The Institute of Navigation’s (ION) Satellite Division presented several annual awards Sept. 25 during the ION GNSS+ Virtual Conference.
Morton Honored with Kepler Award
Y. Jade Morton received the Johannes Kepler Award for advances in scientific and navigation receiver technology, automated data collection, robust carrier phase tracking, remote sensing, and profound impact as an educator and author.
Morton is the director of the Colorado Center for Astrodynamics Research at the University of Colorado, Boulder ,where she mentors students, faculty, staff and an ever-expanding international network of collaborators throughout the world. She is a prolific author with more than 270 publications. She was awarded her Ph.D. in Electrical Engineering at Pennsylvania State University. She has also authored articles for GPS World.
Receiver Technology Pioneer. Morton has made pioneering contributions to the advancement of GNSS receiver technology and utilization of these enhanced capabilities for scientific discovery. Her work brings together scientific rigor with state-of-the-art engineering innovations to simultaneously improve PNT, while revealing remarkable new applications for GNSS.
Morton’s lab-developed event-driven GNSS data acquisition systems (EDAS), designed to capture severe space weather and ionosphere disturbances of GNSS signals, which could not be handled by existing GNSS monitoring receivers. Her lab designed and built remotely-configurable, multi-GNSS, multi-band, SDR hardware using off-the-shelf components; and developed software including machine-learning algorithms for automatic event detection to trigger raw data recording during these events.
Network established. Her lab deployed these receivers worldwide. The network has enabled unprecedented studies and forecasting of ionosphere/space weather phenomena, detection of satellite oscillator anomalies, and development of advanced GNSS receivers for navigation and remote sensing under challenging conditions.
Morton’s group has made groundbreaking advances in GNSS carrier-phase processing and established theoretical performance bounds. Her group developed optimal carrier tracking loop architectures and implementations, and successfully applied the techniques to processing signals experiencing strong ionospheric scintillation for ionosphere and space weather research; radio-occultation signals traversing moist lower troposphere for weather and climate modeling; weak coherent reflected signals from ocean, land, and sea ice for precision altimetry applications; and navigation in urban canyons and on high dynamic platforms.
Morton is an expert on space weather and ionosphere monitoring. Her research findings range from climatology and morphology of ionospheric plasma irregularities to spatial, temporal and frequency domain characteristics; cause-effect relationships between solar-geomagnetic activities and GNSS signal disturbances; and radio wave propagation theory and simulation. The studies, based on data from her GNSS networks, magnetometers, radar and satellite-based measurements, cover the globe from the arctic to the equator and span an entire solar cycle.
Volunteer service. Morton has served numerous organizations with thousands of hours of volunteer service including organizing each of the ION’s large technical conferences and leading over 10 student teams participating in ION’s autonomous lawn mower and snowplow competitions, is credited as one of the co-organizing founders of the ION’s Pacific PNT conference, has served as the ION Satellite Division Chair and is the current ION President. Dr. Morton is a past recipient of the IEEE Kershner Award and the ION’s Burka and Thurlow Awards. She is a Fellow of the ION, RIN and the IEEE.
The Johannes Kepler Award recognizes and honors an individual for sustained and significant contributions to the development of satellite navigation. It is the highest honor bestowed by the ION’s Satellite Division.
Kimia Shamaei Honored with Parkinson Award
ION’s Satellite Division presented Kimia Shamaei with its Bradford W. Parkinson Award Sept. 25 for her thesis, “Exploiting Cellular Signals for Navigation: 4G to 5G.”
The Bradford W. Parkinson Award is awarded annually to an outstanding graduate student in GNSS. The award, which honors Dr. Parkinson for his leadership in establishing both the U.S. Global Positioning System and the Satellite Division of the ION, includes a personalized plaque and a $2,500 honorarium.
Any ION member who is a graduate student completing a degree program with an emphasis in GNSS technology, applications, or policy is eligible for the award. ION thanks the altruistic experts who served on this year’s selection committee.
A version of the NeQuick G algorithm using a new coding approach is now available for download on the GSC website. This version is the result of intensive recoding by engineers at the EU’s Joint Research Centre.
GNSS signals traveling through the ionosphere can be significantly delayed by the electrical charges in this atmospheric layer before reaching the users’ terminal. To compensate for this delay in the signal, Galileo receivers integrate a dynamic model of the ionosphere composition known as the NeQuick G model.
Receiver manufacturers will now be able to benefit from a version of the NeQuick G correction algorithm that implements a new coding approach.
Rigorous testing
The JRC concluded its work recently after successful rigorous testing in the framework of the gLAB tool (GNSS software suite from the Universitat Politecnica de Catalunya). This version of the code has been designed to be highly modular, rendering it more legible for a potential programmer with no specific knowledge about signal propagation in the ionosphere. A library has been also developed to enable its quick integration into existing applications.
This software will be released as free and open source software under the terms of the European Union Public Licence (EUPL), version 1.2.
The open-source code is now ready to be implemented on single-frequency platforms and can be used on a global scale without limitation under the EUPL. This freedom should contribute to a wider adoption of the NeQuick G model at user level.
Space Based Augmentation Systems (SBAS) – known in North America as the Federal Aviation Administration’s (FAA’s) Wide Area Augmentation System (WAAS) – have been fully operational in one form or another for several years. The FAA’s incremental improvements to integrity, accuracy and reliability in WAAS have brought the system to a point where we have precision en-route navigation for aircraft, and we can also land aircraft using WAAS signals at thousands of airports in the US and in Canada.
Why not Mexico, which also benefits from the same WAAS coverage? More on that later, as we piece together the many parts of the complex SBAS mosaic.
SBAS precision approach coverage, May 2016. Graphic: FAA Tech Center, Lockheed Martin, GMV
Europe benefits from high-accuracy en-route navigation, and there are also hundreds of operational approaches using the European Geostationary Navigation Overlay Service (EGNOS) SBAS.
In India, the GPS Aided Geo-Augmented Navigation (GAGAN) system provides accurate en-route navigation and approach capability. However, ionospheric disturbance may limit some aspects of performance.
Japan established the Multi-functional Satellite Augmentation System (MSAS) SBAS, and has benefited from improved en-route navigation, but it’s possible that the more limited geographic distribution of GPS ground reference stations has restricted improvements to approach capabilities.
But what happened to the International Civil Aviation Organization (ICAO) concept from 2007, supported by all the ‘aviation-going’ countries of the world, that SBAS would evolve and eventually multiple national systems would provide coverage around the rest of the world, maybe even by 2016?
Countries in Asia, South America, Africa and the continent of Australia all appear to have looked closely into establishing their own SBAS, but nothing seems to have come out of these investigations. Technical issues, cost, and political obstacles have all hindered global SBAS progress.
The ionospheric challenge. Graphic: GMV and Lockheed-Martin
Technical Issues. Ionospheric scintillation problems around the Equator seem to be at the root of most technical problems for SBAS. Getting to the required level of probable, bounded system error is hugely difficult. The iono disturbance ‘blob’ follows the sun around the Equator and wipes out any chance of satisfactory system performance when it passes over Equatorial countries.
As total electron count (TEC) increases, the ionospheric grid, which most SBAS use to predict ionospheric variation across their geographic area between fixed reference stations, well, it just doesn’t work anymore.
Cost. The capital cost of building a satellite-based augmentation system and the on-going cost of maintaining a bunch of geographically distributed reference sites, building and launching GEO satellites or renting transponders on someone else’s orbiting asset, establishing, operating and maintaining redundant uplink stations, redundant terrestrial data links, and setting up control systems that collect and create the SBAS uplink message — it all adds up. Millions and maybe even billions of dollars or equivalent, in total, have been spent by those select countries who could afford their own SBAS. Others named above have lesser financial resources upon which to draw.
Political Obstacles. One of the trickiest issues is sovereignty: the need for a country to control its own navigation and landing system. This has likely been the source of most resistance to more SBAS systems being set up and shared by bordering countries around the world.
For a large number of smaller countries, SBAS would only make sense if it was shared across a number of neighboring countries, but that means relinquishing sovereignty to some degree. In several regions of the world a number of geographically adjacent countries don’t particularly like each other, never mind thinking of such sharing/collaboration.
National sovereignty, by the way, isone of the main reasons that existing satellite navigation systems underpinning SBAS, such as Galileo, GLONASS, IRNSS (now NAVIC), QZSS and of course BeiDou have all been put in place.
Another problem with potential SBAS sharing across adjacent countries stems from responsibility for liability. Should something not work and an accident ensues from such a malfunction, who’s liable? Mexico seems to have adopted the view that since the US provides WAAS on what could be called an ‘as-is’ basis, then the potential liability issue seems to trump using the system.
Solutions? Technical issues with the ionosphere may soon be resolved by using dual-frequency L1/L5 airborne receivers that directly calculate their own ionospheric corrections, rather than using the computed SBAS iono grid. If we add in dual-frequency E1/E5a signals from Galileo, things start to get even better. New requirements and prototype equipment are already being developed for dual frequency multi-constellation airborne receivers. Airbus anticipates equipping aircraft with such receivers around 2025. Could this solve the SBAS technical issue for Equatorial countries?
ARINC (now a UTC/Rockwell Collins company) and SITA (in Europe) have been providing commercial aircraft with operational communications services on a pay-for-use basis for a number of years, and this is notarized as an accepted means of compliance within ICAO policy/requirements:
From ICAO Doc. 9161, Sec. 3.99: “A group of states or a regional organization might also undertake to operate the augmentation satellite service required, either by themselves or by contracting a commercial or government organization to do so on their behalf.”
ARINC en-route coverage. Graphic: ARINC
Aireon has partnered with NAV CANADA, the Irish Aviation Authority (IAA), Enav, NATS and Naviair, as well as Iridium Communications and Harris Corporation to provide real time ADS-B data (GPS position output from aircraft) to air-traffic control providers. Aireon’s payloads on the new Iridium NEXT Low-Earth Orbit (LEO) satellite constellation will receive aircraft ADS-B messages and relay them to Air Traffic Controllers in real-time.
There are 66 Iridium NEXT satellites in operation, with significant overlap and redundancy built into the system to enable this safety-of-life service to be provided on a pay-for-use basis to the aviation industry. We could at last know the location of every suitably equipped aircraft in the air, in almost real-time. The ICAO requirement is for an update rate of 15 minutes.
Inmarsat ADS-C is a similar service available to aircraft on a contracted, pay-for-use basis via Inmarsat GEO satellites.
Market Solutions. If a substantial company showed up with a worldwide distributed SBAS solution and offered it on a fee for service basis, why wouldn’t countries that are already accustomed to ARINC and SITA pay-for-use communications? The Aireon international aircraft tracking system, to be provided on the same basis, adds to the credibility of such a pay-for-use service.
So why wouldn’t these accepted services demonstrate to those countries concerned about control and national sovereignty that an SBAS service could be provided on this basis?
The liability for provision of service sits with the providers, so user countries/airlines would have someone to turn to about liability issues, and there presumably could be contract terms to provide system performance guarantees.
No huge capital costs, no system to construct, nor staff to operate or maintain, and yet a level of control similar to that which has been around for commercial aircraft communications for decades.
Would this be of interest to countries that have not yet jumped on the SBAS bandwagon? A definite ‘maybe,’ we could imagine? What’s not to like?
The punch line to all this is that Lockheed Martin and GMV (Spain) have teamed to challenge these non-SBAS countries with a solution which may appeal.
Uralla reference test site. Photo: Lockheed-Martin
To present convincing evidence that it would work, a dual frequency GPS (L1/L2) + Galileo (E1/E5a) reference site has been set up in collaboration with Geoscience Australia and Land Information New Zealand. The reference site is located at Uralla, New South Wales on Australia’s East Coast, where it gathers data demonstrating bounded errors within the operational range which could enable GNSS approach capability.
L1 (2006) vs. DFMC (2018) SBAS at Bangkok. Graphic: Lockheed-Martin, GMV
Another test site in Bangkok, Thailand has demonstrated that existing L1-only SBAS in this area cannot manage this performance (all current SBAS are L1 only), but that with dual-frequency multi-constellation (DFMC) GPS L1/L2+Galileo E1/E5a, the required performance limits could be met.
Lockheed Martin has also been using the Uralla uplink site to test the uplink and downlink of dual-frequency SBAS-like test messages.
The Moral of the Story. There are no miracles as yet, but interest in the pay-as-you-go SBAS concept appears to be growing, and the LM/GMV team continues to work to bring their approach to market.
A large number of countries could well benefit from the high accuracy, integrity and continuity of SBAS service if this all comes together.
The next generation of Europe’s satellite navigation overlay service, EGNOS, will combine use of GPS and Galileo signals to improve accuracy and robustness of navigation for air traffic and other uses where lives are at stake.
A contract was signed Jan. 26 at ESA’s technical centre in the Netherlands for the second generation of the European Geostationary Navigation Overlay Service, EGNOS V3, planned to enter service in 2025.
ESA Director of Navigation Paul Verhoef signs the EGNOS V3 contract Jan. 26 with Senior Vice President of Airbus Defence and Space, Mathilde Royer Germain. (Photo: ESA)
ESA Director of Navigation Paul Verhoef signed the contract with the senior vice president of Airbus Defence and Space, Mathilde Royer Germain, in the presence of senior managers of the European Global Navigation Satellite System Agency (GSA) and of the European Commission.
This improved version of the service will take advantage of in-operation Galileo signals as well as new frequencies from an improved class of GPS satellites. Use of the L5 second frequency will improve service robustness against errors and propagation delays caused by the ionosphere, an electrically active outer layer of Earth’s atmosphere.
“This will be the first such regional satellite augmentation systems worldwide to employ dual frequency, GPS and Galileo signals,” comments Didier Flament, overseeing EGNOS development for ESA.
For aircraft with the latest avionics, EGNOS V3 will be able to guide them accurately and safely down to Category 1, a 10 m Vertical Alert Limit (also called Cat1 Autoland capability), while also providing legacy users equipped with current avionics a more robust version of the current LPV200, or 35 m vertical limit vertical guidance service.
As well as improving services for civil aviation, the plan is to introduce new services for other sectors such as maritime navigation and rail, and extend service coverage from the European continent to link up seamlessly with other interoperable augmentation systems worldwide.
EGNOS is Europe’s other satellite navigation system, next to the global Galileo system. Comparable to the US WAAS, the Wide Area Augmentation System, and other regional augmentation systems in the rest of the world, EGNOS is an overlay system based on a network of ground stations and transponders on geostationary satellites. These stations gather data on the current accuracy of US GPS signals and embed correction data in the EGNOS signal, which is uplinked via geostationary satellites to EGNOS users.
The current EGNOS augments the accuracy of GPS signals across Europe and informs users of their current reliability level within six seconds. EGNOS belongs to a family of systems called Satellite Based Augmentation Systems (SBAS); the EGNOS V3 second generation will augment both GPS and Galileo.
Designed against global standards set by the International Civil Aviation Organisation, EGNOS began offering its Open Service for non-safety-of-life uses in October 2009. In March 2011 its Safety-of-Life Service became available for aircraft navigation.
Dozens of European airports are today employing EGNOS for vertical guidance approaches, as an economic alternative to ground-based infrastructure, like Instrument Landing Systems. It is estimated that that around 110 000 aircrafts worldwide are today equipped and using SBAS systems.
The development of satellite-based augmentation systems around the world is being coordinated in particular by the international SBAS Interoperability Working Group, which last week held its 33rd meeting at ESA’s centre in Madrid, chaired by ESA and the US Federal Avigation Authority, joined by current or planned service providers from Africa, Australia, Canada, China, India, Japan, Russia and South Korea.
Initiated by ESA in cooperation with the EU and Eurocontrol, the EGNOS Exploitation phase is managed by GSA and funded by the EU. ESA manages the EGNOS development under a working arrangement signed between GSA and ESA.
The eclipse across America on Aug. 21 was not only a magnificent visual event, it was also observed indirectly by the impact that it had on the propagation of radio signals — including those of global navigation satellite systems.
There was a decrease in the number of free electrons in the part of the Earth’s ionosphere along the eclipse path where sunlight was temporarily blocked by the moon. While not as significant as the daily variation as day turns to night, the effect was clearly seen in the signals received on the ground from GPS satellites.
GPS signals are routinely used to monitor the behavior of the ionosphere. The density of electrons in the ionosphere affects the speed of propagation of radio signals and this effect is slightly different at different frequencies.
By combining measurements made on the L1 and L2 legacy signals transmitted by all GPS satellites using high-grade receivers, scientists and engineers can measure the total electron content (TEC), which is the number of electrons in a column with a cross-sectional area of one meter squared along the path of the signal from satellite to receiver.
This value can then be projected to the vertical direction using a simple equation. Given the large number of electrons in the column, we measure the TEC in TEC units (TECU), where 1 TECU = 1016 electrons per square meter.
TEC time series from two continuously operating GPS monitoring stations near the path of totality, BREW at Brewster, Washington, and NISA at Boulder, Colorado, show a small dip of about 2 TECU or so around 18:00 UTC on Aug. 21, coincident with the timing of the eclipse. These time series are illustrated in FIGURES 1 and 2. Also shown in the figures are the time series for the day before, Aug. 20, which just show the normal diurnal ionospheric variation.
Figure 1. Time series of vertical total electron content observed using all GPS satellites observed at Brewster, Washington, on Aug. 21, 2017, the day of the eclipse (in blue) and the time series from the previous day, Aug. 20., 2017, for comparison (in red).Figure 2. Time series of vertical total electron content observed using all GPS satellites observed at Boulder, Colorado, on Aug. 21, 2017, the day of the eclipse (in blue) and the time series from the previous day, Aug. 20., 2017, for comparison (in red).
The effect of the eclipse was also be seen in the real-time correction data transmitted by the U.S. Wide-Area Augmentation System (WAAS) using geostationary satellites.
WAAS provides enhanced accuracy, integrity and availability for GPS single-frequency users using a network of dual-frequency GPS receivers all across North America. Corrections include a grid of ionospheric propagation delay values, updated every 5 minutes, which are used to account for the delay in receiver measurements.
FIGURE 3 shows part of the grid transmitted by WAAS and the path of totality across the U.S. Three of the grid points are close to the path and the time series of delay values of these points are shown in FIGURE 4.
Figure 3. Map showing the locations of a subset of the grid points used for the WAAS ionospheric delay corrections highlighting the three grid points close to the eclipse path of totality used to examine the effect of the eclipse along with one grid point far removed from the path for comparison.Figure 4. Time series of ionospheric vertical delay values of three WAAS ionospheric grid points along the eclipse path of totality on Aug. 21, 2017, along with the values from a grid point far removed from the path.
We see clear dips in values of up to about 50 centimeters. This is equivalent to what we see in the TEC time series from the BREW and NISA monitor stations since 1 TECU equates to 16 centimeters of propagation delay at the GPS L1 frequency.
Furthermore, the times of the dips correspond to the times of totality as the eclipse quickly moved across the country from west to east. Also shown for comparison in Figure 4 are the delay values for a grid point far removed from the path of totality, which show only the normal diurnal variation.
Not only does a total eclipse mesmerize the general public, it excites many scientists and engineers, too. A number of university research groups organized special eclipse observing campaigns to collect data from GPS receivers as well as other ionospheric monitoring tools to better understand exactly how the ionosphere reacts to a total eclipse of the sun.
And although we expect future analysis of the data will show features of great interest to science, the immediate results from the total eclipse of Aug. 21 show no significant impacts on the position, navigation and timing service GPS provides.
GPS “weathered” the eclipse with flying colors.
(Attila Komjathy, Siddharth Krishnamoorthy, Anthony J. Mannucci, Lawrence C. Sparks, Lawrence E. Young and Giorgio Savastano from the NASA Jet Propulsion Laboratory operated by the California Institute of Technology; Gerald W. Bawden from NASA HQ Earth Science Division; and Hyun-Ho Rho and Richard B. Langley from the University of New Brunswick, Fredericton, Canada, contributed to this article.)
At the end of 2016, the DemoGRAPE consortium observed, for the first time ever, ionospheric scintillations on Galileo signals in Antarctica, using Septentrio’s PolaRx5S GNSS reference receiver.
DemoGRAPE investigates improvement of high-precision satellite positioning with a view to developing scientific and technological applications in Antarctica. At higher latitudes, GNSS signal degradation due to ionospheric activity is more pronounced.
Septentrio’s PolaRx5S reference receiver.
The more precise phase-based positioning modes are particularly vulnerable to ionosphere disturbance such as scintillations. Elevated ionospheric activity can cause a loss of precise-positioning mode or, in more extreme cases, a total loss of signal lock.
Monitoring the movement and evolution of ice shelves and glaciers as well as geodetic prospecting require highly precise positioning. Besides this scientific interest, accurate positioning is important from a safety standpoint.
When visibility is limited and travel is restricted, designated routes between remote locations have to be strictly followed to avoid dangers such as falling into a crevasse during a snowstorm.
DEMOGrape is an international project lead by Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy in partnership with the Politecnico di Torino, the South African National Space Agency (SANSA) and the National Institute for Space Research, São Paulo, Brazil (INPE).
Septentrio’s PolaRx5S is the benchmark for GNSS space weather applications. It provides data for scintillation analysis (I&Q correlations, phase, code and carrier-to-noise) at up to 100 Hz for all GNSS L-band frequencies. SBF, RINEX and BINEX data logging is possible on both a built-in 16 GB memory and on an externally connected device. Up to 24 independent data archives can be defined. Logged data can be accessed via the web UI server or automatically pushed to a FTP server.
“We are really very happy of the fruitful collaboration with Septentrio colleagues that supported our measurements in the extreme environment of Antarctica,” the team said in an article published in Space Weather. “The first Galileo scintillations observed in the DemoGRAPE sites are attracting the attention of space weather communities, also beyond the European borders.” (Alfonsi, L., P. J. Cilliers, V. Romano, I. Hunstad, E. Correia, N. Linty, Fabio Dovis et al. “First Observations of GNSS Ionospheric Scintillations From DemoGRAPE Project.” Space Weather14, no. 10 (2016): 704-709).
“We are really proud to have enabled our colleagues and friends from INGV and the DEMOGrape consortium to make this first of a kind scintillation measurement on the Galileo signals,” said Bruno Bougard, director of R&D at Septentrio. “Galileo added value on high-precision application resides in its ability to increase the position availability and reliability compared to traditional GPS+GLONASS systems. Demonstrating its resilience to scintillation is key for operations at high latitudes.”
How four frequencies help when the ionosphere is disturbed
The authors explore how cycle slips in Galileo carrier-phase measurements can be more effectively detected using four frequencies.
INNOVATION INSIGHTS with Richard Langley
MORE SATELLITES OR MORE SIGNALS? That was the question put to the delegates at GNSS Election ’08, the stimulating and amusing entertainment provided at the GPS World Leadership Dinner held in conjunction with The Institute of Navigation’s meeting in Savannah in September 2008.
During the debate ahead of the election, the Satellite Party advocated that the GNSS user community would be better served by more satellites than more signals. They argued that more satellites (more than those in the operational GPS constellation) would enable more continuous and reliable positioning in cities, mountainous areas and other difficult environments and that the legacy GPS signals were sufficient. Greg Turetsky, one of their candidates, stated, “I would maintain from an economic standpoint that it’s far more cost-effective for our constituents to have more of the same satellites to give them more of the same services that they enjoy today, in more areas, rather than creating new things for which they have no use.”
The Signal Party, on the other hand, advocated for more signals with receivers capable of using them to provide high accuracies for a wide spectrum of GNSS uses. Signal Party candidate Javad Ashjaee opined, “We are the party of building roads, generating accurate maps, growing your food by automating agriculture, synchronizing your power stations. We are even working on automatically landing aircraft to use the air space more efficiently.”
Although contested, the election was won by the Satellite Party, 62 votes to 46. But clearly, both sides offered beneficial advances to the GNSS user community, so why not work together, have the parties enter into an alliance, and provide both more satellites and more signals?
Fast forward to 2016. The alliance has come to pass and we have the best of both worlds. We have two complete GNSS constellations, GPS and GLONASS, with two more, Galileo and BeiDou, on track for completion within the next few years. We also have regional systems either supplying an independent local positioning service or augmenting GPS with NavIC (also known as the Indian Regional Navigation Satellite System) and QZSS, respectively. Not to mention a growing number of satellite-based augmentation system satellites. When I compiled The Almanac for the August issue, there were over 100 GNSS satellites transmitting signals to users. And not only more signals from more satellites, but more technologically advanced signals on more frequencies.
The plethora of signals now being transmitted by GNSS satellites is already leading to further advances in positioning, navigation and timing—even before full constellations transmitting those signals are in place. A good case in point is Galileo’s Open Service, which is transmitted in the E1 and E5 bands. A modified version of binary-offset-carrier (BOC) modulation, called Alternative BOC or AltBOC, is used to generate the wideband E5 signal. Its structure is such that a receiver can track and make measurements on just the lower frequency part of the signal centered on 1176.450 MHz (E5a), just the upper frequency part centered on 1207.140 MHz (E5b), the whole AltBOC signal centered on 1191.795 MHz (E5a+b), or any combination of these including all three. Using all three together with the E1 signal provides us with a four-frequency positioning capability. What’s the benefit of using four frequencies? There are several, but in this month’s column, a recently graduated award-winning Belgian student and her supervisor tell us how cycle slips in Galileo carrier-phase measurements can be more effectively and efficiently detected using four frequencies.
The availability of data offered in the Galileo GNSS Open Service on four carrier frequencies opens the way to new multi-frequency solutions for civil users. In the research reported in this article, we focused on one of the consequences of signal tracking loss, the appearance of cycle slips, and how the use of the four frequencies can help in their detection.
Cycle-slip detection is a key issue for high-precision positioning applications. Any users in need of determining a precise and reliable position must be aware of the potential presence of cycle slips in their data, since they compromise data quality.
Traditionally, two carrier frequencies were used for positioning; for instance, the GPS L1 and L2 frequencies. More recently, three-carrier positioning has allowed enhanced precision and accuracy. Though using a third carrier frequency has allowed us to partially solve the cycle-slip detection issue, existing procedures are still lacking in some aspects. One of today’s main challenges is cycle-slip detection under high ionospheric activity, which is why we focused on this specific case study. And since the use of three frequencies helps to improve reliable cycle-slip detection, might not the use of an additional fourth frequency further improve detection capability? Since Galileo supplies four frequencies in its Open Service, we thought we might be able to improve cycle-slip detection algorithm performance once more.
Framework. In this article, a new quad-frequency cycle-slip detection algorithm is introduced — seemingly, an unexplored track in the literature until now. The algorithm uses undifferenced carrier-phase observations from a single-station static receiver. First developed for post-processing, the algorithm also has been adapted to real-time applications. This algorithm aims to improve cycle-slip detection under high ionospheric activity.
CYCLE SLIPS
Though code (pseudorange) measurements are commonly used for standard positioning, any precise positioning application needs to use carrier-phase measurements, due to their better quality. Unfortunately, the latter are potentially subject to cycle slips, generating a constant bias in data and, if undetected and uncorrected, impacting the inferred positioning.
Carrier-phase measurements are made by observing the beat phase, that is, the difference between the received carrier from the satellite and a receiver-generated replica. At the first observation epoch, only the fractional part of this beat phase can be measured, but the integer offset between the satellite signal and the receiver’s replica is unknown. This integer number of cycles is called the initial phase ambiguity and remains constant during the observation period.
The carrier-phase observable (between a satellite i and a receiver p), in meters, is given by the following equation:
(1)
where the subscript fk indicates the term dependency on the frequency and Φ on the carrier-phase observable. G is the geometric term (that is, a function of the geometric range between the receiver and the tracked satellite, the tropospheric delay, and satellite and receiver clock bias), I is the ionospheric delay, M is the multipath error, HW stands for satellite and receiver hardware delays, c is the vacuum speed of light, N is the initial phase ambiguity, and ε is the random error (also called phase noise).
At the first observation epoch, an integer counter is initialized, and as the tracking goes on, it is incremented by one cycle whenever the beat phase changes from 2π to 0. If the receiver — even briefly — loses track on the signal, the counting is suspended and an integer number of cycles is lost. This loss can result from various causes (signal obstruction, rapid change in the carrier-phase observable, and so on).
In the observation equation, the cycle slip will appear as a change in the value of the initial phase ambiguity. Thus, a one-cycle slip will involve a phase measurement shift of about 20 centimeters (equal to the carrier wavelength), depending on the affected carrier frequency. The cycle-slip size can be any value from one to thousands of cycles.
Ionospheric delay is the only term that could possibly be confused with a small cycle slip. Indeed, during an ionospheric perturbation event, this delay variation between two observation epochs (spaced at 30-second intervals, say) often reaches 20 centimeters (the size of a one-cycle slip in the phase measurement) or more. The ionosphere activity has two main consequences. Firstly, as mentioned before, slips can be hidden in observation noise (including ionospheric variability) and not detected. Secondly, received signal variability can cause loss of lock and thus cycle slips.
A lot of different configurations can arise when the signal is lost. Signal tracking can be interrupted on one single carrier resulting in an isolated cycle slip (ICS) or simultaneously on multiple carriers. In the second case, the slip magnitude on the different carriers can be the same (simultaneous cycle slips of the same magnitude, or SCS-SM) or different (simultaneous cycle slips of different magnitudes, or SCS-DM).
Detection History. The first cycle slip detection algorithm using undifferenced observations, Turbo Edit, was developed in 1990 by Geoff Blewitt. Code and phase measurements from two carrier frequencies are used. It has been implemented in many data preprocessing programs, such as GIPSY-OASIS II, PANDA and Bernese. The Turbo Edit algorithm has been enhanced numerous times. In its latest version, it was adapted to detect cycle slips under high ionospheric activity, but it is still a dual-frequency technique.
Availability of a third, simultaneous signal frequency permits the development of new combinations of observables. A low-noise phase-only combination eliminating geometric as well as first-order ionospheric terms was developed by Andrew Simsky and applied to cycle-slip detection. Studies have also been made to determine the best combinations to be used in triple-frequency positioning, and subsequently in cycle-slip detection and correction algorithms. These algorithms use both code and phase measurements, as well as a triple-frequency method developed by Maria Clara de Lacy and colleagues.
Concern about cycle slips and the relationship with the ionospheric signature in data is trending. In 2011, Zhizhao Liu published a paper on using the rate of change of total electronic content to detect cycle slips. On the other hand, after studying ionospheric cycle slips, Simon Banville and Richard Langley concluded in a paper published in 2013 that the “increased measurement noise associated with an active ionosphere makes correcting cycle slips an ongoing challenge, which requires further investigation,” while Xiaohong Zhang and colleagues, in a paper published in 2014, came to the same conclusion while trying to repair cycle slips during scintillation events. See Further Reading for a list of the highlighted papers in the history of cycle-slip detection and correction.
QUAD-FREQUENCY ALGORITHM
Cycle-slip detection techniques use testing quantities (where the cycle slip is represented by a jump or significant change in the quantity). These are associated with a discontinuity detection algorithm, which aims to locate the jump.
Testing Quantities. Testing quantities are linear combinations of observations. They differ in several aspects: the observables used (in our case, only phase measurements), the number of carrier frequencies used and inner properties of the combination (geometry-free, ionosphere-free and the noise level on the combination).
In our study, we assumed values for the noise on Galileo carrier-phase measurements as given in TABLE 1.
Table 1. Frequencies available in the Galileo Open Service.
Triple-Frequency Simsky Combination. Our algorithm is mainly based on exploiting the triple-frequency Simsky combination. It is a geometry-free and ionosphere-free carrier-phase combination, in meters, as shown in Equation 2.
(2)
When four frequencies are available, four triple-frequency combinations can be computed. Two of them are sufficient to detect slips on any of the four frequencies.
The combination choice must first depend on its precision (given by σS in TABLE 2), obtained by applying the variance-covariance propagation law to raw measurement noise (see Table 1). Precision is not the only factor to be taken into account in the choice of suitable combinations. In each combination, carrier frequencies have different impacts due to their different wavelengths: the impact of a one-cycle-amplitude slip on the E1 frequency will indeed not be the same as the one on E5a, E5b or E5a+b (see Table 2). The smallest impact on a given combination is always the most difficult one to detect.
Table 2. Simsky combinations.
Therefore, the efficiency of a given combination will depend on both the effect of the smallest cycle slip and the combination precision (given by the standard deviation): the higher the ratio between them, the more efficient the combination.
Among the four combination possibilities, the two highest ratios are those formed by the E5a-E5b-E5a+b and E1-E5a-E5b combinations. These will thus be the ones used in our algorithm.
The Simsky combination allows us to detect ICS as well as SCS-DM cycle slips. Nevertheless, this combination is insensitive to SCS-SM slips on all four frequencies (which is a rare phenomenon). We will therefore have to add another testing quantity to our algorithm.
Dual-Frequency, Geometry-Free Combination. The dual-frequency, geometry-free (GF) combination, in meters, allows us to detect SCS-SM slips. It can be computed as follows:
(3)
Unfortunately, the raw dual-frequency, geometry-free combination is affected by ionospheric delay. To mitigate the ionospheric smooth trend, a fourth-order time difference is computed. Still, the result suffers from rapid variations of ionospheric delay.
When four frequencies are available, six dual-frequency combinations can be computed. One is sufficient to detect the presence of simultaneous cycle slips of the same magnitude. The choice will again depend on the ratio between combination precision and the smallest effect of simultaneous one-cycle slips.
On the one hand, differencing the combination results affects precision. On the other hand, the cycle slip, thus the smallest effect to detect, will be amplified by high-order differencing. The best ratio is obtained with a fourth-order difference (see TABLE 3), even if a smooth variation due to the ionosphere is already removed in the second-degree differencing (see Figure 1).
Even if one combination is sufficient, our approach will use two of them to double check their outputs: E1-E5a and E1-E5a+b, since they offer the best ratios.
Detection Method. To detect a discontinuity due to a cycle slip in the testing quantity, it is necessary to establish detection thresholds. Thresholds are one of the key parameters in cycle-slip detection, since they lead to the decision on the presence of a cycle slip or not. If the threshold is too restrictive, some real slips can be missed (a false negative). On the other hand, if it is not restrictive enough, discontinuities that do not match with a cycle slip could be abusively detected (a false positive).
It is important to notice, as our study highlights, that there is no perfect threshold that suits all the needs and constraints. The choice must be made considering the positioning application at hand. Threshold values given in this article are representative and were empirically determined to be optimal with respect to our goal of cycle-slip detection under high ionospheric activity. Results and further discussions about different thresholds can be found in the first author’s thesis (see Further Reading).
Cycle slips will affect the raw Simsky combination by a shift in the mean combination value, whereas the time-differenced one will be affected by a spike.
Detection Using Simsky Combination. Cycle-slip detection on the triple-frequency Simsky combination is performed in two cascading steps (see FIGURE 2).
FIGURE 2. Detection method for the Simsky combination.
The first one uses a time-differenced combination to detect potential cycle slips using a 20-observation-sized forward and backward moving average window, in which the mean and standard deviation statistical parameters are computed. The current epoch is compared to the previous ones to detect a spike, which could correspond to a cycle slip. Two types of thresholds are used: statistical (or relative) and absolute.
As shown in FIGURE 3, using a statistical threshold allows us to adapt detection to the inertia of statistical parameters. Assuming the noise on the observations (here, the Simsky combination results) follows a normal distribution, a confidence interval of 3-sigma around the mean includes 95 percent of the observations. Given the ratio of the two Simsky combinations used (computed earlier), the success rate reaches 100 percent for both combinations, which means any ICS and SCS-DM slips on data will be detected for sure (no false negatives). Nevertheless, false positives may occur because 5 percent of the data is statistically outside the 3-sigma bounds.
FIGURE 3. Statistical and absolute thresholds.
To reduce this rate, an absolute threshold is also applied, equal to 0.4 times the smallest impact of a cycle slip on the combination (see Table 2). If we can take Figure 3 as a suitable example of an extreme ionospheric disturbance leading to unusually high variability in combination results, the absolute threshold will most of the time be far higher than the statistical one and will help to reduce the rate of wrong detections.
As an output of this first step, a flag value is assigned to epochs with larger values than both thresholds, and which are therefore potentially affected by cycle slips.
Once the locations of potential slips are achieved, the second step consists in comparing the mean before and after potential cycle slips for the flagged epochs. A second absolute threshold is applied, equal to 0.8 times the smallest effect. If another potential cycle slip is present in the detection window, the size of the detection window will be reduced to avoid calculation of statistical parameters on partially shifted data.
The goal of the first step is to detect potential slips. Therefore, the priority is to avoid missing a real slip with low threshold values, sometimes leading to false positive detection. On the other hand, the second step aims to separate the potential remaining false positives — outlier spikes in the raw combination — from the real cycle-slip shifts on average. The theoretical performance of this two-step approach is 100 percent: neither false positives nor false negatives should be encountered.
Detection Using Geometry-Free Combination.Since the fourth-order differenced geometry-free combination is affected by a residual ionospheric delay, the previous procedure cannot be applied. Like any time-differenced testing quantity, the slip will appear as a spike in the combination. Therefore, there is no way to distinguish cycle slips from outliers by a mean level comparison (second step).
Consequently, the detection method only consists of a forward-and-backward moving average window, in which a 4-sigma confidence interval is compared to the current epoch combination value. Indeed, in this case, we cannot afford to encounter false positives on 5 percent of epochs (induced by the use of a 3-sigma threshold) since no further step can be set up to eliminate remaining false positives.
The theoretical performances of the geometry-free detection method are also expected to reach 100 percent. Again, neither false positives nor false negatives should be encountered. Note that this calculation only takes ratios into account, neglecting the fact that the geometry-free combination is also sensitive to the variability of the ionosphere.
VALIDATION
We have tested the quad-frequency algorithm on 30-second quad-frequency Galileo observations from stations GMSD (in Nakatane, Japan) and NKLG (in Libreville, Gabon). The GMSD observations were used to test algorithm robustness towards simulated particular cases, whereas the NKLG data were used to assess algorithm behavior for cases met in the equatorial area.
Methodology. Cycle slips were artificially inserted into the GMSD data, simulating the following cycle-slip scenarios: ICS, SCS-DM and SCS-SM. The benefit of such a simulation approach is that the algorithm output can easily be compared to the already-known solution. Moreover, these data had been used to determine whether the use of more carrier frequencies could increase cycle-slip detection performance.
We analyzed a 50-day NKLG dataset, covering observations from Jan. 6 to Feb. 1 and from June 24 to July 19, 2014. This sample is made up of various ionospheric states: calm and extreme days, as well as typical equatorial activity. Since the solar cycle peak happened in 2014, data from that year perfectly fits a study of the effects of high ionospheric activity.
We used NKLG raw data to achieve a dual goal. Firstly, we wanted to determine the proportion of epochs for which small cycle slips (one, two or five cycles) couldn’t be distinguished. This was performed by comparing the impact (in meters) of such scenarios to the instantaneous threshold associated with each epoch. In the case of a high cycle-slip detection threshold, potentially present slips of one, two or five cycles couldn’t be detected. The fraction of epochs in a day for which such small cycle slips would not be detected, for each combination used in the algorithm, seemed to be a suitable indicator of algorithm effectiveness in the equatorial area.
Secondly, we analyzed results by visually assessing algorithm output using combination graphics, and tried to answer the following questions: Do flagged epochs seem to be affected by cycle slips? Are there actual cycle slips that remain undetected?
Results. We looked closely at the results of both our simulations and the analysis of raw data.
Simulation of Particular Cases. Compared to equivalent dual- and triple-frequency methods, our new quad-frequency algorithm gave better results: all inserted cycle slips were successfully detected and no false positive were noticed.
NKLG Raw Dataset Analysis. The validation process using NKLG raw data highlights several trends in algorithm results. First of all, it is interesting to notice that the detection of isolated slips as well as slips of different magnitude (using the Simsky combinations) was guaranteed for every observation epoch of every analyzed day. Indeed, Simsky instantaneous thresholds never exceeded the effect of a slip of one-cycle amplitude.
In addition, in 25 percent of the analyzed days, detection of cycle slips of the same magnitude could also be guaranteed. For the remaining days, detection of simultaneous cycle slips whose amplitudes are less than five cycles could not be guaranteed for a few observation epochs, which can reasonably be neglected because of the very small probability of experiencing such exceptional cases. This is due to the impact of ionospheric variability on the geometry-free combination, inducing high instantaneous threshold values.
However, both the Simsky and geometry-free combinations suffer from false positive detection under extreme ionospheric events: if a cycle slip is detected, it sometimes corresponds to an outlier. This side effect is due to the threshold choices we made to match our initial purpose of detecting all cycle slips for sure, rather than risking missing one of them, even if false positives are part of the results list.
FURTHER IMPROVEMENTS
In addition to post-processing applications, we have also considered a real-time adaptation of the algorithm. The real-time constraint impacts both the Simsky and geometry-free detection methods. In this configuration, the statistical window can indeed only move forward, which neglects cycle-slip detection on the first 20 epochs. Further on, the mean level comparison (see the Simsky detection method described earlier) can no longer be considered because the mean following a potential cycle slip cannot be computed in real-time processing. Even if our quad-frequency detection algorithm suffers from the real-time constraint, it still proves efficient if the latter is taken into account for suitable thresholds choices.
Cycle-slip detection is indeed only a first step, and cycle-slip correction should complete the procedure to avoid discontinuities. It should be pointed out, however, that simply being aware of the presence of a cycle slip in a dataset is precious information for a user, and at the corresponding epoch, the parameters in the solution may be reinitialized.
Enhanced with a suitable cycle slip correction method and a real-time feature, our algorithm could be directly integrated into a software receiver, enabling the supply of continuous and corrected data to the user.
CONCLUSION
In this article, we have introduced the first quad-frequency cycle-slip detection algorithm, with an efficiency that is clearly a step forward.
This innovative detection method opens new doors to numerous research and commercial applications. Every Galileo user, whether civil or military, will be able to benefit from better-quality positioning, especially under harsh ionospheric conditions: not only where the ionosphere is particularly restless such as in the equatorial and polar regions, but also at any latitude during an ionospheric disturbance.
With regard to precise positioning, this is yet another step that reinforces Galileo’s competitiveness against other dual- or triple-frequency systems.
ACKNOWLEDGMENTS
This article is based on the paper “Cycle Slips Detection in Quad-Frequency Mode: Galileo’s Contribution to an Efficient Approach Under High Ionospheric Activity,” the winning submission to the 2014–2015 Students’ Contest of the Comité de Liaison des Géomètres Européens in the Galileo, EGNOS, Copernicus category, which was sponsored by the GSA, the European Global Navigation Satellite Systems Agency.
LAURA VAN DE VYVERE received an M.Sc. in geomatics and geometrology from the Université de Liège, Belgium, in 2015. Her master’s thesis was dedicated to Galileo cycle-slip detection under extreme ionospheric activity. In 2015, she joined M3 Systems Belgium in Wavre as a radionavigation project engineer and is currently involved in GNSS reflectometry and GNSS hybridization projects.
RENÉ WARNANT received an M.Sc. in physics in 1988 and a Ph.D. in physics with a specialty in GNSS in 1996, both from the Université catholique de Louvain, Louvain-la-Neuve, Belgium. He started his career as a geodesist at the Royal Observatory of Belgium in 1988. Since June 2011, he is a full-time professor and head of the Geodesy and GNSS Laboratory at the University of Liège where he is responsible for education in the field of space geodesy and GNSS.
“A New Automated Cycle Slip Detection and Repair Method for a Single Dual-Frequency GPS Receiver” by Z. Liu in Journal of Geodesy, Vol. 85, No. 3, March 2011, pp. 171–183, doi: 0.1007/s00190-010-0426-y.
“An Automated Editing Algorithm for GPS Data” by G. Blewitt in Geophysical Research Letters, Vol. 17, No. 3, March 1990, pp. 199–202, doi: 10.1029/GL017i003p00199.
Cycle Slips and the Ionosphere
“Improved Precise Point Positioning in the Presence of Ionospheric Scintillation” by X. Zhang, F. Guo and P. Zhou in GPS Solutions, Vol. 18, No. 1, Jan. 2014, pp. 51–60, doi: 10.1007/s10291-012-0309-1.
“Cycle Slip Detection and Repair for Undifferenced GPS Observations Under High Ionospheric Activity” by C. Cai, Z. Liu, P. Xia and W. Dai in GPS Solutions, Vol. 17, No. 2, April 2013, pp. 247–260, doi: 10.1007/s10291-012-0275-7.
“Mitigating the Impact of Ionospheric Cycle Slips in GNSS Observations” by S. Banville and R.B. Langley in Journal of Geodesy, Vol. 87, No. 2, Feb. 2013, pp. 179–193, doi: 10.1007/s00190-012-0604-1.
Real-Time Cycle-Slip Detection and Repair
“Real-Time Detection and Repair of Cycle Slips in Triple-Frequency GNSS Measurements” by Q. Zhao, B. Sun, Z. Dai, Z. Hu, C. Shi and J. Liu in GPS Solutions, Vol. 19, No. 3, July 2015, pp. 381–391, doi: 10.1007/s10291-014-0396-2.
“Real-Time Cycle Slip Detection in Triple-Frequency GNSS” by M.C. de Lacy, M. Reguzzoni and F. Sansò in GPS Solutions, Vol. 16, No. 3, July 2012, pp. 353–362, doi: 10.1007/s10291-011-0237-5.
A sea-based test is demonstrating the potential of extending satnav augmentation coverage into north polar regions, offering a safety-of-life standard of navigation performance to users including shipping or aircraft in flight.
Norwegian research vessel Gunnerus, owned by the Norwegian University of Science and Technology, is equipped to pick up satnav signals from GPS and GLONASS as well as augmentation signals specially generated for the test, modeled on Europe’s existing European Geostationary Navigation Overlay System (EGNOS).
Norwegian research vessel Gunnerus, owned by the Norwegian University of Science and Technology. (Photo: ESA)
Gunnerus is making use of the signals during five days of sailing off Trondheim. The demonstration is part of the Arctic Test Bed project, conducted within the European Global Navigations Satellite System Evolutions Programme (EGEP) of ESA.
The ESA-designed EGNOS improves the precision of US GPS signals over most European territory, while also providing continuous and reliable updates on their integrity.
A 40-strong network of ground monitoring stations perform an independent measurement of GPS signals, so that corrections can be calculated and then passed to users immediately via a trio of geostationary satellites. The result is a several-fold increase in precision.
“Simply due to Earth’s curvature, EGNOS signals are not visible above about 70 degrees north, but they are needed to support polar routing,” explains Marco Porretta, overseeing the Arctic Test Bed project.
To investigate possible methods for improving Satellite-Based Augmentation System (SBAS) performance in this Arctic region, the test campaign will assess the benefits of augmentation for various types of satnav signals: single-frequency GPS; dual-frequency GPS; and dual-constellation dual-frequency, where GPS signals are combined with those of its Russian counterpart, thus increasing the number of observations.
“The planned next-decade upgrade of EGNOS, along with other augmentation systems operated over other continents (such as the U.S. equivalent Wide Area Augmentation System, WAAS), will perform multi-constellation augmentation as standard,” adds Marco. “That means data from this test case should be especially valuable to support interoperability between future augmentation systems.”
The Arctic Test Bed makes use of some EGNOS reference stations along the north of Europe, along with additional stations in locations including Greenland, Jan Mayen Island, Spitsbergen and Norway.
Model of the well-known Oct. 30, 2003, Halloween solar storm produced by the MIDAS tomographic ionospheric model from the University of Bath. (Image; ESA)
Marco explains, “These stations will allow specific monitoring of the ionosphere — the electrically active segment of Earth’s atmosphere — in the Arctic region. The ionosphere is significant because it is an important source of satnav signal delay, or in some cases can cause receivers to lose signal lock due to ionospheric scintillations.”
With geostationary satellites out of sight, navigation corrections for the Arctic Test Bed will be transmitted via terrestrial radio. In future, an operational version of the system could either stick with this solution or rely on other satellite-based means of dissemination from non-geostationary orbit.
The all-important generation of the augmentation correction message will take place at a processing center in Hønefoss, Norway, using adapted EGNOS algorithms.
An operational version of the Arctic Test Bed could potentially extend augmentation coverage to as high as 85 degrees north, as high as Greenland, extending to the edge of WAAS coverage.
The Arctic Test Bed project was initiated by ESA, with Kongsberg Seatex serving as prime contractor, GMV Aerospace and Defence, Thales Alenia Space France, Logica, Terma, the Norwegian Mapping Authority, Technical University of Denmark, Septentrio and the University of Calgary.
Using a smartphone for GNSS ionospheric data collection
By Andrew Kennedy, Ryan Kingsbury, Anthea Coster, Victor Pankratius, Philip. J. Erickson, Paulo Roberto Fagundes, Eurico R. de Paula, Kerri Cahoy and Juha Vierinen
INNOVATION INSIGHTS with Richard Langley
DO YOU REMEMBER YOUR FIRST PERSONAL COMPUTER? I do.
It was a Timex Sinclair 1000. Released in 1982, it used a Zilog Z80A processor running at 3.25 MHz and sported a whopping two kilobytes of memory and a wonky membrane keyboard. You had to hook it up to a tape recorder to record and load programs (in BASIC) and it used a TV tuned to channel 3 or 4 as a display device. We’ve certainly come a long way in the past almost 35 years. Now, I have a computer I can hold in my hand with more than one thousand times the computing power and more than one million times the memory and a built-in interactive display. It’s an Apple iPhone 5S smartphone. I am one of the billions of owners of a smartphone. In 2015 alone, almost 1.5 billion smartphones were sold worldwide.
We use our smartphones for a wide range of tasks. Besides voice phone calls, we use them to text, to wake us up, to listen to our tunes, to watch movies, to take photographs and videos, to surf the Web, to navigate. The list goes on and on. In 2015, there were about 1.5 million applications or apps available for both Apple and Android smartphones.Those with the ability can even program their smartphones to perform tasks specific to their lifestyles, hobbies, or professions.
In this month’s column, we take a look at the use of a smartphone app to collect GNSS ionospheric data. Why would you want to do that?
In the experience of the developers of the app, GNSS receivers are often characterized by a complex, proprietary data interface that differs for each manufacturer. In practice, this leads to significant investments in understanding interfaces and software tools. Human operators must familiarize themselves with the commands used to configure each receiver as well as with proprietary graphical user interfaces and tools specific to each receiver. The authors’ app-centric approach provides a software framework and output format that remain the same for different receivers. Receiver-specific commands are configurable within the app, so users can easily attach new receivers while reusing the existing infrastructures for data collection and processing. And smartphones have more than enough power and connectivity to do the job and can be easily moved from site to site.
The smartphone as a handheld device to help scientists study the ionosphere? Probably not even Clive Sinclair foresaw that.
Continuous and high-resolution dual-frequency GNSS observations are required to capture the ionospheric response to external forcing from events such as the 2011 Tohoku-Oki earthquake and tsunami or the 2003 Halloween geomagnetic storms that severely impacted the U.S. Federal Aviation Administration’s Wide Area Augmentation System. These events, as well as other natural and man-made disasters, have been shown to produce structure of various scales in the ionospheric total electron content (TEC). TEC estimates can be directly derived from dual-frequency GNSS observations and so these observations are a valuable source of information about the ionosphere.
However, with the exception of a few areas such as Japan, where the GNSS Earth Observation Network (GEONET) has an average spacing of 5 kilometers, the density of ground-based GNSS sensors needed to capture displacements of the ionosphere is lacking. This is primarily due to data acquisition costs. Networks on the order of 50-kilometer spacing would provide the density of coverage needed to capture the propagation of medium-scale traveling ionospheric disturbances (TIDs), which have horizontal wavelengths of up to hundreds of kilometers and speeds of hundreds of meters per second. Irregularity structures in the polar regions may require even denser networks to capture the fine-grained auroral structures.
The Mahali project, supported by the U.S. National Science Foundation, aims to improve the ability of the GNSS community to perform large-scale science by facilitating increases in the density of required sensors. The “last mile data transport” problem remains critical, and Mahali explores new ways to efficiently and effectively move data from the many types of GNSS receivers deployed across the world to the cloud, at affordable cost. “Kila Mahali” means “everywhere” in the Swahili language, a term that epitomizes the project’s ambitions for data collection.
A short-term objective of Mahali is to demonstrate the utility of mobile phones as low-cost preprocessors and relays that transport TEC observation data to cloud-computing environments for more advanced processing and storage. We eventually envision an “ecosystem” of open-source software, which includes various smartphone tools that aid researchers in interfacing with GNSS sensors.
In this article, we present one such smartphone software application (hereafter denoted “app”) from the Mahali software ecosystem that researchers can install on their Android smartphones. They can then link the smartphone directly to a dual-frequency GNSS receiver over a USB port. Thus, data can be immediately collected, pre-processed on the smartphone, and sent to cloud storage environments like Dropbox whenever an Internet connection is available. This approach tests out a building block for large-scale data-collection networks, which can grow incrementally by adding more GNSS receivers and smartphones.
Smartphones for science
Modern mobile processors offer ever-increasing computing capabilities. For example, Nvidia offers mobile multicore processors with four central-processing-unit cores and more than 60 graphics-processing-unit cores on a single chip.
While most mobile applications are leveraging this power for multimedia, photography or gaming, these hardware capabilities are now available for scientific data processing. Everyday smartphones like the Samsung Galaxy S5 smartphone have a quad-core processor running at 2.5 GHz, 2 GB of random-access memory, and a variety of sensor and network connectivity options.
The smartphone also features reliable backhaul via Wi-Fi and the world’s ever-growing cellular data network, qualities highly relevant for scientific applications. Even in Africa, there was an average of 60 mobile-cellular subscriptions per 100 inhabitants in 2012 according to the International Telecommunication Union. Smartphones therefore have a significant advantage over other platforms for large-scale, distributed applications.
Today’s smartphones are typically only equipped with single-frequency GNSS receivers, and thus it is not yet possible to entirely replace dual-frequency GNSS receivers by smartphones running a data-collection app. To make the necessary scientific measurements to recover TEC, receivers require dual-frequency tracking capability. In the current work, we focus primarily on using smartphones as data-collection and relay devices. We anticipate, however, that future consumer demands, such as precision navigation, will eventually push dual-frequency capabilities into next-generation mobile devices. In that case, our app would not need to be connected to external receivers, but instead would use the smartphone’s internal receiver.
Mahali GNSS Logger App
This section describes the Mahali GNSS Logger App we developed at the Massachusetts Institute of Technology (MIT) to collect data from a GNSS receiver and relay scientific data to the cloud.
Setup. The app interfaces with a GNSS receiver over a USB-to-serial connector, as shown in FIGURES 1 and 2. It collects observation data output from the receiver, and stores it to files on local storage on the smartphone. The app allows the uplink of data files to a cloud-based storage medium available through the Internet, where further data processing and analysis can be performed. For our evaluation, we demonstrate this uplink by interfacing with Dropbox, a widely used cloud data storage service.
Figure 1. Smartphone and a USB-to-serial adapter.Figure 2. Android smartphone connected to a GNSS receiver over a USB-to-serial adapter.
The GNSS data logger app facilitates the process of collecting GNSS data from a variety of commercially available receivers. The app was developed in the Java/Android programming language for deployment on mobile devices running Google’s Android operating system (OS).
Usage scenarios. Figure 3 illustrates the concept of operations for a scenario involving multiple smartphones and GNSS receivers. Data is initially generated by each GNSS receiver (step 1). A smartphone connected to each receiver runs the app (step 2) and gathers the data on local storage. After establishing a connection to a cloud-based server, the app acts as a relay and transmits the local data to the cloud (step 3).
Figure 3. Concept of operations for the Mahali GNSS Logger App involving data collection from multiple receiver types.
The app is intended for usage scenarios in which a particular smartphone connects to a single GNSS receiver. The app collects the data from the serial output of the receiver and forwards that data to a cloud-based storage location for subsequent analysis. We focused primarily on Dropbox for this purpose, used through Android’s “share” interface. In addition, the app can also configure the GNSS receiver by issuing specific commands on the serial port.
User interface. The app was structured to provide a convenient user interface for the quick commencement of data-collection sessions and upload of data files to the cloud. FIGURE 4 illustrates the typical user interface that a scientist would see when starting the app, and FIGURE 5 shows how scientists can configure commands that the smartphone issues to initialize a GNSS device.
Figure 4. Main screen of the Mahali GNSS Logger App.Figure 5. Command configuration screen for the GNSS receiver initialization.
The “Edit GPS Config” button (item 1 in Figure 4) allows access to a basic text editor screen (shown in Figure 5), which lists a series of ASCII character commands that are sent to the GNSS receiver upon commencement of a data-collection session. This approach lets a user configure the app to work with different types of GNSS receivers.
The “Session control” toggle button (item 2 in Figure 4) allows the user to start and stop a data-collection session. When a session is created, it is assigned a file name using a UTC time tag from the smartphone’s clock and a file extension corresponding to the GNSS receiver type. The file is stored in a dedicated directory in the smartphone’s external bulk memory (such as an SD card). This directory location is defined within the app at a set location.
The real-time status display (item 3 in Figure 4) shows the name of the current session file and the number of bytes that have been collected from the receiver interface and saved to the file.
The scrollable “Previous Sessions” display (item 4 in Figure 4) lists all previous session files found in the external storage directory. The user can tap on any session within the list to upload the file to the cloud. The user can delete session files from a submenu accessible through the three dots at the top of the screen.
File formats. The app currently logs data in a binary format that is dependent on the particular GNSS receiver. An example is the “nvd” format shown in Figure 4. Once the data is in the cloud, a variety of software tools are available to convert these files to other formats, such as the widely used RINEX format.
Currently, our post-processing of “nvd” files stored in the cloud is done in a custom Python script that converts them to the RINEX format in batch mode. To validate the generated RINEX format, we use the “TEQC” tool provided by UNAVCO.
Software architecture. The Android OS implements “threads” as a way to let users run multiple tasks at the same time, to manage multiple user interface updates, or to perform various background actions. “Activity” threads handle a user’s interaction with the main screen and GNSS configuration screens. Other app-specific threads are spawned by the main activity thread in response to user prompts. The spawned threads perform specific actions asynchronously in the background, so the user can continue to interact with the app while uploads are in progress.
In particular, the main activity thread handles the user’s interaction with the main screen. The activity calls the appropriate software functions that respond to button taps. The main thread also updates the user interface with the latest status information and manages the creation of new threads for serial input/output (I/O) as well as uploads to Dropbox.
The GNSS receiver config activity thread presents the user with a light-weight text editor, which captures all necessary GNSS receiver configuration commands. In the current version of the app, these commands are permanently stored in the smartphone internal bulk memory using the Android SharedPreferences module. This can be easily extended in the future, such as to store and download command configuration files to and from the cloud.
When a user toggles the “Session Control” button (in other words, a “Start Session” event), the serial I/O manager thread starts storing all the bytes received from a GNSS receiver to a GNSS session file. The bytes are read from the smartphone’s USB serial data interface and written to a file in binary format. The file and its properties are represented internally by a “GNSS Session” object. The file itself is a raw byte file; it is formatted in exactly the same way that the GNSS receiver outputs data. When interfacing with one modern multi-GNSS receiver specifically designed for scintillation studies and TEC monitoring, every hour of data collected took about 18 MB of storage. We have not yet tested the app’s performance at extremely high data output rates from a GNSS receiver, but we expect that it should be able to support all standard serial data rates.
When a user stops the data collection, the main activity updates the “Previous Sessions” list with a new session. The code ensures that at most one serial I/O manager thread is created, that is, a GNSS receiver data stream can only be logged to one session file at a time.
A DB (Dropbox) upload task is created upon user prompt. The task sends the selected session file to a directory within a Dropbox account specified by the user. The first time a user attempts file upload, the app obtains the necessary account authorization from the user.
Testing in the field
To test the app and the Mahali system concept, field trials were conducted from January to February 2015 in Brazil at the sites shown in TABLE 1. Data were collected from one multi-GNSS scintillation receiver, and two older GPS scintillation receivers, using two types of Android smartphones.
Table 1. Summary of app field test sites.
We chose Brazil for our test because it is in a region of significant interest for space weather studies. Manaus, one of the sites visited, is located at the magnetic dip latitude 5.1° N. São José dos Campos, the other site visited, is located south of the geomagnetic equator at a dip latitude of 18.9° S and is within the equatorial, or Appleton, anomaly region. This anomaly region, consisting of enhanced TEC, forms 10 to 20 degrees north and south of the geomagnetic equator due to the well-known “ionization fountain” effect. During geomagnetic storms, electric fields of magnetospheric origin can penetrate into the equatorial region and directly influence ionospheric density, neutral composition and temperature at low latitudes.
Geomagnetic storms generate large-scale gravity waves that propagate from high to low latitudes. Because of Brazil’s location in the tropics, gravity waves associated with tropospheric convection patterns can also propagate upwards producing a myriad of small- to medium-scale TIDs. Finally, it is suspected that the South Atlantic Anomaly, a region of weakened geomagnetic field that falls over Brazil, exerts considerable influence on the development of space weather phenomena in both hemispheres. Large day-to-night and day-to-day variations in TEC are frequently observed in this region. For all of these reasons, having a dense pattern of GNSS observations from this region is of significant scientific interest.
Our test campaign was primarily motivated by a desire to test the utility of the smartphone-based solution and to demonstrate the feasibility of easily setting up remote field sites for the purpose of filling in gaps in data coverage. During this campaign, we collected data at the three sites listed in Table 1 using three different receivers. About 220 MB of data were collected in total at all of the sites.
Table 1 summarizes the relevant information about the field test sites. The first field site visited was the Universidade Luterana do Brasil (ULBRA) campus in Manaus on Jan. 30, 2015. This site is in close proximity to the geomagnetic equator. Because the receiver at the ULBRA campus is involved in ongoing scientific observations, we were only able to collect data for a short period of time, approximately an hour in total. Nevertheless, we were able to attach the smartphone, configure the GNSS receiver to produce the appropriate data products and start data collection all within about 10 minutes. In total, only 20 minutes of GPS data were collected at this location, but the experience demonstrated how quickly the app-based solution can be installed.
The second and third field sites visited were near São José dos Campos on the campuses of the Instituto Nacional de Pesquisas Espaciais (INPE) and Universidade do Vale do Paraíba (UNIVAP). São José dos Campos is well to the south of the geomagnetic equator and in the Appleton anomaly region. We successfully collected observations from both sites, but were only able to conduct long-duration testing at the UNIVAP site (approximately 9.5 hours in total). This data is shown in FIGURE 6, in total electron content units (TECu = 1016 electrons per square meter) in the bottom plot (with the four-letter site name SAUN).
Figure 6. Total electron content in TECu across South America on Feb. 5, 2015, between 19:00 UTC and 19:15 UTC.
The other data in Figure 6 (site names MCL1, RJCG, ONRJ) were processed from GPS receivers operated by Instituto Brasileiro de Geografia e Estatística (IBGE). These observations show a progression of TEC values as a function of latitude and were collected on Feb. 5, 2015, a day of minor geomagnetic activity (the highest value reached of the Kp index, an indication of global geomagnetic activity, was 3.3) and of moderate solar flux (10.7-centimeter solar flux, an indicator of solar activity, was 142). The data collected at UNIVAP covers the period from 10:00 until 20:00 UTC. Note that for the earlier, more geophysically active period shown in Figure 6 between 0:00 and 5:00 UTC, the smartphone did not collect data due to resource limitations on available battery power.
Figure 7. Total electron content in TECu across South America on Feb. 5, 2015, between 19:00 UTC and 19:15 UTC.
FIGURE 7 illustrates the overall geophysical picture. It shows the locations of the data-collection sites overlaid onto vertical TEC estimates obtained separately from the aforementioned Brazilian GNSS receiver network and receivers owned and operated by the Red Argentina de Monitoreo Satelital Continuo (RAMSAC) continuously operating reference station network of the Instituto Geográfico Nacional de la República Argentina and the Low Latitude Ionospheric Sensor Network. This data was averaged over 15 minutes and binned in 1° by 1° bins. The southern Appleton anomaly region clearly appears as a red band that extends diagonally north of São José dos Campos parallel to the geomagnetic equator that dips in this region. Because this day is geomagnetically quiet, São José dos Campos lies in a region of smaller TEC south of the anomaly region. During more geomagnetically active conditions, it can lie directly under the anomaly.
Figure 8. Differential vertical total electron content in TECu reflecting traveling ionospheric disturbances on Feb. 5, 2015, at 19:15 UTC.
By contrast, FIGURE 8 shows data that is neither binned nor averaged over time. This alternate processing method using the same underlying data set reveals TIDs moving across the region. The low concentration of electrons in the region 40°–55° W longitude and 5°–10° S latitude and the high concentration of electrons in the region 40°–55° W longitude and 15°–20° S latitude suggest the shape of a TID. Relevant to the potentials of distributed Mahali sensor systems, better interpretations could be made in this scientific context with more data points.
Both Figures 7 and 8 clearly show the need for more receiver sites to fill in the gaps in data coverage. This is where the Mahali concept can make a real contribution, as it enables receiver deployment in areas with less developed infrastructure such as the Amazon area.
We have also recently undertaken a campaign in Alaska and have reported on that experience elsewhere (see Further Reading).
Conclusion
This article presents one app in the Mahali software ecosystem, designed to directly connect smartphones to GNSS receivers for scientific data collection. The initial Brazil field tests described here have provided a proof of concept that smartphones can be used as versatile relays of data to cloud storage environments. The results have demonstrated that the Mahali concept can make a new and fundamental contribution to observational science by enabling receiver deployment in areas with less developed infrastructure. Observations from these regions contain crucial geophysical information and are at the forefront of geospace scientific research.
The Mahali project is funded by a National Science Foundation Integrated NSF Support Promoting Interdisciplinary Research (INSPIRE) grant. We would also like to acknowledge our collaborators at Boston College, Virginia Tech, Johns Hopkins University, the University of New Brunswick and Colorado State University, as well as the support of UNAVCO for loans of dual-frequency GNSS receivers for use in this project. We also thank Intel for loans of mobile smartphones.
Travel to Brazil was kindly supported by the MIT International Science and Technology Initiatives program. This article is based on the paper “A Smartphone App for GNSS Ionospheric Data Collection: Initial Field Test Results” presented at ION GNSS+ 2015, the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation held in Tampa, Florida, Sept. 14–18, 2015.
Manufacturers
The GNSS receivers used for our tests in Brazil included a NovAtel GPStation-6 GNSS Ionospheric Scintillation and TEC Monitor (GISTM) receiver and earlier generation NovAtel GSV4004B GISTM receivers. We employed Samsung Galaxy S3 and Motorola Moto G smartphones.
ANDREW KENNEDY is a doctoral candidate in the Space, Telecommunications, Astronomy and Radiation Laboratory at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass.
RYAN KINGSBURY is a recent doctoral graduate from the Space, Telecommunications, Astronomy and Radiation Laboratory at MIT.
ANTHEA COSTER is an assistant director and principal research scientist at MIT Haystack Observatory, Westford, Mass., and a co-principal investigator (co-PI) of the Mahali project.
VICTOR PANKRATIUS is a research scientist at MIT Haystack Observatory where he leads the Astro- & Geo-Informatics Group. He also serves as the principal investigator of the Mahali project.
PHILIP ERIKSON is an assistant director and principal research scientist at MIT Haystack Observatory and a co-PI of the Mahali project.
PAULO ROBERTO FAGUNDES is professor at Universidade do Vale do Paraíba, São José dos Campos, Brazil.
EURICO R. de PAULA is a senior researcher in the Aeronomy Division of the Instituto Nacional de Pesquisas Espaciais, São José dos Campos, Brazil.
KERRI CAHOY is the Boeing Assistant Professor of Aeronautics and Astronautics at MIT.
JUHA VIERINEN is a research scientist at MIT Haystack Observatory.
FURTHER READING
Authors’ Conference Papers
“The Mahali Project: Deployment Experiences from a Field Campaign in Alaska” by A. Coster, V. Pankratius, T. Morin, W. Rogers, F. Lind, P. Erickson, D. Mascharka, D. Hampton and J. Semeter in Proceedings of ITM 2016, the 2016 International Technical Meeting of The Institute of Navigation, Monterey, Calif., Jan. 25–28, 2016, pp. 885–892.
“A Smartphone App for GNSS Ionospheric Data Collection: Initial Field Test Results” by A. Kennedy, R. Kingsbury, A. Coster, V. Pankratius, P.J. Erickson, P. Fagundes, E.R. de Paula, K. Cahoy and J. Vierinen in Proceedings of ION GNSS+ 2015, the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation, Tampa, Fla., Sept. 14–18, 2015, pp. 3745–3754.
“The Mahali Space Weather Project: Advancing GNSS Ionospheric Science” by A. Coster, V. Pankratius, F. Lind, P. Erickson and J. Semeter in Proceedings of ION GNSS+ 2014, the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation, Tampa, Fla., Sept. 8–12, 2014, pp. 1213–1221.
“Mobile Crowd Sensing in Space Weather Monitoring: The Mahali Project” by V. Pankratius, F. Lind, A. Coster, P. Erickson and J. Semeter in IEEE Communications Magazine, Vo. 52, No. 8, Aug. 2014, pp. 22–28, doi: 10.1109/MCOM.2014.6871665.
“Automated GPS Processing for Global Total Electron Content Data” by W. Rideout and A. Coster in GPS Solutions, Vol. 10, No. 3, July 2006, pp. 219–228, doi: 10.1007/s10291-006-0029-5.
“Medium-scale Traveling Ionospheric Disturbances Observed by GPS Receiver Network in Japan: A Short Review” by T. Tsugawa, N. Kotake, Y. Otsuka and A. Saito in GPS Solutions, Vol. 11, No. 2, March 2007, pp. 139–144, doi: 10.1007/s10291-006- 0045-5.
“Traveling Ionospheric Disturbances as a Diagnostic Tool for Thermospheric Dynamics” by K.C. Yeh in Journal of Geophysics, Vol. 77, No. 4, Feb. 1972, pp. 709–719, doi: 10.1029/JA077i004p00709.
“Detecting Ionospheric TEC Perturbations Caused by Natural Hazards Using a Global Network of GPS Receivers: The Tohoku Case Study by A. Komjathy, D.A. Galvan, P. Stephens, M.D. Butala, V. Akopian, B. Wilson, O. Verkhoglyadova, A.J. Mannucci and M. Hickey in Earth, Planets and Space, Vol. 64, No. 12, Dec. 2012, pp. 1287–1294.
PlanetiQ has signed a contract with Antrix Corporation Limited, the commercial arm of the Indian Space Research Organization (ISRO), for the launch of PlanetiQ’s first two weather satellites on a Polar Satellite Launch Vehicle (PSLV) during the fourth quarter of 2016.
Ten more satellites are planned for launch in 2017 to complete an initial set of 12 satellites that will dramatically improve global weather forecasting, climate monitoring and space weather prediction, and enable advanced analytics for numerous industries worldwide.
The ISRO’s PSLV is among the world’s most reliable launch vehicles with 30 consecutive successful flights. It has launched 51 satellites for international customers from 20 countries, in addition to 33 Indian national satellites.
“The stellar track record of the PSLV combined with our seven-year satellite design life provides the reliability and data continuity not just desired, but required by the operational weather forecast community,” said Chris McCormick, Chairman and CEO of PlanetiQ. “Within days after launch, we will validate and start delivering high-quality data and services to our customers.”
24 hours of data from 12 PlanetiQ satellites = ~34,000 occultations/day.
Each of PlanetiQ’s 10-kilogram microsatellites will fly PlanetiQ’s Pyxis-RO sensor, an advanced satellite weather sensor in a small package that can penetrate through clouds and storms down to the Earth’s surface. Pyxis-RO uses a technique called radio occultation to track the bending of GPS and other signals as they travel through Earth’s atmosphere, and then converts the bending angle into high-precision measurements of global temperature, pressure and water vapor in the atmosphere, and electron density in the ionosphere.
Pyxis-RO quadruples the data collection capability of radio occultation sensors on orbit today by tracking signals from all four major satellite navigation systems — GPS, Galileo, Beidou and GLONASS. With 12 satellites on orbit, PlanetiQ will collect 34,000 occultations per day, evenly distributed around the globe with high-density sampling over both land and water.
Each occultation is a vertical profile of atmospheric data with high vertical resolution, comprised of measurements less than every 200 meters from the Earth’s surface up into the ionosphere. The data is similar to that collected by weather balloons, but more accurate, more frequent and on a global scale.
“The world today lacks sufficient data to feed into weather models, especially the detailed vertical data that is critical to storm prediction. That’s why we see inaccurate or ambiguous forecasts for storms like Hurricane Joaquin, which can put numerous lives at risk and cost businesses millions of dollars due to inadequate preparation or risk management measures,” McCormick said. “Capturing the detailed vertical structure of the atmosphere from pole to pole, especially over the currently under-sampled oceans, is the missing link to improving forecasts of high-impact weather.”
FIGURE 2. TEC map over São Paulo state as forecast by the CALIBRA model on Sept. 26, 2012, at 2:00 UT. The range of the TEC in the image is from 0 to 90 TEC units (blue to red). The red line is the geomagnetic equator.
Countering Ionospheric Disturbances Affecting GNSS in Brazil
By Marcio Aquino
After 27 months of intense research, the CALIBRA project ended successfully in February 2015, with the project team devising solutions to tackle the effects of perturbations typical of the Brazilian ionosphere on high-accuracy GNSS positioning. CALIBRA was funded by the European Union and the European GNSS Agency.
Kicked off in 2012, CALIBRA first confirmed the vulnerability of GNSS high-accuracy techniques to ionospheric disturbances through a thorough user performance review, where degradation in GNSS Precise Point Positioning (PPP) and real-time kinematic (RTK) positioning was seen to correlate with the occurrence of ionospheric scintillation and high Total Electron Content (TEC) variability. This is especially so in Brazil, because of its geographical location extending across the magnetic equator in one of the most troublesome ionospheric regions of the Earth, qualifying the country as a test-bed for worst-case scenarios.
The team established a suitable metric to characterize these disturbances, which was used in developing the new models and algorithms to counter their effects. The short-term empirical CALIBRA Forecasting Model (CFM) for TEC and scintillation was developed and tested.
To counter scintillation, a number of approaches were proposed and their benefits demonstrated. Building on the project’s success, CALIBRA partner INGV (Istituto Nazionale di Geofisica e Vulcanologia) filed a patent for the CFM and a new spin-off company — SpacEarth Technology — was set up. SpacEarth aims to secure the software’s commercialization for potential applications and services, while also improving and adapting it to evolving market needs.
Another outcome of commercial interest is that project partner Septentrio developed several rover-level mitigation approaches, notably a new model for ionospheric delay estimation.
Monitoring Network. To support the research and operational activities of the project, a dedicated network of ionospheric scintillation monitor receivers (ISMRs) was deployed, forming the CIGALA-CALIBRA network of 12 monitoring stations equipped with PolaRxS receivers. A web interface for data analysis — the ISMR Query Tool — was developed by project partner UNESP (São Paulo State University) and is available for public use, collecting and treating more than 10 million observations of GPS, GLONASS, Galileo, BeiDou and other augmentation systems on a daily basis. Data visualization and data mining techniques support users in data analysis and knowledge extraction.
Finally, two important field trials aiming to validate the new algorithms were carried out in Brazil, involving actual precision agriculture and offshore operations. For the precision agriculture trial, the Brazilian company Agro Pastoril Campanelli provided expert operational environment and support.
The tractor used in the precision agriculture trial at Agro Pastoril Campanelli’s premises.
For the offshore trial, the project counted on the collaboration of the DOF Brasil Group representing Norskan Offshore, a provider of high-end offshore services to the Brazilian oil and gas industry. Detailed results of both trials are in the project’s final report, which can be accessed through the GSA.
The Geograph vessel is operated by DOF Brasil.Setting up the receiver antenna for the offshore trial on board the Geograph vessel.
To provide a glimpse of the performance of the CALIBRA algorithms during the offshore trial, in FIGURE 1 we selected a period when strong scintillation conditions were encountered. In the top plot, two height component time series for kinematic PPP processing are shown, respectively, for the case where no mitigation is applied (black time series) and the case where the CALIBRA algorithm is applied (red time series).
FIGURE 1. Performance of CALIBRA algorithms in the offshore trial.
The bottom plot shows the level of amplitude scintillation (S4 index) affecting the GPS satellites over a 10-degree elevation angle.
The improvement obtained with the CALIBRA solution can be seen in particular during the PPP convergence period (18:00 to 18:30 UT) and during the period of strong scintillation (22:30 to 23:30 UT). As there was no accurate ground truth available, the RMS values with respect to the mean height, taken from the quiet period (between 19:00 and 22:00 UTC), along with the percentage of improvement when applying the CALIBRA mitigation approach are summarized in TABLE 1.
TABLE 1. RMS values with respect to mean height, 19:00–22:00 UTC.
Despite all the successful work carried out by CALIBRA, the team notes that research must be continued to accomplish further improvement in models and algorithms to finally develop processes for real-time operation. The challenge would be to counter these ionospheric threats in the scope of an operational service aimed to provide robust high-accuracy positioning to support user applications.
Furthermore, there were strong indications that the addition of Galileo will assist in mitigating the problems addressed in the project when more signals are available in space.
Marcio Aquino is a Principal Research Fellow at the Nottingham Geospatial Institute of Nottingham University and leader of CALIBRA.
The April 25 magnitude 7.8 earthquake in Nepal created waves of energy that penetrated into Earth’s upper atmosphere in the vicinity of Nepal, disturbing the distribution of electrons in the ionosphere. These disturbances were monitored using GPS signals received by a science-quality GPS receiver in Tibet, a neighboring region to Nepal.
The data show that after the initial earthquake rupture (indicated by the vertical black line on the graphic), it took about 21 minutes for the earthquake-generated ionospheric disturbance to reach a GPS station (LHAZ) about 400 miles (640 kilometers) away from the epicenter in Lhasa, Tibet, China.
Image Credit: NASA/JPL/Ionosphere Natural Hazards Team
The disturbance measurements, known as vertical total electron content (VTEC) (depicted in blue in the upper panel), have been filtered using processing software developed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif., to show wave-like disturbances (circled in red) in the distribution of electrons in the ionosphere. The waves have periods of between two and eight minutes in length. The disturbance measurements following the earthquake rupture are circled in black in the lower panel. The colors represent the relative strengths of the earthquake-induced ionospheric disturbances as captured by the GPS signals, with red being high and blue being low.
Attila Komjathy, a principal investigator of the Ionospheric and Atmospheric Remote Sensing group at JPL and adjunct professor at the University of New Brunswick, is leading this effort. Komjathy is also a GPS World annual award winner and named a Fellow of the Institute of Navigation in January.
The LHAZ GPS station is hosted at the Tibet Autonomous Regional Bureau of Surveying and Mapping Institute. The site collects both GPS and GLONASS (the Russian global navigation satellite system) data at a rate of 1 Hertz and is part of the International GPS Service (IGS).
Scientists study ionosphere-based measurements caused by natural hazards such as earthquakes, volcanic eruptions and tsunamis to better understand wave propagation in the upper atmosphere.The ionosphere is a region of Earth’s upper atmosphere located from about 37 miles (60 kilometers) to 621 miles (1,000 kilometers) above Earth’s surface.
The disturbances caused by earthquakes help scientists develop new first-principle-based wave propagation models. These models may become part of future early warning systems for tsunamis and other difficult-to-detect natural hazards.
Y. Jade Morton received the Johannes Kepler Award for advances in scientific and navigation receiver technology, automated data collection, robust carrier phase tracking, remote sensing, and profound impact as an educator and author.
