High-efficiency Loran-C interference identification by synchronous sampling

A new technique is proposed for estimating the frequencies within carrier-wave interference (CWI) affecting Loran-C receivers. Its novelty lies in that interference samples are taken synchronously with the Loran-C pulses and ensemble-averaged in the time domain prior to conventional spectrum analysis. Each interferer is thus automatically weighted according to its effect on the phase tracking operation of the receiver. The new method substantially increases the efficiency with which the most insidious interferers may be pinpointed. It requires little additional computational power or memory in the receiver.     

Hard Limiting and Sequential Detection Applied to Loran-C

The combination of an antenna, a 100 kHz bandpass filter, a hard limiter, and a sequential detector can supply highly accurate Loran-C data to a digital processor, even under low signal-to-noise-ratio conditions. For such a simple, low-cost receiver, calculations are given for the accuracy of the envelope and phase tracking of the Loran-C signal as a function of the signal-to-noise (Gaussian and atmospheric) ratio, averaging time, and radian speed of the observer with respect to the transmitter. Mentioned are the quasi-noise censoring effects of the hard limiter. Besides the Loran-C application, the hard limiter-sequential detector system can in general be applied for low-cost, synchronous signal detection under poor signal-to-noise ratio.     

Modeling and analysis for the GPS pseudo-range observable

In this paper, a digital system for the Global Positioning System (GPS) pseudo-range observable is modeled and analyzed theoretically. The observable is measured in a GPS receiver by accurately tracking the pseudorandom noise (PRN) code phase of the input GPS signal using a digital energy detector and a digital delay lock loop (DDLL). The following issues are presented: (1) mathematical modeling of the digital PRN code acquisition and tracking system, (2) the closed-form expression derivation for the detection and false-alarm probabilities of the acquisition process and for the variance of code phase tracking error, and (3) the linear and nonlinear performance analysis of the DDLL for optimizing the receiver structures and parameters with tradeoff between the tracking errors due to receiver dynamics and due to input noise     

An Automatic Timing Receiver System Based on the Loran-C Navigational Network

This paper describes some new concepts in dealing with the circuitry for Loran-C automatic timing systems. The conventional analog techniques associated with phase-adjusting networks have been replaced by an incremental digital phase-shifting device. The Loran-C period generator includes facilities for delay compensation by means of an epoch monitor producing a 1-Hz output coincident with the master station TOC (time of coincidence). The required initial time information has to be accurate within ± 20 ms. The automatic format identification and decoding equipment together form a system which takes into account the information of every Loran-C pulse. Owing to the use of digital signal treatment, the synchronization accuracy is limited only by the resolution of the incremental phase shift. The automatic cycle selection device is based on sampling techniques where the derivative of the envelope is calculated. The time of coincidence has to be precalculated and fed into the thumbwheel memory of the epoch monitor, which is automatically initiated when the synchronizing operations are concluded. For VLBI purposes and transcontinental use, the accuracy of this system will be better than 1 ?s when post corrections, supplied by the U.S. Naval Observatory, are taken into account.     

Filtering of discontinuous processes arising in marine integratednavigation systems

A refined stochastic model for the errors of the Loran-C radio navigation aid is described, and it is shown how this model can be used to improve the performance of integrated navigation systems. In addition to the usual propagation errors, Loran-C time of arrival measurements are occasionally plagued with sudden intermittent errors of a particular magnitude and caused by receiver cycle selection errors. These result in sudden large jumps in the calculated position solution. The Loran-C error has been modeled as the sum of a diffusion process, representing the normal propagating errors, and a pure jump process of Poisson type, representing the cycle selection errors. A simple integrated navigation system is then described, based on the Loran-C model and the standard dead reckoning (heading and speed) system model. Assuming that the observed process is governed by a linear stochastic difference equation, a recursive linear unbiased minimum variance filter is developed, from which the Loran-C and dead reckoning errors, and hence position and velocity, can be estimated     

基于apFFT的罗兰-C窄带干扰频域抑制方法

针对现有罗兰-C接收机普遍采用固定陷波电路抑制窄带干扰的情况,提出了基于全相位谱分析(apFFT)的频域精确自适应陷波罗兰-C窄带干扰抑制方法,设计了基于双相移组合全相位法的FIR频域陷波器。在对窄带干扰进行精确谱分析的基础上,设计相应频点的陷波器对多个窄带干扰进行抑制。基于MATLAB的仿真结果表明:该方法能够根据罗兰-C窄带干扰频率的变化,实现频点任意控制的频域自适应陷波,有效恢复出罗兰-C信号,为增强型罗兰-C接收机的设计提供了一种简单有效的窄带干扰抑制方法。     

Analysis of GPS and Loran-C performance for land vehicle navigationin the Canadian Rockies

Loran-C and GPS were assessed for vehicular navigation along selected roads of British Columbia during the winter of 1991. The general topography of this mountainous area is described, together with the specific topographic features and tree coverage characteristics of the 2000 km of roads tested on the mainland and on Vancouver Island. The configuration and characteristics of the Loran C Canadian West Coast chain along the roads used are described. The portable vehicle-mounted system used to collect and analyze the Loran-C and GPS signals along road profiles is described. The performance of Loran-C is analyzed in terms of signal to noise ratios (SNR), field strength, and time-difference distortions, as measured by differential GPS. These distortions, which can reach several hundred meters over distances of less than 20 km, are analyzed in terms of topographic features. The possibility of using these time-independent distortions to calibrate Loran-C for use along the above roads is discussed. Masking of GPS signals due to topographic features and tree coverage along the roads is analyzed. A comparative analysis of both Loran-C and GPS is presented in terms of signal availability and accuracy     

Tracking in clutter with nearest neighbor filters: analysis andperformance

The measurement that is “closest” to the predicted target measurement is known as the “nearest neighbor” (NN) measurement in tracking. A common method currently in wide use for tracking in clutter is the so-called NN filter, which uses only the NN measurement as if it were the true one. The purpose of this work is two fold. First, the following theoretical results are derived: the a priori probabilities of all three data association events (updates with correct measurement, with incorrect measurement, and no update), the probability density functions (pdfs) of the NN measurement conditioned on the association events, and the one-step-ahead prediction of the matrix mean square error (MSE) conditioned on the association events. Secondly, a technique for prediction without recourse to expensive Monte Carlo simulations of the performance of tracking in clutter with the NN filter is presented. It can quantify the dynamic process of tracking divergence as well as the steady-state performance. The technique is a new development along the line of the recently developed general approach to the performance prediction of algorithm with both continuous and discrete uncertainties     

Microminiature Loran-C Receiver/Indicator

A Loran-C Receiver is used as an example to show how an analog system could be converted to a digital one to take advantage of the expanding integrated circuit technology. The digital equivalents of the analog servo elements are described. Criteria for the design of a phase-tracking servomechanism is developed in detail. The Loran performance requirements are used to illustrate their application. The noise performance of a critically damped Type II servomechanism is derived in detail. Since the system will be employed in aircraft, tracking velocity becomes an important consideration. An analysis is made showing that an adaptive control is desirable.     

The Effect of Aircraft Lateral Dynamics on Aircraft Positioning Accuracy When Using Loran-C

The dynamics of an aircraft following a fixed course line using Loran-C for position fixing are shown to interact with Loran-C receiver dynamics to result in cross-track aircraft positioning errors that are smaller than cross-track Loran receiver errors. In a particular case considered, this error reduction is on the order of 50 percent.     

Analysis of an all-digital data-transition tracking loop

An all-digital implementation of the data-transition tracking loop (DTTL) is proposed and analyzed. The input waveform is assumed to be nonreturn-to-zero (NRZ)-coded binary signals. The mathematical analysis of the loop is reduced to the study of a Markov chain which allows the derivation of expressions for the mean square phase error, steady-state probabilities, transient behavior (time to acquire and time to bit slippage), and bit error rate (BER). Theoretical results have been validated using a computer simulation of the loop. The all-digital implementation can take full advantage of advanced technology in memory and processing speeds     

Joint Soviet/American Loran operations: the Bering Sea Chain

The US Coast Guard entered into an agreement with the Soviet Union for the implementation of a mixed Loran-C/Chayka chain in the North Pacific. The similarities and differences of the US Loran-C and USSR Chayka systems are discussed, and the agreed-on design for a Bering Sea Chain is presented. The chain will provide marine and aviation coverage over the five-hundred-mile-wide coverage gap that exists in the North Pacific between the North Pacific chain and the Northwest pacific and Soviet Eastern USSR chains     

Eigen-decomposition techniques for Loran-C skywave estimation

Eigen-decomposition spectral analysis techniques are used to estimate the delays of Loran-C skywaves. Their performance is evaluated and compared with that of Fourier-based techniques. Results using off-air data are presented. This work establishes the basis on which to design a Loran-C receiver capable of adjusting its sampling point adaptively to the optimal value in a constantly changing skywave environment. Such receivers promise to improve significantly the accuracy and reliability of positioning under adverse operational conditions.     

Joint integrated probabilistic data association: JIPDA

A new recursive filter for multi-target tracking in clutter is presented. Multiple tracks may share the same measurement(s). Joint events are formed by creating all possible combinations of track-measurement assignments and the probabilities for these joint events are calculated. The expressions for the joint event probabilities incorporate the probabilities of target existence of individual tracks, an efficient approximation for the cluster volume and a priori probability of the number of clutter measurements in each cluster. From these probabilities the data association and target existence probabilities of individual tracks are obtained, which allows track state update and false track discrimination. A simulation study is presented to show the effectiveness of this approach.     

Precise Decoupling Tracking of Airspeed and Altitude for UAV Based on Causal Solution of Stable Inversion

Use stable inversion to accomplish precise decoupling tracking of airspeed and altitude for conventional takeoff and landing of unmanned aerial vehicles (UAVs) is in essence a non-minimum phase output tracking problem. The main contribution of this article is that a new method to calculate the causal solution of stable inversion is proposed by introducing a well defined perturbed signal to the system's unstable internal dynamics. It is helpful to overcome the pitfalls resulting from non-causality in existing methods. Different from the mathematically accurate offline non-causal solution, the causal solution is an approximation with asymptotically convergent errors. The important merits are: It obviates the needs for the output trajectory to be pre-known time parameterized functions, hence broadening the application of stable inversion; The low computational workload is much more suitable for and beneficial to real-time applications than any existing method based on stable inversion. The output tracking problem is then converted into a state tracking problem based on the causal solution of stable inversion. Precise decoupling tracking of airspeed and altitude is realized by using a feedback controller. Simulations are carried out to verify the viability and low computational workload of the method.     

Transient Response of Tracking Filters with Randomly Interrupted Data

The transient responses during the initialization phase of a first-order ?-? tracking filter and a second-order Kalman filter are evaluated as a function of radar measurement accuracy and the probability of receiving valid data at the prescribed intervals. Monte Carlo simulation results are complemented by analysis of the filtering processes and curves are presented which clearly define the deterioration in filter performance attributable to reduced probabilities of data acquisition. In addition, the responses of ?-? and Kalman filters are shown to be identical when the ?, ? gains are selected optimally.     

Multi-Target Tracking in Clutter without Measurement Assignment

When tracking targets using radars and sonars, the number of targets and the origin of data is uncertain. Data may be false measurements or clutter, or they may be detections from an unknown number of targets whose possible trajectories and detection processes can only be described in a statistical manner. Optimal all-neighbor multi-target tracking (MTT) in clutter enumerates all possible joint measurement-to-track assignments and calculates the a posteriori probabilities of each of these joint assignments. The numerical complexity of this process is combinatorial in the number of tracks and the number of measurements. One of the key differences between most MTT algorithms is the manner in which they reduce the computational complexity of the joint measurement-to-track assignment process. We propose an alternative approach, using a form of soft assignment, that enables us to bypass this step entirely. Specifically, our approach treats possible detections of targets followed by other tracks as additional clutter measurements. It starts by approximating the a~priori probabilities of measurement origin. These probabilities are then used to modify the clutter spatial density at the location of the measurements. A suitable single target tracking (STT) filter then uses the modified clutter intensity for updating the track state. In effect, the STT filter is transformed into an MTT filter with a numerical complexity that is linear in the number of tracks and the number of measurements. Simulations show the effectiveness of this approach in a number of different multi-target scenarios.     

密集杂波环境下的数据关联快速算法

基于联合概率数据互联（JPDA）的思想,提出了一种新的数据关联快速算法（Fast Al-gorithm for Data Association,简称FAFDA算法）.该方法不需象在最优JPDA算法中那样生成所有可能的联合互联假设,因而具有计算量小,易于工程实现的特点。仿真结果表明,与最优JPDA算法相比,FAFDA算法的跟踪性能令人满意,并且在密集杂波环境下可实时、有效地跟踪100批次以上的目标。     

Using GPS to calibrate Loran-C

Various techniques for using simultaneous Global Positioning System (GPS)/Loran data to estimate the propagation uncertainties that limit the absolute accuracy of Loran-C are discussed. Significant improvements in the absolute accuracy of Loran can be achieved with very simple calibrations. The absolute accuracy of Loran in the Gulf of Maine without calibration is presented. The maximum and RMS absolute errors are between 700 and 500 m, depending on the choice of land model. Simple calibrations greatly improve the absolute accuracy of Loran. As shown, if the land conductivities are fixed a priori and a single parameter is optimized, the maximum and RMS absolute errors fall to around 250 and 60 m, respectively. Alternatively, land can be treated as a single conductivity which can be adjusted to reduce offshore additional secondary phase factor errors. The performance of this practice is summarized in tables which show maximum and RMS errors of around 300 to 100 m, respectively     

北斗／罗兰-C组合定位技术

“北斗一号”是我国的卫星导航系统,其有源定位模式使其很难得到广泛应用。为此,提出了一种北斗／罗兰-C组合定位方案以实现无源定位。该方案利用“北斗一号”两颗卫星和地面罗兰-C台站获得的距离差信息实现双曲线相交定位。仿真结果表明,该方案的精度可以满足一般定位要求,具有较强的实用性。