The Navigator GPS Receiver
sensors
The Navigator GPS Receiver (GSC-TOPS-67)
A GPS navigation system built for high Earth orbits
Overview
Until now, Global Positioning System (GPS) receivers, while providing an accurate and inexpensive means of navigation, have been limited to low Earth orbit (LEO) missions. This innovative new receiver technology developed by NASA Goddard Space Flight Center is a leap forward for GPS technology. The Navigator is an autonomous, real-time, fully space-flight-qualified GPS receiver with exceptional capabilities for fast signal acquisition and weak signal tracking. These features enable the use of GPS navigation in high Earth orbit (HEO), geostationary orbit (GEO), and other high altitude applications. The Navigator receiver can quickly and reliably acquire and track GPS signals at 25 dB-Hz and lower.
Because GPS signals at altitudes above the GPS constellation are 10 to 100 times weaker and less densely populated, GPS receivers have not been feasible for use above LEO. The Navigator is a radiation-hardened GPS receiver specifically designed for use in high Earth orbits. It is capable of significantly faster acquisition times and tracking for both strong and weak signals. It requires no external data, and its fast acquisition enables it to be powered down in any orbit until needed.
The Technology
To enable it to acquire GPS signals very quickly and also track weak signals, the radiation-hardened Navigator receiver utilizes a bank of hardware correlators, a ColdFire microprocessor, and a specialized fast acquisition module (see figure 1). The hardware is implemented in VHSIC Hardware Description Language (VHDL) to target radiation-hardened Field Programmable Gate Arrays (FPGA) rather than Application-Specific Integrated Circuits (ASIC), in order to maintain flexibility for growth and design modifications.
The Navigator was designed to operate autonomously to enable the use of GPS for onboard navigation in high altitude space missions. With the exception of GPS signals, Navigator requires no external data (e.g., current time estimate, recent GPS almanac, or converged navigation filter estimate of the receiver dynamics).
By double buffering data up front in 1ms blocks, data can be processed as it is acquired. A discrete Fourier transform (DFT) is used to calculate the 1ms correlations, significantly reducing computing time. Computational efficiency is optimized and tradeoffs among sampling rate, data format, and data-path bit rate are carefully weighed in order to increase performance of the algorithm.
In addition, the Navigators hardware-independent receiver software includes both a hardware interface to perform low-level functions as well as basic navigation. Onboard orbit determination and accurate state estimation/propagation during periods with no GPS access are accomplished by integration with the GPS Enhanced Onboard Navigation System (GEONS).
Exploiting the properties of Fourier transform in a massively parallel search for the GSP signal, the Navigator has been tested and proven capable of acquiring signals at 25dB-Hz and below.
Benefits
- Enables GPS in high Earth orbit: Because it can acquire and track even very weak signals and requires no external data, the Navigator receiver enables use of GPS in HEO, GEO, and other high altitude uses
- Acquires signals faster: By employing efficient Fast Fourier Transform (FFT) algorithms and Field Programmable Gate Arrays (FPGA) to implement a massively parallel search, even weak signals can be acquired thousands of times faster than in traditional searches
- Operates autonomously: With the exception of GPS signals, the receiver requires no external data for operation
- Is robust and reliable: The radiation-hardened receiver can reliably operate in the harsh environment of space
- Improves Use for LEO: When used in LEO, the receivers fast acquisition rate eliminates the approximately 20-minute cold start delay time, acquiring GPS signals in only seconds
Applications
- High Altitude Spacecraft (e.g., Geostationary Operational Environmental Satellite (GOES), Magneto Multiscale Science (MMS), other geostationary orbit (GEO) satellites)
- Low Earth Orbit spacecraft (offers enhanced GPS navigation via Navigators fast-acquisition capability)
Similar Results
Powerline Geolocation
The electrical transmission lines used to transmit power are optimized for 50 to 60 Hz waveforms, but are suitable for waveforms of higher frequencies, into the megahertz range. Therefore, powerline conductors are capable of transmitting signals which could be used for geolocation.
Indeed, frequencies in the 100 kHz range are used for diagnostic purposes in contemporary power grids today. Signals transmitted in this band are used to verify the operation of sites along the power grid, are used to configure the power grid (for example, throw a circuit breaker).
The technology takes advantage of the suitability of electrical conductors employed in power transmission for signal transmission in the sub-megahertz range, and, in the preferred embodiment, utilizes the existing diagnostics signals in this range for geolocation.
Location Corrections Through Differential Networks System
The system is designed to work with any internet enabled mobile device that has access to its GPS pseudorange, code phase and potentially carrier phase measurements. The system makes use of GPS measurements or corrections obtained from a stationary base station to refine its positioning estimate using established computational techniques such as those associated with differential GPS, Real-Time Kinematic (RTK) GPS and/or the Local Area Augmentation System (LAAS). The location information from mobile and fixed base signals can be integrated using established such as those used for differential GPS, Such methods remove errors common to the measurements of both devices (the mobile and base station) thereby increasing accuracy of the mobile device position estimates.
Goddard's Reconfigurable Laser Ranger (GRLR)
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Real-Time Tracking System
The innovation builds upon conventional UWB hardware by incorporating tracking methodology and algorithms in addition to external amplifiers for signal boost. The tracking methodology is a triangulation calculation consisting of Angle of Arrival (AOA) and Time Difference of Arrival (TDOA) using a cross-correlation peak detection method. By directly estimating TDOA information from UWB pulses, the method achieves the high temporal resolution (on the order of picoseconds) needed to measure AOA with extreme precision. The system uses a PC to synchronize and process data in real time from two receivers, or clusters, to display the position of the transmitter-equipped person or object. The interface software enables the PC to access the two data sets simultaneously through separate sockets. In the data collection process, data segments from each receiver are interleaved with those from the other receiver in chronological order of collection. Within the PC, the data segments are stored in a separate buffer; therefore, the contents of the buffers are representations of the same UWB pulse waveform arriving at the two receivers at approximately the same time. This data synchronization provides the separate and simultaneous collection of waveform data that the tracking algorithm requires for accurate real-time tracking.
Lunar Surface Navigation System
NASAs reverse-ephemeris lunar navigation system is a concept for determining position on the lunar surface based on known orbits of satellites. In conventional GPS navigation systems, the GPS satellite transmits ephemeris data to a receiver on earth for determining position at the receiver location. Whereas for the reverse-ephemeris approach the receiver becomes the transmitter, and the satellite instead serves more as a fixed reference position with a known ephemeris. This simplifies the satellite requirements and also mitigates potential navigational disruptions that can otherwise arise in navigation systems that utilize satellite-based communications, for example from interference, jamming, etc.
The design consists of lunar surface S-Band (2,400 2,450 MHz) 10 W transceivers ranging with analog translating transponders on a three-satellite constellation in frozen elliptical orbits to provide continuous coverage with service to 300 simultaneous users over 1.8 MHz of bandwidth at the transponder. Digital bases systems are possible too. As compared to GPS-based navigation requiring four or more satellites costing 100s of millions of dollars, the new NASA concept is based on using only three smallsats.
