David S. De Lorenzo

Compass-M1 Broadcast Codes in E2, E5b, and E6 Frequency Bands

With the launch of the compass-M1 satellite on 14 April 2007, China is set to become the latest entrant into global navigation satellite systems (GNSS). Understanding the interoperability and integration of the Chinese Compass with the current GNSS, namely the U.S. Global Positioning System (GPS), the European Galileo, and the Russian GLONASS, requires knowing and understanding its signal structures-specifically its pseudorandom noise (PRN) codes and code structures. Moreover, the knowledge of the code is a prerequisite for designing receivers capable of acquiring and tracking the satellite. More important is determining if the signal may degrade performance of the current GNSS in the form of interference. Finally, we are eager to learn from the code and signal design of our Chinese colleagues. For this research, we set up a 1.8-m dish antenna to collect the broadcast Compass-M1 signals. Even with the dish antenna, the received signal is still weak and buried in thermal noise. We then apply signal processing and are able to extract the PRN code chips out of the noise in all three frequency bands. The PRN codes are thousands of bits long. In addition, we find that the Compass-M1 PRN codes in all frequency bands are Gold codes. We also derive the Gold code generators to represents thousands of code chips with fewer than a hundred bits. Finally, we implement these codes in our software receiver to verify and validate our analysis.

Design and Implementation of Real-Time Software Radio for Anti-Interference GPS/WAAS Sensors

Adaptive antenna array processing is widely known to provide significant anti-interference capabilities within a Global Navigation Satellite Systems (GNSS) receiver. A main challenge in the quest for such receiver architecture has always been the computational/processing requirements. Even more demanding would be to try and incorporate the flexibility of the Software-Defined Radio (SDR) design philosophy in such an implementation. This paper documents a feasible approach to a real-time SDR implementation of a beam-steered GNSS receiver and validates its performance. This research implements a real-time software receiver on a widely-available x86-based multi-core microprocessor to process four-element antenna array data streams sampled with 16-bit resolution. The software receiver is capable of 12 channels all-in-view Controlled Reception Pattern Antenna (CRPA) array processing capable of rejecting multiple interferers. Single Instruction Multiple Data (SIMD) instructions assembly coding and multithreaded programming, the key to such an implementation to reduce computational complexity, are fully documented within the paper. In conventional antenna array systems, receivers use the geometry of antennas and cable lengths known in advance. The documented CRPA implementation is architected to operate without extensive set-up and pre-calibration and leverages Space-Time Adaptive Processing (STAP) to provide adaptation in both the frequency and space domains. The validation component of the paper demonstrates that the developed software receiver operates in real time with live Global Positioning System (GPS) and Wide Area Augmentation System (WAAS) L1 C/A code signal. Further, interference rejection capabilities of the implementation are also demonstrated using multiple synthetic interferers which are added to the live data stream.

Navigation, Interference Suppression, and Fault Monitoring in the Sea-Based Joint Precision Approach and Landing System

The United States Navy seeks the capability to land manned and unmanned aerial vehicles autonomously on an aircraft carrier using GPS. To deliver this capability, the Navy is developing a navigation system called the Sea-Based Joint Precision Approach and Landing System (JPALS). Because standard GPS is not sufficiently precise to land aircraft on a shortened, constantly moving runway, Sea-Based JPALS leverages dual-frequency, carrier-phase differential GPS navigation. Carrier phase measurements, derived from the sinusoidal waveforms underlying the GPS signal, are very precise but not necessarily accurate unless the user resolves the ambiguity associated with the sinusoids periodicity. Ensuring the validity of ambiguity resolution is the central challenge for the high-integrity, safety-critical JPALS application. Based on a multi-year, multi-institution collaborative study, this paper proposes a navigation and monitoring architecture designed to meet the guidance quality challenge posed by Sea-Based JPALS. In particular, we propose a two-stage navigation algorithm that meets the aggressive integrity-risk requirement for Sea-Based JPALS by first filtering a combination of GPS observables and subsequently exploiting those observables to resolve the carrier ambiguity. Because JPALS-equipped aircraft may encounter jamming, we also discuss interference mitigation technologies, such as inertial fusion and array antennas, which, with appropriate algorithmic modifications, can ensure integrity under Radio Frequency Interference (RFI) conditions. Lastly, we recommend a fault monitoring strategy tailored to the two-stage navigation algorithm. Monitoring will detect and isolate rare anomalies such as ionosphere storms or satellite ephemeris errors which would otherwise corrupt ambiguity resolution and positioning in Sea-Based JPALS.

A Real-Time Capable Software-Defined Receiver Using GPU for Adaptive Anti-Jam GPS Sensors

Due to their weak received signal power, Global Positioning System (GPS) signals are vulnerable to radio frequency interference. Adaptive beam and null steering of the gain pattern of a GPS antenna array can significantly increase the resistance of GPS sensors to signal interference and jamming. Since adaptive array processing requires intensive computational power, beamsteering GPS receivers were usually implemented using hardware such as field-programmable gate arrays (FPGAs). However, a software implementation using general-purpose processors is much more desirable because of its flexibility and cost effectiveness. This paper presents a GPS software-defined radio (SDR) with adaptive beamsteering capability for anti-jam applications. The GPS SDR design is based on an optimized desktop parallel processing architecture using a quad-core Central Processing Unit (CPU) coupled with a new generation Graphics Processing Unit (GPU) having massively parallel processors. This GPS SDR demonstrates sufficient computational capability to support a four-element antenna array and future GPS L5 signal processing in real time. After providing the details of our design and optimization schemes for future GPU-based GPS SDR developments, the jamming resistance of our GPS SDR under synthetic wideband jamming is presented. Since the GPS SDR uses commercial-off-the-shelf hardware and processors, it can be easily adopted in civil GPS applications requiring anti-jam capabilities.

GPS attitude determination for a JPALS testbed: Integer initialization and testing

An attitude-based search algorithm was implemented for initial ambiguity resolution and integer determination, and then tested on a three-element equilateral array with baselines of 0.5m. Running the initialization algorithm at every epoch, it was found that the search algorithm gave efficient and reliable integer solutions for a static antenna array, but was sensitive to errors in a dynamic environment in the absence of multi-epoch filtering or solution checking procedures. Incorporating knowledge of allowable array orientation into the search greatly increased execution speed and reduced spurious integer estimates. This paper reviews the fundamentals of multi-antenna GPS-based attitude determination, develops in detail a simple and efficient 3-D search algorithm, discusses tradeoffs between execution speed (search spacing), signal phase noise, and estimate reliability, and presents results from static and in-motion automotive testing. Based on these findings, the utility of integration between GPS and inertial systems for robust attitude determination cannot be overstated.

Calibrating adaptive antenna arrays for high-integrity GPS

A major challenge in using GPS guidance for aircraft final approach and landing is to reject interference that can jam reception of the GPS signals. Antenna arrays, which use space–time adaptive processing (STAP), significantly improve the signal to interference plus noise ratio, but at the possible expense of distorting the received signals, leading to timing biases that may degrade navigation performance. Rather than a sophisticated calibration approach to remove biases introduced by STAP, this paper demonstrates that a relatively compact calibration strategy can substantially reduce navigation biases, even under elevated interference conditions. Consequently, this paper develops an antenna bias calibration strategy for two classes of adaptive array algorithm and validates this method using both simulated and experimental data with operational hardware in the loop. A proof-of-concept system and an operational prototype are described, which implement the adaptive antenna algorithms and deterministic corrections. This investigation demonstrates that systems with adaptive antenna arrays can approach the accuracy and integrity requirements for automatic aircraft landing, and in particular for sea-based landing on board aircraft carriers, while simultaneously providing significant attenuation of interference. Evidence suggests that achieving these goals is possible with minimal restrictions on system hardware configuration—specifically, limitations on the permissible level of antenna anisotropy and the use of sufficient analog-to-digital converter resolution.

Secure Location Verification

The use of location based services has increased significantly over the last few years. However, location information is only sparsely used as a security mechanism. One of the reasons for this is the lack of location verification techniques with global coverage. Recently, a new method for authenticating signals from Global Navigation Satellite Systems(GNSS) such as GPS or Galileo has been proposed. In this paper, we analyze the security of this signal authentication mechanism and show how it can be used to establish a secure location verification service with global coverage. This new security service can be used to increase the security of various different applications, even if they are not directly connected to navigation or positioning.