Sherman Lo

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.

Authenticating aviation augmentation system broadcasts

This paper studies the feasibility and means by which authentication can be overlaid upon the existing SBAS and GBAS designs. It considers how to achieve the authentication that is compatible with the current augmentation system and its users. It also considers how to perform the security necessary to support authentication within the current NAS framework. One important issue is secure key distribution and the paper presents some options designed to be reasonable for aviation infrastructure and operations. One means is a key distribution protocol that utilizes the operation of the aircraft and air traffic to aid in key verification. This provides to distribute keys and provide some ability to validate them without significant additions to the NAS. Another issue is bandwidth. The paper presents ways of modifying protocols such as TESLA to reduce bandwidth use while maintaining an acceptable level of security. The paper uses the current L1 SBAS and GBAS as case studies. The paper presents reasonable method to provide authentication on the current SBAS using about ten percent of bandwidth. The method is compatible to current SBAS user equipment in that they will not be adversely affected. GBAS can employ similar means. As it has greater data bandwidth, a more critical issue for GBAS is key distribution to the ground stations.

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.

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.

Loran data modulation: extensions and examples

Loran has provided navigation service since 1958. Though not originally designed with data broadcast capabilities, Lorans versatility has enabled data to be broadcast with great benefits. Research in the last two decades has resulted in a tremendous increase in the data capacity of Loran thereby increasing its utility. Currently, a modernized Loran is being evaluated for its capability to backup GPS and data modulation is an integral part of this Loran design. This paper details some recent Loran modulation designs and ideas.

Loran Data Modulation: A Primer [AESS Tutorial IV]

Loran has provided navigation service since 1958. Though not originally designed with data broadcast capabilities, Lorans versatility has enabled data to be broadcast with great benefits. Research in the last two decades has resulted in a tremendous increase in the data capacity of Loran thereby increasing its utility. Currently, a modernized Loran is being evaluated for its capability to backup GPS and data modulation is an integral part of this Loran design. An overview and analysis of Loran modulation techniques is provided.

Improving Loran Coverage with Low Power Transmitters

Enhanced Loran (eLoran) is currently being implemented to provide back up to global navigation satellite systems (GNSS) in many critical and essential applications. In order to accomplish this, eLoran needs to provide a high level of availability throughout its desired coverage area. While the current Loran system is generally capable of accomplishing this, worldwide, there remain a number of known areas where improved coverage is desirable or necessary. One example is in the middle of the continental United States where the transmitter density is not adequate for providing the desired availability for applications such as aviation in some parts. This paper examines the use of lower power, existing assets such as differential GPS (DGPS) and Ground Wave Emergency Network (GWEN) stations to enhance coverage and fill these gaps. Two areas covered by the paper are the feasibility and performance benefits of using the antennas at these sites. Using DGPS, GWEN or other existing low frequency (LF) broadcast towers requires the consideration of several factors. The first is the ability of the transmitting equipment to efficiently broadcast on these antennas, which are significantly shorter than those at a Loran station. Recent tests at the US Coast Guard Loran Support Unit (LSU) demonstrated the performance of a more efficient transmitter. This technology allows for the effective use of smaller antennas at lower power levels. Second is the ability to broadcast a navigation signal that is compatible with the Loran system and the potential DPGS broadcast (when using a DGPS antenna). The paper examines some possibilities for navigation signals. The goal is to develop a suitable low power signal that enhances navigation and is feasible for the transmission system. The second part of the paper examines the benefits of using these stations. The benefits depend on the location of the stations and the ability seamlessly to integrate them within the existing Loran infrastructure. Analysis of these factors is presented and the coverage benefits are examined.

Evaluation & comparison of ranging using Universal Access Transceiver (UAT) and 1090 MHz Mode S Extended Squitter (Mode S ES)

The FAA Alternative Position Navigation and Timing effort is developing technologies to provide navigation service capable of sustaining operations in the event of the loss of Global Navigation Satellite Systems (GNSS). APNT will utilize existing ground infrastructure to support this capability. One effort is to examine the use of the Automatic Dependent Surveillance Broadcast (ADS-B) ground infrastructure for ranging. This paper examines the use the two transmitted ADS-B signals: 1) 1090 MHz Mode S Extended Squitter (Mode ES) and Universal Access Transceiver (UAT). It uses the transmitted, on-air signal to examine multipath, ranging and timing performance.

Security from Location

The emergence of the Internet and personal computers has led to an age of unprecedented information content and access. The proliferation of Internet connectivity, personal computers, and portable, high density data storage has put volumes of data are at one’s fingertips. While the spread of such technology has increased efficiency and knowledge, it has also made information theft easier and more damaging.