GPS World

Tag: Richard Langley

  • Study of Atmospheric ‘Froth’ May Help GPS Communications

    Editor’s note: GPS World Innovation editor Richard Langley has co-authored a study, described below, exploring how irregularities in Earth’s upper atmosphere can distort GPS signals, an important step toward mitigation.

    Source: GPS world staff
    The Aurora Borealis viewed by the crew of Expedition 30 on board the International Space Station. The sequence of shots was taken on February 7, 2012 from 09:54:04 to 10:03:59 GMT, on a pass from the North Pacific Ocean, west of Canada, to southwestern Illinois. Image Credit: NASA/JSC

    News from the Jet Propulsion Laboratory

    When you don’t know how to get to an unfamiliar place, you probably rely on a smartphone or other device with a GPS module for guidance. You may not realize that, especially at high latitudes on our planet, signals traveling between GPS satellites and your device can get distorted in Earth’s upper atmosphere.

    Researchers at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif., in collaboration with the University of New Brunswick in Canada, are studying irregularities in the ionosphere, a part of the atmosphere centered about 217 miles (350 kilometers) above the ground that defines the boundary between Earth and space. The ionosphere is a shell of charged particles (electrons and ions), called plasma, that is produced by solar radiation and energetic particle impact.

    The new study, published in the journal Geophysical Research Letters, compares turbulence in the auroral region to that at higher latitudes, and gains insights that could have implications for the mitigation of disturbances in the ionosphere. Auroras are spectacular multicolored lights in the sky that mainly occur when energetic particles driven from the magnetosphere, the protective magnetic bubble that surrounds Earth, crash into the ionosphere below it. The auroral zones are narrow oval-shaped bands over high latitudes outside the polar caps, which are regions around Earth’s magnetic poles. This study focused on the atmosphere above the Northern Hemisphere.

    “We want to explore the near-Earth plasma and find out how big plasma irregularities need to be to interfere with navigation signals broadcast by GPS,” said Esayas Shume. Shume is a researcher at JPL and the California Institute of Technology in Pasadena, and lead author of the study.

    If you think of the ionosphere as a fluid, the irregularities comprise regions of lower density (bubbles) in the neighborhood of high-density ionization areas, creating the effect of clumps of more and less intense ionization. This “froth” can interfere with radio signals including those from GPS and aircraft, particularly at high latitudes.

    The size of the irregularities in the plasma gives researchers clues about their cause, which help predict when and where they will occur. More turbulence means a bigger disturbance to radio signals.

    “One of the key findings is that there are different kinds of irregularities in the auroral zone compared to the polar cap,” said Anthony Mannucci, supervisor of the ionospheric and atmospheric remote sensing group at JPL. “We found that the effects on radio signals will be different in these two locations.”

    The researchers found that abnormalities above the Arctic polar cap are of a smaller scale — about 0.62 to 5 miles (1 to 8 kilometers) — than in the auroral region, where they are 0.62 to 25 miles (1 to 40 kilometers) in diameter.

    Why the difference? As Shume explains, the polar cap is connected to solar wind particles and electric fields in interplanetary space. On the other hand, the region of auroras is connected to the energetic particles in Earth’s magnetosphere, in which magnetic field lines close around Earth. These are crucial details that explain the different dynamics of the two regions.

    Source: GPS world staff
    CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) is a made-in-Canada small satellite from the Canadian Space Agency. It is comprised of three working elements that use the first multi-purpose small satellite platform from the Canadian Small Satellite Bus Program. Image Credit: Canadian Space Agency

    To look at irregularities in the ionosphere, researchers used data from the Canadian Space Agency satellite Cascade Smallsat and Ionospheric Polar Explorer (CASSIOPE), which launched in September 2013. The satellite covers the entire region of high latitudes, making it a useful tool for exploring the ionosphere.

    The data come from one of the instruments on CASSIOPE that looks at GPS signals as they skim the ionosphere. The instrument was conceived by researchers at the University of New Brunswick.

    “It’s the first time this kind of imaging has been done from space,” said Attila Komjathy, JPL principal investigator and co-author of the study. “No one has observed these dimensional scales of the ionosphere before.”

    The research has numerous applications. For instance, aircraft flying over the North Pole rely on solid communications with the ground; if they lose these signals, they may be required to change their flight paths, Mannucci said. Radio telescopes may also experience distortion from the ionosphere; understanding the effects could lead to more accurate measurements for astronomy.

    “It causes a lot of economic impact when these irregularities flare up and get bigger,” he said.

    NASA’s Deep Space Network, which tracks and communicates with spacecraft, is affected by the ionosphere. Komjathy and colleagues also work on mitigating and correcting for these distortions for the DSN. They can use GPS to measure the delay in signals caused by the ionosphere and then relay that information to spacecraft navigators who are using the DSN’s tracking data.

    “By understanding the magnitude of the interference, spacecraft navigators can subtract the distortion from the ionosphere to get more accurate spacecraft locations,” Mannucci said.

    Other authors on the study were Richard B. Langley of the Geodetic Research Laboratory, University of New Brunswick, Fredericton, New Brunswick, Canada; and Olga Verkhoglyadova and Mark D. Butala of JPL. Funding for the research came from NASA’s Science Mission Directorate in Washington. JPL, a division of the California Institute of Technology in Pasadena, manages the Deep Space Network for NASA.

  • Summer — and the Living Ain’t Easy

    Summer 2014 will be one that the €6.3 billion (US$8.2 billion) Galileo GNSS programme will need to chalk down to experience and hope to move on from. At the time of my last EAGER column, we were starting to get a hint that one of the four in-orbit Galileo IOV satellites was not functioning. We now know that it has suffered a catastrophic power failure.

    And in August, celebrations for a successful launch of the first two Galileo FOC satellites, named Doresa and Milena, quickly went sour when it became clear that they had suffered an anomaly during launch. And a very big anomaly at that.

    We are still awaiting the preliminary results from the inquiry set up by the European Space Agency (ESA), the European Commission, and the rocket’s operator Arianespace. It is hoped that the results of the inquiry board will be available at the end of September.

    ESA spokesperson Franco Bonacina informs me that on the satellite side, ESA’s teams at the ESOC control centre are investigating the possibility of partially raising or modifying the orbit of the two satellites, which are fully under control and in good health. They are also considering performing some software adaptations on board the spacecraft and at ground station level to try and make them “be seen” as operational satellites and able to provide operational services within the Galileo system. All this is still “work in progress” and it will take a few more weeks to be fully evaluated and put into practice.

    So, what went wrong?

    In the absence of firm conclusions from the inquiry, the rumour mill has been running at full throttle with the wide range of theories from cock-up to conspiracy.

    What we do know is that the Galileo craft were supposed to be launched by a Soyuz-Fregat rocket into a circular orbit 23,222 kilometres above the Earth and angled at 56 degrees to the planet’s equator. The initial launch from Kourou, French Guiana, on August 22 seemed to be smooth and nominal, but something went wrong in the final stages of the flight, and the two satellites were placed in an elliptical orbit varying from more than 2,000 kilometres too high to nearly 10,000 kilometres too low and also tilted by about 5 degrees from the intended plane.

    The most plausible explanation for the anomaly is that the Soyuz – Fregat upper stage suffered a control malfunction at some point before its final orbital injection burn. So although the rocket engine seems to have fired correctly, the craft wasn’t pointing in the right direction.

    In fact, I am told that a likely mechanical root cause for the anomaly in the Fregat stage has been identified by the inquiry committee; however, more information is being gathered and further technical analysis is required to verify this.

    Can anything be done?

    Doresa and Milena do not have enough fuel on board to achieve the correct orbit for full Galileo operations.

    Some interesting solutions have been proposed to launch a rescue mission to drag the errant satellites into their correct orbit, but that could be a slow, risky and expensive exercise. So could the satellites be useful in situ?

    For most GNSS uses, the answer is probably no. Despite the fact that the satellites themselves are apparently working perfectly, in a safe state, correctly pointing towards the Sun, properly powered and fully under control of the ESA team, their elliptical orbit does not conform with Galileo’s standardized data format. For example, the value that represents the shape of the satellites’ orbit is too big to be expressed within the allotted bit limit for that parameter.

    Marco Falcone, ESA’s Galileo system manager, says his team have been working intensely to determine if the satellites can be at least partially recovered. Among the considerations are the flight dynamics of moving the two spacecraft and the impact of the radiation they are experiencing in their current location, which can shorten the satellite’s lifetime. “It’s very dangerous for the satellite,” admits Falcone.

    Another unknown is the timing performance of the satellites’ rubidium frequency and hydrogen maser given the relativistic effects of their orbit. Signal issues, such as the navigation message almanac, also must be considered before the FOC satellites can be introduced into operation, Falcone said.

    Good for geoscience?

    But according to fellow GPS World contributor Richard Langley of the University of New Brunswick, the situation is more hopeful for scientists wanting to use Galileo-derived data in their research. Quoted in an article in leading science journal Nature, Richard says that researchers tracking GNSS satellites via the IGS global network of ground stations combine that information with the timing data transmitted by the satellites themselves and could use it to measure changes in the position of points on the ground much smaller than the one-metre margin of error for standard navigation-system receivers. This level of precision is good enough to detect millimetres of movement in tectonic plates, for example.

    This reminded me of a presentation by a team of Italian scientists I saw recently. The VADASE (Variometric Approach for Displacements Analysis Stand-alone Engine) project uses a novel strategy consisting of an algorithm able to perform real-time retrieval and estimation of displacement and waveforms based on high-frequency (1 Hz or more) carrier phase observations collected by a stand-alone GNSS receiver. The algorithm works with broadcast data (satellite clocks and orbits), requires very simple hardware, and has demonstrated potential application in real-world situations such as earthquake risk assessment (and related early-warning systems for tsunamis) and structural monitoring (see citations below). VADASE had already achieved a Galileo-only displacement solution using the four IOV satellites in orbit.

    I contacted Gabriele Colosimo at Rome’s “Sapienza” University, and he confirmed that, although the satellites could not be used for direct gravimetric studies that require a very low orbit (below 1,000 km), the data from Doresa and Milena could be used to estimate displacements of GNSS receivers. He and the VADASE team think that a slight adjustment in their algorithm might be needed depending on the exact orbit parameters of the satellites, but the data would be useable without any significant change in orbit being required.

    Gabriele also thinks that the data could be used to usefully contribute to studies in fields such as troposphere studies and GNSS reflectometry, as well as for geodynamic and seismic monitoring using GNSS.

    Munich Masters

    Hopefully the mystery of Doresa and Milena’s anomaly will have been resolved by October 23, when the 2014 European Satellite Navigation Competition (ESNC) awards ceremony takes place in Berlin in conjunction with the two-day Satellite Masters Conference.

    Gabriele and the VADASE team are in the running for a prize — so we wish them well.

    But the real prize for the European GNSS community would be a full explanation of the recent Galileo issues, and how they are being resolved, and a clear statement and timeline on the future deployment and implementation of this flagship programme.

    A bientôt – as they say in these parts

    Citations

    G.Colosimo, M. Crespi and A.Mazzoni, “Real-time GPS Seismology with a stand-alone receiver: A preliminary feasibility demonstration,” Journal of Geophysical Research, vol 116, doi: 10.1029/2010JB007941

    M. Branzanti, G.Colosimo, M.Crespi and A.Mazzoni, “GPS near-real-time coseismic displacements for the great Tohoku-oki earthquake,” IEEE Geoscience and Remote Sensing Letters, vol 99, doi: 10.1109/LGRS.2012.2207704

  • Canadian Science Minister Announces Grant to Langley’s UNB Lab

    Canadian Science Minister Announces Grant to Langley’s UNB Lab

    Professor Langley (center) discusses the UNB geodesy program with Canadian Science Minister Ed Holder (second from left.)
    Professor Langley (fourth from left) discusses the UNB geodesy program with Canadian Science Minister Ed Holder (third from left.)

    The Canadian Minister of State for science and technology, Ed Holder, visited the University of New Brunswick on July 28 to announce the awarding by the Natural Sciences and Engineering Council of $2.4 million to 28 UNB researchers.

    He was joined by Keith Ashfield, member of Parliament for Fredericton, where UNB is based, and Craig Leonard, the New Brunswick Minister of Energy and Mines.

    A highlight of the visit was a tour of the Department of Geodesy and Geomatics Engineering to see the work of Prof. Richard Langley and his students. Langley received $170,000 in Natural Sciences and Engineering Research Council (NSERC) funding in the competition. The funding will support the work of his group in improving augmented multi-constellation satellite-based precise positioning in a wide range of environments. Langley is GPS World’s Innovation editor, a post he has held since the magazine’s inception.

    Canadian Science Minister Ed Holder looks at GPS World magazine, which has featured Innovation columns edited by Richard Langley for more than two decades.
    Canadian Science Minister Ed Holder looks at GPS World magazine, which has featured Innovation columns edited by Richard Langley for more than two decades.

    Although GPS was the first widely available satellite navigation system, it has now been joined by the Russian GLONASS system, and will soon be accompanied by the European Galileo system, the Chinese BeiDou system, and the Japanese QZSS — all of which have test satellites now in orbit. There are interesting problems to be solved in gaining maximum benefits from this plethora of GNSS for precise positioning and navigation, and Langley and his team will address a number of them.

    The team is also involved in the analysis of data from the GPS-based instrument on the Canadian CASSIOPE scientific satellite launched at the end of September 2013. The instrument, which precisely determines the position of the satellite and provides information on the state of the Earth’s ionosphere, was designed at UNB.

    The NSERC Discovery Grants Program is an integral component of the government’s efforts to develop, attract and retain the world’s most talented researchers at Canadian universities. The program funds discovery research in a multitude of scientific and engineering disciplines, which builds a broad base of research capacity across the country.

    Professor Langley gave the following presentation at the NSERC Discovery Grants Scholarships Rollout Announcement at UNB on July 28:

  • The Halloween Storms: When Solar Events Spooked the Skies

    The Halloween Storms: When Solar Events Spooked the Skies

    Photo: Hathaway/NASA/MSFC
    Photo: Hathaway/NASA/MSFC

    Ten years ago, scientists watching the skies experienced a Halloween fright of cosmic proportions, when space weather degraded GPS signals, affecting land and ocean surveys, and commercial and military aircraft navigation.

    The most extreme of what became known as the Halloween Storms hit on October 30, 2003 — ten years ago today. According to the National Oceanic and Atmospheric Agency, the Earth could experience a repeat performance this Halloween, with a 35 percent chance of a major storm at high latitudes.

    The U.S. Geological Survey describes the cause of the 2003 storms:

    In mid-October 2003, a bundle of concentrated magnetic energy emerged from the Sun’s interior, forming a large sunspot, a site of seething activity. Enormous solar flares soon followed.

    Then, on October 28, the sunspot abruptly ejected a concentrated mass of electrically conducting solar wind, flinging it out into interplanetary space toward the Earth. Less than a day later, on October 29, a geomagnetic storm was initiated as the solar wind disrupted the Earth’s protective magnetosphere.

    Over the next three days, the “Halloween magnetic storm” would evolve and grow to become one of the largest such storms in half a century. Magnetic storms are global phenomena, and their effects can be easily seen around the world. During the Halloween storm, for example, magnetic direction in Alaska quickly changed by more than 20 degrees. In other words, the storm was so large that it could be measured with a simple compass. The Halloween magnetic storm also produced spectacular aurora, with green phantom “northern lights” seen as far south as Texas and Florida.

    “The aurora was exciting,” said Richard Langley, GPS World’s Innovation editor. “I’ve never seen a better one since.”

    This full-sky aurora was observed near Fredericton, New Brunswick, Canada (46 degrees north latitude) on October 31, 2003. (Photo courtesy of Richard Langley.)
    This full-sky aurora was observed near Fredericton, New Brunswick, Canada (46 degrees north latitude) on October 30, 2003. (Photo courtesy of Richard Langley.)

    Langley explained the effect of the phenomenon in his introduction to the October 2004 Innovation article, “Combating the Perfect Storm: Improving Marine Differential GPS Accuracy with a Wide-Area Network.”

    It was previously thought that the mid-latitude North American ionosphere was reasonably benign, with minimal storm effects of relevance for marine DGPS users. However, during ionospheric storms in May and October, 2003, [single-frequency] marine DGPS horizontal position accuracies were degraded by factors of 10–30.These degraded accuracies persisted for hours and were well beyond system tolerances specified for marine DGPS users. Such ionospheric activity is not unusual during the years following solar maximum, and is expected to persist for several years.

    Langley provides background on what scientists learned from the Halloween Storms in his February 2011 Innovation column, “GNSS and the Ionosphere: What’s in Store for the Next Solar Maximum?”:

    The current solar cycle is referred to as cycle 24. During the last solar cycle, cycle 23, the GNSS community was alert and aware of what could happen, and therefore many events were observed and analyzed. Among the most well-known events is a sequence of storms during October and November 2003, commonly referred to as the Halloween Storms.

    The most extreme was the storm on October 30, 2003, which resulted from a CME on October 29 at 20:49 UTC, which subsequently impacted Earth’s magnetic field at 16:20 UTC on October 30 and produced a great geomagnetic storm, which lasted for many hours.

    Effects on GPS positioning of this storm have been documented by the GNSS research group of the Royal Observatory of Belgium, where kinematic analyses of data from 36 GNSS stations in Europe showed position errors of more than 10 centimeters in the horizontal and up to 26 centimeters in the vertical between 21:00 and 22:00 UTC on October 30. The position errors were largest for locations in northern Europe including Sweden and Norway. The data analysis was carried out using high-quality carrier-phase data, and the processing was based on using an ionosphere-free linear combination of observations from the L1 and L2 frequencies, whereby the first-order effect of the ionosphere is removed from the results. The position errors are thus caused by mainly higher order ionospheric effects.

    For navigation-grade GPS positioning, a U.S. National Atmospheric and Oceanic Administration technical memorandum reported that the Wide Area Augmentation System (WAAS) vertical error limit of 50 meters was exceeded for a period of about 11 hours on October 30, 2003. This means that, in practice, WAAS was not available for precision aircraft approaches during that time. The European Geostationary Navigation Overlay Service (EGNOS) was not transmitting during the storm, but simulations carried out later by ESA showed that the boundary regions of the EGNOS coverage area would have been especially affected by a reduction in service availability of about 20–60 percent during that day.

    The simulations also showed, however, that in the center of the EGNOS coverage area (in the vicinity of northern Italy), the effect would have been much smaller with a reduction in service availability of only 5–6 percent over the day.

    Such large storms are also often accompanied by displays of aurora (aurora borealis and aurora australis) at lower latitudes than normal.

    15.trimmed
    Another shot of the Halloween 2003 aurora, as seen near Fredericton, New Brunswick. (Photo courtesy of Richard Langley)

    Other Innovation columns assessing the ionosphere’s effect on GPS include: