Acta Astronautica Vol. 20, pp. 197-202, 1989 0094-5765/89 $3.00 + 0.00 Printed in Great Britain Pergamon Press plc

LAND MOBILE SATELLITE SERVICES IN EUROPE

P. Bartholom6*, G. B e r r e t t a * * and R. Rogard* European Space Agency

Abs t rac t

The demand f o r land-mob i le communications on a Europe-wide bas is is an important and p ress ing problem. The pan-European c e l l u l a r network now in the p lann ing stage w i l l be slow in coming and i t w i l l have i t s l i m i t a t i o n s . A reg iona l s a t e l l i t e system f o r Europe to complement the c e l l u l a r network i s the on ly p r a c t i c a l way to s a t i s f y a a p e c i a l i s e d market tha t encompasses a popu la t i on o f seve ra l hundred thousand mobi les, i nc l ud ing road v e h i c l e s , merchant sh ipp ing , f i s h i n g boats , and t r a i n s . The deployment o f a reg iona l system would take p lace in a number o f phases, the f i r s t be ing based on a s imple payload embarked on a host s a t e l l i t e be long ing to a European o r g a n i s a t i o n . Fur ther phases w i l l i n vo l ve the development o f more advanced payloads on ded icated s a t e l l i t e s . For the long- te rm f u t u r e , the use o f s a t e l l i t e s in h i g h l y i n c l i n e d o r b i t s i s being cons idered as a means o f improving t h e i r v i s i b i l i t y and hence the se rv i ce q u a l i t y .

I n t r o d u c t i o n

For many years , mobi le communications remained a marg ina l a c t i v i t y in the o v e r a l l telecommunica- t i o n s i n d u s t r y and t h i s i s r e f l e c t e d in the r e l a t i v e l y small amount o f f requency spectrum tha t i s a l l o c a t e d to mobi le se rv i ces in the Radio Regu la t ions o f the I n t e r n a t i o n a l Telecommunica- t i o n s Union. Between 0 and lO00 MHz f o r ins tance , on ly 6g o f the spectrum i s a v a i l a b l e to mobi le se rv i ces , compared wi th 50g f o r b roadcas t ing . Since, in the absence o f s u f f i c i e n t spectrum space, the p o s s i b i l i t i e s o f expansion were l i m i t - ed, t h i s s t a t e o f a f f a i r s tended to pe rpe tua te i t s e l f . A breakthrough was achieved ten years ago wi th the i nven t i on o f the c e | l u l a r network concept which a l l ows , in p r i n c i p l e st l e a s t , un l im i t ed reuse o f the same f requenc ies . This t r i g g e r e d a renewal o f i n t e r e s t in t e r r e s t r i a l mobi le communi- c a t i o n s , which i s now a very f as t growing market almost everywhere in the World.

In the wake o f the exp lod ing a c t i v i t y tha t i s c u r r e n t l y going on in the c e l l u l a r f i e l d , many o r g a n i s a t i o n s are focuss ing t h e i r a t t e n t i o n on the poss ib l e a p p l i c a t i o n s o f s a t e l l i t e s , as i t i s a l ready obv ious tha t t e r r e s t r i a l systems cannot p rov ide a s o l u t i o n to a l l problems. In Europe in p a r t i c u l a r , the European Space Agency is e x p l o r i n g the r o l e s tha t s a t e l l i t e s could p lay in mobi le communications, to complement the t e r r e s t r i a l c e l l u l a r networks.

= ESA/ESTEC, Noordwi jk , The Nether lands * * ESA Headquar ters , Pa r i s , France

The Pan-European C e l l u l a r Network

Fo l low ing the example set by the four Nord ic coun t r i es (Sweden, Norway, Denmark, F i n l and ) , which share a common rad io c e l l u l a r system (NMT), severa l o the r coun t r i es are equipp ing themselves wi th c e l l u l a r networks or are p lann ing to do so in the near f u t u r e . Most o f these systems, u n f o r t u - n a t e l y , have d i f f e r e n t c h a r a c t e r i s t i c s , which make them incompat ib le , so that the European mo to r i s t who crosses the border o f h is home count ry d i sco - vers tha t h is rad io te lephone has gone s i l e n t .

Under pressure from the Commission o f the Euro- pean Communities, f ou r teen governments have r e c e n t l y agreed to adopt the same standard f o r the next genera t ion o f c e l l u l a r system, the s o - c a l l e d "pan-European" (PE) system, tha t w i l l e v e n t u a l l y cover most o f the con t i nen t . The p r i n c i p a l m i l e - stones f o r i t s implementat ion are the coverage o f major c i t i e s and a i r p o r t areas by 1993, and the main roads connect ing those c i t i e s by 1995. T h e r e a f t e r , the network w i l l cont inue to expand at a r a te tha t w i l l depend on the p r e v a i l i n g c i rcum- stances in each count ry . Given the magnitude o f the investments requ i red f o r the i n f r a s t r u c t u r e o f t h i s p r o j e c t (es t imated at USD 3200 m i l l i o n over the f i r s t f i v e yea rs ) , i t can be expected tha t the coverage, as a percentage o f the t o t a l geograph i - cal area, w i l l s t i l l f a l l we l l shor t o f tOOg by the year 2000. In f a c t , in the leas t popula ted reg ions o f the con t i nen t , the investment w i l l p robab ly never be j u s t i f i e d .

Un l i ke i t s predecessors, which a l l use analogue techn iques, the PE system w i l l be e n t i r e l y d i g i - t a l . The access techniques f o r the mobi le users w i l l be based on s o - c a l l e d "narrow-band TDMA". The main c h a r a c t e r i s t i c s w i l l be as f o l l o w s :

Frequency p lan: 124 c a r r i e r s at 200 kHz i n t e r v a l in two pa i red bands (890-915 and 935-960 MHz)

Transmission ra te : 270.833 k b i t / s

TDMA frame: 4.615 ms d i v ided i n t o 8 t ime s l o t s

Modu la t ion : Gaussian Minimum S h i f t Keying (GMSK)

Speech encoding: 13 k b i t / s regu la r pulse e x c i t a t i o n wi th long- term p r e d i c t i o n (RPE-LTP)

197

198 P. B~THOLO~ et al.

Although the PE c e l l u l a r system no doubt has a b r i g h t f u t u r e and w i l l be welcomed by m i l l i o n s o f users , i t w i l l have i t s l i m i t a t i o n s . The f i r s t w i l l be i t s l i m i t e d coverage, which by necess i t y w i l l be r e s t r i c t e d to areas that generate enough t r a f f i c to j u s t i f y the investment. The second w i l l be i t s cos t , which is c e r t a i n to esca la te from the cur ren t o p t i m i s t i c f o r e c a s t s , as the development progresses and p r a c t i c a l problems o f implement- a t i o n are d iscovered . The t h i r d l i m i t a t i o n i s tha t the system i s geared towards the p r o v i v i o n o f a h i g h - q u a l i t y pub l i c se rv i ce and w i l l t h e r e f o r e not be the most economical way to p rov ide serv ices r e q u i r i n g on ly low data ra tes , nor lend i t s e l f to the wide v a r i e t y o f requirements tha t come from s p e c i f i c sec to rs o f business and governmental a c t i v i t i e s .

Requirements not covered by the PE System

As w i l l be seen l a t e r , the re are d e f i n i t e needs f o r Europe-wide se rv i ces tha t requ i re data t r a n s - miss ion at low ra tes and that the PE system cannot s a t i s f y economica l ly . The f o l l o w i n g are t y p i c a l examples:

- paging - message hand l ing (data and vo ice ) - t e l e x - p o l l i n g - q u e r y - r e p l y - data c o l l e c t i o n - p o s i t i o n r e p o r t i n g - l o c a l i s a t i o n .

Many o r g a n i s a t i o n s a lso requ i re c losed networks f o r t h e i r own exc lus i ve use, ra the r than se rv i ces as such. The i r communications problems are very s p e c i f i c and they wish to r e t a i n con t ro l over t h e i r own system. There again the PE system would not p rov ide an adequate answer, even i f i t s coverage were s u f f i c i e n t . What these o r g s n i s a t i o n s need i s the equ iva len t o f P r i va te Mobi le Radio (PMR) w i th Europe-wide coverage. As connect ion w i th the pub l i c network is not necessary or even not d e s i r a b l e in such cases, d i f f i c u l t i n t e r f a c e problems do not a r i s e . I t can c o n f i d e n t l y be p red i c ted tha t the s a t e l l i t e w i l l f i n d i t s best a p p l i c a t i o n s in decen t ra l i sed networks used f o r business mobi le se rv i ces , as is a l ready the case f o r f i x e d se rv i ces .

S a t e l l i t e s to complement the PE System

To many o f the p o t e n t i a l users who t r a v e l across Europe, the PE c e l l u l a r network would not be an a t t r a c t i v e p r o p o s i t i o n because o f i t s l i m i t e d coverage. The i n t e g r a t i o n o f a s a t e l l i t e system wi th the t e r r e s t r i a l network would have a number o f advantages, the most obvious being that

i t would o f f e r quas i -complete European coverage r i g h t from the ou t se t , t h e r e f o r e adding another a t t r a c t i v e f e a t u r e to a system designed to meet the needs o f the i n t e r n a t i o n a l t r a v e l l e r . But the s a t e l l i t e could a lso be en t rus ted wi th s p e c i f i c f unc t i ons w i t h i n tha t system, such as Europe-wide paging a n d the l o c a t i n g o f mobi le subscr ibers t r a v e l l i n g f a r from t h e i r home base.

When a s a t e l l i t e i s used as par t o f the pub l i c network, the problem f o r the system des igner has a q u i t e d i f f e r e n t na tu re . The se rv i ces to be p r o v i - ded by s a t e l l i t e must be o f the same q u a l i t y as those o f the assoc ia ted t e r r e s t r i a l network. Furthermore, space and t e r r e s t r i a l l i n k s have to be i n t e r f a c e d in such a way tha t the complete network is p e r f e c t l y t r anspa ren t , to the po in t tha t users remain unaware o f the type o f channel being used at any p a r t i c u l a r t ime to re l ay t h e i r communications. There are atso problems o f a less t echn i ca l nature such as charg ing, harmonisat ion o f t a r i f f s and, l a s t but not l e a s t , f i n d i n g an adequate amount o f f requency spectrum. Assuming tha t a l l these problems can be reso lved in due course, i t seems l i k e l y tha t a s i zeab le market w i l l develop in the long term f o r pub l i c mobi le communications by s a t e l l i t e , as a complement to the t e r r e s t r i a l c e l l u l a r se rv i ce .

Review o f Market Survey Resul ts

Var ious sec to rs o f business a c t i v i t y have been exp lo red wi th a view to determin ing the nature o f t h e i r needs f o r mobi le communications and assess- ing the s i ze o f the market in terms o f te rm ina l popu la t i on , namely :

- road haulage - in land-waterway nav iga t i on - merchant sh ips and f i s h i n g boats - ra i lways

The sec to r tha t appears to have the g rea tes t and most urgent needs i s the road t r anspo r t i ndus t r y [ 1 ] . There are more than 5000 companies in Europe tha t might be i n t e r e s t e d in mobi le communications by s a t e l l i t e , f o r the f o l l o w i n g reasons: - they opera te a la rge f l e e t o f veh ic les (more than 50) end hence have high ope ra t i ng cos ts , which they t r y to reduce by app ly ing more e f f i - c ien t f l e e t management techniques; and - t h e i r a c t i v i t i e s are i n t e r n a t i o n a l , i . e . t h e i r area o f ope ra t i on inc ludes the whole o f Western Europe, and sometimes extends i n t o p e r i p h e r a l areas such as Eastern Europe, the Middle East and North A f r i c a .

In-depth i n v e s t i g a t i o n s o f t h i s market have led to the conc lus ions shown in the t a b l e below.

Table I The road t r anspo r t market in Europe

I n t e r n a t i o n a l haulage companies Companies tha t ca r r y t h e i r

own goods Fre igh t fo rwarders Companies in p e r i p h e r a l coun t r i es

Number o f Number o f Number of companies veh i c l es s a t e l l i t e i n t e r e s t e d ( thousands) channels

2300 260 2600 300 45 450

140 85 850 90 10 100

Tota l 2800 400 4000

Land mobile satellite services in Europe 199

The c o n t r i b u t i o n o f nav iga t i on on in land waterways to i n t e r n a t i o n a l t r a n s p o r t in the European Community i s roughly equal to tha t o f the road, namely 40~ o f the o v e r a l l tonnage, the remain ing 20 • being c o n t r i b u t e d by the r a i l . The t o t a l number o f vesse ls i s r e l a t i v e l y smal l , however, a t about 12,000. The i r needs in terms o f mobi le communications a re not so p ress ing as in the r o a d - t r a n s p o r t sec to r because sh ippers u s u a l l y a l ready have some means o f radio-communicat ions at t h e i r d i sposa l . The p o t e n t i a l market in t h i s sec to r i s t h e r e f o r e f a i r l y smal l .

On the o ther hand, the l a rge number o f merchant sh ips and f i s h i n g boats s a i l i n g around the Europe- an coasts , in the North Sea and in the M e d i t e r r a - nean, a popu la t i on o f severa l m i l l i o n s , leads to an es t imate o f at l eas t 100,000 subscr ibers f o r t e l e x - d a t a se rv i ces by 1995, and a s i m i l a r number f o r te lephony [ 2 ] . Taken in combinat ion wi th the h igh- tonnage p leasure boats , f o r which the pene- t r a t i o n o f s a t e l l i t e t e rm ina l s i s expected to be s i g n i f i c a n t , i . e . between 35 and 50 X depending on the cos t , the mar i t ime community in Europe rep re - sents a p o t e n t i a l market o f 350 to 650 thousand u n i t s by 1995. I t i s t h e r e f o r e comparable to the r o a d - t r a n s p o r t market. However, whereas the road-based community cons is t s main ly o f numerous c losed user groups whose needs are best met by p r i v a t e (PMR) networks, the mar i t ime sec tor inc ludes a l a rge percentage o f i s o l a t e d users who would p r e f e r access to a pub l i c se rv i ce .

As f a r as ra i lways are concerned, s tud ies o f the requi rements in France, I t a l y and Spain i n d i c a t e that s a t e l l i t e - b a s e d techniques could be ins t rumen ta l in i nc reas ing the e f f i c i e n c y , s a f e t y and r e l i a b i l i t y o f the se rv i ce . One s p e c i f i c problem i s the ! o c a l i s a t i o n o f t r a i n s , which s t i l l r e l i e s on a t r a f f i c - c o n t r o l method developed in the l a s t cen tu ry . The t rack i s segmented i n t o sec t ions a few k i l ome t res long which must not con ta in more than one t r a i n at a t ime. The c o n t r o l system ensures tha t no t r a i n can enter a sec t ion un less t h i s has f i r s t been vacated by any prece- d ing t r a i n . The p r e c i s i o n in the l o c a l i s a t i o n i s t h e r e f o r e very crude and, as the f requency and speed o f t r a i n s inc reases , t h i s method begins to show i t s l i m i t a t i o n s . Due to pressure o f compet i - t i o n from o ther modes o f t r a n s p o r t , p a r t i c u l a r l y road and a i r l i n e s , th~ ra i lway companies are being fo rced to look f o r more e f f i c i e n t management and c o n t r o l techn iques. A s a t e l l i t e communications system o f f e r s them the p o s s s i b i l i t y o f implement- ing na t i on -w ide c o n t r o l systems based on an accura te p o s i t i o n r e p o r t i n g technique.

Voice communications by s a t e l l i t e f o r both r a i lway company and passenger use i s a lso being cons idered in l a rge coun t r i es such as I t a l y , where the c o r r e l a t i o n between the road and r a i l network topo logy is low, so that the c e l l u l a r network would not g e n e r a l l y be a v a i l a b l e f o r communica- t i o n s wi th moving t r a i n s . At the European l e v e l , the r a i l market i s es t imated to be severa l tens o f thousands o f u n i t s .

Ou i te apar t from the va r ious b u s i n e s s - a c t i v i t y sec to r s , s a t e l l i t e s are a lso being cons idered by governmental agencies concerned wi th t r a n s p o r t s a f e t y and env i ronmenta l p r o t e c t i o n . The number o f

AA 20--N

road v e h i c l e s , t r a i n s and boats t r a n s p o r t i n g dangerous cargoes i s es t imated to be between 10 and 50 thousand in a count ry such as France. On a European sca le , t h e r e f o r e , the number o f mobi les rep resen t i ng a p o t e n t i a l r i s k to pub l i c s a f e t y i s seve ra l hundred thousand, i . e . f a r more than can p o s s i b l y be moni tored by conven t iona l means. S a t e l l i t e s o f f e r the on ly p r a c t i c a l s o l u t i o n i f an e f f i c i e n t and safe s u r v e i l l a n c e system i s to be implemented.

F i n a l l y , mention should a lso be made o f another f i e l d o f a p p l i c a t i o n , namely emergency communica- t i o n s . In case o f a na tu ra l d i s a s t e r , s a t e l l i t e mobi le te rm ina ls may prov ide the on ly means o f r e s t o r i n g communications wi th a f f e c t e d areas when a l l conven t iona l means have f a i l e d .

The PRODAT System

In order to t es t the market f o r mobi le communi- ca t i ons by s a t e l l i t e in Europe and to help po ten- t i a l users de f i ne t h e i r requi rements b e t t e r , ESA has developed a demonstrat ion system c a l l e d PRODAT. This system, descr ibed e x t e n s i v e l y e l s e - where [ 3 ] , p rov ides two-way data t ransmiss ion at low ra te us ing the MARECS s a t e l l i t e s t a t i oned over the A t l a n t i c Ocean ( F i g . l ] . I t s main f ea tu res are as f o l l o w s :

(a ) The access technique on the re tu rn l i n k s (mobi le to hub s t a t i o n ] i s CDMA. One o f i t s advantages i s the ease o f random access, which i s r e f l e c t e d in the s i m p l i c i t y o f the user te rm ina l and o f the e n t i r e network. I t a lso p rov ides f o r e f f i c i e n t spectrum use and p r o t e c t i o n aga ins t i n t e r f e r e n c e , both w i t h i n the system i t s e l f and w i th o ther systems ope ra t i ng in the same f requency band (e .g . INMARSAT Standard-A] .

(b) The t ransmiss ion technique i nvo l ves an o r i g i n a l coding scheme which i s adap t i ve and is much more e f f i c i e n t than t r a d i t i o n a l c o n v o l u t i o n a l V i t e r b i techniques in a land-mobi le envi ronment, where the s a t e l l i t e l i n k i s cons tan t l y a f f e c t e d by obs tac les such as b u i l d i n g s , b r idges , t r e e s , e tc .

(c ) The p r o t o c o l s a l low severa l modes o f opera - t i o n : - t ransmiss ion o f messages between f i x e d and

mobi le users and between mobi le users them- se lves

- t ransmiss ion o f messages to mobi les in the broadcast mode

- q u e r y - r e p l y - p e r i o d i c p o l l i n g - paging.

F i e l d t r i a l s are c u r r e n t l y in progress to eva lua te the system's performance and the users ' r eac t i ons under o p e r a t i o n a l c o n d i t i o n s . They i n v o l v e more than 30 te rm ina l s i n s t a l l e d on road v e h i c l e s , smal l boats and a i r c r a f t . Six o f the land-mob i le t e rm ina l s are f i t t e d to the t rucks o f a French company s p e c i a l i s e d in the i n t e r n a t i o n a l t r a n s p o r t o f refrigerated goods. Two concrete r e s u l t s have a l ready been observed. F i r s t l y , the a v a i l a b i l i t y of PRODAT has reduced the use o f te lephone communications to almost zero. Secondly, c o n t r a r y to expec ta t i ons , th ree qua r te rs o f the messages are generated by the d r i v e r s , and on ly one qua r te r by the main o f f i c e , i l l u s t r a t i n g the degree o f acceptance o f the system by u n s k i l l e d personnel .

200 P. BART~OLOM~ et al.

DTE

DTE

DTE

TTY

TTY

• PACKET SWITCHED PUBLIC DATA NETWOI

x25

;ATEWAY L___

TELEX NETWORK

MOBILE NETWORK

Fig. 1 - A r c h i t e c t u r e o f the PRODAT demonstrat ion system

Future Plans

The f e a s i b i l i t y o f e x p l o i t i n g the PRODAT system on an o p e r a t i o n a l bas is beyond the demonstrat ion phase i s c u r r e n t l y being examined. This may r e s u l t in an i n t e r i m se rv i ce being p rov ided by a European ope ra to r l eas ing capac i t y on MARECS. For the longer term, however, the PRODAT-MARECS network c o n f i g u r a t i o n i s not optimum because i t does not meet the requirements o f the p o t e n t i a l users f u l l y . On the one hand the s a t e l l i t e , located over the A t l a n t i c , i s not we l l p o s i t i o n e d to serve Europe. I t s e l e v a t i o n above the hor izon is too low, p a r t i c u l a r l y as seen from h i g h - l a t i t u d e c o u n t r i e s . The best o r b i t a l arc f o r t h i s purpose would be 10 to 20" East, where the EUTELSAT s a t e l l i t e s are s i t u a t e d . Moreover, the t r a f f i c must t r a n s i t through an ear th s t a t i o n loca ted in Spain, f a r from many business cen t res . The d i s - advantages o f t h i s s i t u a t i o n are a l ready being f e l t by users, who complain about de lays in message t ransmiss ion caused by blockages in the t e r r e s t r i a l network. F i n a l l y , the system does not p rov ide v o i c e communication, which market surveys show to be an abso lu te necess i t y .

To progress the technology so tha t i t responds more f u l l y to the needs o f the user community, ESA i s c u r r e n t l y d e f i n i n g a payload that would support the ope ra t i on o f VSAT networks i n v o l v i n g both f i x e d and mobi le s t a t i o n s , the f i r s t work ing at 14/12 GHz and the second at 1.611.5 GHz. The use o f a f u t u r e EUTELSAT s a t e l l i t e as a host f o r t h i s payload i s being discussed w i th that o r g a n i a a t i o n .

With the l i m i t a t i o n s imposed on the s i ze o f t h i s payload (80 kg, 200 W), i t i s p o a l i b l e to envisage a s a t e l l i t e EIRP o f 42 dBW at 1.5 GHz at the edge o f a beam cover ing Europe on ly (F ig . 2) .

Two k inds o f networks could be supported by t h i s payload:

(a ) Networks f o r pub l i c access f o r l ow - ra te da ta - t ransmiss ion se rv i ces such as PRODAT and, to a l i m i t e d ex ten t and on exper imenta l bas is , t e l e - phony o f t o l l q u a l i t y .

(b) P r i v a t e Mobi le Radio f o r business se rv i ces to c losed user groups (F ig . 3) .

The data se rv i ces to be prov ided w i t h i n the pub l i c networks would c a l l f o r a PRODAT-type mobi le te rm ina l as PRODAT (G/T: -24 dB/K). PMR networks, on the o ther hand , would r equ i r e enhanced te rm ina l s (G/T: -15 dB/K) to support d i g i t a l te lephony at 4 .8 kb/s . The f i x e d hub s t a t i o n s would use antenna d ishes va ry ing in d iameter from 1.8 m fo r PMR c losed networks, to 3.5 m f o r pub l i c data networks. To ta l capac i t y would be at l eas t 10 independent data networks each serv ing up to 10,000 mobi les, and 300 PMR channels each se rv ing 100 mobi les. This would leave some capac i t y to experiment w i th t o l l - q u a l i t y te lephony f o r pub l i c use.

To conso l i da te the se rv i ce and accommodate the t r a f f i o growth tha t would be generated in the va r ious market sec to rs descr ibed above, s a t e l l i t e payloads w i th much l a r g e r c a p a c i t i e s w i l l have to be developed and p laced in o r b i t . Several concepts i n v o l v i n g mul t ip le -beam antennas f o r increased EIRP and spectrum reuse are under s tudy, and p lans are being prepared f o r i n - o r b i t v a l i d a t i o n on exper imenta l f l i g h t s . P re l im ina ry i n v e s t i g a t i o n s have a lso been i n i t i a t e d o f systems based on non -geos ta t i ona ry s a t e l l i t e s , which may become the u l t i m a t e s o l u t i o n in the long term.

Land mobile satellite services in Europe 201

Fig . 2 - Coverage o f the proposed European system

ELEVATION ANGLE EFFECT OF OBSTRUCTIONS

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Fig. 3 - Closed networks f o r business se rv i ces

F ig. 4 - E f f ec t o f o b s t r u c t i o n s

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Fig . 5 - TUNDRA o r b i t s u b - s a t e l l i t e t r a c k

Non-Geosta t ionary S a t e l l i t e s

For mobi le communications in h i g h - l a t i t u d e reg ions , 9 e o s t a t i o n a r y s a t e l l i t e s are not the i d e a l s o l u t i o n . They are very low over the hor izon and are e a s i l y ec l i psed by mountains and b u i l d i n g s (F ig . 4 ) . A l though i t i s q u i t e f e a s i b l e to opera te a da ta - t r ansm iss i on or message-handl ing system desp i t e constant i n t e r r u p t i o n s , as demonstrated by PRODAT, a voice-communicat ions se rv i ce would not t o l e r a t e breaks in conversa t ion unless very excep t i ona l or due to p r e d i c t a b l e causes (e .g . tunne ls and b r i d g e s ) . Many d i f f e r e n t types o f o r b i t can be cons idered to r e a l i s e the concept o f a q u a s i - g e o s t a t t o n a r y s a t e l l i t e hover ing over a h i g h - l a t i t u d e reg ion . Two o f them, the s o - c a l l e d TUNDRA and MOLNYA o r b i t s , are o f p a r t i c u l a r i n t e r e s t . Both are h i g h l y eccen t r i c and i n c l i n e d by about 63' to the Equator. With t h i s i n c l i n a - t i o n , the apogee-per igee ax i s remains f i x e d in the o r b i t a l p lane at an angle o f 27" w i th the Nor th - South d i r e c t i o n .

The TUNDRA is a 24-hour o r b i t wh i le the MOLNYA i s a 12-hour o r b i t . Typ ica l va lues o f apogee and pe r igee d is tances are :

TUNDRA : 25,000146,000 km MOLNYA : 1,500139,000 km

F igures 5 and 6 i l l u s t r a t e the p r o j e c t i o n o f such o r b i t s onto the Ea r t h ' s su r face .

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Fig. 6 - MOLNYA o r b i t s u b - s a t e l l i t e t rack

To ensure u n i n t e r r u p t e d se rv i ce over Europe wi th TUNDRA o r b i t s , at l eas t two s a t e l l i t e s are necessary. They must be placed in two o r b i t s i n c l i n e d symmet r i ca l l y w i th respect to the Nor th - South ax i s and phased such tha t one is at apogee when the o the r i s at pe r igee . The i r s u b s a t e l l i t e t r acks are then i d e n t i c a l and they f o l l o w each o the r at an i n t e r v a l o f 12 hours. Each s a t e l l i t e p rov ides a 12 hour se r v i ce cyc le .

202 P. BARTHOLOM~ et al.

With MOLNYA o r b i t s , at leas t th ree s a t e l l i t e s are necessary. For them to descr ibe the same ground t r ack , they must be p laced in th ree o r b i t s separated by 120' and phased such that they f o l l o w each o ther at 8 hours i n t e r v a l .

The r e l a t i v e mer i t s o f these two types o f o r b i t are c u r r e n t l y being assessed. The t h r e e - s a t e l l i t e MOLNYA system p r o v i d e s a b e t t e r coverage and s l i g h t l y h igher e l e v a t i o n angles than the two- s a t e l l i t e TUNDRA c o n f i g u r a t i o n . Moreover the TUNDRA system could on ly serve Europe, whereas the MOLNYA-baaad system could in p r i n c i p l e p rov ide the same s e r v i c e in the P a c i f i c reg ion , as shown in Figure 6. There are , however, many o ther aspects to be cons idered in a comparat ive eva lua t i on .

Problems remaining to be s tud ied in d e t a i l are the i n j e c t i o n i n to o r b i t , the phasing between the d i f f e r e n t o r b i t s , the p o i n t i n g and zooming of the s a t e l l i t e antenna beam, and the procedures f o r handover from one s a t e l l i t e to the o the r . Both systems would have the advantage over the gaosta- t i o n a r y s a t e l l i t e tha t the mobi le te rm ina ls could use a 7 to 8 dB gain antenna wi th f i x e d p o i n t i n g , d i r e c t e d at the zen i t h .

Conclusions

The needs f o r Europe-wide land-mobi le se rv ices by s a t e l l i t e are mani fes t and the cur ren t f i e l d t r i a l s w i th the PRODAT system are p r o v i d i n g encouraging r e s u l t s in terms o f both system performance under o p e r a t i o n a l cond i t i ons and i t s acceptance by the users . However, the data se r - v ices that the PRODAT-MARECS network can p rov ide are on ly an i n i t i a l and p a r t i a l answer to the problem. ESA t h e r e f o r e cons iders i t app rop r i a te that a reg iona l m o b i l e s a t e l l i t e system be deployed to complement the pan-European c e l l u l a r network now in the p lann ing stage. The combinat ion o f space and t e r r e s t r i a l techniques is the on ly way to meat the s p e c i f i c needs o f the European community.

References

[1] I n t e r e s t o f Road Hau l ia rs in a S a t e l l i t e - Ass is ted system for communications wi th Veh ic les Sate l Consai l and ITA, ESA Contract 6023•84 Rider No . l , August 1987

[ 2 ] Land Mobi le Communications S a t e l l i t e Miss ions Sata l Conse i l , ESA Contract 6023184, May 1986

[3] Land Mob i le S a t e l l i t e Serv ices in Europa - Resul ts o f Market Studies and I n i t i a l System Demonstrat ions Bartholomb P., 3ongajans A. , Lo isy C. and Rogard R. AIAA 12th I n t . Com. Sat. Systems Conference, March 1988, Paper 88-0847