Laurea Specialistica in Ingegneria Spaziale2° anno

Corso ‘ Stazioni di Terra’

Appendice 7Stazioni per lo sfruttamento delle telecomunicazioni

Terminali per TLC via satelliti in GEO e LEO

Giorgio PerrottaAnno Accademico 2007

Sistemi che operano con satelliti geostazionari

Applicazioni principali:

1) data gathering da boe o piattaforme disseminate nel territorio o in mare. I terminali (vengono chiamati cosi’ gli apparati ricetrasmittenti degli utenti del Servizio) sono generalmente piccoli e di caratteristiche modeste, il che significa che l’onere del radiocollegamento è spostato dal lato satellite;

2) broadcasting (radiodiffusione) TV : i terminali utente devono essere piccoli – tipicamente 60-90 cm di diametro, per motivi di estetica urbana- e quindi anche in questo caso l’onere del radiocollegamento è spostato dal lato satellite;

3) collegamento singoli punto –punto: tipicamente le larghezze di banda trattate sino corrispondenti ad una frazione di transponder standard (cioè qualche MHz) e le antenne del satellite hanno guadagni medio-bassi per conseguire coperture significative cdel territorio servito. In tal caso le antenne dei terminali - trasportabili /semidissi o fissi – hanno diametri compresi tra 1 e 3 metri ;

4) collegamenti punto-multipunto: è il caso tipico di affasciamenti di canali telefonici che ‘salgono’ verso il satellite ed occupano larghezze di banda piuttosto ampie, per cui le antenne del trasmettitore centralizzato deve essere abbastanza grande. A bordo del satellite il traffico viene partizionato e rediretto verso le destinazioni con affasciamenti di minor larghezza di banda, il che consente di utilizzare terminali di utente con antenne non eccessivamente grandi.

Influenza del tipo di ripetitori del satellite (1)

Ripetitore trasparente: - tecnologia molto semplice ancora molto utilizzata. - la trasparenza del ripetitore lo rende molto adatto ad operare con segnali in transito

caratterizzati da modulazioni analogiche (AM, FM, ecc); - richiede che il C/N realizzato nella tratta in salita ( terra> satellite) sia circa 10 dB maggiore

del C/N realizzato nella tratta in discesa (satellite>terra) per contenere il fenomeno del ‘noise power robbing’. Ciò ha un notevole impatto nel dimensionamento dei terminali terrestri.

- la ‘trasparenza’ del ripetitore implica una grande flessibilità operativa circa le caratteristiche di modulazione e codifica dei segnali che possono essere scelte liberamente dall’utente purchè compatibili con le larghezze di banda istantanee assegnate ;

- la canalizzazione del ripetitore viene effettuata nel dominio della frequenza e ciò comporta una certa rigidità operativa a meno di non complicare il progetto realizzando sistemi ad agilità ( limitata ) di frequenza;

- il problema della canalizzazione in frequenza si sente maggiormente nel caso di un ripetitore trasparente associato ad un’antenna con più spot beam in una configurazione di sistema con accesso multiplo.In questo caso risulta conveniente associare tecniche di accesso TDMA a tecniche FDMA;

Influenza del tipo di ripetitori del satellite (2)

Ripetitore rigenerativo: - un po’ più complesso del ripetitore trasparente- adatto per operare su segnali caratterizzati da modulazioni digitali , purchè il tipo di

modulazione sia noto e adatto ai demodulatori del ripetitore. Esistono tuttavia demodulatori programmabili che si possono ‘adattare’ a diverse modulazioni digitali in funzione dei requisiti dell’Utenza;

- la rigenerazione del segnale digitale disaccoppia, di fatto, l’up dal down link, pertanto il fenomeno del ‘noise power robbing’ è assente e ciò consente risparmi notevoli sul dimensionamento dei terminali trasmittenti di terra;

- il fatto che a valle della demodulazione a bordo si recuperi il segnale digitale in banda base consente, se opportuno, di rimodulare il segnale video- sulle portanti radio da ritrasmettere a terra - con tipi di modulazione diversi da quelli del segnale ricevuto;

- il fatto di disporre del segnale digitale demodulato consente di realizzare, a bordo del satellite, operazioni lineari e non lineari sui flussi dati quali: compressione, thresholding e mascheramenti, ri-quantizzazione, immagazzinamento in memorie con velocità di scrittura/lettura diverse;

- il fatto di disporre di flussi dati digitali rigenerati facilita parecchio, in sistemi operanti in regime di accesso multiplo TDMA e caratterizzati da antenne multifascio in ricezione e/o trasmissione, l’instradamento del traffico minimizzando la complessità del ripetitore. Per

queste funzioni complesse di instradamento si aggiungono matrici di commutazione di tipo ‘space stages’ o ‘time-space-time stages’

Come si presentano i vari terminali terrestri ?Boe :

Terminali di utente per distribuzione TV o per cominicazio

Ci sono migliaia di boe marine o anche semplici piattaforme di raccolta dati terrestri che hanno il compito di campionare periodicamente l’ambiente e memorizzare i dati di diversi sensori specializzati per poi ritrasmetterli in tempo differito ad uno o più centri di raccolta ed elaborazione dati. Le antenne dei ricetrasmettitori dei dati da/al satellite sono tipicamente poco direttive e le potenze trasmesse sono limitate dalla capacità di autoalimentazione degli apparati che vengono lasciati non attesi per lunghi periodi di tempo. Pertanto il satellite geostazionario che riceve i segnali emessi da questi mini-ripetitori, deve avere elevati valori di G/T e, se trasmette informazioni verso le piattaforme, anche discreti valori di EIRP.

Tra le boe si possono includere anche stazioni galleggianti, fisse o trasportabili adibite alla raccolta dati di interesse industriale o di utilizzazione delle risorse terrestri.

Queste piccole stazioni hanno requisiti di comunicazione, ad 1 o 2 vie , più impegnative delle boe sopra descritte, ma ciò non cambia sostanzialmente l’impatto sul dimensionamento del satellite che deve svolgere quel servizio di comunicazione.

Terminali d’Utente per la ricezione TV o per voce-dati punto-punto

La figura illustra una serie di prodotti commerciali – ci sono parecchi produttori di questi apparati- di cui la parte più ‘vistosa’ è l’antenna, con diametri che vanno da 5 m (a sinistra) per collegamenti a banda relativamente larga in applicazioni di tipo ‘business’, a poco meno di 1 metro per la ricezione individuale o comunitaria di segnali TV diffusi via satellite.

Per i satelliti geostazionari questi terminali sono privi, di norma, di sistemi di trcking e quindi sono ‘fixed-mounted’. A seconda dell’ambiente di lavoro possono essere equipaggiati con apparati di de-icing o radomes come protezione contro vento, pioggia, sabbia... Terminali professionali per traffico di tipo ‘trunking’ o comunque a banda larga

Questi terminali sono spesso trasportabili su ruote : l’elettronica trova posto in un ‘van’ equipaggiato con una o due postazioni di lavoro per gli operatori della Stazione mobile. L’antenna - eventualmente ripiegata o disassiemata se le sue dimensioni eccedono i limiti imposti ai trasporti su strada- viene eretta o ri-assiemata una volta raggiunto il sito di stazionamento. Il puntamento dell’antenna è aiutato dal GPS e bussole di precisione.

Questa foto illustra un classico prodotto militare per radiocollegamenti satellitari sicuri nelle bande X o Ku .In questo caso il terminale è trasportabile ma non carrato: viene trasportato ripiegato e messo in opera mediante un sistema meccanico con gambe regolabili. La forma a ‘diamante’ del paraboloide serve a ridurre i lobi laterali lontani emessi dall’antenna.

Schema a blocchi generico di un terminale ricetrasmittente

FDC Frequency Down Converter; FPS Front End Power Supply ;

FUC Frequency Up-Converter ; RFR Radio Frequency Receiver :

RFT Radio Frequency Transmitter

Lo schema, che rappresenta sinteticamemte le funzionalità dell’apparato, non cambia molto da un tipo di applicazione ad un altro. Cambiano invece: la tecnologia, le dimensioni, il grado di sofisticazione della componentistica, il livello di ridondanza messo in opera, la presenza di dispositivi accessori (p.e. monitorie e circuiti di guardia /allarme); o sistemi di alimentazione elettrica (p.es. da batteria, da rete, da accumulatori, da celle solari, ecc)

Terminali per la sola ricezione TV

Nel caso del broadcasting TV da satellite i terminali di Utente hanno la sola funzione di ricezione e pertanto sono ulteriormente semplificati: a valle dell’antenna vi è un ricevitore a basso rumore, ed un convertitore di frequenza che trasla il segnale ricevuto dalla banda Ku alla banda L, dopodichè il segnale viene ulteriormente amplificato e convertito in frequenza nella cosidetta ‘rooftop box’: da qui il segnale esce su una portante che cade nella banda UHF utilizzata dai normali canali televisivi.

L’offerta della TV interattiva e dell’uso di Internet via satellite sta facendo cambiare ancora la tecnologia dei terminali TV d’utente, aggiungendo una capacità trasmissiva, a banda relativamente stretta ma che utilizza bande di frequenze molto alte (banda Ka ed oltre), alla funzionalità di ricezione a banda larga.

Questa trasformazione è dovuta alla lentezza e scarsa capacità di traffico delle linee telefoniche proposte in un primo tempo per il canale di ritorno (Utente verso Service Provider) necessario per realizzare l’interattività ovvero la comunicazione bidirezionale tra Service Provider ed Utenti.

Tuttavia la trasformazione tecnologica è solo agli inizi e si è ben lungi dall’essere andati a regime.

satellite

TV uplink Station

Internet Service Ptovider

Public phone switching network

home phone

PC and TV set

satellite

home receiver

PV and TV srt

satellite

TV uplink station

Internet Service Provider

in verde: il canale di ritorno per l’interattività in tempo reale . Userebbe la banda Ka

L’evoluzione dell’uso delle frequenze e delle comunicazioni nella TV interattiva e Internet via satellite

Le coperture dei satelliti adibiti ai servizi di comunicazione d’area

Il sistema di comunicazioni europeo più diffuso ed utilizzato è il sistema Eutelsat che si avvale di una serie di satelliti in GEO posizionati in corrispondenza dell’arco orbitale di maggiore interesse per i paesi europei.

La copertura del territorio comprende sia l’Europa che parte dell’Africa del nord , della penisola arabica e, con alcuni satelliti più spostati verso est, parti del continente asiatico e di zone non contigue all’area di Servizio primaria

Nel tempo la tecnologia dei satelliti Eutelsat si è affinata parecchio ed oggi è competitiva o superiore alla tecnologia adottata nei satelliti commerciali prodotti da Aziende degli Stati Uniti. In particolare c’è da notare il largo ricorso all’uso di fasci d’antenna sagomati che consentono di concentrare l’EIRP del satellite dove ce ne è più bisogno, e l’impiego di tubi amplificatori ad onda progressiva, operanti in banda Ku e Ka , che rappresentano lo stato dell’arte.

Eutelsat wide European beam

Notare la copertura delle Canarie , della penisola Arabica e del medio Oriente

Sistema Eutelsat Satellite W5 posizionato a 70.5 ° est Esempio di copertura, mediante spot beam ripuntabile, della Asia peninsulare ed insulare

Copertura in downlink : notare gli alti valori dell’EIRP satellite

Copertura in uplink

Le ultime innovazioni del sistema EUTELSAT: Skyplex

Eutelsat developed a new Ka-band version of LinkStar for a broadband system called Skyplex, aimed at the market for high-speed multimedia communications. This terminal was custom-designed for Eutelsat's new Skyplex satellite, the first in the world to feature on-board multiplexing of digital television, radio, and data. The system is able to collect uplink signals from many sources, in many locations, package them on-board the satellite and deliver them to digital receivers on the ground. Higher frequency Ka-band satellites promise smaller, lower-cost terminals and reduce satellite air-time costs, while customers avoid long video backhauls to a central location and smaller or regional content pr oviders find it easier to contribute programming.

The system operates on a multiple access mode or with permanent bandwidth in order to offer maximum flexibility and cost-efficiency and can use 90cm terminals with 2 Watt amplifiers priced at less than €4,000, making it a highly competitive solution for broadband private meshed networks. In addition to LAN to LAN interactivity for applications such as file transfer and rich media, Skyplex Data is powerful enough for transmitting MPEG4 television channels through small uplink terminals for applications such as business TV.

The release states that the service is also being adapted for broadcasting DVB/MPEG2 television.Eutelsat’s new service will be offered through up to four Ka-band transponders on its Hot Bird 6 satellite located at 13 degrees East. The terminal is a compact unit (30cm wide and 4.5 cm high) able to receive a downlink stream of up to 38 Mbps from as many as six uplink carriers each with a net bit rate of approximately 6 Mbps, or 18 carriers each with a net bitrate of 2 Mbps. Eutelsat has recently completed validation of Ka-band terminals from ViaSat for the indoor unit and Patriot Antennas for the outdoor unit.

INMARSAT (1)

Inmarsat is the pioneer of global mobile satellite communications. Today it stands at the forefront of 3G wireless telephony, capitalizing on almost a quarter of a century's experience to deliver broadband communications solutions to enterprise, maritime and aeronautical users around the globe. Inmarsat operates a constellation of geostationary satellites that extend mobile phone, fax and data communications to every part of the world, except the poles.

The Inmarsat network

To deliver its services, Inmarsat calls and data connections are linked into the terrestrial telecoms infrastructure through a worldwide network of more than 29 land earth stations (LESs) located in 29 countries. A call from an Inmarsat mobile terminal goes directly to the satellite overhead, which routes it back down to an LES. From there the call passes into the public phone network for connection to home or office. The flow of communications traffic through the Inmarsat network is monitored and managed by the Network Operations Centre (NOC) at Inmarsat HQ.

The NOC is supported by network co-ordination stations (NCS). Their primary role is to help set up each call by assigning a channel to the mobile terminal and the appropriate LES. There is one NCS for each ocean region and for each Inmarsat system (Inmarsat A, B, C, etc). Each NCS communicates with all the land earth station operators in its ocean region, the other NCSs and the NOC, making it possible to distribute operational information throughout the system. For Regional BGAN, access to the network is provided through the gateway station, called an Satellite Access Station (SAS), in Fucino, Italy.

INMARSAT (2)

Il sistema INMARSAT supporta 5 categorie di terminali standard che vengono realizzati da produttori diversi secondo tecnologie più o meno avanzate , con ovvi riflessi sui prezzi. I terminali che mirano a servire ambienti d’uso particolari, quali il marino e l’avionico, vengono prevalentemente realizzati da Case costruttrici dotate di tecnologie più sofisticate.

INMARSAT (3)

Inmarsat I-4: Gateway to Broadband

Eighty-five per cent of the world's total landmass is now covered by Inmarsat's next-generation satellite system - the Inmarsat-4 (I-4) series. The first two of three I-4 satellites are now in commercial operation in Inmarsat's Indian and Atlantic ocean regions, with coverage extending across North and South America, Europe, Africa, Asia and the Far East.

Inmarsat's fourth-generation I-4 spacecraft (per ulteriori informazioni su questo nuovo e potente satellite si rimanda all’Appendice 1) are among the largest commercial communications satellites ever launched. They replace their highly successful predecessors - the Inmarsat-2 and Inmarsat-3 spacecraft - as the pillars of Inmarsat's new Broadband Global Area Network (BGAN) services. Together they deliver simultaneous voice and data at speeds of about half a megabit per second.

The Inmarsat-4s, like their predecessors, are equipped with a single global beam that covers up to one-third of the Earth's surface, apart from the poles. Each satellite also generates 19 wide spot-beams that provide continuous coverage across the same region for Inmarsat's existing high-end services. New to the I-4s are an additional 228 narrow spot-beams, designed to form the backbone of Inmarsat's broadband services, including the Broadband Global Area Network (BGAN), which was launched at the end of 2005. BGAN delivers Internet and intranet content and solutions, video-on-demand, videoconferencing, fax, e-mail, phone and LAN access at speeds of up to 492kbps.

INMARSAT (4)

In-orbit operations

The Inmarsat satellites are positioned in geostationary orbit. The satellites are controlled from the Satellite Control Centre (SCC) at Inmarsat HQ in London, which is responsible for keeping the satellites in position and for ensuring the onboard systems are fully functional at all times. Data on the status of the Inmarsat satellites is supplied to the SCC by four tracking, telemetry and control (TT&C) stations located at Fucino, Italy; Beijing in China; Lake Cowichan, western Canada; and Pennant Point, eastern Canada. There are also back-up stations at Eik in Norway and Aukland, New Zealand.

A call from an Inmarsat mobile terminal goes directly to the satellite overhead, which routes it back down to a gateway on the ground called a land earth station (LES). From there the call is passed into the public phone network.

With the launch of BGAN, two new gateways, called Satellite Access Stations (SASs), have been introduced. Both are owned by Inmarsat. The first, in Burum, The Netherlands, is operated by Inmarsat partner Stratos / Xantic, and the other, in Fucino, Italy, by another partner, Telespazio.

Sembra molto probabile che il sistema Inmarsat, specialmente con l’introduzione del BGAN, comporti una ulteriore riduzione della già marginale quota di mercato assorbita dai sistemi LEO

Il Sistema INTELSAT (1)

Intelsat è un operatore internazionale che gestisce circa 20 satelliti in GEO operanti parte in banda C e parte in banda Ku. Si avvale di una rete terrestre per il telecontrollo dei satelliti , di teleporti e di stazioni per il monitoraggio in orbita delle prestazioni dei satelliti (In-Orbit Tests > IOT) e della qualità dei segnali. I servizi resi vanno dall’affitto di un’antenna al lancio e al supporto – per mezzo del suddetto netwirk - delle operazioni orbitali iniziali, alle prove in orbita e al monitoraggio delle comunicazioni.

Più in dettaglio, il ground network consiste di 7 stazioni di TT&C, in Clasksburg, Fucino, Kumsan, Paumalu, Perth, Pretoria a Riverside. Questi siti sono equipaggiati di terminali operanti in banda C e Ku ma sino disponibili, via accordi con partner, terminali operanti nelle bande S ed X.

Le stazioni per l’IOT sino installate a Pechino, Clarksburg, Fucino, cin la possibilità di operazioni in remoto dall’Intelsat Control Center in Washington .

Ci sono teleporti a Funchsstadt, Hong Kong, Mountainside e Riverside che consentono l’accesso a Clienti via satellite o rete terrestre da ogni punto del mondo. Inoltre ci sono diverse stazioni di monitoraggio della qualità del servizio

Il Sistema INTELSAT (2)

Specifiche delle antenne dell’Intelsat Ground Network

C-band full performance antenna diametro: 11-32 m

frequenze: Tx: 5850-6425 MHz / Rx: 3625-4200 MHz

Polarizzazione: circolare e lineare

G/T : 31-41 dB/K

Tracking: monopulse e programmabileKu_band full performance antenna diametro: 11-14 m

frequenze: Tx: 13750-14500 MHz / Rx: 10950-12750 MHz

Polarizzazione: circolare e lineare

G/T : > 35 dB/K

Tracking: monopulse e programmabile

Per quanto riguarda i terminali dei clienti – adibiti alla ricetrasmissione dei segnali di comunicazione, Intelsat facilita il procurement e la produzione delle antenne avendo definito una serie di standard per i tipi: A,B,F,H,C,E,K

Il Sistema INTELSAT (3)

I satelliti

Intelsat's fleet of over 20 satellites provides connectivity virtually anywhere on earth for a variety of communications needs. The Intelsat satellites occupy prime orbital locations which assures a superior positioning that, in combination with flexible coverage options, enables providing a variety of connectivity solutions for global, regional and in-country communications requirements.

Intelsat's operational excellence continues to be an important selling features; the hallmark of the Intelsat satellite system is its transponder reliability, with an average transponder availability rate of 99.99% since 1985

Intelsat supervises every step of a spacecraft's life - from design through construction, testing, launch and operations. Once the satellites are operational, Intelsat flies and controls each of them from the satellite control center in Washington, 24 hours a day.

Sistemi che operano con satelliti in LEO

Applicazioni principali

1) Data gathering e data distribution, incluso servizi di e-mail (p.es. sistema Orbcomm)2) trasporto e consegna files elettronici e dati in modalità store and forward (tipo ‘cassetta

postale’), in special modo tra postazioni non servite, o servite male, dai servizi di comunicazione convenzionali;

3) comunicazioni con mezzi mobili, terrestri, marini, aeronautici per servizi a banda non molto larga, in particolare per servire zone territoriali servite male da infrastrutture terrestri o dei satelliti geostazionari;

4) comunicazioni di emergenza, servizi di comunicazione per ‘distress relieving’ ( p.es. sistema COSPAS-SARSAT)

I servizi di tipo Broadcasting sono praticamente incompatibili con i satelliti in LEO, anche se fanno parte di costellazioni di satelliti.

I servizi di tipo punto –multipunto, a parte il broadcasting TV, possono esser considerati ma richiedono la messa in opera di costellazioni di numerosi satelliti interconnessi tramite intersatellite links : il che aggiunge complessità , rischi tecnici e costi sia a bordo che a terra.

In ogni caso sistemi di comunicazione appena un po’ impegnativi ed ambiziosi richiedono lo sviluppo e la messa in opera di un segmento terrestre ben più complesso e costoso di quello che verrebbe richiesto per una soluzione basata su satelliti geostazionari.

I sistemi di comunicazione in LEO : IRIDIUM (1)

The Iridium satellite constellation is a system of 66 active communication satellites and spares around the Earth. The system was originally to have 77 active satellites, and was named for the element iridium, which has atomic number 77. Iridium allows worldwide voice and data communications using handheld devices. The service is interdicted for political reasons in North Korea and Northern Sri Lanka.

Iridium communications service was launched on November 1, 1998 and went into Chapter 11 bankruptcy on August 13, 1999. Its financial failure was largely due to insufficient demand for the service. The increased coverage of terrestrial cellular networks (e.g. GSM) and the rise of roaming agreements between cellular providers proved to be fierce competition. The cost of service was also prohibitive for many users, despite the continuous world-wide coverage of the Iridium service. In addition, the bulkiness and expense of the handheld devices when compared to terrestrial cellular mobile phones discouraged adoption among users. Mismanagement has also been cited as a major factor in the program's failure. The initial commercial failure of Iridium has had a dampening effect on other proposed commercial satellite constellation projects, including Teledesic. Other schemes (Orbcomm, ICO Global Communications, and Globalstar) followed Iridium into bankruptcy protection, while a number of proposed schemes were never constructed.

The Iridium satellites, however, remained in orbit, and their services were re-established in 2001 by the newly founded Iridium Satellite LLC, owned by a group of private investors.

I sistemi di comunicazione in LEO : IRIDIUM (2)

Present status

The system is being used extensively by the U.S. Department of Defense for its communication purposes through the DoD Gateway in Hawaii. The commercial Gateway in Tempe, Arizona provides voice, data and paging services for commercial customers on a global basis. Typical customers include maritime, aviation, government, the petroleum industry, scientists, and frequent world travelers. Iridium Satellite LLC claims to have approximately 137,500 subscribers as of September 30, 2005, which is a 22% increase from the third quarter 2004. Revenue for the nine months ended September 30, 2005 was up 24% over the nine months ended September 30, 2004.

Phone rates from land lines to Iridium phones are $3 to $14 per minute, from Iridium to land lines about $1.50 per minute and between Iridium phones less than $1 per minute. Iridium and other satellite phones may be identifiable to the listener because of the particular "clipping" effect of the data compression and the latency (experienced as a noticeable lag or time delay) due to the electronic equipment used. Iridium operates at a data rate of 2400 baud, which requires very aggressive voice compression and decompression algorithms.

I sistemi di comunicazione in LEO : IRIDIUM (3)

The constellation

The Iridium system requires 66 active satellites in orbit to complete its constellation, with spare satellites in orbit to fill in case of failure. Satellites are in low Earth orbit at a height of approximately 485 miles. Satellites communicate with neighbouring satellites via intersatellite links. Each satellite can have four intersatellite links: two to neighbors fore and aft in the same orbital plane, and two to satellites in neighboring planes to either side. The satellites orbit from pole to pole with an orbit of roughly 100 minutes. This design means that there is excellent satellite visibility and service coverage at the North and South poles, where there are few customers.

Because satellites use an over-the-pole orbital constellation design (known as a "Walker star") there is a "seam" where satellites in counter-rotating planes next to one another are travelling in opposite directions. Cross-seam intersatellite-link handoffs would have to happen very rapidly and cope with large Doppler shifts; Iridium only supports intersatellite links between satellites orbiting in the same direction.

The cellular lookdown antenna has 48 spot beams arranged as 16 beams in three sectors. The four intersatellite cross links on each satellite operate at 10 Mbit/s. The cross links were originally envisioned to be optical.

I sistemi di comunicazione in LEO : IRIDIUM (4)

The satellites

The satellite contains seven Motorola PowerPC 603E processors running at roughly 200 MHz. Processors are connected by a custom backplane network. One processor is dedicated to each cross-link antenna ("HVARC"), and two processors ("SVARC"s) are dedicated to satellite control — one being a spare. Late in the project an extra processor ("SAC") was added to perform resource management and phone call processing.

The original design envisioned a completely static 1960s "dumb satellite" with a set of control messages and time-triggers for an entire orbit that would be uploaded as the satellite passed over the poles. It was found that this design did not have enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. Therefore, the design was scrapped in favor of a design that performed dynamic control of routing and channel selection late in the project, resulting in a one year delay in system delivery.

Earth base-stations

Iridium routes phones calls through space. There are four earth stations and the space-based backhaul routes phone call packets through space to one of the downlinks ("feeder links").

Station-to-station calls can be routed directly through space with no downlink. As satellites leave the area of an Earth base station the routing tables change and frames are forwarded to the next satellite just coming into view of the Earth base station.

I sistemi di comunicazione in LEO : IRIDIUM (5)

Sintesi delle caratteristiche di IRIDIUM

N° di satelliti: 66 più 6 come in-orbit sparespiani orbitali: 6altezza orbitale: 780 km inclinazione dei piani orbitali: 86.4°periodo orbitale: 100 minuti, 28 secondipeso del satellite: 689 Kgspot beams: 48 per satellite margine del collegamento: 16 dB (in media !) vita del satellite: 7-9 anni

Bande di frequenza:- telefonia e massagistica: 1616-1625.5 MHz (L_band)- collegamenti di servizio e - intersatellite links: 23.18-23.38 GHz (Ka_band) - Collegamenti col segmento terrestre:

downlinks: 19.4 – 19.6 GHz ( Ka_band) uplinks : 29.1- 29.3 GHz ( “ “ )

I sistemi di comunicazione in LEO : IRIDIUM (6)

Il telefono di IRIDIUM

Che sviluppo potrà avere IRIDIUM ? Riporto qui una predizione ed il relativo commento di un noto osservatore del mercato internazionale:

“"Iridium will succeed because every time we estimated the growth of cellular phones, we were LOW by a factor of four" - Bary Bertiger of Motorola, system inventor.

In fact, Iridium is a "fill-in system" which depends for its success on these incorrect estimates of terrestrial cell phone growth. So when the inventor of Iridium used this argument to justify the system, he was actually predicting its imminent demise.

I sistemi di comunicazione in LEO: Globalstar (1)

System architecture. Globalstar differs from Iridium in several important ways: Globalstar satellites are simple bent pipe repeaters; there is no inter-satellite linking. A

network of ground gateway stations provides connectivity from the 48 satellites to the public switched telephone network; users are assigned telephone numbers on the North American Numbering Plan in North America or the appropriate telephone numbering plan for the country that the overseas gateway is located in. Because there is no inter-satellite linking, a satellite must have a gateway station in view to provide service to any users it may see. Because there is no gateway stations to cover certain remote areas (such as oceans far from land), no service can be provided in those areas even though the satellites fly over them.

Globalstar orbits have an inclination of 52 degrees, compared to the near-polar 86.4-degree orbits used by Iridium. Globalstar also does not cover polar areas due to the lower orbital inclination.

The Globalstar system uses the Qualcomm CDMA air interface; however, handsets use standard GSM SIMs.

Despite (or because of) these limitations, Globalstar’s operational costs were significantly cheaper and can support somewhat higher data rates than Iridium. However, such cost differences later proved meaningless when both companies shed their multi-billion dollar debts through bankruptcy

I sistemi di comunicazione in LEO: Globalstar (2)

System deployment

Like Iridium, Globalstar received its U.S. spectrum allocation from the FCC in January 1995, and continued to negotiate with various other sovereign nations for rights to use the same radio frequencies in their countries.

The first satellites were launched in February 1998, but system deployment was delayed through a series of embarrassing and costly launch failures, notably the September 1998 loss of 12 satellites in a launch by the Russian Space Agency. In February 2000, it launched the last of 52 satellites: 48 satellites and four spares (reduced from the original plan of eight spares).

The first call on the Globalstar system was placed on November 1, 1998, from Irwin Jacobs (chairman of Qualcomm) in San Diego to Bernard Schwartz (CEO and chairman of Loral Space and Communications) in New York.

In the Fall of 1999, the system began limited commercial service with 36 of 48 planned satellites. In March 2000, it began full commercial service in North America, Europe and Brazil. Initial prices were $1.79/minute vs. $9/minute for Iridium. In 2005, some of the satellites began to reach the limit of their operational lifetime of 7.5 years. In December of 2005, Globalstar began to move some of its satellites into a graveyard orbit above LEO.

I sistemi di comunicazione in LEO: Globalstar (3)

I sistemi di comunicazione in LEO: Globalstar (4)

I sistemi di comunicazione in LEO: Globalstar (5)Le Gateways

Il terminale Globalstar multifunzionale

QUALCOMM's Globalstar Tri-Mode Phone compliments existing fixed and cellular telephone networks by switching from terrestrial cellular telephony to satellite telephony as required. Globalstar Tri-mode portable phone provides clear digital voice and data communications worldwide by using QUALCOMM's Code Division Multiple Access (CDMA) technology combined with a 48 Low-Earth Orbit (LEO) satellite constellation. The Globalstar phone is designed to provide a low cost solution in areas where land line or cellular phones are inaccessible due to incompatible cellular technologies or lack of coverage.

Using LEO satellites allows the Globalstar System to maintain lower power consumption and propagation delay, which results in a smaller sized handset with clear voice quality, comparable, and in some cases, superior to today's digital cellular networks

Globalstar Gateways are an integral part of the Globalstar ground segment, which also includes Ground Operations Control Centers (GOCCs), Satellite Operations Control Centers (SOCCs), and the Globalstar Data Network.

Each Gateway, which is owned and managed by the service provider for the country in which the Gateway is located, receives transmissions from orbiting satellites, processes calls, and switches them to the appropriate ground network.

A Gateway may service more than one country. Gateways consist of three or four dish antennas, a switching station and remote operating controls. Because all of the switches and complex hardware are located on the ground, it is easier for Globalstar to maintain and upgrade its system, than it is for systems which handle switching in orbit.

Gateways offer seamless integration with local and regional telephony and wireless networks. The Gateway connects the Globalstar satellite-based wireless network with the Public Land Mobile Network (PLMN), such as AMPS and GSM. It also connects directly into the local Public Switching Telephone Network (PSTN). As such, the Gateway is the termination point for network transmission and network signaling.

I sistemi di comunicazione in LEO: Globalstar (6)

Ground Operations Control Centers (GOCC)

The Ground Operations Control Centers (GOCC) are responsible for planning and controlling the use of satellites by Gateway terminals and for coordinating this utilization with the Satellite Operation Control Center (SOCC). GOCCs plan the communications schedules for the Gateways and control the allocation of satellite resources to each Gateway.

Satellite Operations Control Center (SOCC)

The Satellite Operations Control Center (SOCC) manages the Globalstar satellite constellation. The SOCC tracks satellites, controls their orbits, and provides Telemetry and Command (T&C) services for the constellation. Globalstar satellites continuously transmit spacecraft telemetry data that provides on-board health and status reports for the satellite. The SOCC also oversees satellite launch and deployment activities. The SOCC and GOCC facilities remain in constant contact through the Globalstar Data Network (GDN).

Globalstar Data Network (GDN

The GDN is the connective network which provides wide-area intercommunications facilities for the Gateways, the Ground Operations Control Centers, and the Satellite Operations Control Centers.

The Gateway is connected to the existing PSTN using standard E1/T1 trunk supporting a variety of signaling protocols. Inter-operability between the Globalstar system and telephone/cellular companies enables the subscriber to maintain a convenient single point for billing. Encoding ensures voice and signaling security for individual transmissions.

Vista dell’antenna di una Gateway del sistema Globalstar . L’antenna è motorizzata per un puntamento dinamico ed è dotata di sistema di tracking per inseguire il moto dei satelliti in LEO . Il diametro dell’antenna è compreso, tipicamente, da 3.5 a 5 metri

Il sistema ORBCOMM (1)

Overview

The ORBCOMM system provides global, two-way, data communication services to a wide variety of applications. Subscriber communicators (SCs) pass data messages to and from Gateway Control Centers (GCC) over ORBCOMM satellites and GCCs route messages to users over the internet or dedicated delivery lines.

SCs are highly versatile communications devices available in a variety of configurations to suit different applications. They can support relatively simple, fixed-site, prescribed-content, interval-based messaging as well as complex, mobile, sensor-integrated, event-driven reporting. Common to all SCs is the VHF communication interface to ORBCOMM satellites. SCs communicate directly with satellites using ORBCOMM’s packet-switched protocol, and the system offers full transmission acknowledgement. The satellites provide SCs with system information and serve as the communication link between SCs and the ORBCOMM terrestrial network.

Messaging traffic flows between the satellites and a GCC through tracking stations called Gateway Earth Stations (GESs) that connect with satellites as they pass overhead. When a satellite is not connected to a GES, it can still support SC messaging in a store-and-forward mode. The system’s space segment consists of 30 operational satellites distributed around the globe in a low Earth orbit (LEO) constellation, with a license for up to 47 satellites in seven orbital planes at an altitude of 825 km (515 miles).

Il sistema ORBCOMM (2)

There are currently 12 GES facilities on four continents, maintaining satellite-GCC connectivity and near-real-time messaging capabilities for users throughout much of the world. Message traffic passed down to the GESs is directed over dedicated lines to GCCs for processing and delivery to end users.

The internet is commonly used for final transmission to the user, although more secure, dedicated delivery methods are also available. Users are able to manipulate received data as needed and often have back-office applications that assist with the presentation of, or automated responses to, incoming data. Outbound messaging to an SC follows the reverse transmission path.

Satellite Communication Frequencies and Data Rates Satellite Uplink Frequency: 148.00-150.05 MHz

Satellite Downlink Frequency: 137.00-138.00 MHz

Subscriber Uplink Rate/Mod: 2.4 kbps / SDPSK

Subscriber Downlink Rate/Mod: 4.8 kbps / SDPSK

Gateway Uplink Rate/Mod: 57.6 kbps / OQPSK

Gateway Downlink Rate/Mod: 57.6 kbps / OQPSK

Il sistema ORBCOMM : the Ground Segment (3)

Gateway Control Center

The GCC provides switching capabilities to link mobile subscriber communicators (SCs) with terrestrial-based customer systems. Interfaces to the GCC enable reliable, efficient and cost effective integration of the ORBCOMM system into existing or new customer information systems. A GCC controls one or more GES located within a licensed region, with multiple GCC deployed worldwide.

Interfaces to the GCC include both SMTP and web-based access via leased line, dial-up modem, public or private data networks or the Internet.

Monitors performance of regional GES. Monitors the status of the ORBCOMM message switch and related software. GCCs are located in Brazil, Italy, Japan, Korea, Malaysia and the United States.

Network Control Center

The NCC is located at ORBCOMM's headquarters in Dulles, VA. In addition to serving as the U.S. GCC, it is responsible for managing the constellation of ORBCOMM satellites. The NCC is staffed seven days a week, 24 hours a day by ORBCOMM certified controllers.  

Provides tools for the command, control, and analysis of the ORBCOMM satellites. Monitors the performance of ORBCOMM satellites.

Il sistema ORBCOMM : the Ground Segment (4)

The Gateway Earth Station

They are located in multiple locations worldwide and provide the following functions:

Tracks and establishes two way communication with the ORBCOMM satellites. Relay messages and telemetry between the satellites and the GCC.

Monitor the status of local GES hardware.

Contains two high gain VHF antenna systems, which function independently providing additional throughput and redundancy.

Current Planned

 

New York KazakhstanWashington South AfricaGeorgia GhanaArizona AustraliaMalaysia TurkeyCuracaoBrazilArgentinaItalyMoroccoKoreaJapan

Il sistema ORBCOMM : the User equipment (5)

Gateway Earth Stations location

For fixed and mobile two-way data and messaging

Units use economical VHF electronics. Simple antenna design and small package offer installation flexibility. Low power electronics enable extended operations using batteries, solar panels or

available power.

Subscriber Communicator Specifications (Typical)Transmission Power: 5 WattsReceive Dynamic Range: -116 to -80 dBmSensitivity Performance: BER of 10-5 at -116 dBm inputOperating Temperature: -30 to +60 oC

Power Consumption (at +12VDC) Receive: 100 mA Transmit: 2 A

 Sleep: < 1 mA

Satelliti geostazionari INMARSAT della generazione I-4

Constructed of flight-proven technology that promises at least 10 years of service life, the satellites will sit in geostationary orbit. They will bring about a 16-fold increase in the traffic-bearing capacity of the Inmarsat network - and will extend high-speed data into space, creating a truly global broadband network. Much of the I-4 traffic will be carried as Internet Protocol (IP) packet-switched data - the 'language' of the Internet. But the network will also be powerful enough to support circuit-switched applications, such as voice, ISDN and fax.

All the benefits of broadband will be open to Inmarsat users, including simultaneous voice and data, web browsing, e-mail, file transfer and access to local area networks (LANs).

Aside from their raw power, the uniqueness of the I-4s lies in their ability to generate hundreds of high-power spotbeams. These can quickly be reconfigured, and focused anywhere on Earth to provide extra capacity where needed.

The contrast with their predecessors, the Inmarsat-3s, is massive. Each I-4 will generate 19 wide beams and more than 200 narrow spotbeams, compared with only seven wide beams on the I-3. It will also bathe the Earth in a single global beam, which provides an initial signalling link for all services.

Every aspect of the spacecraft has been tailored to meet the demands of 24/7 operations in the harsh vacuum of space, where temperatures can fluctuate within minutes from -150 degrees centigrade in solar eclipse to +150 degrees in the full heat of the Sun.The satellite

A multi-national team of space technologists from the UK, France, Germany the USA and Canada has collaborated on the Inmarsat I-4 project. In May 2000, Inmarsat contracted European satellite specialist EADS Astrium as the lead contractor to build three spacecraft. The contractor's tried-and-trusted Eurostar series provided the blueprint for the Inmarsat I-4. The main body of the satellite was constructed in Britain. The bus, the onboard rocket engine that positions the spacecraft in orbit (also known as the service module), was built at the EADS Astrium facility in Stevenage. The payload - the satellite's communications powerhouse - was assembled at the company's facility in Portsmouth. The other main elements of the spacecraft - the antenna, the solar arrays and the 9-metre reflector - were manufactured in Canada, Germany, and the USA and integrated in the EADS Astrium facility in Toulouse.

The payload

It will handle massive volumes of traffic, and deliver voice and broadband services via the Inmarsat network. The payload incorporates advanced software and systems, enabling the I-4 to exploit fully those areas of the radio-frequency spectrum allocated to Inmarsat services. This optimizes network capacity and ensures the most efficient and economic use of resources. The satellite controllers' ability to reconfigure the I-4's powerful digital signal processor (DSP) in real time, to meet changing traffic needs, will also contribute to overall efficiency and performance.

The bus

This is based on EADS Astrium's Eurostar 3000 generic service module. It draws on the long and successful track record of the Eurostar series. A core element is the plasma propulsion subsystem (PPS), which provides thrust to keep the satellite on the correct orbital station. The PPS contains four Russian-designed stationary plasma thrusters. These flight-proven units are supplied by xenon gas and work by electrostatically accelerating plasma ions through a discharge chamber. The PPS is effectively two power-processing units in one. Each contains one prime thruster and one back-up. A separate chemical propulsion system will be used to establish the I-4 in its final orbit, for some in-orbit manoeuvres, and to back up the PPS.

Reflector

Astro Aerospace (an affiliate of Northrop Grumman Space Technology of the USA) built the reflector, which is based on a similar device successfully launched in 2000. But, the I-4 reflector has been redesigned to be smaller and lighter, and to be stowed into a space about the size of a large refrigerator. A deployment boom - unique to the I-4 - will unleash the reflector and cause it to bloom like a giant flower once the satellite achieves final orbit. The Canadian division of EMS Technologies supplied 120 helix antenna feed elements for the 2.5-metre antenna array. This structure enables the I-4 to generate complex spotbeam patterns using the spacecraft's 9-metre reflector. In tandem with the spacecraft's DSP, it will optimize bandwidth and channels to provide varied and high-quality broadband services.

Digital signal processor

A digital signal processor (DSP) that will govern the antennas, beam forming, gain control, switching and channel allocation. The use of digital beam forming gives Inmarsat a flexible system, and allows each individual spacecraft to be used at any orbital location. The beams can be changed at Inmarsat's convenience and be continuously adapted to the needs of system traffic as they evolve. The DSP has the ability to combine 100 kHz channels to provide wider bandwidths, if required.

Solar array

ADS Astrium designed a solar array for the I-4 that uses solar cells provided by RWE of Germany. The company created two types of solar panel - one containing gallium arsenide (GaAs) cells and the other high-grade silicon (Si) cells. The former are more efficient and are highly resistant to heat and radiation damage, and so ideally suited to use in space. The latter are lighter, more widely available, and proven to be reliable and cost-effective.

Launch vehicles

At six metric tons, the Inmarsat-4 is the largest commercial communications satellite ever launched-The satellite has been designed for compatibility with several tried-and-tested heavy lift vehicles, including Atlas V, Delta 4, Proton Breeze M and Zenit 3SL. After a thorough assessment process, International Launch Services (ILS) and Sea Launch were selected by Inmarsat to provide launch services for the Astrium-managed I-4 launch programme.

International Launch Services

ILS was founded in 1995 to provide launch services using US Atlas and Russian Proton vehicles. Inmarsat has contracted with ILS for an Atlas V vehicle to launch the first I-4 satellite. ILS is a joint venture between Lockheed Martin of the USA and the Russian companies Khrunichev State Research and Production Center and RSC Energia. It  uses launch sites at Cape Canaveral in Florida, and Baikonur in Kazakhstan.

The Atlas series has become the 'workhorse' of the US space programme, with 586 launches since the first Atlas mission in 1957.

Sea Launch

The Sea Launch partnership was formed in 1995. It provides a unique service by launching spacecraft from a floating platform in the middle of the Pacific Ocean.

The partnership includes Boeing Commercial Space Company of the USA; RSC Energia of Russia; SDO Yuzhnoye/PO Yuzhmash of Ukraine; and Kvaerner ASA of Norway. Operating from its home port of Long Beach California, it uses the Zenit 3SL launch vehicle.

Before launch, the floating platform Odyssey is accompanied to a launch zone near the equator by assembly and command ship Sea Launch Commander. The launch location provides the most direct route to orbit, offering maximum lift capacity from the 'sling-shot' effect of launching near the Equator.