On any given day — especially cloudy winter days — most people may not give a lot of thought to the Sun.

More than 150 million kilometers (93.2 million miles) away, its remoteness belies the enormous forces at work in this yellow dwarf star with a mass 330,000 times that of Earth. If someone asked us to name its harmful effects, we might come up with sunburn, heat stroke, skin cancer.

But many in the GNSS community know better.

On any given day — especially cloudy winter days — most people may not give a lot of thought to the Sun.

More than 150 million kilometers (93.2 million miles) away, its remoteness belies the enormous forces at work in this yellow dwarf star with a mass 330,000 times that of Earth. If someone asked us to name its harmful effects, we might come up with sunburn, heat stroke, skin cancer.

But many in the GNSS community know better.

Every second, the Sun fuses 620 million metric tons of hydrogen into helium — a far-from-placid process that waxes and wanes in solar cycles roughly 11 years long.

As the sun enters a period of increased activity it produces sunspots, solar flares, and coronal mass ejections (CMEs) — bursts of charged particles and magnetic field that stream out from the sun at more than one million miles an hour. These phenomena travel to the Earth where they can create profound effects: radio bursts, geomagnetic storms, and colorful auroras.

Like its planetary counterpart, space weather can have widespread and damaging impacts on human activity. The so-called Carrington Event of 1859, for example, caused telegraph systems worldwide to go haywire. Operators reported sparks from their telegraphs and sections of wire were seen to catch on fire. Even when telegraphers disconnected the batteries, aurora-induced electric currents in the wires still allowed messages to be transmitted.

Although it’s unknown when the next Carrington-level storm may occur, the space weather community does agree that the effects of such an event would be crippling to our critical infrastructure today, including GNSS.

The current solar cycle — the 24th since 1755, when recording of sunspot activity began — is expected to reach its peak, the so-called solar max, in May 2013.

Three space weather phenomena account for most GNSS errors: total electron content (TEC)–induced delays, scintillation, and solar bursts.

As GPS signals propagate through the ionosphere, the speed and direction of the GPS signal are changed in proportion to the varying electron density or TEC along the line of sight between the receiver and the satellite. This in turn affects the GPS range observable:a delay is added to the code measurements and an advance to the phase measurements.

When a radio wave crosses through the ionosphere, it results in a distortion of phase and amplitude. These fluctuations are called scintillation and create rapid variations in signal power, reducing the received power and phase coherence of the GNSS signals, which can cause a loss of lock on the signal.

Solar radio bursts begin with a solar flare that injects high-energy electrons into the solar upper atmosphere.Radio waves are produced, which propagate to Earth and cover a broad frequency range, acting as RF noise that can degrade GNSS signals.

Since the last solar maximum in 2001, societal dependence on GNSS has increased substantially. This situation has brought increasing attention to the subject of space weather and its effects on GNSS systems and users.

To help us explore the subject further, we called on Dr. Genene Fisher, now senior policy advisor with NOAA’s National Weather Service. With a doctorate in atmospheric and space sciences from the University of Michigan, Dr. Fisher served as a policy Fellow for the American Meteorological Society’s Atmospheric Policy Program for more than 10 years.

In 2010 she helped organize an AMS workshop on the subject, editing a report on the discussions there that was published last March: Satellite Navigation & Space Weather: Understanding the Vulnerability & Building Resilience.

IGM: Some research suggests that the current solar cycle will be the least active of those that have occurred for more than 100 years. Does that mean GNSS users and manufacturers don’t need to worry about space weather in the near future?

FISHER: GNSS users and manufacturers need to be concerned about space weather. The next sunspot maximum, currently predicted for May 2013, is expected to be a relatively weak maximum in terms of sunspot number. However, many of the most intense solar storms have occurred during below-average solar cycles.

Moreover, the underlying processes driving the solar cycle are still not fully understood, and any projections of the size of the next solar cycle, even this one, carry a great deal of uncertainty.

The 1859 Carrington Event, which was the largest solar flare recorded, actually took place during a less than average solar cycle.

The current solar cycle will fully test for the first time much of the GPS-based technology installed since the last solar maximum. So, GNSS users cannot rely on the last solar cycle as an indication of what may happen during future years.

IGM: What can GNSS service providers do to mitigate the effects of space weather on users?

FISHER: Companies that offer services to a broad user base should be integrating space weather products into their operations. Currently, NOAA’s Space Weather Prediction Center (SWPC) offers several products to assist GNSS users, including nowcasts of space weather conditions over North America. SWPC products include near real-time total electron content (US-TEC), designed to estimate the signal delay for single- and dual-frequency GPS applications, updated every 15 minutes.

SWPC also provides the planetary K-index plot, showing the occurrence of any magnetic field disturbances during the previous 72 hours. SWPC’s space weather scales describe conditions on a scale from 1 (minor) to 5 (extreme). For a geomagnetic storm (G3), SWPC alerts users to such effects as intermittent satellite navigation problems, including loss-of-lock and increased range errors.

For a G4 storm, satellite navigation may be degraded or inoperable for hours, and under G5 conditions GNSS signals may be degraded or unavailable for days.

The full inventory of SWPC services and products can be found online at www.spaceweather.gov. By using these products, and others available from commercial space weather providers, users can take mitigating actions.

IGM: Which improvements in space weather prediction activities and products seem to offer the greatest cost/benefit value?

FISHER: The NOAA/SWPC recently transitioned the WSA-ENLIL model, the first large-scale, physics-based space weather prediction model that provides forecasts of the solar wind structures and CMEs. Magnetic storms can occur on Earth 1–4 days after a CME erupts from the Sun. Before development of this model, forecasters could predict timing of such impacts within a 30-hour window, on average.

The new model allows forecasters to narrow that window to 12 hours. Oil drilling, mining, surveying, aviation, and other operations that rely on GPS equipment – which can be made unreliable by space weather – can avoid conditions that might put operators at risk.

IGM: Traditionally, GNSS operators and receiver manufacturers incorporate ionospheric models into the design of their systems and product. How can these models be improved and incorporated into GNSS equipment specifications?

FISHER: GNSS equipment manufacturers are using ionospheric models to make their receivers “solar max proof.” But much of the limitations of receivers is based on the current knowledge of space weather. Many of the models are climatology-based with crude, broad measure assimilation schemes. It will require much better real-time assimilation to capture the true temporal and spatial nature of the ionospheric disturbances.

As the research community continues to make advances in space weather and heliophysics, improved ionospheric models and receivers should be forthcoming. 

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Articles about the would-be wireless broadband company that wants FCC approval to set up a terrestrial network transmitting on frequencies next door to GPS and other GNSS L1 signals garnered the most page views on our website for 6 out of 12 months.

If the readership of insidegnss.com news is any indication, 2011 was the year of LightSquared.

Articles about the would-be wireless broadband company that wants FCC approval to set up a terrestrial network transmitting on frequencies next door to GPS and other GNSS L1 signals garnered the most page views on our website for 6 out of 12 months.

Only such matters as Wikileaks, a Supreme Court case, the Compass/BeiDou ICD, or Galileo’s first IOV launch could break the spell of the continuing LightSquared vs. GPS saga. We reprise those popular news items below. 

JANUARY
OHB-System Disclaims Wikileaks Report of CEO Comments on Galileo
January 16, 2011 – OHB-System AG, builder of Galileo navigation satellites, has issued a statement from its chairman denying a WikiLeaks report that the German company’s CEO had told U.S. embassy officials that the European GNSS program was a "stupid idea" and "a waste of EU tax payers money.

FEBRUARY
GPS/GNSS Community Feels New Telecom Interference Threat from LightSquared
January 30, 2011 – In his State of the Union address on January 25, President Obama held up the Global Positioning System as a prime example of government providing "cutting-edge scientists and inventors with the support they need" to create world-leading, job-creating innovation.

MARCH
LightSquared GPS Working Group Sends FCC Its First Report on Interference Assessment
March 16, 2011 – Yesterday a working group mandated by the FCC waiver to address the potential LightSquared/GPS interference issue delivered its first monthly report required by the FCC.

APRIL
U.S. Department of Justice Seeks Supreme Court Review of GPS Surveillance Issue
April 16, 2011 – On April 15, 2011 the Obama administration asked the U.S. Supreme Court to take up the question of whether prolonged police surveillance using GNSS-aided tracking of a suspect requires a search warrant under the provisions of the Fourth Amendment to the U.S. Constitution.

MAY
Air Traffic Control Modernization: FAA, NextGen, GNSS, and Avionics Equipage
April 30, 2011 – In between partisan confrontations around the 2011 federal budget and raising the U.S. debt limit, prospects are improving for federal legislation that would provide the Federal Aviation Administration (FAA) with a regular full-year budget for the first time since Fiscal Year 2007 – including support for completing the transition to a GNSS-driven air traffic control (ATC) system known as NextGen and a "public-private partnership" to equip aircraft with the needed avionics.

JUNE
Test Results: LightSquared Would Cause Serious Interference to GPS Receivers
May 28, 2011 – Tests of GPS receiver vulnerability to transmissions from terrestrial base stations in the proposed LightSquared 4G/LTE broadband service were scheduled to wrap up on May 27, but results as we post this message clearly indicate substantial adverse effects on GPS user equipment.

JULY
LightSquared GPS Interference Report and Recommendation to FCC Fuels the Fight
July 1, 2011 – LightSquared submitted a 318-page report to the Federal Communications Commission (FCC) yesterday on the months-long effort of a technical working group (TWG) to investigate the effects of the would-be cellular broadband company’s terrestrial transmitters on GPS receivers operating in an adjacent RF band.

AUGUST
GPS Interference Test Results May Not Slow FCC Decision on LightSquared Deal
July 29, 2011 – Having apparently exhausted themselves debating a subject that a majority of Americans considered a non-priority – raising the debt ceiling – the U.S. Congress adjourned for summer recess on August 1, automatically cancelling two House hearings (discussed later in this article) that had been scheduled on the LightSquared/GPS interference issue.

SEPTEMBER
LightSquared Would Jam GPS, Defense Officials Tell Congress
September 15, 2011 – Amidst a storm of political controversy and the conspicuous absence of a key administration witness, Department of Defense (DoD) officials told members of Congress today that a proposed broadband wireless service would degrade or render useless billions of dollars of equipment essential to military operations.

OCTOBER
Galileo: At Long Last, Launch
October 14, 2011 – Launch of two Galileo in-orbit validation (IOV) satellites next week will bring Europe’s long-awaited (and much-delayed)GNSS program into a new phase. 

NOVEMBER
GPS Timing Used in Experiment to Measure ‘Faster Than Light’ Particles
November 17, 2011 – No sooner had GPS positioning helped validate key aspects of Einstein’s general theory of relativity in the Gravity Probe-B program than GPS timing has been cited as helping bring his special theory of relativity into doubt. 

DECEMBER
China launches 10th Compass Beidou-2 Navigation Satellite into Geosynchronous Orbit
December 2, 2011 – China launched its 10th Compass/ BeiDou-2 GNSS satellite this morning. This is the third Compass satellite launched this year and the fifth placed into an inclined geosynchronous orbit. 

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