Editing Satellite Internet access (section)

===Signal latency===
[[Latency (engineering)|Latency]] (commonly referred to as "ping time") 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 it is received at its destination.

A radio signal takes about 120 milliseconds to reach a geostationary satellite and then 120 milliseconds to reach the ground station, so nearly 1/4 of a second overall. Typically, during perfect conditions, the physics involved in satellite communications account for approximately 550 milliseconds of latency round-trip time.

The longer latency is the primary difference between a standard terrestrial-based network and a geostationary satellite-based network. The round-trip latency of a geostationary satellite communications network can be more than 12 times that of a terrestrial based network.<ref>{{cite web |last1=Golding |first1=Joshua |title=Q: What is the difference between terrestrial (land based) Internet and satellite Internet |url=http://www.niasat.com/q-what-is-the-difference-between-terrestrial-land-based-internet-and-satellite-internet-service/ |website=Network Innovation Associates |access-date=August 6, 2021 |archive-url=https://archive.today/20130630011719/http://www.niasat.com/q-what-is-the-difference-between-terrestrial-land-based-internet-and-satellite-internet-service/ |archive-date=2013-06-30 |date=August 9, 2011 |url-status=dead}}</ref><ref>{{cite web |title=Latency- why is it a big deal for Satellite Internet? |url=http://www.vsat-systems.com/satellite-internet-explained/latency.html |website=VSAT Systems |access-date=August 6, 2021 |archive-url=https://archive.today/20141021141107/http://www.vsat-systems.com/satellite-internet-explained/latency.html |archive-date=2014-10-21 |url-status=dead}}</ref>

Satellite latency can be detrimental to especially time-sensitive applications such as on-line gaming (although it only seriously affects the likes of [[first-person shooter]]s or [[Sim racing|racing simulators]] while many [[Massively multiplayer online game|MMOGs]] can operate well over satellite Internet<ref>Tom’s Hardware [http://www.tomshardware.co.uk/forum/page-99005_25_0.html "How much latency is too much for Online Gaming?"]. Accessed 23 January 2009. Internet Forum {{webarchive |url=https://web.archive.org/web/20110719162241/http://www.tomshardware.co.uk/forum/page-99005_25_0.html |date=19 July 2011 }}</ref>), but IPTV is typically a [[Simplex communication|simplex]] operation (one-way transmission) and latency is not a critical factor for video transmission.

The effects of this delay may be mitigated using data compression, TCP-acceleration, and HTTP pre-fetching.<ref>Newtec Productions NV [http://www.newtec.eu/fileadmin/user_upload/product_leaflets/sat3Play/Sat3Play_Ku-band_Terminal_TP210_R1.pdf "TP210 Sat3Play Broadband Terminal"] (Version R2/06.2010). Satellite Internet Modem factsheet {{webarchive |url=https://web.archive.org/web/20101117191319/http://www.newtec.eu/fileadmin/user_upload/product_leaflets/sat3Play/Sat3Play_Ku-band_Terminal_TP210_R1.pdf |date=17 November 2010 }}</ref>

====Geostationary orbits====
A [[geostationary orbit]] (or geostationary Earth orbit/GEO) is a geosynchronous orbit directly above the Earth's equator (0°&nbsp;latitude), with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero (i.e. a "circular orbit"). An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers. Launchers often place communications satellites and weather satellites in geostationary orbits, so that the satellite antennas that communicate with them do not have to move to track them, but can point permanently at the position in the sky where the satellites stay. Due to the constant 0°&nbsp;latitude and circularity of geostationary orbits, satellites in GEO differ in location by longitude only.

Compared to ground-based communication, all geostationary satellite communications experience higher latency due to the signal having to travel [[1 E7 m|{{convert|35786|km|mi|0|abbr= on}}]] to a satellite in geostationary orbit and back to Earth again. Even at the [[speed of light]] (about 300,000&nbsp;km/s or 186,000 miles per second), this delay can appear significant. If all other signaling delays could be eliminated, it still takes a radio signal about 250 milliseconds (ms), or about a quarter of a second, to travel to the satellite and back to the ground.<ref>{{cite web|url= http://www.its.ohiou.edu/kruse/publications/aiaa96.pdf|title= Data Communications Protocol Performance on Geo-stationary Satellite Links (Hans Kruse, Ohio University, 1996)|website= ohiou.edu|access-date= 28 March 2018}}</ref> The absolute minimum total amount of delay varies, due to the satellite staying in one place in the sky, while ground-based users can be directly below (with a roundtrip latency of 239.6&nbsp;ms), or far to the side of the planet near the horizon (with a roundtrip latency of 279.0&nbsp;ms).<ref>Roundtrip latency numbers are from RFC 2488, Section 2: Satellite Characteristics</ref>

For an Internet packet, that delay is doubled before a reply is received. That is the theoretical minimum. Factoring in other normal delays from network sources gives a typical one-way connection latency of 500 to 700&nbsp;ms from the user to the ISP, or about 1,000 to 1,400&nbsp;ms latency for the total round-trip time (RTT) back to the user. This is more than most dial-up users experience at typically 150–200&nbsp;ms total latency, and much higher than the typical 15 to 40&nbsp;ms latency experienced by users of other high-speed Internet services, such as [[cable Internet access|cable]] or [[very high bit rate digital subscriber line|VDSL]].<ref>{{Cite web|url=https://www.thetripleplay.net/att-internet/|title=AT&T Internet Plans & Packages &#124; AT&T Internet Service Deals, Prices|website=www.thetripleplay.net}}</ref>

For geostationary satellites, there is no way to eliminate latency, but the problem can be somewhat mitigated in Internet communications with [[TCP acceleration]] features that shorten the apparent round trip time (RTT) per packet by splitting ("spoofing") the feedback loop between the sender and the receiver. Certain acceleration features are often present in recent technology developments embedded in satellite Internet equipment.

Latency also impacts the initiation of secure Internet connections such as [[Secure Sockets Layer|SSL]] which require the exchange of numerous pieces of data between web server and web client. Although these pieces of data are small, the multiple round trips involved in the handshake produce long delays compared to other forms of Internet connectivity, as documented by Stephen T. Cobb in a 2011 report published by the Rural Mobile and Broadband Alliance.<ref>{{cite web|url= http://www.rumbausa.net/|title=RuMBA White Paper: Satellite Internet Connection for Rural Broadband |author=Stephen Cobb |website=RuMBA – Rural Mobile & Broadband Alliance |access-date=22 March 2019 |archive-url=https://web.archive.org/web/20120729082818/http://www.rumbausa.net/ |archive-date=2012-07-29|url-status=usurped }}</ref> This annoyance extends to entering and editing data using some Software as a Service or [[SaaS]] applications as well as in other forms of online work.

Functions, like live interactive access to a distant computer—such as [[virtual private network]]s, can be affected by the high latency. Many TCP protocols were not designed to work in high-latency environments.

====Medium and low Earth orbits====
Medium Earth orbit (MEO) and low Earth orbit (LEO) satellite constellations do not have such long delays, as the satellites are closer to the ground. For example:
* The current LEO constellations of [[Globalstar]] and [[Iridium Satellite LLC|Iridium]] satellites have delays of less than 40&nbsp;ms round trip, but their throughput is less than broadband at 64&nbsp;kbit/s per channel. The Globalstar constellation orbits 1,420&nbsp;km above the Earth and Iridium orbits at 670&nbsp;km altitude.
* The [[O3b]] constellation orbits at 8,062&nbsp;km, with RTT latency of approximately 125&nbsp;ms.<ref>{{cite journal|arxiv=1407.2521|last1=Wood|first1=Lloyd|title=Revisiting elliptical satellite orbits to enhance the O3b constellation|journal=Journal of the British Interplanetary Society|volume=67|pages=110|last2=Lou|first2=Yuxuan|last3=Olusola|first3=Opeoluwa|year=2014|bibcode=2014JBIS...67..110W}}</ref> The network is also designed for much higher throughput with links well in excess of 1&nbsp;[[Data-rate units#Gigabit per second|Gbit/s]]. The next generation [[O3b mPOWER]] constellation shares the same orbit and delivers from 50&nbsp;Mbit/s to multiple gigabits per second to a single user.<ref>{{cite press release|url=https://boeing.mediaroom.com/2020-08-07-Boeing-to-Build-Four-Additional-702X-Satellites-for-SES|title=Boeing to Build Four Additional 702X Satellites for SES's O3b mPOWER Fleet|publisher=Boeing|date=7 August 2020|access-date=25 April 2021}}</ref>
* A study in 2021 showed that the [[Starlink]] satellites orbit at 550 km altitude, with an average RTT latency of 45&nbsp;ms.<ref>{{cite news|url=https://www.zdnet.com/article/starlink-is-better-than-its-satellite-competition-but-not-as-fast-as-landline-internet/|title=Starlink is better than its satellite competition but not as fast as landline internet|date=5 August 2021|access-date=26 March 2022}}</ref> Another study in 2022 showed that the latency of the Starlink network is 1.8&nbsp;ms to 22.8&nbsp;ms more than the latency of terrestrial networks according to a measurement study conducted in the Metro Vancouver area. <ref>{{Cite arXiv |last1=Ma |first1=Sami |last2=Chou |first2=Yi Ching |last3=Zhao |first3=Haoyuan |last4=Chen |first4=Long |last5=Ma |first5=Xiaoqiang |last6=Liu |first6=Jiangchuan |date=2022-12-27 |title=Network Characteristics of LEO Satellite Constellations: A Starlink-Based Measurement from End Users |class=cs.NI |eprint=2212.13697 }}</ref> Note that the exact measurement results can differ as the deployment of Starlink infrastructure varies across time and locations.

Unlike geostationary satellites, LEO and MEO satellites do not stay in a fixed position in the sky and from a lower altitude they can "see" a smaller area of the [[Earth]], and so continuous widespread access requires a constellation of many satellites (low-Earth orbits needing more satellites than medium-Earth orbits) with complex constellation management to switch data transfer between satellites and keep the connection to a customer, and tracking by the ground stations.<ref name="McK">[https://www.mckinsey.com/industries/aerospace-and-defense/our-insights/large-leo-satellite-constellations-will-it-be-different-this-time ''Large LEO satellite constellations: Will it be different this time?''] McKinsey & Company, 4 May 2020, Accessed 25 April 2021</ref><ref>[https://spacenews.com/divining-what-the-stars-hold-in-store-for-broadband-megaconstellations/ ''LEO and MEO broadband constellations mega source of consternation''] SpaceNews, 13 March 2018, Accessed 25 April 2021</ref>

MEO satellites require higher power transmissions than LEO to achieve the same signal strength at the ground station but their higher altitude also provides less orbital overcrowding, and their slower orbit speed reduces both [[Doppler effect|Doppler shift]] and the size and complexity of the constellation required.<ref>[http://interactive.satellitetoday.com/via/march-2019/the-gravity-of-space-debris/ ''The Gravity of Space Debris''] Via Satellite. March 2019, Accessed 25 April 2021</ref><ref>{{cite web |title=Satellites: LEO, MEO & GEO |url=https://www.atlantarf.com/Download_LEO_MEO_GEO.php |website=Atlanta RF |access-date=August 6, 2021 |archive-url=https://web.archive.org/web/20141003022149/https://www.atlantarf.com/Download_LEO_MEO_GEO.php |archive-date=2014-10-03 |date=October 29, 2013 |url-status=dead}}</ref>

Tracking of the moving satellites is usually undertaken in one of three ways, using:
* more diffuse or completely omnidirectional ground antennas capable of communicating with one or more satellites visible in the sky at the same time, but at significantly higher transmit power than fixed geostationary dish antennas (due to the lower gain), and with much poorer signal-to-noise ratios for receiving the signal 
* motorized antenna mounts with high-gain, narrow beam antennas tracking individual satellites
* [[phased array]] antennas that can steer the beam electronically, together with software that can predict the path of each satellite in the constellation.

In May 2022, [[Kazakhstan|Kazakhstani]] mobile network operator, [[Kcell]], and satellite owner and operator, [[SES (company)|SES]] used SES's [[O3b]] MEO satellite constellation to demonstrate that MEO satellites could be used to provide high-speed mobile internet to remote regions of Kazakhstan for reliable video calling, conferencing and streaming, and web browsing, with a latency five times lower than on the existing platform based on [[geostationary orbit]] satellites.<ref>[https://www.commsupdate.com/articles/2022/05/26/kcell-ses-demo-o3b-satellite-enabled-remote-mobile-services/ ''Kcell, SES demo O3b satellite-enabled remote mobile services''] Comms Update. 26 May 2022. Accessed 30 May 2022</ref><ref>{{cite press release|publisher=SES|date=25 May 2022 |url=https://www.ses.com/press-release/kcell-and-ses-successfully-demonstrate-cellular-network-connectivity-kazakhstan|title=Kcell and SES Successfully Demonstrate Cellular Network connectivity in Kazakhstan|access-date=30 May 2022}}</ref>

====Ultralight atmospheric aircraft as satellites====
A proposed alternative to relay satellites is a special-purpose [[high altitude platform station]]s 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.

For example, the United States [[Defense Advanced Research Projects Agency]] [[DARPA Vulture|Vulture]] project envisaged an ultralight aircraft capable of station-keeping over a fixed area for a period of up to five years, and able to provide both continuous surveillance to ground assets as well as to service extremely low-latency communications networks.<ref>{{cite press release |url=http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=1800 |title=DARPA's Vulture Program Enters Phase II |date=September 15, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20121017002043/http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=1800 |archive-date=2012-10-17 |access-date=2021-08-06}}</ref> This project was cancelled in 2012 before it became operational.{{citation needed|date=June 2019}}

Onboard batteries would charge during daylight hours through solar panels covering the wings and would provide power to the plane during night. Ground-based satellite internet dishes would relay signals to and from the aircraft, resulting in a greatly reduced round-trip signal latency of only 0.25 milliseconds. The planes could potentially run for long periods without refueling. Several such schemes involving various types of aircraft have been proposed in the past.