellite. ‘l’his revn1utioti:try sysiciii also uses [hand for communicating with intibiles and Ka-band (Xl/SO GI&) lol. links to gr-ound stations. Iridium is Ihe h s l : coiiiinercial satellite systrm to use on-board swiichirig with intcr-satellite links (in thc 23 CHz wil(ci* vapo11r absorptioii baud) so iha i linlts bctwccn mobiIcs need not. iisc krrcstrial gateways.

Althnup;h thc Iridiuin syslein is pritiinrily ititcnded LO support personal comt~i i in icah~s , aeronautical tcrininals with one to eight chaiiticls arc behg develnpctl for the Iriditun system by AllicdSignal Aerospace. ’Ilw per~orm;lnce ol thcsc tcrminais is currenlly licing assessed by [Ili;kA for usc on UK rrrililary transport a iraafl. and mi AlliedSign n I tc rm i 11 al is CII r re d y undergoing trials on nn liAb’ Bcrculcs aircriill (Fig. 2).

Other low-earlh-orbit (LIM) pcrsonal cotnmunicat ion systcrns (PCSs) such as GlobalStar, and mcdiiiiii-carllI- nrliit (MEO) systcms such as IC0 arc duc to corne oil-line in [lie ncnr €uIurc (:ilhmgh ai [lie h e of writing a11 threc PCSs arc ex~ieriericinl: hnmcial and/or technical dilficiili.ies) I ‘[’heir dcvclopincnl is being closely monitored.

4 The airframe-antenna placement

Onc of Ihc h s i . prflblenis mcoutitcrcd wheii considering an airborne satconi tcr~ninail is whcre lo site the aritcrma on llie airframc so as to gain an uriiriterrupterl path to thc satellite. On smaller aircrah and hclicoptcrs, iu particular, space is at a prcrnirim, with h e ’bed.’locations likcly to be iisccl for key tactical systcins such :IS indar and antcnnas lur dcct.ronic support measures. Hclicoptcrs in particular pr-wcnl a. unique cliallengc due tu thcir rotors and body shape, which makc it difficult to find an rininterrupted vicw of Ihc satcllitc €or all orienlnl.ioirs.

By way of cxamlile, Fig. 3 shows tlic blockage (sometimes rcfcrrctl to as ‘wooding’) Tor 1 WO Iiypotlwtical anlentis locations on the Apache alhck hclicoptcr, which is due t.0 enter scrvicc with thc Ariny in 2000. Here, Ihc hloclcage has been cnmpukil by projccling h e s i lho~ie~k nf tlic airframc arid tnniti rotor, as illuminated from 1hc anteiitia site, onlo it hemisplierc. This techniqii~. is applic;il)le to any airfrainc/aotenna. combinatioti, providcrl the antciina is much smaller than lhc aircraft. Most appai’enl frnm these diagratns is fhe cxtciit ol Ihe shntlowing by thc main rotor black, which wises uiiwatitd aniplitiidc and frcqucncy tnorlulalion (see Irclow). With ilie nnienna mouiitccl on the helicnpter spinc just

behind Ihe engine, the silhouettrs nf the cockpit (forc) ~ind tail (aft) arc vcry appareul. Moving the ariteiinn to onc of the two stiibliy ‘wiiigs’, which srippurt (lip armainent, provides better fnrc and aft covcragc hut surrcrs blockage 011 uiit‘ side due to h e Cuselage.lkn atitcntias, n ~ i c tlii cadi wing rvoulcl provitlc Ille mcessxy hemisphcricai coverage. Not stirprisingly, the ‘best’ Incatinn for a satcoil? niitcnna on the Al~i~chc is above he centre of l l ~ main rotor on a de-spun nnisl - ihe location Cor ihp Ixln.&ow radar!

5 Themodem

It1 atldii.ioii to engine/rotor tnndulation, nicnlionccl above, the inodulaIor/demodulalor (nintlctn) in an ;tirhorile satcoin tcriiiinal has to be designed to nccntnmatlak the cffcct uf Doppler shirt orilie IW sigrral due to tlic aircraCl’s spcctl and acceleration. I’atentially, this speed could be up to 601) tti/s rclalivc [U the sakllile. T h e nrnniiitt, of llopplcr shift iticrcascs with increasing radial velocily ancl RI? carrier frccliiciicy.

Ati airbortic lactical rnoclcrn has Ihc rollowinf; requiren1errls:

. . ~

. . . . . . . ’ . : . . . . . .. . , . ., . ::- :..,I

- i

Fig. 2 (a) AlliedSignal Iridium terminal currently undergoing trials on (b) an RAF Hercules aircraft

li[l<CTRONICS & COMMUNICA’t[ON ENGINEERING JOURNAL l~’ISI<RlJARY 2000 17

. . . .

. .

~

. .

a b

. . Fig. 3 3D polar plots of the blockage for a satcom antenna mounted on an AH64 helicopter (with the antenna located at the origin): (a) indication of the helicopter model attitude; (b) blockage with the antenna mounted behind the motor, on the spine; (c) blockage with the antenna mounted on the right-hand 'wing'. The dashed line indicates the extent of the partial blockage caused by the main rotor blades.

. . . . .

. . . .

. .

. .. -- --:, . ../,

>.' / . . '

. .

C

lo be efficienl in tcrms of carricr-lo-noise density (C/N,) for a givcn bit crror ratio (REI<)

a 10 operate under high Doppler offsets and rates of chanKe, due k~ platform motion

0 to opcrak with minimal degradation in the presencc of multipath pmpagdion and occasioiial signal loss due to airframc blockagc ancl antenna switching rapid acquisition of cominunicalions following signal loss lo operate with minimal degradation in the prcsencc oC rotor mfidulation lo comlily with internatinnal rcgirlations on power spectral density and adjaccnt satcllito interference

Table 2: Required SHF modem performance for large and rotary wing aircraft

Parameter Performance

Data rate CM0

Sensitivity Mode 1 50 bitls 23 dB Hz (asynchronous data) 75 bith 25 dB Hz

300 biVs 31 dBHz

Sensitivity Mode 2 2-4 kbit/s 40 d B Hz (synchronous voice) 4-8 kbit/s 43 dBHz

Max. multipath -10 dB

Max. Doppler offset & kHz

Max. Doppler rate of change 500 Hz/s

Max. rotor modutation degradation in UNo

1 dB

to iinplcincnl cledronic pmkc( inn measures, as required.

Thomson CSF aiirl 13lSRAare jointly developing a proof-of- principle SHI; modem to meetthcscl rcquirctnciits in large and roiary-winK aircrdl, the outline specificatioii frir which is given in Table 2.

To illuslralc the combined eCkcts oC I)oppler, rotor modulation ancl multipath, wc show in Pig. 4 the scatleririg fiincljoii for communications lo a mcdiuni-sized hclicopkr. Here, the sill.com anlermn is moiinted behind thc main cngincs 011 ihc ccnlral Itiselage below the niain rotor.

Thc scattei+ing hinction is based upnn cross-ambiguity analysis, a tool commonly used in radar. 1i.k essentially the output of a 'filter' tlini is matched to the uncnrruptcd kansniilktl S ig r id (&[). The sigiid filteritig and pt'occssing incorporated in il satcorn i.eceiveris rinrinally a very close approximahm to that required 10 perform matcliccl filtering of tlic rccciverl signal from the satellite. Thc tnagriitudc of the sca th ing [unction, illustrated in 13~. 4, is it ineasni-e UT the interrereace presetit in Ihc rcccivcd signal (3, which comprises h e vector sum of scvwil sigiial paths, each siiuullaneonsly nl'fset in frequency v;I) arid delayed in titric (7). Ideally, the niaklicd 'filter' oiilpiil: [or zero delay (multipaUi) and frcyucncy shirt (Ihpplei9 would be ;I 'spilte' cerilred upon am Doppler and xero delay (broadened by thc finite time-bmtlwidlh product). Significant signal loss ariscs when the received signal sca1tcriti.q funclinn is 'stncarctl' in time and lretluency by Thppler and multipath. Smearing is non-syminekic due to the aircraft's relative inotion.

18 ELIIL"I<L"CS & COMMUNICATION ENGIN WRING JOURNAI, lTJ3RLJAItY 2000

'1he rcceivetl signal undergoes both amplihitle niid I

phasc inorlitlation :is a coIIscqllc11w nl r-otor modulation. I'erindic fading rpsdts from rotor Iilndc obscuration, and deep nulls (in exccss of IS dI<) arise due to cnmplcx coupling nt phasc ancl lhpplcr shift. '[he resnltant scattering ftiiirtion in this caw (Fig, 4b) is depcndmt upon inany factors, including thc aspect angle l o Ihe rotors and aircrafi, rrom thc tcrininal and

Table 3: Required SHF antenna performance (R(L) HCP = right (left) handed circular polarisation)

~~

Parameter Low-gain antenna Medium-gain antenna

Transmit gain Receive gain Transmit polarisation Receive polarisation Frequency

Axial ratio Power handling R m x isolation Steerable Packaging

6 dai 6 dBi RHCP LHCP

R X 7.25-7.75 GHz TX 7.9-8-4 GHZ

6 dB 40 W

N/A (different antennas) switched

aircraft blade radome 1 Tx and 1 Rx blade

16 dBi 15-5 dBi RHCP LHCP

7.25-8.4 GHz

3 dB

20 dB

aperture less than 10 cm fully contained within 12 cm

3aw

fully

rccciver, the tiurnbcr of rotor bladcsanil slriictui-alcomposiliuii, ~~i~clian~!tc,flcxirig;u~d aircraft atitciiiia size (aperhtrc) cat1 IIC tlctcrmined. hiili rotalion rates. Rotor modulation t:lius pnscs scrious l<xperieiicc shows lhal anleiinn requirements, fnr itsc with prnbicrns for corivmtional satcorn link acquisition, RF genstatintiary sp;icccraCt, fdl into three cakgorics: carrier tracking (for cninpciisation of aircraft rnotioi~} ; ~ n d link retention. With tlic correct signol proccssing, (U) low gain for positicm reporting and rncssagiti.g howcvcr, we believe that thc effects nT rotor Inodulalion (b) mcdium Rail1 €or voice arid low-&~;i-r:ite can bc reduced to <l dR powcr pt'nalty.

(c) high j y i t i for applicalions such as file-transfcr, 6 Theantenna srti~v~illance/reco~inaissaiict. data, etc.

Having detcrrnincd the required cornmuuications data r a k arid the available s;itellitc: power to suyport it, tlic

cornmutiicahn and

Thc rcquit'emenis for low- and niccliiirr-gain antennas arc summarised in Table 3.

I cross-ambigulty ScatterinQ function: 1 volociiy distribution Fig. 4 (a) Sources of multipath. rotor modulation and Doppler with antenna on the central fuselage of a helicopter; (b) example

4 - - . . -. . , . . . - - - - - % modulation for a medium-

* sized helicopter SI

scatteiing function with rotor

n= I

advancing '\ f - ' d~-.. 3 . I blade U-

a scaucrlrrg luncllon-single caiiior + m!ritipalll

Doppler ottsel frequency, Hz

b

ELECTRONICS & COMMUNICATION ICNGINEBRtN(.; JOLJI<NAL FEBRUARY 2000 I9

mtd beam IOW beam high beam

---F switch

a

b

C

Fig. 5 Axial-symmetric, switched 3-beam, 5HF low-gain antenna: (a) schematic diagram; (b) antenna patterns; (c) practical realisation

The ‘holy grail’olairbnrne nntctmas has long bccti thin, conforriial phased arrays which talw the shapc of the aircraft’s sltin. However, for satcorn applications, thc sizc, per Comarice (aperture efficiency, mise tenipcraturc) ancl cost ol currenl phased-at ray technologies severely limit their usciulness. Our overall philosophy has been ha1 airborne satcoin antennas should be low cost.‘lb this end, DERA has rleveluped conipatct proof-of-conc~pI low- and meilioin-gain airborne SIIF salcoin anletmas using more conventional technologies.

bw-gnita atiteman A number ol approaches have been investigated for n

low-gain antenna, with the additional goal of reducing the complexity of the a n t ” pointing mctliod to fiirtticr

rniriimisc cosk. 111 particular the m e ola nuinbcr of sialic, switchcd-bcam antcnnas with azimulh-indcpendeii1 gain has Iiccn piirsucd. Thc minimuin iiutnher of independent bcains rcquircrl to cover a given solid angle can be infcrrcrl from the gain rcquited for B singlc beam aizil l’ablc 4 givcs the t.hhcorcLica1 diredivity uf switched-beam antcnnas for various numbers oC beams. In practice, antenna gain will be lowcr than thc directivity chic to fcccl lnsscs, ctc., coriscqucntly thc rninimiim number of Iieams togiiaratiteeagaiti nfatleast(idEic is3 Note thattlicpoor ilireclivily ol the low-gain aritenna necessitates the use of alternulive techniques to redirce 11 nwanled interference, nokldy spectral spreading.

Apractical rcalisation oEa switched three-bcam low-gain anknna is shown in Fig. 5. Each beam is omnidirecliorial in azimuth will] overlapping elevation coverage, as shown. Thc thrcc clcnicnls (high, middle and low) arc swikhed according lo the aircraft’s attihide. The preferred configuration (to achieve high isolation) is to haw one transmit and nnc rcccivc antcnna, swikhctl in taiidcm, cnch containing thrcc bcams in a so-callccl ‘bladc’housing (radome) siinilar to [:hat shnwi in Fig, 2.

M&~H W - ~ U U ?~t.t?t HU The bitlirecl.innnl, mediunz-gain antenna Cor vnice/low-

data-rate cntrimuriicatinns nriginally emliloyccl a corriigated circular horn (Big. tju), which has gond cross- polarisation and low sidelobe pei-hrinat~ce~. However the swept volume oC this nntmria is principally tlelermitied by its lenglh atid not its aperture, with the consequence that R retatively large ratlome is reqiiired ~ with a resultatit impact on aerodyniiinic clrag. Altcrnativc solutinns were therefore investigated and the short ‘back-fire’ antcnna (Fig. 6b)7 appeared to offer a more cnmpact mcctianical soliition. Howcvcr, tticrc is littlc cvidcricc in thc litcratiirc of dital circularly polarised short back-lire antcnnas operating in X-band with 15% fractional bandwidth. 1;ollowing an initial design phase a protnlype was built ancl evalualed with eiicoui-aginq results and Pur1he1- work on the feed nelworlr resulkcl in n n antetmn which inet both tlw mcclianical ancl electrical rcquircments, with >16 dnic gain.

?‘he final back-fii-c antcniia has a vcry high apcrturc efficiency (tlie ratio nf antenna gain to that of a pcrfcct antcnna ofthc sainc apcrlurc arca) of arniind unity, partly as a rcsult of its scmircsonant structiirc. It also has i t vcry small swept volumc - csscnlially limited by its alicrturc (-90 min diainctcr-jtistovcr2w~vclcn~ths).

7 Platform dynamics and antenna pointing

Having sited the antenna on the airframe it is necessary to erisure h t it continues 10 maintain the Iransmilted and received signals nver a given link within specified liinik, regardless of ilie motion of the aircran. ~ which, Cor military aircraft, can be severe. Signal (pointing) loss is nn unwanted conseqiience of the inherent uncerlainty in the tracking process and i s affected by:

aiigular pointing offscts and drift

antenria bcamwirlih platform stability (dcpentlerit upon airframe, fliglil mil n agem e11 t , wc atlicr il nd all i I 11 tl r) s h k and dynamic loading (dependent oil stores/ payload configuration anti wcapons) brain squint rcsulliiig Croni ncar-ficld rlistorlions and mull’lpath (racloinc diIfractinti/rcfractioii) altitwrlc-scnsor ta la accuiwy atid i ipclnk rate r i m - atid lrrcquency dependrnt colliin a I’ ion errors (Iroposphcric refraction anti scintillation).

For a given positional accutwy, the signal loss diie to anlentla pniriling is intrinsically proportional to the anlent-ia beatnwidtti. This, iii tnrn, is inversely ptaportional 10 thc sclirarc root of ihe nritcrina gain. High- gain antennas ilierelnrc rcyuirc. high-accui-acy pointing or kacking systetns. Thc iradc-dls 1)etwecn a closccl-loop lracltiiig system and an opcn-loop pointing systcni arc complcx. Ship-borne satellite tctminals have geiicrally arlnptccl closed-loop tracking, whilst the Nimrod M KA4, is expected adnpt an opcn-loop poiiiting systcni. However, it is likely that a closcd-loop Ir;ickirig systcrn within an open-Ioai) [iointing systcrn will be required to rncct the pointing recluirerncnts for iriture hi~.li-gain/narrow-bcam airboriic terminals.

Most modern aircraCt incorporale an inertial naviEalion syslcm (INS) or similar integrated i~~vigntioti systcm thal can polenlially proviclc the inlornuttion 011 the aircrdt allihde to allow ttie a t i t ” l o tie steered towards the satellite. Experiencc has shown, however, that connccliiig to an existing rnililary aircrafl’s INS c m bc prohil~itively expensive; iiirlccd, [he inost si~niGcant hiirtlle towards providitw ainililar y satcorii capability to aflee1.a faircraft is the high cnst ni inlegralian. l’urtticrtnoi-c, the data obhiiietl from thc INS is not. always suitalde lor S ~ I ~ C U I ~ I

antctiiiapoini:ingrlue toiriatictl1ialcupd;ite raks or flcxing of Ihc aircraft struclure bctwccn the INS sensors arid thc antenna’s locat ion.

The authors’ approach has therehre bccn to cxploit INS d a h whew avaiiahlc litit to concallrate on thc use uC

Table 4 Minimum antenna directivity for different numbers of switched beams

Beams per hemisphere Directivity per beam

3 dBi 6 dBi 8 dBi 9 dBi

10 dBi

autonoinous allilude ;mil liending rcfcrcncc systclns (AHKS) built into thc satcorii terminal, as closc tu the antcnna as is foasiblc. Tu his er id we have invcstigatcd a riutnhcr of low-cost AHRS system inkridecl for usc in rintnanncd acrial vchicles (UAVs). Two such systcms are shnwo iti Fig. 7.

‘lhc Watson C304 unit (Fig. 70) is a small, lighhvcight strap-down AHRS hasccl on a solid-slatt: three-axis gyro cluster, a pair n€ liquid anglc sensors anrl a thl-ec-axis magnelomeler. ‘111~ nttitiirlc is dcrived by integrating thc signals from the triaxiid gyro. ‘These are tlicti refcrcnced lo the carlh axes via h e angle seiisnrs, which are eingloycd as ttic gravikitional reference, and thc triaxial inagiielo~nctcr.

’Ihe ‘lrirnblc Aclwticcrl Navigation Sensor (TANS) Vectnr (Fig. 7h)is a solid-skte attitude-deterrniriatinn and position-location syslcm which uses the carrier phase difference of ’global posilioning systcin (WS) signals lrom 1111 to six satellites, received at lour scpat’ale antctitias. Uniqiicly, the TANS Veclor rcquircs n o attitude or position initialisalioii.

The complete autonmnoLis antciina pointing aribsyskm (Fig. 7c) coiriprises ;I GPS rcccivcr; AII IE, antetina positiuncr anrl coinguLer-lrasecl atitcnna coiilrol unit. The basic subsystcin employs an opeti-loop pointing. system, which in its simplest lorin prcdicts thc antenna poin[irig angles based on satcllitc ephemeris dah, air platform location antl attitude.

’I’hc accurxy and upd;ik ralc of lhcsc low-cost AIIRS systctns with medium-gain antennas arc, in €act,

a b

Fig. 6 Medium-gain antennas: (a) corrugated horn; (b) compact ‘short back-fire’ antenna

KIBLTRONICS &i COMlvlVrIJNICATION RNGINl<lCl<lNG JOLJRNAI, l~ISIil~IJAl<Y 2ocIo 21

a

.. ..

.- . . . . . . . . . . . . . . - b

G@ allllude and )loading Irl;a5\ raw nlatform I i\

tenna sitionm

a +

calculation algorithms

I *U?W* f-----

elevation t t Rx signal levet satetlite ephemeris

E

I

Fig. 7 Low-cost/autonomous positioning systems: (a) Watson AHRS; (b) Trimble TANS Vector; (c) control system schematic diagram

considered adequate Tor all but thc tnnst rlcmandinx air platlorins. Fig. 8 shows llie relative pointing losses associakd wilh a ~-ange ol'111atCorn1s [or I" and 10" antenna <J dB) bcaniwidths with balh low-cost and high-grade AHRS. Pointing ci-rui-s arc most significant [or satconis with thc narrowest bcamwidi-lis oil-board Llie most agile platforms. This impacts must niarkcrlly 011 the use ol'lSIiF satcoins on fastjcts (sec below).

Fillcrinfi. of tho low-cost AHRS signals and antenna. pnititing-anglc predictions can improve salcorn perfnrmanco. For cxxatnplc, a convcntiond Kdman filter based approach ~iiovidcs aii 'optimum' angular prrtlictiori vector (and covariance). Such a filter takes into account the known errors in the nieasiircmc~it systcms and plal.form-relaled disturbances and appropriate systcin noisc aiid cyuipmenl process models. However, different airfraintls and rolcs rcquirc development 01 a modified I<almaii filter for cach case.

A truly adaptive controllcr would coiitinuc to provide tiear-optimal antenna pointing - cvcn whcii stores and weapon config;urations chaiige Ihroughoul il mission and equiuiptnent life cyclc - by instigatiiig changcs to ils ow11 structurc (self-organising). So-called furwlogic controllers arc tnorc amcnalilc than Kalman filers 10 adaptive control in which thc structurc of thc control loop itself is changed. Purthcr bciicfits include the relative ease with which ad hoc control laws (such as thc inclusion of airlrame dependent 'wnodctl' regions ancl cmission reskictions) may be incorporated atid the ability lo clarify thcir aclapt.ivo aiid learning reasoning (tinlike artificial ncural networks) . Tlic use oC himy-logic and neural-fumy filterin2 is thereforc being iiivcstig;ilcd lo improve the performance, especially robustness, of satcoin aiitciiiia

pointing systems for use oti n wivitlc range of air p h i Corms.

6 Low-data-rate and voice-capable 5HF satcom terminal

I I I X 4 has tlcvclnpcd lwo variants of it proof-of-concept airborne SB1; satcam tcrtnind (Fig. 9) basctl arouiid a modrtlnr ('plug and work') approach, whcrcby tlic terniinal caii be configurd to npcrato in various moclcs by connecting motlulcs (antctmas, AHRS, handsrt ck.) to t i

cntnmm contrdlcr, Tlicsc terminals may be rcconfigurctl by means of tuiich-scrccn menus xd are Ttdly autonomotis, requiring only 1)C power from tlic host platform.

Tlic majority of llic terminal lunctions arc implemented in soflwarc. Thc principal ogeralirig iiiotles or these terminals arc:

Mode 1: world-witlc Iposition rcpor Ling aiicl messaging Mode2:voicc comrtiiinicatioiis (in satellite spot beams)

0 Mode 3: voice plus position rcporting and messaging.

l l ic SHF terminal niodulesarc:

0 the attitude and heading unit, comprising thc AHRS 0 thc atitcnna unit, comprising either thc switched low

Eain or stecrahle higll-gaiii antenna (degending on

coiifiguration), togcthcr wilh a. lownoise amplifirr (LNA), hig11-pnwcr atnglificr (IIPA) and diplcxrt- fillers thc tcrininal cnritml mil, dcveloped in corijiiiiclion with Ilella Communication, which codains thc RI; u p and dawn-cnnvcrlcrs, the GPS rccoivcr, ihe voice coder- decoder (vcicuiler) and a l"l4 cinbetiderl l'ctcntiuin PC (and evenlually thc mudern) the remote display mil, which prnvidcs h e operator intcrhace atid cnmpriscs an eriibetldccl PC with a touch scrisitive I ,Cl ) display.

'l'hesr: SHF lerminals arc currently bcing cvaluatetl Ihi*on& an cxtcnsive set of trials

9 Millimetre-wave satcoms: the future

'17ic cxisling Skynet 4 cotistcllation is dric to bc replaced by 2007.'lIie ncxtgcneratinti nf satcllites, Slcynet 5, duc to be iizlrorlucccl from 2005, is currcntly (he suhjcct of a series or 'I'ihlic Finance 1nit:iativc' studies. One of the options bcing activcly considered Cor Slync l 5 is the provision oC nn on-boartl-llrucesse[l, cxtrcmcly-hi& Crcrlucncy (IC1113 pay-load similar to that iisccl on the US Mi 1 star cn nstcllatio n:$.

'Ihc Milstar 13111: systcni opctates in thc 20.2-21.2 GI-Iz (dowi-link)/43.5455 CH:! (uplink) bantis aiicl was originally dcvcloped to provide post-nuclL.ar-sli'ike cornmancl and conlrnl facilitics. Specifically, Milshr was designed to provide:

enhancctl rcsishnce to jammiiiK enhnnccd irnmunily tri ionospheric scintillation (siich as that fnllowing a Iiigli-altitutlc nuclcar explnsiori) reduced probability oC iril~rccption/tleteclinn.

Mvlilstar aclchicvcs these goals though a coriibitiation of Cow factors:

use of an cxlreindy high frcclucncy (TCHF) for carlh-lo- salellitc linlts use of a highly robust c o m t n u n i c a h ~ wavcform use of sophisticated on-board grocessitip; (dcmotlirla- tinti/sprcacling, switchinK arid rcmodulation/ sprcading) usc oC cross-salcllitc linlts (iti thc 60 GITz nxypm absorption batid).

Withthcentl ofthc Colclwar, ~ecentMilst;lrsa~ellitesl~avc been aclaptcd io prnvidc tactical iisers with up to 1.5 M l W s of {slklitly lcss robiist) h igh-da l~a le communications eacll'J. Thri iisc of timc division mulliplcxing O M ) 011 thc clown-link contributes lo increasccl cfiiciency Cor lii&dak1-rate serviccs arid r'einoves tlw problem of ititermotlulatioii with frequency division multiplcxing; (IJIIM). On-board ~c~cncral.ion/switcl~itig also allows dircct cotniniiriicntinn bctwccn socalled disadvantaged users (thosc with small tcrminals), such as aircrafi-, wiihotd tlic iiccd tu use a1arg.c (and p o i ~ ~ ~ ~ i a l l y v u l i i ~ r ~ b l ~ ) gi*ound station AH a hub.

Thc iise of 1?RF Cor military satcoins lias bccn

mdl iet 'medium let

,,/large !t roiaty win^

Fig. 8 Relative pointing antenna pointing errors for various manoeuvres on different platforms: (a) 1" antenna beamwidth, high-grade AWRS; (b) lo antenna beamwidth, low-cost AHRS; (c) I O " antenna beamwidth, low-cost AHRS

invcstigat.ed by I3INAfor more than a decadc. All Slcyne(. 4, shgc 1 satellites (Slryiicl 4 A, H aiid C) have expctimcnhl I F I l ? rcccivcrs operating a144 GHz, iiscd Cor propaigal.ion and antcnoa poinling studies. Any 20/44 GI1z EllP payload sclcctcrl for Skyiiet 5 is cxpcctctl lo be rcquired lo bc compalible with (enhancctl) Milstar wavvehi-Ins so as to enslire ititeropcral~ilily with the US equipment.

lligli antenna gains are often citcrl as a henelit of operation nt EHlVrcqnencies; howcvcr tlw bcamwidtlis of satcl1il.e antennas arc genernlly rlctcrmiticd by the gcogtajiliical xovcragc required (such as catth cover), with l h e co~isequcnc~ h a t all antennas with sirnitar coverage also hnvc similar gain, rwarrllcss ofircquency. A hurther imgorl:atit cotiscquence or this fixed covcrxge is that, in the ahscnco of atmospheric attcnnation, link pcrhx~maricc for a given satcnin tcrtninal aperture is inlrinsically intlcpciident or frequencyIO.

'I'he principal appeal of the highcr. Prequency bands is llieteforc largcly clue to ihe iticrcascd speciriim available. Wider spcct.rum allnws cithcr accommodation of inore users/scrviccs or greater explnihtion 01 spread-spcctriim

ELECTRONICS & COMMIJNLCATION ENGINEERING JOURNAI, FEBRUAtlY 2000 23

Fig. 9 Proof-of-concept airborne SHF satcom terminal control unit

tcchniqucs to counter jarnrning and to reduce i:he prnbahility of interceptinti. Increased protect.ion against jamming and interccption through the use of spread- spectrum is usually characterised in terms oC the coding: gain - the ratin nC the spread bandwidth t.o the iirisprend bandwidth. 'I'he cadinggairi of an l i H F signal occiipyirig a -2 GHzs~,readba;otlwidlt~is thus I3 dI<grealerthnn thatuC an SH li'signal using a spread bandwidlh ol -100 M W ! In theory litis advantage against jamming is compktely offset by ihe higher gain of a jammer antenna of fixccl sim a l 1WI' comparctl lo SHE Whcthcr such n gain improvement can be achieved in practice is ;1 matter for debate", however the increasetl spat.ial selectivily at EIIF pcrmits mnre el'rective use nr a n m n a niilling u11 the satellite tn defeat jamming.

For the airborne user ihe primary advaniage oC El-IF frcqiiencies is reduced lilselitmd oC inlercel)tion due to the stnallcr antenna bcamwirlths for ;I given d a h rate. Dccpstriltc aircrait ancl athclc helicopters can operate hrindrcds of kilomctrcs bchind cncmy lines h clisnble the enemy's infrastructiirc and supply lines. Lnw probahility

oldeteciion (LPD) is vital on such missions. yet largctitw and threat data need to be tipdaletl coiistaiitly while the aircraft is en route lo its mission ancl surveillarice by the aircran relayed back.

Again, incre;sed spectrum for iisc by sprcad- sgeclriirn techniques works in favour of the aircr-idl- and against thosc trying to detcct its tr;insmissions since ii detector must scarch a larger spectral space to dctcct ttw aircra€t signal. The dctcctioii raiigc for a sitnplc radiometer-type dctcctoi- dc]icrids itivciscly on thc square root of thc bandwidth that the intcrccptor must monitor for a tratisiiiission,

A fiirtticr (Ll'D) advatitage resirlls froin the narrower beninwidih oC the receiver's ;mtciina; for the same receiver sensitivity a dctcctor will take longer to sweep Ihe same solid angle for the signal.

These armmcnts arc sntncwhat - oversiinplified, however the combination of wider spread- spectrum bandwidth and iiarrowcr atitcnna bcatnwidth (Cor llie same qm-iiire) at BBF result in a significant irnproveinenl in an a i rcd t ' s LPD - at the expense of reduced l o l c r " to antcnna pointinE ci-rnrs.

Nie most obvivious disadvantage of npcmting at BH1J frcqiicncics is iiicrcascrl attnosphcric attenuation in Ihe Iroposphcrc, as show~i in Fig. IO. The familiar absorption pcaks occur at -60 GHz and -118 GHz due to gaseous oxyEren and at -23 GHz and -1 83 GMx due to wtcr vapour. Continuum a t k n u a h n above 10 GIla also iiicrt.;tscs significaiitly in (hc pi+csmcc of rain and miti-bearing clouds as the signal wavclcngth bccnrnes conipnr+able with M I C S ~ ~ L of watcr tlroplcts. Rain f a h of30 dB or mnre can occiir due to a cornliination of absorption arid scattering - -alt:Iwiig.h onc arlvatitagc of thc increased atmospheric attenuation at sea lcvcl is likcly to be that terrestrially based intetecpiioii will Iic more severely affected by ainiospberic attenuation than airliornc satcoms.

Riither ford AIJplelon Taboratorics (CLRC l a ) arid Illl:l{A have develo~)ed a. versatilc attcnuation Inodcl to

1000

i ao

+ 10 s"

B % = I

0 1

I I I 1 10 1 OD 1000

frequency, GHZ

cakulatc scasonal maps of the link margin (in dccihcls) rcyuircd tn cnsure a given avai 1 ability for II:HII' communications at variotis a1titiicW2. The model employs lhc standard tnternaliond Telccommunications Union rccomtncndatiotis (ITtJ-R) to inotlcl attcnuation due to miti,

but pretliclions for absorption by cloud aiid atmospheric gascs are based on radiosondo data ancl cloud-cover obscrvatinns. Examples of lhcsc maps arc shown in Fig. 11, which shows 44

Ihc cficcls oI attcnuation arc gt‘catcst, to 20MO ft (60‘36 m), wherc thc attcnuation is almost ncgligilh.

It1 tlic humid, rainy climales near t:he cquat:nr, a very high litilr margin o f some 45 (ID would be requircil at sca level in order I O eiistire e v ~ 99% glolial availalility (at 44 G h ) ! 1;rom ‘Ltblc 5 it caii bc scui that a n availability n l 99% corresponds to just. 3% days oirtagc pcr ycnr. ilr-opying Ihe avaiktbility lo 95% incrcascs thc outaKc to 18 (lays pcrycar. A 15 dB powcr margin at lid GHz would yirielrl at lcast 95%availaliilityat sea lcvcl in all climatic rcginns of Lhc worlcl. Morc spricifically, the same niargiti woii td also yicld 99% nvailability al sen level iu 1i:tlropt..

For aircrdt flying above tlie freezing layer (abovc which rain liiriis lo ice) Ihe liiili margins arc considc.rably rcclucccl. I11 this caw ;i mal-gin of under ti d U nn the aircraft 1111-link will givc greater Uian 99% global availahiliLy.

Atttiosphcric attetiualinn is a major coricerii Cor users OC ICHB but it is riot the ntily problem lo affect ope~*ai.ioIi a( thcsc frcqiiencics. Aiiteimn pninting arid air lratne iiitcgratinn arc lrey issues which we likely lo become cveri ~iioi-c difficult as wc innve towards 11101’~ agile, stealthy aircraft such as the future oifensive air system (I<OAS)l:~.

I’mvirling highly rnbus1, Milslat.-coniIialible, l?HP satcoins to a highly agile, st.eallhy, aii+-pIalCorm is it

Table 5: Outage time per year against availability

Availability Annual outage

99.9% 8X hours 99% 3% days 95% 18 days 90% 36 days

sigtiificanl. challenge. MakinE it afforclablc is an evcti greater one. I3y clcvclopi~ig nnrl evaluating key tccliiiologics in airbortic EH1; kriuinal design, I3KRA is abic to iiivcstigatrt the praclical isstirs ;tssociatccl with these and other pokniial KIII’ optioiis (such as thc iisc c i f military Ka-baud) h r Skynct 5 and iniiiitnise Ihe risks assockiled with thc procurcIncri1 of llic: Moll’s hitiire ;tilborne satcom terminals.

Acknowledgments

Many pcople havc contributed to the worlc ilescribetl within this papcr, parlicularly 1. I<ariisden, I) . I,ynes, 1-1 Wclls, A. Moore, S. Jones, M. I-Iollow~y, ‘K Yalt.s,A. Brown, A. Smith, M. Sidford (DlSNA), C. Wreiich (RAT,), I? l’ierrot

a d

b e

C

Alenuation, d0

Fig. 11 European EHF propagation winter maps at sea level (a, 4, 10000 ft (b, e) and 20000 ft ( E , f ) . The maps a t the left (a, b, c), which show aircraft-to-satellite link path attenuation levels exceeded far 1 % of time, are for 99% availability The maps a t the right (d, e, fj, which show aircraft-to-satellite link path attenuation levels exceeded for 0.1 % of time, are for 99.9% availability. The link frequency is 45.5 GHz.

0

RT.I?CIXONICS & COMMUNICATION ENGINEERING JOUIWAL IXBIWARY 2000 25

Stuart Nichol gaiiied a BEiu: Honours degree iii Wcctrical ant1 lhlectroiiic Ewininecririg from the llniveriity uf I’nisley, atid n inastcis rlcjiret iii Microw:ivc Solid Slate Physics from Ihc Uiiivcrsity of Portsmouth. Since 1997 lie has workcd for DBlIA, wherehis niaiii field of wlivily has bem airbornu satcoins. Hc has worltetl nii system and Lcmiinal tledgii and bceii irivolvetl i t i il numbcrofmobilc wtcntnlrids,

IMowinK aradrraliorr with an lioriours ilccrcc i n fkcl roriic Enginccciny!’ iti 14M Garry Wtilbm joined thc Marcuiii Coinpniiy, wlicrc hc ivorkcil on design and dcvolopment nf surveillancc ~ n r l sateliite coniiiiiiiiica~iais ctliiipineiit. Bctwecn 1987 and 1094 Iic wurkckcil rxi a nuinher of Eurupcdii sl)acc nEency and coinincl-cia1 snace wouainincs. Sinct: 1994 lie . has l m t i iiti[iertakiiig rcscwch into saiclliic corntnuriicatioiis a t DEW, l)cllortI.‘l’he focus of this irrsearcli Iias liecn ilinicrl at provirliiig rcliablc salclliic coinmuuications iinlts lo agile platforms. Acirlili~ial areas of rcscwcli iiavc indudet1 ilsscssiiis Ihc clkcls of weather oii tlic availability of t i i i l l i i i ie l rc~~qucricy salcllite linlts withiii thc inulilc eiivironiiicnt. IIc 1x1s also uiidertnlten rcsc:irch i i i h sale1lii.e terminal lcclinoliigy aimed at reducing tlir: ilt:ptr~de~ice of a n airlmrrie satellite teriniiial o n t11c host platform. TIC currcniiy lcads !hcairhrne satcom rcsuarcli (cain willijn 1)IMA.

19% hc joinctl tlic I>c[cncc Kvaluiition antl lteesearch k c n c y (DERAI. €IC wiis. unlil receiilly, Project Man:igcr fur Airlmrnc Satoliile CoIntnunicnlions Research a i llic DISRA Satellite Coinmtinicarioiis Ccntrc :it Dcl‘turrl. He is ~ i n w Pcnject Managcr for Sntellitc Sysicin MariMclnent Research.

Ewtoil: ldrvrslbmolcOtIera.g.nv.uk

Scicnlist iind diarturcti etigineer. IIc liolds a i~iinibcr ul ~lalc~ils :ind has had ~iaiiers ~iiitiiislied on relatcd iircas ulI)hasctl ways, qmai-nptica, rlcteciion, tr:tclsing (inclurlitig lrarl<-beforedetccl processing), iiiimjnilsc rlzmodulnlio~i, atlnptivc statc cstiinatiri~i, prcdicliori and coiilrol npplietl tu rarlar, arid satellite cumrrulnicnt ions.

(+Iliomson CSF), K. Keet1 (Keen Associates), R. Heron, Ii. Hodson (Delta Conmimication), R. Willer, (Watson Inclustrics), antl J . McCarthy (DST0,Auslralia).

Anyviewsexpressed arc ttwsc of the authors antl do ntit necessarily reflect thosc of DERA, Moll or ntiy nlhcr UX p;ovcrnmcn t deparlmeti t.

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2 FRANKR, IC., and VAAL, G.: ‘Link riiar~ii i for IJHF airborne salcllile communic;ilions (SAXOM) operation’. 11511$ Natioiidl Aerosp:icc and E1cch“cs Conference, NAIXON 1996, Dayton. Ohio, 20lh-22ntl May 1996, I, pp. 90-37

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il I<ccslruciurcd MIISlT& proceeds’, h t . Dei R m , 1st January

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12 WIUENCM, C., DAVIDS, E G., ;old IUMSLIIIN, J.: ‘GloImI prediction ti[ slant path sttenu;dion un e;irth spacc litiks at HIP’, htzr.J Satell, ConamuM., 1999, 17, pp. 177-186

13 MORROCCO, J.: ‘IOAS eyes ratitlgr: nf tcclinoloRies’,Aviafioii Week oad Space Techno!., 7th Se1)l.cnibcr 1998, p. 93

1993, p. 15

@Crown Copyriglil 2000. Pul~lished w i ~ h the ljcrmission of the 1)elence Rcscai-ch & Lduation Agency on beliall of HMSO. lkceivctl 14th Scptcinber 1999

26