Sana Ullah Qaisar

Partial Differential Postcorrelation Processing for GPS L2C Signal Acquisition

L2C is the second civilian signal introduced on the modernized block of GPS satellites. The two PRN sequences employed in L2C, named civil moderate (CM) and civil long (CL), have periods of 20 ms and 1.5 s, respectively. Stemming from the fact that using a full code period (CM or CL) for signal acquisition in GPS L2C receivers might not be necessary in normal situations (e.g. outdoor, light indoor, etc.), in this paper, we introduce a partial acquisition architecture using specially-designed matched filters (MFs) in order to relieve the computational complexity of the acquisition stage. The partial correlation loss is compensated by differential postcorrelation techniques. Three techniques, namely conventional differential combination (CDC), generalized differential combination (GDC), and modified generalized differential combination (MGDC), are investigated in terms of detection probabilities and mean acquisition time leading to the selection of MGDC as the most suitable technique for the L2C partial acquisition. By using this technique, a 2-dB sensitivity improvement with respect to the conventional noncoherent combination and a 94.5% reduction in mean acquisition time in comparison with the full code acquisition are shown for 1-ms partial correlation.

Assessment of the GPS L2C Code Structure for Efficient Signal Acquisition

L2C is the second Global Positioning System (GPS) civilian signal to become available over the full constellation. Because of the unusually-long ranging codes used in L2C signal, the search space and the computational complexity for signal acquisition become huge. The time-multiplexed nature of L2C code can, however, be exploited to design smarter replica codes that expedite the signal search and reduce the computational complexity. In this paper, various designs of L2C replica code are assessed for efficient signal acquisition, and the search capabilities and potential application scenarios for each of the assessed designs are identified. The nonreturn-to-zero civil moderate (NRZ CM) replica code design is exploited to develop a chipwise (CW) correlation strategy. In this strategy, the received L2C code samples are accumulated across each chip period, and the correlation is then performed at the L2C code chipping rate, reducing both the code search space and the computational complexity significantly. The detection performance of CW strategy is assessed as equivalent to that of the full-rate correlation, irrespective of the phase of the received L2C code.

Assessing design dependencies in modular systems

Modular system design offers several benefits such as flexibility, upgrade, extension and inter-operability across multiple vendors. The level of system modularity is strongly driven by design inter-dependencies. Several academic models have been proposed for assessing these inter-dependencies. However, there is lack of consistency across these models in terms of evaluation metric and scope of application. In this paper, by measuring system pleiotropy, we compare three existing models to investigate essential elements of reliable assessment methodology. The three models are applied on a computer mouse architecture. It is identified that by including module composition, inter-module interface complexity and functional dependencies, a credible measure of system design dependencies can be obtained.

Minimizing pleiotropic effects in modular and hierarchical system architectures

Complex systems are distinguished by high-pleiotropy interactions where the effect of a change in one physical element ripples throughout the system. Previous studies on this subject conclude that system pleiotropy, which measures such inter-dependencies across a system, varies inversely with the level of modularity adopted, suggesting maximum modularity is useful for minimizing the pleiotropic interactions in a system. Utilising the generalized NK modelling framework with a basic symmetrically distributed system model, in this paper, we demonstrate that there exists an optimal level of modularity as opposed to maximal modularity and an optimal level of hierarchy that minimizes the system pleiotropy in single-layered and hierarchically structured system architectures respectively. It is also identified that, proportional to system size, pleiotropy becomes sensitive to optimal levels of modularity and hierarchy. The assessment approach and analytical derivations presented in the paper provide a baseline for further research on restricting pleiotropic effects in practical systems.

Processing Cost of Doppler Search in GNSS Signal Acquisition: Measuring Doppler shift in navigation satellite signals

To acquire a global navigation satellite system (GNSS) signal, the receiver must determine the Doppler offset experienced by the signal. The time and resources consumed in the Doppler search contribute to the cost and efficiency of the receiver, which is fundamentally dependent on signal processing techniques adapted for the search.

A novel efficient signal processing approach for combined acquisition of GPS L1 and L2 civilian signals

L1-L2C is the first dual-frequency pair of civilian GPS signals, expected to become available over the full constellation by 2019. Pre-operational CNAV broadcast is made available to support development of L2C signal processing capability in modernized GPS receivers. The transmission of synchronized L1 C/A and L2C codes by Block IIR-M and follow-on generations of GPS satellites allows GPS receiver designers to develop correlators for combined acquisition of the two signals to achieve additional detection sensitivity when compared to processing an individual signal. One of the key challenges in this joint acquisition is the processing complexity or cost of Doppler search to be conducted over the synchronized data-bit intervals of 20 milliseconds in each of the L1 and L2C channels. To date, researchers have addressed this problem by limiting the coherent correlation interval to a 1ms period, thereby reducing the Doppler search space while combining the outcomes of 20 coherent correlations through non-coherent and differentially-coherent processing techniques to compensate the effect of residual Doppler appearing across the data-bit interval in each of the L1 and L2C channels. These solutions however compromise the signal-tonoise- ratio when compared to fully coherent processing of signals over 20 milliseconds. In this paper, we propose complex-averaging of L1 and L2C signals at the IF stage to reduce the sampling rate of the Doppler signal, and accomplish the Doppler search at a low processing cost without compromising the detection performance of individual channels. The scheme is evaluated theoretically as well as validated through experiments on real GPS signals broadcast by Block IIR-M satellites. The proposed processing approach allows GPS receivers to detect weaker signals through low-power processors.

Affordable processing for long coherent integration of weak debris-scattered GNSS signals with inconsistent Doppler

Space Surveillance and Tracking is the capacity to identify and anticipate the kinematics of space debris in orbit around the Earth. It is important for avoiding collisions with the International Space Station, operational satellites and other spacecraft. GNSS signals have been proposed as illumination sources for passive radar to track space debris. Detecting space debris by scattered GNSS signals requires extremely long integration to gather sufficient energy. Such long coherent processing is computationally expensive, and non-uniformities in the signal phase variations confound many existing processing aids, such as the Fast Fourier Transform. In this paper, we propose a novel multi-step processing strategy which is shown to be able to reduce the processing burden to an affordable level. The first step correlates the received signal with a replica of the expected signal, followed by an integrate and dump. This reduces the bandwidth of the signal to Audio frequencies, taking care that the target uncertainty volume is preserved. In the final step, the full length coherent integration is formed by summing the audio samples with the necessary phase adjustments, for example allowing the residual phase to vary a second order non-linear function over time. This processing technique is demonstrated using real data collected from GPS satellites, highlighting the ability to synthesize coherent integrations within a reasonable uncertainty volume at low cost using just the audio rate signal.

System construction cost of hierarchical-modular structures

Modularity and hierarchy are sometimes used interchangeably. In this paper we provide a novel definition of modularity and hierarchy, as well as hierarchical-modularity. The effects of hierarchical-modularity on system characteristics including constructability is understudied. This paper presents a simple model for system constructability based on the construction cost estimated via a Baysian estimation method. We derive the construction cost of a hypothetical systems of nodes and links, under different scenarios. These scenarios include various linking success probabilities and various hierarchical configurations, including the number of hierarchical levels and number of modules within each hierarchy.