GPS World

Tag: space navigation

  • Research roundup: Advancing space and lunar navigation

    Research roundup: Advancing space and lunar navigation

    SpacePNT and European Engineering and Consultancy (EECL) delivered the final presentation of the European Space Agency (ESA)-funded project, “Earth Moon GNSS Spaceborne Receiver for In-Orbit Demonstration.” This project aims to develop the NaviMoon GNSS receiver for lunar applications. (Photo: SpacePNT)
    SpacePNT and European Engineering and Consultancy (EECL) delivered the final presentation of the European Space Agency (ESA)-funded project, “Earth Moon GNSS Spaceborne Receiver for In-Orbit Demonstration.” This project aims to develop the NaviMoon GNSS receiver for lunar applications. (Photo: SpacePNT)

    GNSS researchers presented hundreds of papers at the 2024 Institute of Navigation (ION) GNSS+ conference, which took place Sept. 16-20 in Baltimore. The following papers focused on lunar and space applications. The papers are available here.

    Clock and Orbit Determination for LEO Satellites

    More than 50 years after the Apollo program, there is a growing interest in establishing a sustainable human presence on the moon, with various missions being planned in different lunar orbit regimes to support lunar exploration. To address the challenges of navigation in the lunar environment, researchers have proposed a technique leveraging time-differenced carrier-phase (TDCP) measurements from GPS satellites, which offer millimeter-level accuracy when integer ambiguities are correctly fixed.

    The proposed framework utilizes an extended Kalman filter that combines intermittently available terrestrial GPS TDCP values with gravitational accelerations predicted by an orbital filter. To handle the unique challenges of the lunar environment, such as weak gravity and strong third-body perturbations, the researchers implement an adaptive state noise compensation algorithm and introduce an augmented state vector to address time correlations across TDCP measurements. Through Monte Carlo simulations of lunar satellites in various orbits, the technique demonstrates improved positioning and onboard timing accuracy compared to pseudorange-only navigation solutions.

    Keidai Iiyama, Sriramya Bhamidipati and Grace Gao, “Precise Positioning and Timekeeping in a Lunar Orbit via Terrestrial GPS Time-Differenced Carrier-Phase Measurements.”

    Satellite Ephemeris Parameterization for Lunar Navigation

    This paper explores the development of satellite ephemeris parameterization methods for lunar navigation systems. As space agencies plan to establish satellite networks around the moon for communication and positioning, navigation and timing (PNT) services, the authors investigate optimal techniques for efficiently and accurately broadcasting satellite ephemeris data to lunar users. They propose a framework that directly approximates satellite position and velocity in the inertial frame, using signal-in-space-error requirements as constraints to guide the search for the best ephemeris parameter set.

    The study evaluates different methods based on ephemeris prediction precision, fit interval and message size. It demonstrates the framework’s ability to approximate satellite ephemeris for both low lunar orbits and elliptical lunar frozen orbits while meeting signal-in-space-error requirements. The research considers polynomial and Chebyshev basis types for surrogate models and evaluates performance based on precision and orbital coverage. By quantifying the broadcast message’s fit interval and size, the authors aim to guide the selection of optimal parameterization methodologies for lunar navigation systems.

    Marta Cortinovis, Keidai Iiyama and Grace Gao, “Open Access Satellite Ephemeris Parameterization Methods to Support Lunar Positioning, Navigation, and Timing Services.”

    Improving Navigation Accuracy in GEO

    The authors introduce a new approach to improving the accuracy of satellite position determination in geostationary equatorial orbit (GEO). They propose integrating a regional navigation satellite system (RNSS) with GNSS. Specifically, they suggest using RNSS signals, such as those from the Quasi-Zenith Satellite System (QZSS), to complement the signals provided by GNSS for GEO satellites.

    The research addresses the challenges faced by GEO satellites in using GNSS signals, including poor dilution of precision (DOP) and significant radial errors due to limited observability. By incorporating RNSS signals, the researchers aim to improve the diversity of signal directions and enhance navigation precision. The paper demonstrates the feasibility of receiving QZSS signals across a substantial range in GEO through link budget analyses. Two comprehensive simulations were conducted: a point solution and an extended Kalman filter-based orbit determination. The results confirm the anticipated improvement in navigation precision indicated by the DOP analysis.

    While RNSS signals can be received from any longitude in GEO, enhanced navigation precision depends on the distance between the satellite and the RNSS. The authors suggest that this concept can be adapted to various longitudes within GEO by selecting appropriate RNSS options and promoting stable, high-precision navigation.

    Yu Nakajima and Toru Yamamoto, “Enhancing Navigation Accuracy in a Geostationary Orbit by Utilizing a Regional Navigation Satellite System.”

    Integrating Orbit and Attitude Precision for CubeSat Positioning

    This research paper addresses ways to enhance CubeSat capabilities for demanding missions, particularly in low Earth orbiting positioning, navigation and timing (LEO-PNT) systems. The authors propose an array-aided combined precise orbit and attitude determination model that offers an optimal solution to improve orbital accuracy and provide reliable attitude information. By utilizing multi- and affine-constrained models for precise attitude determination and reconstructing highly precise observations for an antenna array, the method addresses the challenges of higher orbital accuracy and reliable attitude information required for advanced applications.

    The authors recorded significant improvements in orbital accuracy and attitude determination. Validation results show that reconstructed observations outperform original ones, leading to improved orbital components with a three-dimensional root mean square (RMS) of 4.1 cm. Additionally, observation residuals are smoother, with an RMS of 6 mm, half of that obtained via a single antenna. The results show a promising solution for enhancing CubeSat capabilities, particularly for applications requiring high-precision orbit and attitude information.

    Amir Allahvirdi-Zadeh and Ahmed El-Mowafy, “Array-Aided Precise Orbit and Attitude Determination of CubeSats using GNSS.”

  • NASA wants to use GPS at the Moon for Artemis missions

    NASA wants to use GPS at the Moon for Artemis missions

    News from NASA’s Goddard Space Flight Center

    GPS could be used to pilot in and around lunar orbit during future Artemis missions.

    A team at NASA is developing a special receiver that would be able to pick up location signals provided by the 24 to 32 operational GPS satellites. Such a capability could soon also provide navigational solutions to astronauts and ground controllers operating the Orion spacecraft, the Gateway in orbit around the Moon and lunar surface missions.

    The advanced GPS receiver would be paired with precise mapping data to help astronauts track their locations in space between the Earth and the Moon, or on the lunar surface.

    Artist’s concept of NASA’s Magnetospheric Multiscale mission consists of four identically equipped observatories that rely on Navigator GPS to maintain an exacting orbit that is at its highest point nearly half-way to the Moon. (Image: NASA)
    Artist’s concept of NASA’s Magnetospheric Multiscale mission consists of four identically equipped observatories that rely on Navigator GPS to maintain an exacting orbit that is at its highest point nearly halfway to the Moon. (Image: NASA)

    Navigation services near the Moon have historically been provided by NASA’s communications networks. The GPS network could help ease the load on NASA’s networks, freeing up that bandwidth for other data transmission.

    “What we’re trying to do is use existing infrastructure for navigational purposes, instead of building new infrastructure around the Moon,” said engineer and principal investigator Munther Hassouneh at Goddard Space Flight Center in Greenbelt, Maryland.

    NASA has been working to extend GPS-based navigation to high altitudes, above the orbit of the GPS satellites, for more than a decade. The agency now believes its use at the Moon, which is about 250,000 miles from Earth, can be done.

    “We’re using infrastructure that was built for surface navigation on Earth for applications beyond Earth,” said Jason Mitchell, chief technologist for Goddard’s Mission Engineering and Systems Analysis Division. “Its use for higher altitude navigation has now been firmly established with the success of missions like Magnetospheric Multiscale mission (MMS) and the Geostationary Operational Environmental Satellites (GOES). In fact, with MMS, we’re already nearly halfway to the Moon.”

    Navigator GPS

    The team developing a GPS receiver for use in and around lunar orbit (from left): Jason Mitchell, Luke Winternitz, Luke Thomas, Munther Hassouneh and Sam Price. (Photo: NASA/T. Mickal)
    The team developing a GPS receiver for use in and around lunar orbit (from left): Jason Mitchell, Luke Winternitz, Luke Thomas, Munther Hassouneh and Sam Price. (Photo: NASA/T. Mickal)

    The lunar GPS receiver is based on the Goddard-developed Navigator GPS, which engineers began developing in the early 2000s specifically for NASA’s MMS mission, the first-ever mission to study how the Sun’s and Earth’s magnetic fields connect and disconnect. The goal was to build a spacecraft-based receiver and associated algorithms that could quickly acquire and track GPS radio waves even in weak-signal areas. Navigator is now considered an enabling technology for MMS.

    Without Navigator GPS, the four identically equipped MMS spacecraft couldn’t fly in their tight formation in an orbit that reaches as far as 115,000 miles from Earth’s center — far above the GPS constellation and about halfway to the Moon.

    “NASA has been pushing high-altitude GPS technology for years,” said Luke Winternitz, the MMS Navigator receiver system architect. “GPS around the Moon is the next frontier.”Extending the use of GPS to the Moon will require some enhancements over MMS’s onboard GPS system, including a high-gain antenna, an enhanced clock and updated electronics.

    “Goddard’s IRAD (Internal Research and Development) program has positioned us to solve some of the problems associated with using GPS in and around the Moon,” Mitchell said, adding that a smaller, more robust GPS receiver could also support the navigational needs of SmallSats, including a new SmallSat platform Goddard engineers are now developing.

    Building on NavCube

    NavCube, which will be tested aboard the International Space Station later this year, is being used as a baseline for a lunar GPS receiver. (Photo: NASA/W. Hrybyk)
    NavCube, which will be tested aboard the International Space Station later this year, is being used as a baseline for a lunar GPS receiver. (Photo: NASA/W. Hrybyk)

    The team’s current lunar GPS receiver concept is based on NavCube, a new capability developed from the merger of MMS’s Navigator GPS and SpaceCube, a reconfigurable, very fast flight computer platform. The more powerful NavCube, developed with IRAD support, was recently launched to the International Space Station where it is expected to employ its enhanced ability to process GPS signals as part of a demonstration of X-ray communications in space.

    The GPS processing power of NavCube combined with a receiver for lunar distances should provide the capabilities needed to use GPS at the Moon. Earlier this year, the team simulated the performance of the lunar GPS receiver and found promising results. By the end of this year, the team plans to complete the lunar NavCube hardware prototype and explore options for a flight demonstration.

    “NASA and our partners are returning to the Moon for good,” Mitchell said. “NASA will need navigation capabilities such as this for a sustainable presence at the Moon, and we’re developing enabling technologies to make it happen.”

  • NASA to hold workshop on autonomous navigation, GNSS, PNT

    The National Aeronautics and Space Administration (NASA) published a notice Jan. 26 in the Federal Register on a planned space navigation workshop.

    The Workshop on Emerging Technologies for Autonomous Space Navigation is sponsored by NASA Space Communication and Navigations (SCaN) Program. The Feb. 17 workshop is intended to inform industry on evolving positioning, navigation and timing (PNT) technologies and techniques being developed to enhance the operational efficiency and flexibility of future missions.

    The workshop will include optional one-on-one discussions with industry participants on a space-available basis on Friday, Feb. 17. NASA is soliciting information from all interested U.S. private sector enterprises only.

    Navigation topics to be discussed during the workshop include:

    • Emerging GNSS applications, including the development and use of GNSS in high-altitude applications in the Space Service Volume (SSV), protecting and enhancing the GPS SSV, developing a multi-GNSS SSV, NASA’s current and future missions employing GNSS in the SSV, and GNSS receiver developments within NASA.
    • Emerging navigation technologies, including PNT capabilities envisioned for the Next Generation Broadcast Service (NGBS), innovative timing system developments and techniques such as the Deep Space Atomic Clock (DSAC), optical navigation capabilities and techniques that support rendezvous, landing on objects (near Earth or solar system objects) or docking to vehicles, and navigation and PNT capabilities supporting proximity operations, satellite servicing and formation flying.
    • Other advanced topics to be addressed include the use of optimetrics from laser communications systems to support precise PNT solutions, on-board navigation software and filters, such as the Goddard Enhanced Onboard Navigation System (GEONS), and x-ray navigation capabilities and techniques.

    Registration

    The workshop will be held at NASA Headquarters Auditorium (west lobby) 300 E Street SW., Washington, D.C. 20546.

    U.S. participants will register for the navigation workshop at the door on Feb. 16. To RSVP for the follow-on one-on-one meetings scheduled for Feb. 17, RSVP to James J. Miller by Feb. 8 at [email protected] or 202-358-4417.

    Reservations must be received no later than 5:00 p.m. EST on Wednesday, Feb. 8. A confirmation email will be sent to acknowledge your requested participation. Companies will be notified on or before Friday, Feb. 10 of their assigned one-on-one meeting time.

    Agenda

    The agenda for the workshop and industry meetings is as follows:

    Workshop, Thursday, Feb. 16

    8:30 a.m. to 9:30 a.m., Networking Opportunity

    9:30 a.m. to 12:30 p.m., Introductions & Emerging GNSS Applications

    12:30 p.m. to 1:30 p.m., Lunch Break

    1:30 p.m. to 3:30 p.m., Next-Generation Developing Technologies

    3:30 p.m. to 5:30 p.m., Game-Changing Initiatives

    5:30 p.m. to 6:00 p.m., Wrap-Up

    6:00 p.m. to 8:00 p.m., Networking Opportunity

    One-on-One Industry Meetings with NASA, Feb. 17

    9:00 a.m. to 5:00 p.m., 45-minute information-exchange/discussion

    Attendance limitations: The Navigation Workshop and One-on-One Meeting attendees is strictly limited to four (4) persons per company.

    One-on-One Meeting Description

    To facilitate interactive communication with industry, NASA SCaN representatives will be available for one-on-on emeetings to exchange ideas on areas of synergy and potential collaboration. NASA will hold one-on-one meetings with industry on Friday, Feb. 17, 2016, from 9 a.m. to 5 p.m. EST, to discuss space navigation technologies and techniques as related to Nav Workshop presentations. The meetings will be held with interested parties at scheduled times provided in response to the RSVP on a space available basis. NASA will attempt to prioritize non-local companies with One-on-One meeting times.

    The one-on-one meetings are intended to be private question-and-answer and information-gathering sessions based on industry developments that align with NASA investments for enhanced autonomous space navigation capabilities. Industry presentation packages are acceptable and will be held in accordance with any proprietary or business confidential markings as annotated on the chart package. Meetings will not exceed 45 minutes in length. One appointment per hour will be scheduled. Additional separate meetings can be scheduled later if demand exceeds capacity.

     

  • Galileo: Are We There Yet?

    Galileo: Are We There Yet?

    Europe’s ninth and tenth Galileo satellites being fueled by technicians in protective SCAPE suits within the Guiana Space Centre’s 3SB preparation building on 24 August. This left them ready to be attached to their launcher upper stage in preparation for launch. (Photo:ESA)
    Europe’s ninth and tenth Galileo satellites being fueled by technicians in protective SCAPE suits within the Guiana Space Centre’s 3SB preparation building on Aug. 24. (Photo:ESA)

    It has been a good late summer for the European Galileo programme. The latest launch on the night of 10 and 11 September has got the number of orbiting satellites in the EU’s GNSS constellation into double figures at last, and one-third of the way towards the ultimate target of 30.

    The European Space Agency’s (ESA) press releases around the launch were positively euphoric, and there were many pictures of smiling ESA launch teams. And so there should be. The two new satellites (the fifth and sixth fully operational capability (FOC) versions named Alba and Oriana) will now inch their way towards their operational orbits and will soon be joined by two more satellites to be launched in December.

    However, as we already know, one of the in-orbit validation (IOV) satellites (Sif) is not very well, having suffered a power failure in late May, and the first two FOC satellites (Doresa and Milena) ended up in the wrong orbit. At the considerable expense of a significant part of their fuel payloads, these two craft are now established in a more useful orbit and are the current subject of testing to determine the exact contribution they can make to the Galileo services.

    The Commission and ESA are convinced that the outcome will be positive, with Doresa and Milena able to contribute to the network — or at least not degrade the network’s navigation performance. A final decision on if and/or how these two satellites integrate into the system will be made sometime next year.

    In any case, they will be used for the provision of Galileo’s Search and Rescue services. And they are also to be made available for scientific research. One possible science area that has been discussed is to ‘repurpose’ the satellites to measure the slow down of time due to the Earth’s gravitational field as predicted by Einstein’s theory of relativity.

    However, more worryingly, there are rumours of various glitches and performance issues with other in-orbit members of the constellation. Hopefully, they are just rumours; only time will tell.

    Position Paper

    Not surprisingly, those wanting to use the system are getting a tad frustrated. On Sept. 1, Galileo Services, a non-profit organisation involving 180 members including most of the active players in the EU GNSS industry, published a position paper entitled “Europe Must Succeed in the Global Navigation Market Race.”

    The organisation’s aim is to foster an end-to-end vision of the Galileo system that can fully respond to user and market needs. The paper looks at the options to strengthen the competitiveness of the European GNSS downstream sector in the global market and calls for better coordination between the public and private sectors to develop new technologies, applications, services and industries in Europe as a key factor for success.

    In particular, the paper stresses the necessity to urgently establish a European strategic plan to enhance Europe’s GNSS downstream industry’s competitiveness and to foster the uptake of the European GNSS, Galileo and the European Geostationary Navigation Overlay Service (EGNOS), with the aim to corner 33 percent of the global GNSS downstream market for Europe by 2025.

    Galileo Services argues that unless an effective and long-term strategy is in place during the Galileo early services exploitation phase — from 2016, the current official start date for services — the window of opportunity for European industry will be closed. Europe’s goal of achieving GNSS autonomy is also at risk. The paper warns that Galileo is just one of three new GNSS solutions, along with the Russian GLONASS and Chinese BeiDou, that are complementing the U.S.’s GPS — and most applications do not require four GNSS constellations.

    The target of European autonomy will be achieved if and only if Galileo is widely used with equipment designed and manufactured in Europe, as well as applications and services developed in Europe, concludes the paper.

    More R&D Support

    Part of the strategy should be enhanced support for EU GNSS technologies and applications. The European GNSS Agency (GSA) has just launched a new research support channel for GNSS chipset and receiver technologies in Europe.

    The Fundamental Elements programme has a projected budget of EUR 100 million over the period 2015 to 2020 and is part, says the GSA, of an overall strategy of market uptake initiatives in accordance with EU regulations. “For the first time, EU regulation provides a financing tool for the market uptake of European GNSS chipsets and receivers,” said GSA Executive Director Carlo des Dorides in launching the new programme.

    The Fundamental Elements programme complements the EU’s current Horizon 2020 research programme that focuses on adoption of Galileo and EGNOS via content and application development.

    Photo: Horizon 2020 research programme

    Two types of financing will be available via the Fundamental Elements programme: grants and procurement. Grants will be provided to cover up to 70 percent of funding requirements for a project, and intellectual property rights will stay with the beneficiary under the condition that the developed product is actively commercialised.

    Procurement (at 100 percent funding) will be used only in cases where keeping intellectual property rights allow for the better fulfilment of the programme’s overall objectives. For example, by licensing it to different potential manufacturers rather than creating a monopoly supplier.

    Meanwhile, EGNOS Continues

    Of course, one EU GNSS, EGNOS, is operational. The GSA proudly announced that after extensive testing, the latest space segment — the SES-5 GEO satellite — is now fully functional. This will ensure the long-term service of EGNOS until at least 2026 and enable a range of performance improvements, including greater stability during periods of high ionospheric activity.

    The SES-5 is a first step in the complete renewal of the EGNOS Space Segment, including the transition to dual-frequency, multi-constellation services. The renewal will be completed by the introduction of the ASTRA-5B signals and the procurement of a new EGNOS payload, both planned for 2016.

    In parallel, the GSA and ESA have met formally to launch activities to develop the system further following the signing of a working agreement for EGNOS in July. Under the agreement, ESA will be responsible for the development and procurement of future EGNOS evolutions, such as the forthcoming release (V2.4.2), and a new generation of the EGNOS system (V3).

    SES-5 GEO satellite (artist’s depiction, ILS/Loral).

    JOHAN Sports Tracker

    One of the annual gatherings of the whole European GNSS value chain will take place in October in Berlin with the Satellite Masters conference and awards ceremony. We can be sure that comforting words will be spoken by persons from the Commission, the GSA and ESA about their future plans and present progress. But the real buzz of this event is from the showcase of new ideas and applications for Galileo and EGNOS from pretty much every corner of Europe and beyond.

    Despite the uncertainties expressed by some big industrial players, and slow progress in establishing the actual infrastructure, there is still an entrepreneurial enthusiasm from the ‘small guys’ to get involved in this space-based business.

    I have attended these events for a few years now. One of the most enthusiastic winners of recent years is JOHAN, a sports application named after renowned Dutch soccer player and now sport commentator Johan Cruyff.

    The application is the brainchild of Dutch graduate Jelle Reichert, whom I first met when he won the 2013 European Satellite Navigation Competition with this innovative EGNOS-enhanced tracking idea. “We are now operational with our first four clients! And in a final testing phase we are making the system ready for a commercial launch at the beginning of 2016,” he tells me. “We also just have an investor on board, which allows us to hire personnel and take the final steps to become really commercially ready.”

    In just 18 months, Jelle’s idea has been brought into life with support from GSA and ESA. The JOHAN sports tracker and application helps improve teams by monitoring on-field performance. The system’s small, lightweight trackers, or pebbles, use GNSS technology such as EGNOS to ensure reliability and precision.

    The trackers are small and light so they can fit into training vests worn by players across a variety of field sports, though early adopters have all been football teams so far. The trackers measure location, speed, distance, acceleration and orientation statistics, which are then visualized in an online data platform for coaches and players.  This allows coaches to monitor workload and performance, and get tactical information and event analysis and ensure players’ strengths are used to the whole team’s advantage. Players can spot weaknesses and improve their individual game over time.

    “You can see who is training too hard and who has a higher chance of injury, as well as who is strong in which performance aspects, such as endurance, sprint, agility or recovery,” explains Jelle.

    I look forward to hearing about lots more grassroots GNSS innovation in Berlin.

    And Finally … An Out-of-This-World App?

    Take me to the moon! And why not, indeed? It appears that Galileo could be a vital part of an interplanetary navigation system. Or at least it could help (with GPS) spacecraft to routinely navigate to the moon.

    A paper in Acta Astronautica highlights the strict requirements in terms of performance, flexibility and cost for all the spacecraft subsystems required to navigate to the moon. GNSS could introduce an easier way to provide an autonomous orbit determination system using an on-board GNSS receiver. While GNSS receivers have already been used successfully to pilot craft in Low Earth Orbit (LEO), their use for very High Earth Orbit (HEO) up to and including the Moon is an active research area.

    The study from researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) made use of the Spirent GSS8000 multi-GNSS constellation simulator, which supports simultaneously the GPS and Galileo systems with L1, L5, E1 and E5 frequency bands. It showed that GNSS signals can be tracked up to the Moon’s surface, but would need new, more sensitive GNSS receiver technology. The paper describes a possible navigation solution that uses a double constellation GPS-Galileo receiver aided by an on-board orbital filter system to improve the accuracy of the navigation solution and achieve the required sensitivity. Without the filter, position error below 700 metres is possible, but the orbital filter increases the position accuracy to within about 100 metres.

    Vincenzo Capuano from the EPFL team tells me that a further paper on the use of an GPS L1 C/A based orbital filter for Moon transfer orbits will be published soon, which also shows an achievable accuracy of a few hundred meters. So who needs expensive tracking stations for a flight to the moon?

    But the work also has a very practical down-to-Earth application. The EPFL team is developing more sensitive GNSS receivers to pick up these weak signals, and the new receivers could find applications on Earth where current receivers often struggle to get a location, such as inside buildings or in built-up areas, where signals are weak.

    A bientȏt, as they say in these parts.

  • Receivers Reach New Heights

    Receivers Reach New Heights

    The four MMS spacecraft host the highest ever operational GPS receivers in space. (artist’s rendition, credit: NASA)
    The four MMS spacecraft host the highest ever operational GPS receivers in space. (artist’s rendition, credit: NASA)

    Editor’s Note: See additional coverage of the MMS mission here.

    September Marks Start of Magnetosphere Mission, but Navigators Already Perform

    A NASA mission to explore magnetic reconnection also made GPS history this spring. The Magnetospheric Multiscale (MMS) mission, led by NASA’s Goddard Space Flight Center in Greenbelt, Md., is flying four identically equipped spacecraft in a tight formation to take measurements 100 times faster than any previous space mission.

    Each of the four spinning MMS spacecraft — roughly the size of a ballpark once eight booms deploy — is equipped with 25 sensors and other components provided by more than 40 partner institutions in the U.S., Europe and Japan. One key component is a GPS receiver dubbed Navigator.

    Magnetic reconnection is a fundamental, yet poorly understood process.While reconnection occurs throughout the universe when magnetic field lines within plasma connect and disconnect, it can impact our technological society, since it drives virtually all space weather events that can disrupt low-Earth-orbiting spacecraft and lead to GPS, communications and power blackouts on Earth.

    During the mission’s first phase, which begins in September, the spacecraft will travel through reconnection sites on the sun-side of Earth, where the orbit extends out toward the sun to around 47,500 miles. One year later, ground controllers will move the spacecraft to Earth’s night-side or magnetotail where the magnetic fields also reconnect — an orbit that extends away from Earth to almost 99,000 miles, nearly halfway to the moon.

    However, science operations can’t begin before the four move into a highly elliptical orbit and assume their pyramid-shape formation that places the spinning spacecraft just 6.2 miles apart. It required a breakthrough to accomplish such an exacting formation, and the Goddard-developed Navigator GPS provided the solution.

    Begun in the early 2000s as an enabling technology for MMS-type missions, the Navigator receiver and associated algorithms can quickly acquire and track GPS radiowaves even in weak-signal areas well above GPS’s 30-plus-satellite constellation positioned about 12,550 miles above Earth. In addition to continuously tracking weak signals, the Navigator also must operate as the spacecraft spin at three revolutions per minute. As a result, each MMS satellite is equipped with two Navigator receivers (primary and redundant), with four antennas placed around the perimeter of each, assuring continuous contact with the tracked GPS satellites

    “Spinning adds a whole new dimension to trying to figure out where you are,” said Ken McCaughey, MMS GPS Navigator Product Development Lead at Goddard. “As the spacecraft rotates we have an algorithm running that allows us to hand off from one antenna to the next without losing the signal.”

    Robust Receivers. To the satisfaction of the technology’s architect, Goddard technologist Luke Winternitz, the receivers have proven very robust. Shortly after the GPS receivers were powered on after the launch, Navigator became, at more than 43,500 miles above Earth’s surface, the highest ever operational GPS receiver in space. “We’re tracking up to 12 GPS satellites at maximum altitude and track on average about nine,” Winternitz said. “We’re really excited about their performance so far.”

    Even if the receiver were to lose all GPS signals for part of the orbit, Navigator is specifically designed to handle such dropouts. By gathering as many observations as possible, integrated software called GEONS — Goddard Enhanced Onboard Navigation System — can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.

    The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)
    The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)

    This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 95,000 miles from Earth.

    “It’s going to be very interesting to see how far out MMS can still receive signals,” said Mission Deputy Project Manager Brent Robertson. “But Navigator has already far exceeded expectations.”

    Almost all activities associated with operating the mission depend on where the satellites will be positioned a few days hence. That includes everything from determining the best time to downlink telemetry and scientific data to calculating when ground controllers would command the firing of the satellites’ onboard thrusters, which move and help maintain their orbital formation — an exercise that will happen at least once every couple weeks.

    “I think there’s a good chance we’ll end up being able to use GPS and save us some of the expense of using ground observations,” Robertson said.

    While Navigator technology and GPS receivers were previously flown for testing and to help navigate a low-earth-orbit mission, this is the first time that the complete Navigator package has been used to actively navigate a high-altitude mission. Now that the team knows it works so well, Navigator can be used for other missions that travel in similar high orbits.

    The four MMS observatories are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville, Fla. The MMS mission launched March 12, 2015. (Credit: Ben Smegelsky/NASA)
    The four MMS observatories are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville, Fla. The MMS mission launched March 12, 2015. (Credit: Ben Smegelsky/NASA)

    Navigator Highlights

    • At the highest point of the MMS orbit, at more than 43,500 mile above the surface of the earth, Navigator set a record for the highest ever reception of signals and onboard navigation solutions by an operational GPS receiver in space.
    • At the lowest point of the MMS orbit, Navigator set a record as the fastest operational GPS receiver in space, at velocities over 22,000 miles per hour.
    • At the farthest point in its orbit, some 43,500 miles away from Earth, Navigator can determine the position of each spacecraft with an uncertainty of better than 50 feet.
  • NASA Goddard Team Sets High-Flying Record with Use of GPS

    NASA Goddard Team Sets High-Flying Record with Use of GPS

    The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)
    The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)

    Editor’s Note: See more about the MMS mission in our August issue. The article is also posted here.

    News courtesy of NASA

    After years of hard work building a spacecraft, a mission team anxiously awaits after a launch — will the instruments they’ve crafted all work as well as planned? This is all the more true when flying new hardware, such as the onboard navigation tool on the four spacecraft that make up the Magnetospheric Multiscale, or MMS, mission, which launched on March 12. This navigation system had never before flown on a spacecraft with an orbit traveling so far from Earth — but if it worked, it would provide the precision navigation needed for MMS.

    And the results are now in: Not only has the MMS Navigator system exceeded all of the team’s expectations, it has set the record for the highest GPS use in space.

    • At the highest point of the MMS orbit, at more than 43,500 mile above the surface of the earth, Navigator set a record for the highest-ever reception of signals and onboard navigation solutions by an operational GPS receiver in space.
    • At the lowest point of the MMS orbit, Navigator set a record as the fastest operational GPS receiver in space, at velocities over 22,000 miles per hour.

    A precise tracking system is crucial for MMS, which requires extremely sensitive position and orbit calculations. The four spacecraft must fly in a tight pyramid formation to gather science data as they move through Earth’s magnetic environment. The formation is required to obtain three-dimensional observations of a phenomenon called magnetic reconnection that occurs when magnetic fields from the sun connect and disconnect with magnetic fields of Earth, which can allow energy and solar material to funnel into near-Earth space. With its instrument booms deployed, each spacecraft is the size of a baseball field — while flying as close as six miles from each other.

    Artist's concept of the MMS (courtesy of NASA).
    Artist’s concept of the MMS (courtesy of NASA).

    “Demonstration airplanes like the U.S. Navy’s Blue Angels fly in closer formations, but those planes are also much, much smaller and the pilots are always controlling the movements,” said Brent Robertson, deputy project manager for MMS at NASA Goddard Space Flight Center in Greenbelt, Md. “We have four giant spacecraft each with its own unique orbit that we make maneuvers on about every two weeks. It’s quite challenging to control this formation.”

    Tracking spacecraft can be done by radar stations from the ground, but it’s much more expensive and takes longer than an inflight system. However, using GPS as is typically done on Earth by such things as cars, boats and smartphones isn’t nearly as simple for something like MMS. For one thing, the bulk of its highly-elliptical orbit occurs above where the GPS transmitters orbit. So MMS must have specialized, extremely sensitive receivers to capture GPS signals transmitted from the far side of Earth. In addition the MMS spacecraft spin; each one makes three revolutions per minute.

    “Spinning adds a whole new dimension to trying to figure out where you are,” said Ken McCaughey, MMS GPS Navigator Product Development Lead at Goddard. “We have four GPS antennas on each spacecraft. As the spacecraft rotates we have an algorithm running that allows us to hand off from one antenna to the next without losing the signal.”

    In the first month after launch, the MMS team began turning on and testing each instrument and deploying booms and antennas. During this time, the team compared the Navigator system with ground tracking systems and found it to be even more accurate than expected. At the farthest point in its orbit, some 43,500 miles away from Earth, Navigator can determine the position of each spacecraft with an uncertainty of better than 50 feet.

    What’s more, the receivers on MMS have turned out to be strong enough that they consistently track transmissions from eight to 12 GPS satellites — excellent performance when compared to pre-flight predictions that there might be frequent drop outs during each orbit.

    Even if the receiver were to lose all GPS signals for part of the orbit, Navigator is specifically designed to handle such dropouts. By gathering as many observations as possible, integrated software called GEONS — for Goddard Enhanced Onboard Navigation System — can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.

    This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 95,000 miles from Earth.

    “It’s going to be very interesting to see how far out MMS can still receive signals,” said Robertson. “But Navigator has already far exceeded expectations. I think there’s a good chance we’ll end up being able to use GPS and save us some of the expense of using ground observations.”

    While Navigator technology and GPS receivers were previously flown for testing and to help navigate a low-earth-orbit mission, this is the first time that the complete Navigator package has been used to actively navigate a high-altitude mission. Now that the team knows it works so well, Navigator can be used for other missions that travel in similar high orbits.

    — By Karen Fox, NASA Goddard Space Flight Center