Introduction to Broadband and Convergence
How to Choose the Right Communication Bearers
Pros and Cons of Satellite
Sometimes the nature of the terrain or the remoteness of the location or the sheer expanse of the area or the challenge of communicating with vessels at sea is too much even for LMR’s solid coverage. In such cases satellite communications is an option.
A communications satellite, parked in an orbit above the Earth, receives radio signals transmitted from a ground station which has a parabolic dish antenna aimed at it. The satellite amplifies the signals and re-transmits them to be received by appropriately aimed dishes located at multiple ground stations back on Earth.
A comms satellite can create a vital communications link between widely spaced locations on Earth, thus providing high-bandwidth channels to carry voice, data, video or internet. Mining production, oil and gas exploration, and national security have all used satellite in conjunction with other technologies to bring communications to locations that are otherwise difficult or prohibitively expensive to reach using terrestrial technologies.
The satellites themselves can be parked in various types of orbits above the Earth. In a geosynchronous (GEO) orbit the satellite follows a high-altitude path (35,786 km (22,236 miles)) with a speed matching the earth’s rate of rotation. This allows the satellite to appear to be fixed over the same location on Earth.
A geostationary orbit is a special case of this type, where the path is directly above the Earth’s equator. Comms satellites are commonly placed in geostationary orbits since the satellite dishes on the ground can simply point to the same fixed position in the sky and don’t need constant re-alignment to track the satellite. GEO satellites provide unrivalled comms coverage, requiring only 3 or 4 to cover most of the Earth.
Closer to Earth, Middle Earth Orbit (MEO) satellites – circling at between 2,000 km (1,243 miles) and GEO altitudes – and Low Earth Orbit (LEO) satellites at still lower altitudes – need constant automatic tracking by earthbound dishes or require users to wait for the next available satellite to pass overhead.
Communications satellites can use assorted radio and microwave frequencies, but international agreements aimed at minimizing communications chaos restrict their operation to specific bands (e.g. C-band (4-6 GHz), Ku (12-18 GHz) and Ka (26-40 GHz) bands).
Higher frequencies such as those in the Ka band provide greater bandwidth and better resolution but are more susceptible to ‘rain fade’ i.e. the absorption of radio signals by rain or snow in the atmosphere. Hence, in locations with regular rainfall or blizzards, the favored option is to use C-band frequencies despite their reduced bandwidth. This has made C-band almost too popular, with congestion in lower frequencies a major issue for satellite providers. (A less favored option is to use higher Ka or Ku frequencies but with more power to compensate for the signal loss due to rain fade.)
Bouncing a signal up to a satellite and down again takes time – up to half a second for a round trip for a GEO satellite and slightly less for MEO or LEO satellites closer to Earth. This high network latency, which affects all satellite communications, can rule out some applications, such as remote real-time control of machinery.
Although satellite networks are themselves extremely reliable (often quoting 99.7% availability), a satellite nevertheless represents a single point of failure, vulnerable to damage from orbiting debris or solar flares which can be difficult to repair. MEO and LEO satellites are more exposed to space debris and, unlike GEO satellites, experience atmospheric drag which causes their orbits to decay and leads to loss of altitude.
The rapid expansion of satellite communications has also created a growing problem of interference between adjacent satellites in an increasingly crowded sky. Where possible, a satellite-based system design should include provision for redundancy or alternative terrestrial communications to ensure access to communications is always available.
Portable Satellite Antenna and Modem
Satellite services such as BGAN (Broadband Global Area Network) offer briefcase-size portable battery-powered terminals into which a user can plug a phone, laptop or other smart device to make telephone calls or access applications on the Internet via theBritish company Inmarsat’s constellation of geosynchronous satellites. U.S. company Iridium offers similar services and products.
The BGAN plug and play terminals are easy to operate, needing only to be pointed in the general direction of the right satellite to get a connection at speeds for both uplink and downlink of up to 492 kb/s. While this may be a bit sluggish for web surfing, it is good enough to support basic Internet access, set up a VPN connection, send email, have two-way voice conversations, transfer files, and even some video conferencing and streaming IP.
The big issue with using services like these is cost, since you are paying either by flat rate or by the minute or by the megabyte. There are options for controlling cost, with companies offering different charging models, but to get the best of them it is necessary to have a clear idea of where satellite comms should be deployed as opposed to terrestrial alternatives.
Alternatively, VSAT (Very Small Aperture Terminal) satellite communications systems can be set up with a two-way ground station connected to a dish antenna that points to or tracks a particular satellite.
VSAT systems, which may be either fixed or mobile, can provide open standard IP-based Internet services such as email, web access, streaming video, voice over IP telephone, telemetry data transfer, or connection to business networks. VSAT data rates vary between 4 kb/s and 16 Mb/s, but unlike LMR, most VSATs are capable of supporting broadband data applications. VSAT sites can be networked together to connect back to a network operations center, command center or corporate headquarters.
VSAT equipment is available through a number of companies. VSAT is relatively inexpensive to buy, can be installed at a fixed site or in a vehicle, can be re-located quickly when needed, and is easy to operate. Most VSAT terminals operate in the more cost-effective Ku band or, increasingly, the Ka band.
Part of the reason for this lies in the convenience of their smaller dish antennas (74-120 cm. diameter). In contrast, C-band VSAT terminals can have dishes up to 4 meters in diameter – a bit too large for operation out of a typical vehicle!
However, using VSAT requires a clear line of sight between the dish antenna and the satellite, which can be an issue for system design and operation. On a vessel rocking in a stormy sea, line-of-sight can be a struggle to achieve. Gyrostabilized antennas help, but in moderately rough conditions a lot of comms errors and retries can accumulate, degrading the usefulness of the connection. On land, a heavy tree canopy, new buildings, mine dumps, changes in local topography, or simply knocking the dish out of true can impede line-of-sight and thereby disrupt a VSAT connection.
To provide wide-area linking across multiple sites (e.g. exploration camps) VSATs can be directly connected to each other in a mesh configuration. Or for centralized management and control each VSAT can be joined to a central hub ground station in star configuration. Both designs give additional routes for communication in case one or more of the VSATs fail or are unavailable.
By combining VSAT with other network bearers such as WiFi, cellular, LMR, or IP over ethernet, a variety of fully-functioned communication systems can be designed for remote locations without sacrificing connection to the rest of a critical communications system. Security is provided by encrypting voice and data communications as well as the ability to set up virtual private networks.
The big issue with using services like BGAN or Iridium or VSAT is cost, since you are paying either by a flat rate or by the minute or by the megabyte. The cost per megabyte is very high compared to terrestrial technologies. There are options for controlling cost, with companies offering different charging models, but to get the best of them it is necessary to have a clear idea of where satellite comms should be deployed as opposed to terrestrial alternatives.
Nevertheless, adding satellite as a bearer to a multi-technology solution, brings a whole new level of additional coverage, accessibility and communications flexibility.
Kuunganisha na mawasiliano ya mtandao: Jinsi ya kuchagua vyombo vya mawasiliano sahihi - Faida na hasara za satellite - Wakati mwingine asili ya eneo au umbali wa eneo au eneo kubwa au changamoto ya kuwasiliana na meli baharini ni kubwa sana hata kwa chanjo ya LMR. Katika visa hivyo mawasiliano ya satelaiti ni chaguo. Satellite ya mawasiliano, iliyowekwa katika mzunguko juu ya Dunia, hupokea ishara za redio zinazopitishwa kutoka kituo cha ardhi ambacho kina antena ya parabolic dish inayolenga. Satellite huongeza ishara na kuzirudisha ili zipokewe na sahani zilizoelekezwa vizuri zilizo katika vituo vingi vya ardhi duniani. Satellite ya mawasiliano inaweza kuunda kiungo muhimu cha mawasiliano kati ya maeneo yaliyo mbali sana duniani, na hivyo kutoa njia za upana wa bendi ya juu kubeba sauti, data, video au mtandao. Utoaji wa madini, uchimbaji wa mafuta na gesi, na usalama wa kitaifa wote wametumia satelaiti pamoja na teknolojia nyingine kuleta mawasiliano kwa maeneo ambayo vinginevyo ni ngumu au ghali sana kufikia kwa kutumia teknolojia za ardhi. Satelaiti zenyewe zinaweza kuwekwa katika aina mbalimbali za mizunguko juu ya Dunia. Katika mzunguko wa geosynchronous (GEO), satelaiti inafuata njia ya urefu wa juu (35,786 km) na kasi inayofanana na kiwango cha kuzunguka kwa dunia. Hilo hufanya satelaiti ionekane kuwa imewekwa mahali palepale duniani. Mzunguko wa geostationary ni kesi maalum ya aina hii, ambapo njia ni moja kwa moja juu ya ikweta ya Dunia. Satelaiti za mawasiliano kwa kawaida huwekwa katika mizunguko ya geostationary kwani sahani za satelaiti kwenye ardhi zinaweza tu kuelekeza kwa nafasi ile ile ya kudumu angani na hazihitaji usawa wa mara kwa mara ili kufuatilia satelaiti. Satelaiti za GEO hutoa chanjo ya mawasiliano isiyo na kifani, ikihitaji 3 au 4 tu kufunika sehemu kubwa ya Dunia. Satellite za Mzunguko wa Kati wa Dunia (MEO) zinazozunguka kati ya urefu wa kilomita 2,000 na urefu wa GEO na satelaiti za Low Earth Orbit (LEO) zilizo katika urefu wa chini zaidi zinahitaji kufuatilia mara kwa mara na sahani za ardhi au zinahitaji watumiaji kusubiri satelaiti inayofuata inayopatikana kupita juu. Satellite za mawasiliano zinaweza kutumia masafa mbalimbali ya redio na microwave, lakini makubaliano ya kimataifa yanalenga kupunguza machafuko ya mawasiliano yanapunguza utendaji wao kwa bendi maalum (kwa mfano, bands za redio). C-band (4-6 GHz), Ku (12-18 GHz) na Ka (26-40 GHz) bands. Frequencies ya juu kama vile wale katika Ka bendi kutoa bandwidth kubwa na azimio bora lakini ni zaidi nyeti kwa <unk>mvua fade<unk> i.e. Utoaji wa ishara za redio na mvua au theluji katika anga Kwa hiyo, katika maeneo yenye mvua au dhoruba za theluji kwa ukawaida, njia bora ni kutumia masafa ya bendi ya C licha ya upana wao wa bendi uliopunguzwa. Hii imefanya C-bandi karibu sana maarufu, na msongamano katika masafa ya chini suala kubwa kwa watoa satellite. (Uchaguzi chini favored ni kutumia juu Ka au Ku masafa lakini na nguvu zaidi ya fidia kwa ajili ya hasara ya ishara kutokana na mvua fade.) Kuingilia kwa ishara hadi satellite na chini tena inachukua muda wa nusu sekunde kwa safari ya pande zote kwa satelaiti ya GEO na kidogo chini kwa satelaiti za MEO au LEO karibu na Dunia. Hii high mtandao latency, ambayo huathiri mawasiliano yote ya satelaiti, inaweza kuondoa baadhi ya maombi, kama vile mbali muda halisi kudhibiti mashine. Ingawa mitandao ya satelaiti yenyewe ni ya kuaminika sana (mara nyingi inataja upatikanaji wa 99.7%), satelaiti bado inawakilisha hatua moja ya kushindwa, inayoweza kuathiriwa na uharibifu kutoka kwa mabaki ya mzunguko au milipuko ya jua ambayo inaweza kuwa ngumu kurekebisha. MEO na LEO satelaiti ni zaidi ya wazi kwa takwimu za anga na, tofauti na GEO satelaiti, uzoefu drag angahewa ambayo husababisha orbits yao kuharibika na inaongoza kwa hasara ya urefu. Upanuzi wa haraka wa mawasiliano ya satelaiti pia umesababisha tatizo la kuongezeka kwa kuingiliana kati ya satelaiti zilizo karibu katika anga lenye watu wengi zaidi na zaidi. Wakati inawezekana, satellite-msingi mfumo wa kubuni inapaswa kujumuisha utoaji kwa redundancy au mbadala mawasiliano ya ardhi ili kuhakikisha upatikanaji wa mawasiliano ni daima inapatikana. Antenna ya Satellite ya Portable na huduma za Satellite za Modem kama vile BGAN (Broadband Global Area Network) hutoa vituo vya kubebeka vya kubebeka vya kubebeka vya kubebeka ambavyo mtumiaji anaweza kuunganisha simu, kompyuta ndogo au kifaa kingine cha akili kufanya simu au kupata maombi kwenye mtandao kupitia kundi la satelaiti za Inmarsat. Kampuni ya Marekani ya Iridium hutoa huduma na bidhaa kama hizo. BGAN Plug & Play ni rahisi kutumia, na inahitaji tu kuwa na mwelekeo wa jumla wa satellite ya haki ili kupata uhusiano kwa kasi ya uplink na downlink ya hadi 492 kbps. Ingawa hii inaweza kuwa kidogo polepole kwa ajili ya kuvinjari mtandao, ni nzuri ya kutosha kusaidia msingi Internet upatikanaji, kuanzisha uhusiano VPN, kutuma barua pepe, kuwa na mazungumzo ya sauti mbili njia, faili uhamisho, na hata baadhi ya mikutano ya video na mkondo IP. Kwa sababu ya gharama kubwa ya huduma, unaweza kulipa kwa kiwango cha gorofa, kwa dakika, au kwa megabyte. Kuna chaguzi za kudhibiti gharama, na makampuni yanatoa mifano tofauti ya malipo, lakini kupata bora yao ni muhimu kuwa na wazo wazi la ambapo mawasiliano ya satelaiti yanapaswa kupelekwa kinyume na mbadala za ardhi. Vile vile, mfumo wa mawasiliano ya satellite wa VSAT (Very Small Aperture Terminal) unaweza kuanzishwa na kituo cha ardhi cha njia mbili kilichounganishwa na antena ya sahani ambayo inaelekeza au kufuatilia satelaiti fulani. VAT inaweza kuwa ya kudumu au ya rununu, inaweza kutoa huduma za mtandao za kawaida za IP kama vile barua pepe, upatikanaji wa wavuti, video ya utiririshaji, sauti juu ya simu ya IP, uhamisho wa data ya telemetry, au uhusiano na mitandao ya biashara. Kiwango cha data cha VSAT kinatofautiana kati ya 4kbps na 16Mbps, lakini tofauti na LMR, VSAT nyingi zina uwezo wa kusaidia maombi ya data ya bendi pana. Vituo vya VSAT vinaweza kuunganishwa pamoja ili kuunganisha tena na kituo cha shughuli za mtandao, kituo cha amri au makao makuu ya kampuni. Vifaa vya VSAT vinapatikana kupitia makampuni kadhaa. VAT ni rahisi kununua, inaweza kuwekwa katika eneo la kudumu au katika gari, inaweza kuhamishwa haraka inapohitajika, na ni rahisi kuendesha. Wengi VSAT vituo kazi katika zaidi ya gharama nafuu Ku bendi au, zaidi na zaidi, Ka bendi. Sehemu ya sababu ya hii iko katika urahisi wa antenna zao ndogo za sahani (74-120 cm). kipenyo). Kinyume chake, C-band VSAT terminals inaweza kuwa na sahani hadi mita 4 katika kipenyo <unk> kidogo kubwa sana kwa ajili ya operesheni nje ya gari ya kawaida! Hata hivyo, kutumia VSAT inahitaji mstari wazi wa kuona kati ya antenna ya sahani na satellite, ambayo inaweza kuwa suala kwa ajili ya kubuni mfumo na uendeshaji. Kwenye meli inayotembea katika bahari yenye dhoruba, inaweza kuwa vigumu kupata mwelekeo wa kuona. Antenna gyrostabilized kusaidia, lakini katika hali ya wastani mbaya mengi ya makosa comms na retries inaweza kukusanya, degrading manufaa ya uhusiano. Katika nchi kavu, canopy nzito ya miti, majengo mapya, dumps mgodi, mabadiliko katika topography ya ndani, au tu knocking sahani nje ya kweli inaweza kuzuia line-of-sight na hivyo kuvuruga uhusiano VSAT. Kutoa pana-eneo kuunganisha katika maeneo mbalimbali (kwa mfano VATs zinaweza kuunganishwa moja kwa moja kwa kila mmoja katika muundo wa mesh. Kwa ajili ya usimamizi wa kati na udhibiti, kila VSAT inaweza kuunganishwa na kituo cha ardhi cha kituo cha nyota. Vifaa vyote viwili vina njia za ziada za mawasiliano ikiwa moja au zaidi ya VSATs itashindwa au haipatikani. Kwa kuchanganya VSAT na vyombo vingine vya mtandao kama vile WiFi, LMR, au IP-over-Ethernet, aina mbalimbali za mifumo ya mawasiliano ya kazi inaweza kubuniwa kwa maeneo ya mbali bila kutoa sadaka ya uhusiano na sehemu nyingine ya mfumo muhimu wa mawasiliano. Usalama hutolewa kwa encrypting sauti na mawasiliano ya data kama vile uwezo wa kuanzisha mitandao ya kibinafsi virtual. Kwa mfano, kwa kutumia huduma kama vile BGAN, Iridium, au VSAT, unaweza kulipa kwa kila megabyte au kwa kila dakika. Gharama kwa megabyte ni ya juu sana ikilinganishwa na teknolojia za ardhi. Kuna chaguzi za kudhibiti gharama, na makampuni yanatoa mifano tofauti ya malipo, lakini kupata bora yao ni muhimu kuwa na wazo wazi la ambapo mawasiliano ya satelaiti yanapaswa kupelekwa kinyume na mbadala za ardhi. Hata hivyo, kuongeza satellite kama carrier kwa ufumbuzi wa teknolojia mbalimbali, huleta ngazi mpya ya chanjo ya ziada, upatikanaji na mawasiliano ya kubadilika.