Brent Ledvina | Virginia Tech (original) (raw)

Papers by Brent Ledvina

IEEE Transactions on Aerospace and Electronic Systems, 2000

Advances in Space Research, 2005

Agu Fall Meeting Abstracts, Dec 1, 2006

GPS satellites transmit at two frequencies, L1 and L2, enabling the calculation of TEC from the d... more GPS satellites transmit at two frequencies, L1 and L2, enabling the calculation of TEC from the dispersion in the two signals. Unfortunately the only code on the L2 signal is encrypted and it must be compared to the same encrypted code on L1. The most effective comparison is called semi-codeless tracking which yields TEC in quiet conditions for stationary receivers. When the ionosphere becomes active or when the receiver is in an accelerated environment, semi-codeless tracking fails. Fortunately there is a solution emerging to correct these short comings. The new GPS satellites will transmit an unencrypted signal on L2, called L2C, that can be compared with the unencrypted signal on L1. This comparison will be much more robust and easier to implement. The first GPS satellite with the L2C signal was launched in 2005 and another GPS satellite with the L2C signal should be launched in the Fall of 2006. In this paper we describe two techniques to use the new L2C signal. The first technique is a digital storage receiver that captures data and stores it on a PC. The signals can then be "received" and TEC determined at a later time. This approach allows extracting the maximum possible information from the signal. Applications for the digital storage receiver include ionospheric tomography inside a sounding rocket trajectory or high resolution measurements of mesospheric temperatures with a drop sphere. The second technique is a real-time software receiver that is implemented on a digital signal processing chip. This approach allows for the mass production of inexpensive TEC and fast amplitude and phase scintillation receivers that can be easily accessed through the internet. In future years these approaches can be expanded with the Galileo signals although larger bandwidths will be required.

Ieee Transactions on Aerospace and Electronic Systems, 2010

Abstract A large set of equatorial ionospheric scintillation data has been compiled, used to char... more Abstract A large set of equatorial ionospheric scintillation data has been compiled, used to characterize features of severe scintillation that impact Global Positioning System phase tracking, and used to develop a scintillation testbed for evaluating tracking loops. The data ...

ABSTRACT A conceptual method is presented for upgrading exist-ing GPS user equipment, without req... more ABSTRACT A conceptual method is presented for upgrading exist-ing GPS user equipment, without requiring hardware or software modifications to the equipment, to improve the equipment's position, velocity, and time (PVT) accuracy, to increase its PVT robustness in weak-signal or jammed environments, and to protect the equipment from coun-terfeit GPS signals (GPS spoofing). The method is em-bodied in a device called the GPS Assimilator that cou-ples to the radio frequency (RF) input of an existing GPS receiver. The Assimilator extracts navigation and timing information from RF signals in its environment—including non-GNSS signals—and from direct baseband aiding pro-vided, for example, by an inertial navigation system, a frequency reference, or the GPS user. The Assimilator optimally fuses the collective navigation and timing infor-mation to produce a PVT solution which, by virtue of the diverse navigation and timing sources on which it is based, is highly accurate and inherently robust to GPS signal ob-struction and jamming. The Assimilator embeds the PVT solution in a synthesized set of GPS signals and injects these into the RF input of a target GPS receiver for which an accurate and robust PVT solution is desired. A proto-type software-defined Assimilator device is presented with three example applications.

Methods are explored for efficiently mapping GNSS signal processing techniques to multicore gener... more Methods are explored for efficiently mapping GNSS signal processing techniques to multicore general-purpose proces- sors. The aim of this work is to exploit the emergence of multicore processors to develop more capable software- defined GNSS receivers. It is shown that conversion of a serial GNSS software receiver to parallel execution on a 4-core processor via minimally-invasive OpenMP direc- tives leads

Proceedings of the 2012 IEEE/ION Position, Location and Navigation Symposium, 2012

ABSTRACT Background theory, a reference design, and demonstration results are given for a Global ... more ABSTRACT Background theory, a reference design, and demonstration results are given for a Global Navigation Satellite System (GNSS) interference localization system comprising a distributed radio-frequency sensor network that simultaneously locates multiple interference sources by measuring their signals' time difference of arrival (TDOA) between pairs of nodes in the network. The end-to-end solution offered here draws from previous work in single-emitter group delay estimation, very long baseline interferometry, subspace-based estimation, radar, and passive geolocation. Synchronization and automatic localization of sensor nodes is achieved through a tightly-coupled receiver architecture that enables phase-coherent and synchronous sampling of the interference signals and so-called reference signals which carry timing and positioning information. Signal and cross-correlation models are developed and implemented in a simulator. Multiple-emitter subspace-based TDOA estimation techniques are developed as well as emitter identification and localization algorithms. Simulator performance is compared to the Cramér-Rao lower bound for single-emitter TDOA precision. Results are given for a test exercise in which the system accurately locates emitters broadcasting in the amateur radio band in Austin, TX.

IEEE Transactions on Aerospace and Electronic Systems, 2000

Advances in Space Research, 2005

Agu Fall Meeting Abstracts, Dec 1, 2006

GPS satellites transmit at two frequencies, L1 and L2, enabling the calculation of TEC from the d... more GPS satellites transmit at two frequencies, L1 and L2, enabling the calculation of TEC from the dispersion in the two signals. Unfortunately the only code on the L2 signal is encrypted and it must be compared to the same encrypted code on L1. The most effective comparison is called semi-codeless tracking which yields TEC in quiet conditions for stationary receivers. When the ionosphere becomes active or when the receiver is in an accelerated environment, semi-codeless tracking fails. Fortunately there is a solution emerging to correct these short comings. The new GPS satellites will transmit an unencrypted signal on L2, called L2C, that can be compared with the unencrypted signal on L1. This comparison will be much more robust and easier to implement. The first GPS satellite with the L2C signal was launched in 2005 and another GPS satellite with the L2C signal should be launched in the Fall of 2006. In this paper we describe two techniques to use the new L2C signal. The first technique is a digital storage receiver that captures data and stores it on a PC. The signals can then be "received" and TEC determined at a later time. This approach allows extracting the maximum possible information from the signal. Applications for the digital storage receiver include ionospheric tomography inside a sounding rocket trajectory or high resolution measurements of mesospheric temperatures with a drop sphere. The second technique is a real-time software receiver that is implemented on a digital signal processing chip. This approach allows for the mass production of inexpensive TEC and fast amplitude and phase scintillation receivers that can be easily accessed through the internet. In future years these approaches can be expanded with the Galileo signals although larger bandwidths will be required.

Ieee Transactions on Aerospace and Electronic Systems, 2010

Abstract A large set of equatorial ionospheric scintillation data has been compiled, used to char... more Abstract A large set of equatorial ionospheric scintillation data has been compiled, used to characterize features of severe scintillation that impact Global Positioning System phase tracking, and used to develop a scintillation testbed for evaluating tracking loops. The data ...

ABSTRACT A conceptual method is presented for upgrading exist-ing GPS user equipment, without req... more ABSTRACT A conceptual method is presented for upgrading exist-ing GPS user equipment, without requiring hardware or software modifications to the equipment, to improve the equipment's position, velocity, and time (PVT) accuracy, to increase its PVT robustness in weak-signal or jammed environments, and to protect the equipment from coun-terfeit GPS signals (GPS spoofing). The method is em-bodied in a device called the GPS Assimilator that cou-ples to the radio frequency (RF) input of an existing GPS receiver. The Assimilator extracts navigation and timing information from RF signals in its environment—including non-GNSS signals—and from direct baseband aiding pro-vided, for example, by an inertial navigation system, a frequency reference, or the GPS user. The Assimilator optimally fuses the collective navigation and timing infor-mation to produce a PVT solution which, by virtue of the diverse navigation and timing sources on which it is based, is highly accurate and inherently robust to GPS signal ob-struction and jamming. The Assimilator embeds the PVT solution in a synthesized set of GPS signals and injects these into the RF input of a target GPS receiver for which an accurate and robust PVT solution is desired. A proto-type software-defined Assimilator device is presented with three example applications.

Methods are explored for efficiently mapping GNSS signal processing techniques to multicore gener... more Methods are explored for efficiently mapping GNSS signal processing techniques to multicore general-purpose proces- sors. The aim of this work is to exploit the emergence of multicore processors to develop more capable software- defined GNSS receivers. It is shown that conversion of a serial GNSS software receiver to parallel execution on a 4-core processor via minimally-invasive OpenMP direc- tives leads

Proceedings of the 2012 IEEE/ION Position, Location and Navigation Symposium, 2012

ABSTRACT Background theory, a reference design, and demonstration results are given for a Global ... more ABSTRACT Background theory, a reference design, and demonstration results are given for a Global Navigation Satellite System (GNSS) interference localization system comprising a distributed radio-frequency sensor network that simultaneously locates multiple interference sources by measuring their signals' time difference of arrival (TDOA) between pairs of nodes in the network. The end-to-end solution offered here draws from previous work in single-emitter group delay estimation, very long baseline interferometry, subspace-based estimation, radar, and passive geolocation. Synchronization and automatic localization of sensor nodes is achieved through a tightly-coupled receiver architecture that enables phase-coherent and synchronous sampling of the interference signals and so-called reference signals which carry timing and positioning information. Signal and cross-correlation models are developed and implemented in a simulator. Multiple-emitter subspace-based TDOA estimation techniques are developed as well as emitter identification and localization algorithms. Simulator performance is compared to the Cramér-Rao lower bound for single-emitter TDOA precision. Results are given for a test exercise in which the system accurately locates emitters broadcasting in the amateur radio band in Austin, TX.