The Navigator GPS Receiver
A GPS navigation system built for high Earth orbits
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 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, and other high altitude applications. The Navigator receiver can quickly and reliably acquire and track GPS signals at 25 dB-Hz and lower.
| Technology Details
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.
How it works
In order to determine positioning using GPS, a receiver must first acquire the GPS signals and then track those signals simultaneously. When tracking the signal, the receiver holds and extracts data, making range measurements from each satellite. Those measurements are processed to determine the position of each satellite and then extrapolate the receiver’s position.
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.
One of the Navigator’s two design principles was autonomous operation to promote the feasibility of using GPS for onboard navigation of geostationary (GEO) or other high altitude space missions. With the exception of GPS signals, Navigator requires no external data (e.g., current estimate of time, recent GPS almanac, or converged navigation filter estimate of the receiver dynamics).
Data processing software
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 1 ms correlations. Time is significantly reduced by using a FFT algorithm to compute these DFTs. 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 Navigator’s hardware-independent receiver software includes both a hardware interface to perform low-level functions, such as controlling the acquisition engine and tracking loops as well as basic navigation software. The navigation software runs on the Nucleus real-time operating system and forms measurements; provides standard position, velocity, and time-point solutions (when four or more satellites are being tracked); and handles commanding and telemetry messages. It also is capable of determining attitude when it is set up with an appropriate antenna configuration. 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).
Why it is better
Because of the weakness of GPS signals at altitudes above HEO, there are currently no GPS receivers for use in HEO. The standard acquisition approach is to use a serial search of the two-dimensional signal parameter space (code phase and Doppler), using the same hardware that is used for tracking. Because a serial search cold start acquisition, which means there is no prior information about visible GPS signals, can take as long as 20 minutes for even strong GPS signals, weak signals are essentially impossible to acquire. For example, a signal that is 10 times weaker will require 10 times as much data. But, when using serial search methods, acquisition times increase quadratically making the 20-minute search increase to 33 hours.
Navigator exploits the properties of Fourier transform in a massively parallel search for the GSP signal. Navigator has been tested and proven capable of acquiring signals at 25 dB-Hz and below.
NASA Goddard Space Flight Center has patented this technology (U.S. Patent #7,548,199)
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| Licensing and Partnering Opportunities
This technology is part of NASA’s Innovative Partnerships Program Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the Navigator GPS Receiver (GSC-14793) for commercial applications.
| For More Information
If you are interested in more information or want to pursue transfer of this technology, please contact:
Innovative Partnerships Program Office