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Sun outage
Sun outage
Sun outage
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A Sun outage, Sun transit, or Sun fade is an interruption in or distortion of geostationary satellite signals caused by interference (background noise) of the Sun when it falls directly behind a satellite which an Earth station is trying to receive data from or transmit data to. It usually occurs briefly to such satellites twice per year and such Earth stations install temporary or permanent guards to their receiving systems to prevent equipment damage.

Sun outages occur before the March equinox (in February and March) and after the September equinox (in September and October) for the Northern Hemisphere, and occur after the March equinox and before the September equinox for the Southern Hemisphere. At these times, the apparent path of the Sun across the sky takes it directly behind the line of sight between an Earth station and a satellite. The Sun radiates strongly across the entire spectrum, including the microwave frequencies used to communicate with satellites (C band, Ku band, and Ka band), so the Sun swamps the signal from the satellite. The effects of a Sun outage range from partial degradation (increase in the error rate) to the total destruction of the signal. The effect sweeps from north to south from approximately 20 February to 20 April, and from south to north from approximately 20 August to 20 October, affecting any specific location for less than 12 minutes a day for a few consecutive days.

Effect on Indian stock exchanges

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In India, the BSE (Bombay Stock Exchange) and NSE (National Stock Exchange) use VSATs (Very Small Aperture Terminals) for members (e.g. stockbrokers) to connect to their trading systems. VSATs depend upon satellites for connectivity between the terminals/systems. Hence, these exchanges are, with considerable predictability, affected by sun outages.

Until 2009, both BSE[1] and NSE[2] interrupted trading sessions due to sun outages.[3] As of 2009, this practice ended,[4][5] with the stock exchanges urging members to "review their current connectivity set up" and making "alternative arrangements for continuing trading without any disruption".

Other locations

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Saint Helena suffers from island-wide loss of Internet and telecommunications connections during Sun outages because all telecommunications traffic to and from the island is carried on a single satellite link. Sun outage times are published in local newspapers.

As the majority of rural Alaska is served by satellite, population centers like Utqiaġvik, Kotzebue, and Nome suffer from this as well. Nome is the terminus of the annual Iditarod Trail Sled Dog Race, and due to its timing, announcements of the finishers are often delayed during these Sun outages.

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sun outage

View on Grokipedia
from Grokipedia
A sun outage, also known as sun interference or sun transit, is a brief interruption or degradation of geostationary signals received at stations, caused by the Sun's intense radiation overwhelming the much weaker satellite transmissions when the Sun, satellite, and receiving antenna are aligned along the same . This phenomenon increases the noise temperature at the receiver, reducing the carrier-to-noise ratio and potentially leading to total signal loss or visual/audio distortions in services like television broadcasts, radio, , and data communications. Sun outages occur predictably twice each year, centered around the vernal (spring) and autumnal equinoxes in March and September, respectively, when the Sun crosses the equatorial plane occupied by geostationary satellites; these periods typically span 3 to 21 days, with the exact timing varying by location relative to the satellite's orbital position. Each daily event lasts only a few minutes—often 1 to 15 minutes at peak—depending on factors such as the Earth station's antenna size, gain-to-noise-temperature ratio, operating frequency band (e.g., C-band or Ku-band), and the Sun's radio flux density, which can be exacerbated during periods of high solar activity like sunspot maxima every 11 years. The severity is greater for smaller antennas and higher-frequency bands, where the Sun's apparent size relative to the satellite beam is larger. Impacts are most noticeable in fixed satellite services (FSS), including direct-to-home broadcasting and , but geostationary Earth orbit (GEO) systems are designed with margins to tolerate these natural events, treating them as accepted phenomena rather than faults. Mitigation strategies include scheduling non-critical operations around predicted outages using tools like sun interference calculators, employing larger antennas for better signal margins, or switching to backup diversity sites with offset alignments. While outages are geographically specific—primarily affecting stations in spring mornings and afternoons in fall—the global reliance on GEO satellites makes advance prediction essential for maintaining service reliability.

Fundamentals

Definition

A sun outage, also known as sun transit or sun fade, is a temporary disruption in satellite-based communications where the sun's intense radio emissions interfere with the receiver's ability to detect signals from a geostationary . This occurs when the sun, the , and the ground station antenna become aligned in the receiver's , causing the solar noise to overwhelm the much weaker satellite signal and degrade the carrier-to-noise ratio. Unlike other satellite interferences such as , which involves signal due to atmospheric , or solar flares, which produce unpredictable bursts of high-energy emissions, sun outages are predictable and stem from the steady of the quiet sun during specific geometric alignments. affects both uplink and downlink through absorption and scattering, whereas sun outages primarily impact the downlink by elevating the system's without significant . Solar flares, by contrast, can cause widespread ionospheric disturbances lasting minutes to hours, but sun outages are localized to the alignment period and recur predictably near the spring and autumn equinoxes. The phenomenon was first documented in the 1970s during the early operations of geostationary satellite systems, as operators of initial commercial networks like began experiencing these alignment-based disruptions in radio communications. These early observations highlighted the need for mitigation strategies in satellite design and operations.

Physical Causes

Sun outages in satellite communications arise primarily from the Sun's role as a powerful source of radio emissions, which generate significant noise across microwave frequencies utilized by geostationary satellites, including the C-band (approximately 4 GHz) and Ku-band (11-14 GHz). The Sun's radio output consists of thermal emissions from its quiet , slowly varying components linked to activity, and occasional bursts from solar flares, all contributing to a high that can reach tens of thousands of in these bands—for instance, around 21,000 K at 4 GHz for the quiet Sun (based on the model with γ = 0.5), decreasing with higher frequencies such as to approximately 10,000 K at 12 GHz. This noise overwhelms the weaker satellite signals when the Sun aligns with the receiver's , elevating the system's and degrading the carrier-to-noise ratio. The interference mechanism is predominantly a direct line-of-sight overload, where solar radiation enters the receiving antenna's main beam alongside the desired signal, as the Earth's atmosphere and impose minimal or on these frequencies under clear conditions. Gaseous absorption by oxygen and is low (typically less than 1 dB for zenith paths in C- and Ku-bands), and ionospheric effects like Faraday or scintillation are negligible for frequencies above 1 GHz, allowing the full intensity of solar noise to reach ground stations without substantial modification relative to the downlink. This results in the antenna being unable to distinguish from the communication signal, leading to temporary signal degradation. Geostationary satellites in the equatorial plane at an altitude of approximately 35,786 km, maintaining a fixed position relative to Earth's surface due to their synchronous period matching Earth's 24-hour sidereal day. This orbital configuration enables predictable solar alignments twice annually, occurring around the vernal and autumnal equinoxes when the Sun's aligns with the equatorial plane, positioning the Sun directly behind the as viewed from Earth-based receivers for several minutes each day over a period of about two weeks.

Occurrence and Prediction

Geometric Conditions

A sun outage in satellite communications arises from specific geometric alignments where the Sun, a geostationary , and a are positioned such that the Sun falls within the antenna's beam directed toward the satellite. This alignment requires the angular separation between the satellite and the Sun, as viewed from the , to be minimal—typically within 0.5 to 2 degrees, encompassing the antenna's half-power beamwidth (often 0.1° to 1° depending on antenna size and ) plus the Sun's apparent angular radius of approximately 0.25°. For precise outages, the separation must be small enough for solar emissions to enter the , with interference scaling based on the exact overlap. These geometric conditions manifest primarily around the vernal (March) and autumnal (September) equinoxes, when the Sun's aligns closely with the equatorial plane of geostationary satellites, enabling twice-yearly transits. In the , outages typically occur from late to early and mid- to early , while in the , the periods shift to early and late to early , respectively. Each event lasts 1 to 10 minutes daily, with the Sun crossing the beam once per day, and the overall seasonal period spans up to 3 to 9 days, depending on the rate of change (about 0.4° per day near equinoxes). The total alignment window is limited to roughly 21 days centered on each , beyond which the offset exceeds the beam tolerance. The severity and duration of these outages are modulated by the satellite's angle as seen from the and the station's . Lower angles (e.g., below 20°) prolong outage durations because the Sun's apparent has a reduced perpendicular component relative to the beam, extending the time the Sun remains within the alignment zone—potentially doubling durations compared to high- (near 90°) scenarios. influences both the timing and extent: higher-latitude stations experience earlier or later onsets relative to the (e.g., starting up to 10-15 days before at 50° versus on the at equatorial sites) and slightly longer seasonal windows due to greater geometric offsets, though equatorial stations face peak alignments directly at . These factors underscore the positional prerequisites, where precise collinearity in , , , and dictates the outage's occurrence and impact.

Calculation and Forecasting

Predicting sun outages requires calculating the precise alignment between the sun, a geostationary , and an station antenna, typically using solar data to determine the sun's position in equatorial coordinates. The angular separation θ between the sun and the , as viewed from the station, is computed using the θ = arccos(cos δ_sun cos(α_sun - α_sat)), where δ_sun is the sun's , α_sun is the sun's , and α_sat is the 's (with δ_sat ≈ 0° for geostationary orbits). Outage start and end times occur when θ falls below the antenna's half-power beamwidth, typically ranging from 0.5° to 2° depending on antenna size and frequency band. Solar ephemeris data, such as that provided by NASA's JPL Horizons , supplies accurate values for α_sun and δ_sun, enabling predictions for specific dates and locations. Commercial tools integrate these data for user-friendly forecasting; for instance, Intelsat's Sun Interference requires inputs like earth station coordinates, , antenna diameter, and band to output outage times and durations. Similarly, SES provides a Sun Outage that follows comparable steps, incorporating ephemeris-based computations to estimate interference periods for their fleet. These tools often reference ITU-R models for beamwidth and alignment thresholds, allowing operators to plan around predicted events twice annually. Prediction accuracy depends on factors like antenna gain patterns and variations in solar . Antenna gain over the sun's disk (approximately 0.53° angular diameter) must be modeled precisely, as can extend outage durations beyond simple beamwidth estimates; S.1525 outlines integration methods for gain G(θ, φ) across the solar disk to refine increases. Seasonal variations in solar , peaking during the 11-year , affect interference intensity, with higher (e.g., up to 300 SFU at 10.7 cm) prolonging severe outages; models adjust for this using real-time data from sources like NOAA's Space Weather Prediction Center.

Effects

Signal Interference Mechanisms

During a sun outage, the primary mechanism of signal interference arises from the sun's intense radio , which acts as an additional noise source superimposed on the desired signal. This solar radiation significantly increases the system noise temperature TsysT_{sys} of the receiving earth station, thereby degrading the carrier-to-noise ratio (C/N). The degradation can be quantified by the equation CN=CkB(Tsys+Tsun)\frac{C}{N} = \frac{C}{k B (T_{sys} + T_{sun})} where CC is the received carrier power, kk is Boltzmann's constant, BB is the receiver bandwidth, TsysT_{sys} is the baseline system (typically 150–300 for satellite links), and TsunT_{sun} represents the solar contribution, which can reach up to 10,000 during peak events in higher frequency bands. As the sun aligns closely with the in the receiver's beam, this added overwhelms the signal, leading to increased bit error rates and potential loss of lock in demodulators. The severity of the interference exhibits dependence, with more pronounced effects in higher bands such as Ku (11–14 GHz) compared to lower bands like C (4–8 GHz), primarily due to the relative strength of solar emission within the operational spectrum of these systems and the tighter link budgets typical of Ku-band applications. Polarization effects remain minimal, as the sun's radio emissions are largely unpolarized and thus impact both linear and circular polarizations equally without significant or isolation advantages. Signal blackout occurs when the combined noise exceeds the receiver's threshold margin, typically resulting in complete loss of the carrier for the duration of the alignment. Recovery is gradual, as the sun's position relative to the and earth station shifts due to and orbital geometry, allowing the noise contribution to diminish progressively over minutes, restoring the C/N to nominal levels as the solar flux falls outside the antenna's main beam. These solar emissions, stemming from processes in the sun's atmosphere, briefly elevate the overall during the twice-yearly periods.

Impacts on Communication Services

Sun outages significantly disrupt broadcasting by overwhelming the weak downlink signals with solar radio noise, leading to , frozen images, and audio distortions that can last from a few seconds to several minutes per event. These interruptions occur primarily during the periods when the sun aligns closely with geostationary satellites, affecting direct-to-home and cable TV services reliant on C- and Ku-band frequencies. For instance, viewers may experience macro-blocking or complete signal loss as the interference peaks, rendering programming unwatchable for brief durations. Sun outages primarily affect geostationary and data links, which suffer from latency spikes and during sun outages, as the increased noise-to-signal ratio degrades throughput and connection stability. In VSAT-based services, these effects can manifest as intermittent connectivity issues, with rates rising proportionally to the duration of solar alignment, potentially disrupting remote data transmissions. Such degradations stem from the sun's radio emissions overpowering the satellite's carrier signal in the affected frequency bands. Satellite telephony services face call drops and voice quality degradation during peak sun outage periods, particularly in systems using narrowband links vulnerable to signal fading. These disruptions arise when solar interference elevates the noise floor, causing bit errors that interrupt ongoing connections in geostationary-based mobile satellite networks. The economic implications of sun outages include downtime costs for , where even seconds of signal loss can affect millions of viewers, leading to lost and viewer dissatisfaction. In data-intensive sectors, prolonged may result in operational delays, though overall financial impacts are mitigated by the predictable and short-lived nature of these events. VSAT networks in remote or underserved areas exhibit heightened vulnerability to sun outages due to their reliance on small, high-gain antennas pointed at single geostationary satellites, which offer limited link margins against solar noise. Conversely, GPS services experience minimal interference, as their L-band frequencies (around 1.5 GHz) encounter lower solar radio brightness compared to the higher C- and Ku-bands used in communication satellites, reducing the risk of significant signal degradation.

Notable Incidents

Financial Sector Disruptions

Sun outages have notably disrupted operations in the financial sector, particularly in stock trading systems reliant on satellite communications for high-speed data transmission. In , the (BSE) and National Stock Exchange (NSE) historically depended on (VSAT) networks, which utilize geostationary s such as INSAT series operated by the Indian Space Research Organisation and international providers like , to facilitate real-time feeds, order routing, and connectivity between trading terminals across the country. These satellite links are essential for disseminating live quotes, executing trades, and maintaining synchronization during peak hours, but sun outages cause temporary signal interference, leading to delayed or lost data packets that can result in erroneous quotes, failed order executions, or complete trading halts. During the and early , sun outages frequently interrupted trading on BSE and NSE, especially in and periods around the equinoxes when alignment conditions peak. For instance, from September 25 to October 9, 2007, NSE suspended trading daily between 11:25 a.m. and 12:05 p.m. due to VSAT disruptions, affecting approximately 40 minutes of activity per session over two weeks. Similarly, in 2008, both exchanges revised timings, halting operations from 11:45 a.m. to 12:25 p.m. to mitigate signal loss, with trading resuming at 12:30 p.m. and extended closing hours to 4:15 p.m. for compensation. These interruptions, occurring during peak trading volumes, often coincided with broader market volatility; for example, during the March 8-16, 2000, sun outage, the Nifty index declined by 104.05 points (from 1,666.25 to 1,562.20), and the Sensex fell 409.01 points (from 5,511.42 to 5,102.41), amplifying trader losses amid halted connectivity. By the late , NSE began transitioning away from full suspensions, notifying members of potential connectivity issues instead, as seen in alerts for the September 24 to October 8, 2013, period. Since 2011, NSE and BSE have ceased suspending trading during sun outages, instead issuing alerts to members about potential connectivity issues. In response to these recurrent disruptions, the Securities and Exchange Board of (SEBI) has mandated enhanced resilience measures for stock exchanges, emphasizing backup systems and business continuity protocols. Post-incident reviews in the prompted SEBI to require exchanges to implement redundant communication infrastructures, such as leased lines alongside links, to minimize . More recently, SEBI's 2023 and 2024 circulars outline standard operating procedures (SOPs) for handling outages, including notifications to market participants within 15 minutes, activation of disaster recovery sites, and provisions for alternative trading venues—such as designating BSE and NSE as backups for each other effective April 1, 2025—to ensure seamless operations and limit financial impacts during events like sun outages. These guidelines have reduced the severity of sun-related halts, though brief connectivity glitches persist in satellite-dependent setups.

Other Regional Examples

In the 1980s, sun outages frequently disrupted television signals in , particularly affecting early cable systems that distributed programming via geostationary satellites. These predictable events, occurring twice yearly around the equinoxes, caused signal fading or complete blackouts lasting up to 15 minutes daily for about two weeks, impacting millions of viewers and highlighting the challenges of nascent satellite TV infrastructure. In , sun outages have notably affected broadcasting during equinox periods, leading to temporary loss of video and audio quality across satellite TV platforms like and , with disruptions peaking for several minutes each day over a week. In , outside of , sun outages have caused media blackouts in urban centers and operational failures in remote areas; for instance, a 2012 event in led to widespread disruptions in cable TV channels, while in , similar incidents have repeatedly affected VSAT networks used in operations, interrupting links critical for remote site management. These cases underscore the regional variability in impact, with Singapore's blackout affecting major providers like and for brief periods during peak viewing times, and Australian mining VSAT systems experiencing signal loss that can halt safety monitoring and equipment control.

Mitigation Strategies

Technical Measures

Technical measures to mitigate sun outages in satellite communications emphasize hardware and link solutions that enhance resilience to solar interference, which can increase the system by several thousand during alignment. Antenna design is fundamental to reducing sun entry into the receive beam. Antennas with higher sidelobe suppression limit extraneous solar radiation capture when the sun approaches the main lobe edges, thereby constraining overall elevation. Tracking antennas further aid by enabling precise beam adjustments to offset the sun's position relative to the , minimizing direct intrusion into the main beam. Larger parabolic antennas, with their narrower beamwidths (e.g., 0.17° at 3 dB for an 11 m dish at 11 GHz), also shorten outage durations to as little as 2.5 minutes at peak, compared to longer periods for smaller dishes. Frequency band selection and modulation schemes provide additional robustness. Operating in lower bands such as C-band (4-8 GHz) over Ku-band (12-18 GHz) reduces outage severity, as solar radio noise increases with frequency, placing K-band and higher systems at greater risk than C-band equivalents. Adaptive coding and modulation (ACM) can dynamically adjust coding rates and modulation orders to boost link margins during noise spikes, helping maintain service continuity in fixed satellite services. Backup systems offer for uninterrupted operations. Satellite diversity employs dual-beam antennas to switch to alternate geostationary offset from the primary, avoiding simultaneous sun transits and preventing total link loss. For mission-critical applications, terrestrial optic links serve as unaffected alternatives, integrating into hybrid networks to bypass satellite vulnerabilities entirely during outages.

Operational Protocols

Operators schedule non-critical communications around predicted sun outage windows to minimize disruptions, using tools like online calculators to forecast exact times based on earth station location and position. For instance, broadcasters often pause live feeds and switch to pre-recorded content or queued programming during these periods, ensuring continuity for viewers while avoiding signal loss. This proactive planning is essential twice annually during seasons when alignments are most frequent. Real-time monitoring employs spectrum analyzers to detect rising noise levels indicative of impending sun interference, allowing operators to implement immediate workarounds such as signal adjustments. International coordination through the (ITU) facilitates shared awareness in satellite bands, where operators exchange predictions to avoid conflicts in frequency allocations affected by solar emissions. These alerts help in early detection across global networks. Following a sun outage, operators log detailed records of the event, including duration, affected services, and signal metrics, for internal analysis to refine future predictions. , satellite providers of qualifying services such as must report significant outages via the FCC's Network Outage Reporting System (NORS) within 72 hours if they impact at least 900,000 user minutes, aiding regulatory oversight and service improvement. These protocols ensure that even brief disruptions, which can interrupt communication services, contribute to long-term operational resilience.

References

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  2. https://my.intelsat.com/resource/si/Sun_Interference_Background.pdf
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  26. https://www.deccanherald.com/business/nse-not-suspend-trading-during-2396234
  27. https://www.sebi.gov.in/legal/circulars/jan-2023/standard-operating-procedure-for-handling-of-stock-exchange-outage-and-extension-of-trading-hours-thereof_67125.html
  28. https://www.sebi.gov.in/legal/circulars/may-2024/standard-operating-procedure-for-handling-of-stock-exchange-outage-and-extension-of-trading-hours-thereof-in-commodity-derivatives-segment_83588.html
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  39. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-4/subject-group-ECFRb4cb67a8301113e
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