The System: NASA's Vision for Space
September 1, 2008 By: James J. Miller, AJ Oria GPS WorldThe U.S. Space Exploration Policy, unveiled in 2004, seeks to extend human exploration beyond the low-Earth orbit (LEO) environment of the International Space Station (ISS) and Space Shuttle towards a new generation of missions to the moon, Mars, and beyond. Implementing this ambitious Project Constellation by establishing a lunar outpost, and possibly subsequent human spaceflight to Mars, requires construction of an entirely new generation of space transportation vehicles known as the Ares I (crew) and Ares V (cargo) launchers and the Orion crew capsule. We must also establish a cost-effective, synergistic communications and navigation infrastructure that optimizes the capabilities of the existing and planned NASA networks to interact with the Constellation vehicles as well as other NASA missions.
Tracking Space Missions
NASA uses a global array of ground stations, antennas, and satellites known as the Ground Network (GN), Deep Space Network (DSN), and Space Network (SN) for communications and navigation. The SN is better known as the NASA global Tracking Data and Relay Satellite System (TDRSS), a network of geosynchronous communications satellites. A combination of these terrestrial networks and satellites communicates with and tracks assets in space depending on mission phase. Some missions, such as those in deep space, will continue to navigate using communication-channel tracking from ground-based antennas as in the earlier days of space exploration, as well as using newer inertial and optical sensors (star trackers). Other missions, such as those in Earth orbit, will mostly use the space network. In the past few years there has been a growing movement to add the very precise signals of GPS to space operations.
NASA Use of GPS
The GPS Terrestrial Service Volume provides service from the surface up to 3,000-kilometer altitude. NASA has engaged with the Department of Defense (DoD) to define the performance parameters to support navigation services in a Space Service Volume (SSV) designated from 3,000 kilometers to GEO altitude to approximately 36,000 kilometers. This type of navigation requires specialized software to process the side-lobes of GPS signals coming over the earth’s limb, as well as the increased attenuation and tracking of a very few satellites at a time. Once tracking is initiated. however, one can begin to imagine a future where GPS-in-space may also include syncing GPS positioning and timing with spacecraft and beacons broadcasting other “GPS-like” signals near celestial bodies such as the moon and Mars.
GPS Precise Positioning Service (PPS) receivers have been certified for Space Shuttle navigation. GPS receivers for human spaceflight include the Miniaturized Airborne GPS Receiver (MAGR-S) on the Space Shuttle and the Space Integrated GPS/INS (SIGI) receiver on the International Space Station (ISS). Orbiters Discovery and Atlantis have a single GPS receiver in addition to retaining its triple-redundant Tactical Air Navigation (TACAN), while orbiter Endeavour has had its TACAN removed and replaced by a triple-redundant GPS system. GPS was taken to navigation during the critical re-entry mission phase for the first time on STS-115 / OV-104 Atlantis and GPS--only navigation was used for the first time on STS-118 / OV-105 Endeavour. The SIGI receivers were tested on shuttle flights prior to deployment on ISS. The ISS has an array of four antennas on the T1 truss assembly for orbit and attitude determination.
The Jet Propulsion Laboratory (JPL) has developed the GPS Blackjack family of science receivers for science observations such as, for example, use of GPS signals for atmospheric research using occultation measurements through the Earth’s atmosphere, and observations of GPS signals reflected off the Earth’s surface. The latest generations of NASA GPS science receivers are software-programmable units that include the capability to receive the second GPS civil signal. These successes have since led to NASA plans for GPS equipage on the new Ares and Orion vehicles, as well as an agreement between the Air Force and NASA to transition to a space-based launch range by 2011. A space-based range, otherwise known as GPS Metric Tracking (GPS-MT), will enable GPS telemetry data to be transmitted down to mission controllers rather than relying solely on positioning via radar networks. Besides improved precision and latency, this should also reduce infrastructure costs.
Transition from terrestrial-based radar tracking of space vehicles to space-based radiometric data from GPS is well underway at NASA. Simulations demonstrate GPS Navigator receiver applications could be performed almost to the moon. An ongoing effort is developing the TDRSS Augmentation Service for Satellites (TASS) to disseminate differential corrections from the Global Differential GPS (GDGPS) network to users in LEO. The Communication, Navigation, Networking, reConfigurable Testbed (CoNNeCT) on the ISS will use software-defined radios to process GPS/GNSS signals and waveforms.
Adoption of GPS-in-space is by no means limited to NASA, as evidenced by the European Space Agency’s (ESA’s) use of GPS on the Automated Transfer Vehicle. I discussed Space X plans with company founder Elon Musk, who confirmed that all of their space vehicles will be equipped with GPS receivers. I suspect that most emerging commercial space ventures have similar equipage plans.
Not all agree that GPS is necessary for spaceflight. Some say that since we didn’t need GPS in the Apollo years, we don’t need it now. But as any pilot or astronaut knows, it is best to take advantage of every tool available to ensure the safest, most precise journey possible.
NASA scientists have used GPS for science applications for several decades, especially from such contributions as the International GNSS Service (IGS). As these applications continue to evolve with space exploration, the NASA Space Communications and Navigation (SCaN) program is organizing how GPS and other positioning, navigation, and timing (PNT) services are used by NASA missions via domains or functional areas. While GPS will remain a core component of most spaceflight missions from launch on up to orbit, the limits of GPS signals will require other combinations of navigation sensors as missions transverse these bounds. If Galileo and other PNT service providers follow suit, space applications will benefit from the additional robustness intended for the growing ranks of space users.
Figure 1. Navigation functional areas
Figure 2. PNT services by functional area
Integrated Network
A NASA systems engineering core team is mapping current network comm and nav capabilities with future space architecture needs to optimize operations while synergizing network capabilities to reduce unnecessary costs. GPS will be a cornerstone to any future space architecture for two primary reasons:
- Use of passive, one-way GPS positioning and time data reduces the burden and costs on two-way communications using ground networks and the TDRSS.
- Over the next three decades, approximately 95 percent of all nations’ future space missions will still operate below GEO altitude, even as NASA and other nations aim towards the moon and beyond.
To provide the necessary performance for space applications, both PNT services and cross-cutting technologies must be integrated throughout various functional areas. These functional navigation areas broadly include:
- Earth surface and atmosphere
- Earth orbit
- Lunar space
- Interplanetary space
- Mars space (Figure 1).
These areas are defined based on the users and type of navigation service they require. Figure 2 depicts the PNT services available throughout these functional areas. Some users’ requirements span several areas; a human mission to the moon will traverse launch, Earth orbit rendezvous for docking, transfer into a lunar trajectory, lunar orbiting and landing for a 90-day stay, and safe return to Earth. A future human Mars mission will span across launch and Earth-orbit for assembly, a nearly year-long Earth-Mars transfer, and either a 40-day or 400-day stay on Mars depending on whether a conjunction or opposition trajectory is selected.
Cross-cutting technologies enable and enhance uniform satisfaction of PNT services across the functional areas. These cross-cutting processes and technologies include improved flight dynamics modeling; improved geodetic references; autonomous navigation systems; integrated communication and navigation receivers; and space-qualified atomic clocks (Figure 3).
Figure 3. Cross-cutting enablers and enhancers
For example, autonomous navigation systems include inertial navigation systems and celestial tracking (optical visible, optical infrared, x-ray pulsar, and so on) that provide services to multiple functional areas. Improved geodetic references include combined technologies from laser retro-reflectors on GPS satellites and using NASA’s laser-ranging network to improve reference-frame precision, accurate maps, feature recognition technologies, and gravity field mapping (Figure 4).
Figure 4. Integrated PNT architecture
Near-Earth Nav Component
The surface and launch functional area includes PNT services for NASA applications such as monitoring and geodesy, aeronautical application, and launch-vehicle tracking and telemetry. Monitoring activities on the surface include tracking of signals-in-space, crustal movement, tropospheric and ionospheric monitoring, and other parameters that affect the use of navigation signals.
Aeronautical applications include Radio Navigation Satellite Service (RNSS) frequency band protection, GPS receiver design, and World Radiocommunication Conference participation. Launch vehicle tracking includes implementation of GPS metric tracking from January 2011.
Most space users are located in the region of Near Earth, from the surface up to an arbitrary point in cislunar space between GEO altitude and the lunar sphere of gravitational influence. Figure 5 shows navigation services provided by the GPS and NASA’s Space and Ground Networks. GPS can meet many space-user requirements below GEO altitude in real time while offloading NASA network burdens.
NASA seeks to develop a fully integrated comm/nav system leveraging observables used for navigation. Fusion of independent tracking data provides a robust approach for navigation while reducing burdens on NASA networks and valuable bandwidth. Figure 6 shows a notional Integrated Near-Earth Navigation concept; communications channel tracking infrastructure is also used for:
- broadcast of global differential GPS corrections to space users in low-Earth orbit
- download of GPS metrics to the Flight Dynamics Facility for integration with tracking observables;
- upload of navigation data to human missions.
Figure 5. Navigation services near Earth
Figure 6. Integrated Near-Earth navigation
Summary
Worldwide government and commercial spacecraft launch projections over the next three decades show that 60 percent of future spacecraft will be in low-Earth orbit and 95 percent of missions will have perigees below GEO altitude. The majority of space users may be able to meet real-time navigation needs autonomously through GPS or other PNT systems, reducing network time and costs.
NASA network integration, paired with data standardization, could evolve into a “space internet” integrating comm and nav throughout the solar system. Interoperability with GLONASS, Galileo, and COMPASS will go far in creating an international environment to support such stretches of the imagination. We welcome the challenges of the U.S. Space Exploration Policy, and we appreciate the opportunities that the 2004 PNT Policy has provided to create the synergy and energy for the future of spaceflight.
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