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The Artemis generation of lunar explorers will establish a sustained human presence on the Moon, prospecting for resources, making revolutionary discoveries, and proving technologies key to future deep space exploration.
To support these ambitions, NASA navigation engineers from the Space Communications and Navigation (SCaN) program are developing a navigation architecture that will provide accurate and robust Position, Navigation, and Timing (PNT) services for the Artemis missions. GNSS signals will be one component of that architecture. GNSS use in high-Earth orbit and in lunar space will improve timing, enable precise and responsive maneuvers, reduce costs, and even allow for autonomous, onboard orbit and trajectory determination.
Expanding the Space Service Volume
Beyond 1,800 miles in altitude, navigation with GNSS becomes more challenging. This expanse of space is called the Space Service Volume, which extends from 1,800 miles up to about 22,000 miles (36,000 km), or geosynchronous orbit. At altitudes beyond the GNSS constellations themselves users must begin to rely on signals received from the opposite side of the Earth.
From the opposite side of the globe, Earth blocks much of the GNSS signals, so spacecraft in the Space Service Volume must instead “listen” for signals that extend out over the Earth. These signals extend out at an angle from GNSS antennas.
Formally, GNSS reception in the Space Service Volume relies on signals received within about 26 degrees from the antennas’ strongest signal. However, NASA has had marked success using weaker GNSS side lobe signals — which extend out at an even greater angle from the antennas — for navigation in and beyond the Space Service Volume.
Since the 1990s, NASA engineers have worked to understand the capabilities of these side lobes. In preparation for launch of the first Geostationary Operational Environmental Satellite-R weather satellite in 2016, NASA endeavored to better document side lobes’ strength and nature to determine if the satellite could meet its PNT requirements.
Navigation experts at Goddard reverseengineered the characteristics of the antennas on GPS satellites by observing the signals from space. By studying the signals satellites received from GPS side lobes, engineers pieced together their structure and strength. Using this data, they developed detailed models of the radiation patterns of GPS satellites in an effort called the GPS Antenna Characterization Experiment.
While documenting these characteristics, NASA explored the feasibility of using side lobe signals for navigation well outside what had been considered the Space Service Volume and in lunar space. In recent years, the Magnetospheric Multiscale Mission (MMS) has even successfully determined its position using GPS signals at distances nearly halfway to the Moon.
GNSS at the Moon
To build on the success of MMS, NASA navigation engineers have been simulating GNSS signal availability near the Moon. Their research indicates that these GNSS signals can play a critical role in NASA’s ambitious lunar exploration initiatives, providing unprecedented accuracy and precision.
“Our simulations show that GPS can be extended to lunar distances by simply augmenting existing high-altitude GPS navigation systems with higher-gain antennas on user spacecraft,” said NASA navigation engineer Ben Ashman. “GPS and GNSS could play an important role in the upcoming Artemis missions from launch through lunar surface operations.”
While MMS relied solely on GPS, NASA is working toward an interoperable approach that would allow lunar missions to take advantage of multiple constellations at once. Spacecraft near Earth receive enough signals from a single PNT constellation to calculate their location. However, at lunar distances GNSS signals are less numerous. Simulations show that using signals from multiple constellations would improve missions’ ability to calculate their location consistently.
To prove and test this capability at the Moon, NASA is planning the Lunar GNSS Receiver Experiment (LuGRE), developed in partnership with the Italian Space Agency. LuGRE will fly on one of NASA’s Commercial Lunar Payload Services missions. These missions rely on U.S. companies to deliver lunar payloads that advance science and exploration technologies.
NASA plans to land LuGRE on the Moon’s Mare Crisium basin in 2023. There, LuGRE is expected to obtain the first GNSS fix on the lunar surface. LuGRE will receive signals from both GPS and Galileo, the GNSS operated by the European Union. The data gathered will be used to develop operational lunar GNSS systems for future missions to the Moon.
By Danny Baird NASA’s Space Communications and Navigation program office. www.nasa.gov
President Donald J. Trump has issued Space Policy Directive-7 (SPD-7), the United States Space-Based Positioning, Navigation, and Timing Policy. Recognizing that space-based positioning, navigation, and timing (PNT) systems are increasingly critical to the American way of life, SPD-7 directs the pursuit of multiple and varied sources of PNT. SPD- 7 directs an increase of cybersecurity for the Global Positioning System (GPS) and GPS-enabled devices, and acknowledges the potential for GPS to contribute to in-space applications. This is the first update to the United States policy on space-based PNT in more than 16 years.
The multi-use PNT services provided by GPS are integral to United States national security, economic growth, transportation safety, and homeland security. These services are essential but largely invisible elements of worldwide economic infrastructures.
The goal of SPD-7 is to maintain United States leadership in the service provision and responsible use of global navigation satellite systems, including GPS and foreign systems.
To achieve this goal, SPD-7 outlines several objectives, including:
• Provide continuous worldwide access to United States space-based GPS services and government-provided augmentations free of direct user fees.
• Operate and maintain the GPS in accordance with United States law to satisfy civil, homeland security, and national security needs.
• Improve the performance of United States space-based PNT services, including developing more robust signals that are more resistant to disruptions and manipulations.
• Improve the cybersecurity of GPS, its augmentations, and United States Government-owned GPS-enabled devices, and foster private sector adoption of cybersecure GPS-enabled systems.
• Protect the spectrum environment used by GPS and its augmentations.
SPD-7 complements the Executive Order on Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing Services, clarifying the pursuit of multiple and varied alternative sources of PNT for critical infrastructure.
On June 30, 2017, President Donald J. Trump issued Executive Order 13803, reviving the National Space Council “in order to provide a coordinated process for developing and monitoring the implementation of national space policy and strategy.” In addition to Space Policy Directive-7, the President has signed six previous Space Policy Directives and the National Space Policy to restore American leadership in areas of civil, commercial, and military space. www.whitehouse.gov
The United Kingdom will no longer participate in the European Galileo or Egnos programs but can continue, in principle, with Copernicus and remain member of the European Space Agency (ESA), the British Government said.
On its official www.gov.uk website, the Government lists the Brexit transition: new rules for 2021”.
According to the site, the UK will “not use Galileo (including the future Public Regulated Service (PRS)) for defence or critical national infrastructure; have access to the encrypted Galileo Public Regulated Service; be able to play any part in the development of Galileo; be able to play any part in the development of EGNOS; be able to use the EGNOS SoL and EGNOSWorking Agreements (EWAs), which will no longer be recognised by the EU; be able to access or use EDAS”.
Furthermore, from 1 January 2021, “the UK will no longer participate in the EU Space Surveillance and Tracking (EUSST) programme. The UK will however continue to have access to EUSSTservices as a non- EU country”, the Government says.
However, the UK can continue to participate in Copernicus. “The UK welcomes the agreement in principle to continue to participate in the Copernicus component of the EU Space Programme as a third country for 2021-27,” it says. “If the UK confirms in early 2021 to participate in Copernicus, we expect UK-based businesses, academics and researchers will be able to bid for future Copernicus contracts tendered through the EU”.
“Similarly, we expect UK users will also be able to access most of the Copernicus data and services as now.” “(D) evices that currently use Galileo and EGNOS, such as smart phones, will continue to be able to do so”, the Government says.
Also, “the UK’s membership of the European Space Agency (ESA) is not affected by leaving the EU as it is not an EU organisation.” The UK will thus continue to participate and be able to bid for ESA programs. https://spacewatch.global/2020/12/ uk-loses-galileo-and-egnos-but-cancontinue- with-copernicus-and-esa
The EU-funded ENSPACE project has made quantum leaps in the development of a software GNSS solution that supports Galileo and is especially aimed at the small satellite market sector, one of the fastest growing in the ‘New Space’ age. Software-based GNSS receivers enable a new concept for Space. This is an activity that project coordinator Qascom initiated with NASA in 2016.
The experiment was based on the use of a NASA’s software-defined radio platform called SCaN, attached to the exterior of the International Space Station (ISS). A first, the SCaN Testbed provided an orbiting laboratory on the space station for the development of software-defined radio technology for improved navigation and communication experimentations. “In ENSPACE, we evolved this concept and invested in a new software GNSS solution that has been installed in commercial off-theshelf hardware and is also compatible with other system-on-chip components,” notes Samuele Fantinato, head of the Advanced Navigation Unit at Qascom.
In Space, GNSS receivers need to operate in quite different environments from those of ground-based receivers. “Precisely determining satellite position in Space is quite easy for those flying in low Earth orbits. In higher altitudes, such as in geostationary orbits or in interplanetary missions, signal variability becomes prominent. Adding new constellations could increase accuracy in these orbits,” explains Fantinato. Project members have proposed novel techniques for enhanced navigation, positioning and security in Space. Charged particles and gamma rays are another concern for GNSS receivers. The ENSPACE software GNSS solution integrates techniques and logic redundancy that offer a more robust positioning accuracy in case of radiation events. https://cordis.europa.eu
Lockheed Martin Space has released the GPS-III Satellite phase center and intersignal bias data for SVN-74 and SVN- 75. The information has been posted on the U.S. Coast Guard Navigation Center’s website underneath the IIR/ IIR-M antenna pattern data files.
The phase center and inter-signal bias data included in this new release provides additional information that supplements the antenna gain pattern data previously available.
The phase center offset data locates the electrical center of the GPS transmit antenna. This data set also includes the inter-signal corrections as measured in the factory. The modernized satellites provide multiple civil signals in addition to the legacy L1 C/A signal, enabling robust dual-frequency or triple-frequency receiver tracking. These techniques can, for example, help eliminate ranging errors due to ionospheric path delays. But proper operation of a multi-frequency receiver requires accommodation of intersignal biases that arise from differences in the transmit time of each signal type.
Note that the GPS-III SVs also broadcast the Inter-Signal Corrections (ISCs) in the various LNAV/CNAV messages in accordance with all the external IS/ICDs. The value that is being broadcast by the on-orbit constellation is not the factory measured ISCs but the ISCs estimated on-orbit by the Stanford Research Institute (SRI).