Dennis M. Akos

Direct bandpass sampling of multiple distinct RF signals

A goal in the software radio design philosophy is to place the analog-to-digital converter as near the antenna as possible. This objective has been demonstrated for the case of a single input signal. Bandpass sampling has been applied to downconvert, or intentionally alias, the information bandwidth of a radio frequency (RF) signal to a desired intermediate frequency. The design of the software radio becomes more interesting when two or more distinct signals are received. The traditional approach for multiple signals would be to bandpass sample a continuous span of spectrum containing all the desired signals. The disadvantage with this approach is that the sampling rate and associated discrete processing rate are based on the span of spectrum as opposed to the information bandwidths of the signals of interest. Proposed here is a technique to determine the absolute minimum sampling frequency for direct digitization of multiple, nonadjacent, frequency bands. The entire process is based on the calculation of a single parameter-the sampling frequency. The result is a simple, yet elegant, front-end design for the reception and bandpass sampling of multiple RF signals. Experimental results using RF transmissions from the US Global Positioning System-Standard Position Service (GPS-SPS) and the Russian Global Navigation Satellite System (GLONASS) are used to illustrate and verify the theory.

Design and implementation of a direct digitization GPS receiver front end

A direct digitization approach greatly reduces the hardware requirements in traditional front end design. Further, the hardware that has been eliminated is typically the source of a number of potential difficulties including age-based, temperature-based, and/or nonlinear performance. This paper presents a case study on the design and implementation of direct digitization Global Positioning System (GPS) receiver front end. First, sensitivity and dynamic range issues for a generic front end are discussed with particular attention given to the unique requirements in the direct digitization approach. Second, two GPS front end implementations are compared. The first is the direct digitization of the input signal at radio frequency (RF) as is the case in the true digital receiver or software radio. The second uses a more standard approach of downconverting the input signal to an intermediate frequency (IF) for further processing of digitization. Experimental data is presented which characterizes the relative signal-to-noise ratio for both implementations as well as the results of initial acquisition processing of true GPS data.

Carrier loop architectures for tracking weak GPS signals

The performance of various carrier recovery loop architectures (phase lock loop (PLL), Doppler-aided PLL, frequency lock loop (FLL), and Doppler-aided FLL) in tracking weak GPS signals are analyzed and experimentally validated. The effects of phase or frequency detector design, oscillator quality, coherent averaging time, and external Doppler aiding information on delaying loss of lock are quantified. It is shown that for PLLs the metric of total phase jitter is a reliable metric for assessing low C/N performance of the tracking loop provided the loop bandwidth is not too small (~> 5 Hz). For loop bandwidths that are not too small, total phase jitter accurately predicts carrier-to-noise ratio (C/N) at which loss of lock occurs. This predicted C/N is very close to the C/N predicted by bit error rate (BER). However, unlike BER, total phase jitter can be computed in real-time and an estimator for it is developed and experimentally validated. Total phase jitter is not a replacement for BER, since at low bandwidths it is less accurate than BER in that the receiver loses lock at a higher C/N than predicted by the estimator. Similarly, for FLLs operating at small loop bandwidths, it is found that normalized total frequency jitter is not a reliable metric for assessing loss of lock in weak signal or low C/N conditions. At small loop bandwidths, while total frequency jitter may indicate that a loop is still tracking, the Doppler estimates provided by the FLL will be biased.

A Methodology for the Evaluation of a GPS Receiver Performance in Telematics Applications

This paper presents a methodology to evaluate the position availability of automotive grade global positioning system (GPS) receivers intended for Telematics applications utilizing a multichannel GPS satellite signal simulator in a controlled laboratory environment. Initially, field testing of two distinct GPS receivers was conducted in an urban canyon environment and a foliage environment to assess each receivers position availability performance. Test scenarios were then developed on a multichannel GPS satellite signal simulator in order to create controlled and repeatable stimuli to the GPS receivers. The scenarios take into account the actual satellite constellations at the same day, time, and locations of the field data collections. Furthermore, the number of visible satellites and power levels was adjusted in order to stimulate the hardware tracking sensitivity, hardware acquisition sensitivity, dynamic range, and navigation filter design, all of which impact position availability for GPS receivers. Quantitative results demonstrated good correlation between the results obtained using the developed test scenarios and the results from the field testing. The proposed methodology will result in reducing validation cost and time to market for automotive Telematics products

GPS C/N/sub 0/ estimation in the presence of interference and limited quantization levels

The carrier-to-noise density ratio (C/N0) is considered an important parameter describing the GPS receiver performance. This paper compares the performance of two popular coarse-acquisition (C/A) C/N0 algorithms appearing in literature: the variance summing method (VSM) (Psiaki et al., 2003, Psiaki, 2001), and the power ratio method (PRM) (Van Dierendonck, 1996, Sayre, 2003), in terms of their estimates in 1) additive white Gaussian noise (AWGN), 2) narrowband continuous wave interference (CWI), 3) their response to quantization and saturation effects, and their 4) dynamic range. The algorithms were implemented as a part of a software receiver. Two LI GPS data sets are examined; one was obtained from a GPS raw data collection setup, while the other was obtained from a GPS signal simulator. The collected set was stored with almost constant C/N0 level while the simulated one contained variable C/N0 levels. The effect of adding AWGN on the C/N0 estimate was directly proportional with the noise power. The C/N0 estimates suffered more when the CWI frequency was closer to the IF of the receiver. The PRM suffered from saturation at higher C/N0 levels. The VSM showed good tracking at high C/N0 levels and better immunity to limited quantization levels, while its C/N0 estimate suffered from rapid fluctuations in power levels when sudden power steps occurred

Airborne GNSS-R Wind Retrievals Using Delay–Doppler Maps

Global navigation satellite system (GNSS) reflectometry has emerged recently as a promising remote sensing tool to retrieve various geophysical parameters of the Earths surface. GNSS-reflected signals, after being received and processed by the airborne or spaceborne receiver, are available as delay correlation waveforms or as delay-Doppler maps (DDMs). In the case of a rough ocean surface, those characteristics can be related to the rms of the L-band limited slopes of the surface waves and, from there, to the surface wind speed. The raw GNSS-reflected signal can be either processed in real time by the receiver or recorded and stored on board and postprocessed in a laboratory. The latter approach leveraging a software receiver allows more flexibility while processing the raw data. This work analyzes DDMs obtained as a result of processing of the data collected by the Global Positioning System (GPS) data logger/software receiver on board the National Oceanic and Atmospheric Administration Gulfstream-IV jet aircraft. Thereafter, the DDMs were used to retrieve surface wind speed employing several different metrics that characterize the DDM extent in the Doppler frequency-delay domain. In contrast to previous works in which winds have been retrieved by fitting the theoretically modeled curves into measured correlation waveforms, here, we do not rely on any model for the determination. Instead, the approach is based on a linear regression between DDM observables and the wind speeds obtained in simultaneous GPS dropsonde measurements.

L and S bands spectrum survey in the San Francisco bay area

The Global Positioning System (GPS) is a radio frequency (RF) communication system that consists of transmitters on the satellites and receivers on the ground. Because of substantial path loss, the received signal power from satellites is extremely weak and even below the thermal noise floor and as such is very sensitive to changes in the underlying noise floor. The goal of this work is to investigate the radio spectrum environment in the GPS band along with two additional bands, the Unified-S band and 2.4 GHz Industrial Scientific and Medical (ISM) band. The spectrum survey was conducted at various locations in the San Francisco Bay area including various urban, rural areas and airports and harbors which are operational significant to GPS. The measurement data collected in this study will provide a more accurate representation of the current status and the characteristics of the spectrum environment. Geographical variation within the sites will reveal correlation between the spectrum environment and the level of urbanization and also a comparative study on the frequency bands under different level of regulations can be used to investigate the effectiveness of the current spectrum policy.

Monocular Camera/IMU/GNSS Integration for Ground Vehicle Navigation in Challenging GNSS Environments

Low-cost MEMS-based IMUs, video cameras and portable GNSS devices are commercially available for automotive applications and some manufacturers have already integrated such facilities into their vehicle systems. GNSS provides positioning, navigation and timing solutions to users worldwide. However, signal attenuation, reflections or blockages may give rise to positioning difficulties. As opposed to GNSS, a generic IMU, which is independent of electromagnetic wave reception, can calculate a high-bandwidth navigation solution, however the output from a self-contained IMU accumulates errors over time. In addition, video cameras also possess great potential as alternate sensors in the navigation community, particularly in challenging GNSS environments and are becoming more common as options in vehicles. Aiming at taking advantage of these existing onboard technologies for ground vehicle navigation in challenging environments, this paper develops an integrated camera/IMU/GNSS system based on the extended Kalman filter (EKF). Our proposed integration architecture is examined using a live dataset collected in an operational traffic environment. The experimental results demonstrate that the proposed integrated system provides accurate estimations and potentially outperforms the tightly coupled GNSS/IMU integration in challenging environments with sparse GNSS observations.

A direct RF sampling multifrequency GPS receiver

The future satellite positioning/navigation systems (both GPS and Galileo) will provide civil signals on multiple frequencies, similar to that currently available only for military use. The multiple distinct frequencies will provide many advantages to users of the navigation systems. This paper presents a direct RF sampling front end design well suited for multiple frequency satellite navigation receiver design. No frequency down conversion is necessary, rather the particular frequency bands of interest are intentionally aliased using a wide band ADC. The resulting samples are passed, via a doubling buffering FPGA design, to the memory space of a host PC for storage as well as eventually processing of the multiple frequency transmissions. This paper describes the design of the front-end, validates its concept with collected data, and discusses the variations on the design of a generic multiple frequency GPS front end.

Precise phase calibration of a controlled reception pattern GPS antenna for JPALS

The Joint Precision Approach and Landing System (JPALS) is being developed to provide navigation to support aircraft landings for the U.S. military. One variant of JPALS is the Shipboard Relative GPS (SRGPS), which will be implemented on an aircraft carrier. In order to meet strict accuracy, integrity, continuity, and availability goals in the presence of hostile jamming and in a harsh multipath environment, advanced technologies are required. One of those being studied is a controlled reception pattern antenna (CRPA) array with beam steering/adaptive null forming capabilities. The Stanford University GPS Laboratory has developed a software tool to study CRPA algorithms and their effects on GPS signal and tracking characteristics. A testbed has been constructed to investigate hardware issues including the phase center offset of the antenna elements and mutual coupling effects. This testbed consists of a 3 element antenna array with a baseline of 1m, using high-quality survey-grade or lower-quality patch antennas. Data has been taken using this array in conjunction with sufficient satellite constellation and antenna array motion to ensure complete azimuth and elevation signal coverage. A carrier phase-based attitude determination algorithm was used to generate inter-antenna bias residuals, allowing characterization of the virtual phase center of the array. Repeating the testing procedure both with survey-grade antennas, for which the phase center characteristics are well known, and with a patch antenna possessing unknown phase center behavior, allows characterization of the azimuth- and elevation-dependent properties of the patch antenna phase center. In addition, mutual coupling effects have been investigated by adding inactive patch elements around the active patch antenna. All results are compared to predictions from detailed simulation of the patch antenna used using an EM modeling software package.