Satellite Internet services are used in locations where terrestrial Internet access is not available, and also for users who move frequently.
Broadband Internet access via geostationary satellite is available almost worldwide, including vessels at sea and mobile land vehicles. Similar, but slower Internet service is also available through Low Earth Orbit (LEO) satellites, however their coverage areas also include the polar regions at extreme latitudes, making them truly global.
End users must be aware of the different types of satellite communication systems and the technical issues involving each, such as latency and signal loss due to precipitation, in order to make informed decisions on which system will serve them best.
Mechanics and limitations of satellite communication
Signal latency
Latency is the delay between requesting data and the receipt of a response, or in the case of one-way communication, between the actual moment of a signal's broadcast and the time received at its destination. Compared to ground-based communication, all geostationary satellite communications experience high latency due to the signal having to travel to an altitude of 35,786 km (22,236 mi) above sea level (from the equator) out into space to a satellite in geostationary orbit and back to Earth again. This latency problem with satellite communications can be mitigated with TCP acceleration features that shorten the round trip time (RTT) per packet by splitting the feedback loop between the sender and the receiver. Such acceleration features are present in recent technology developments embedded in new satellite Internet services like Tooway.
The signal delay can be as much as 250 milliseconds to 900 milliseconds (one way), which makes this service unusable for applications requiring real-time user input, such as online games or remote surgery. This delay can be irritating with interactive applications, such as VoIP, videoconferencing, or other person to person communication. The functionality of live interactive access to a distant computer can also be subject to the problems caused by high latency. However these problems are more than tolerable for basic email access and web browsing, and in most cases are barely noticeable.
For geostationary satellites there is no way to eliminate this problem. The delay is primarily due to the great distances travelled which, even at the speed of light (about 300,000 km/second or 186,000 miles per second), can be significant. Even if all other signalling delays could be eliminated it still takes electromagnetic radio waves about 250 milliseconds, or one quarter of a second, to travel from ground level to the satellite and back to the ground, a total of over 71,400 km (44,366 mi) to travel from the source to the destination, and over 143,000 km (88,856 mi) for a round trip (user to ISP, and then back to user—with zero network delays). Factoring in other normal delays from network sources gives a typical one-way connection latency of 500–700 ms from the user to the ISP, or about 1,000–1,400 milliseconds latency for the total Round Trip Time (RTT) back to the user. This is far worse than most dial-up modem users' experience, at typically only 150–200 ms total latency.
However, Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) satellites do not have such great delays. The current LEO constellations of Globalstar and Iridium satellites have delays of less than 40 ms round trip, but their throughput is less than broadband at 64 kbps per channel. The Globalstar constellation orbits 1,420 km above the earth and Iridium orbits at 670 km altitude. The proposed O3b Networks MEO constellation scheduled for deployment in 2010 would orbit at 8,062 km, with RTT latency of approximately 125 ms. The proposed new network is also designed for much higher throughput with links well in excess of 1 Gbps (Gigabits per second).
A proposed alternative to geostationary relay satellites is a special-purpose solar-powered ultralight aircraft, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20,000 meters. Onboard batteries would be charged during daylight hours by solar panels covering the wings, and would provide power to the plane during night. Ground-based satellite dishes would relay signals to and from the aircraft, resulting in a greatly reduced round-trip signal latency of only 0.12 milliseconds. Several such schemes involving various types of aircraft have been proposed in the past.
Rain fade
Satellite communications are affected by moisture and various forms of precipitation (such as rain or snow) in the signal path between end users or ground stations and the satellite being utilized. The effects are less pronounced on the lower frequency 'L' and 'C' bands, but can become quite severe on the higher frequency 'Ku' and 'Ka' band. For satellite Internet services in tropical areas with heavy rain, use of the C band (4/6 GHz) with a circular polarisation satellite is popular. Satellite communications on the Ka band (19/29 GHz) can use special techniques such as large rain margins , adaptive uplink power control and reduced bit rates during precipitation.
"Rain margins" are the extra communication link requirements needed to account for signal degradations due to moisture and precipitation, and are of acute importance on all systems operating at frequencies over 10 GHz.
The amount of time during which service is lost can be reduced by increasing the size of the satellite communication dish so as to gather more of the satellite signal on the downlink and also to produce a more intense transmission on the uplink.
Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost.
Large commercial dishes of 3.7m to 13m diameter are used to achieve large rain margins and also to reduce the cost per bit by requiring far less power from the satellite.
Modern download DVB-S2 carriers, with RCS feedback, are intended to allow the modulation method to be dynamically altered, in response to rain problems at a receive site. This allows the bit rates to be increased substantially during normal clear sky conditions, thus reducing overall costs per bit.
Line of sight
Typically a completely clear line of sight between the dish and the satellite is required for the system to work. In addition to the signal being susceptible to absorption and scattering by moisture, the signal is similarly impacted by the presence of trees and other vegetation in the path of the signal. As the radio frequency decreases, to below 900 MHz, penetration through vegetation increases, but most satellite communications operate above 2 GHz making them sensitive to even minor obstructions such as tree foliage. A dish installation in the winter must factor in plant foliage growth that will appear in the spring and summer.
Fresnel zone
The radio signal width between two ground satellite dish receivers is not perfectly straight and uniform, as if it were a beam of light. Instead as the signal propagates away from the transmitting dish, it widens towards the centerpoint between the two dishes and then narrows again as it approaches the receiving dish. This is known as the fresnel zone, and limits the usefulness of satellite dishes in locations where there is extremely limited open sky for signal reception. The signal path through space must be clear not only for direct line of sight, but must also be clear for the expanding fresnel zone, which may be several meters larger in diameter than the ground-based satellite dish.
Two-way satellite-only communication
Two-way satellite Internet service involves both sending and receiving data from the remote VSAT site via satellite to a hub teleport, which then sends relays data via the terrestrial Internet. The satellite dish at each location must be precisely pointed to avoid interference with other satellites. Some providers oblige the customer to pay for a member of the provider's staff to install the system and correctly align the dish—although the European ASTRA2Connect system encourages user-installation and provides detailed instructions for this. Many customers in the Middle East and Africa are also encouraged to do self installs. At each VSAT site the uplink frequency, bit rate and power must be accurately set, under control of the service provider hub.
There are several types of two way satellite Internet services, including time division multiple access (TDMA) and single channel per carrier (SCPC). Two-way systems can be simple VSAT terminals with a 60–100 cm dish and output power of only a few watts intended for consu
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