Performance Evaluation of IFFT Technique for Skywave Detection in Loran-C Receivers

Adaptive skywave estimation techniques that allow Loran-C receivers to adjust their sampling points in real-time would let them minimize the effects of skywave interference on the one hand, and noise and interference on the other. This paper investigates the effects of noise and filtering on the skywave delay estimation performance of the Inverse Fast Fourier transform (IFFT) skywave detection technique. Its performance is assessed using computer simulation under a range of realistic conditions, and by the use of off-air signals. The simulation results show that, even with poor signal-to-noise ratios, the time delays of skywave components may be measured successfully. The paper also presents for the first time skywave delay and amplitude estimates made by the IFFT algorithm on off-air signals. INTRODUCTION Most current Loran-C receivers determine the arrival times of groundwave and skywave pulses by sampling a fixed zero-crossing on their leading edges. If the sampling time is set too late, such receivers suffer unacceptable skywave errors; if too early, they suffer low signal-to-noise ratio (SNR) and signalto-interference ratio (SIR). At previous ILA Conventions and in recent publications [1-5] we have demonstrated the feasibility of Fourier-based and other high-resolution techniques in estimating the skywave delays of Loran-C signals. In these papers, we have shown that the high-resolution algorithms, notably AR, ARMA and MUSIC, yield more precise skywave delay estimates in the time domain than do Fourier-based algorithms [3,4]. This improvement, however, is achieved at the expense of higher computational complexity. In this paper we revisit the IFFT technique, carrying out simulations to evaluate the parameters that determine its performance. In particular, we investigate the effects of noise and filtering. We show that by careful optimization of a windowing filter, much improved skywave delay estimates can be obtained. The performance of the IFFT technique is assessed by theoretical analysis, by computer simulation under a range of realistic conditions, and using off-air signals. SKYWAVE REJECTION BY LORAN-C RECEIVERS In a perfect world, Loran-C signals would travel from transmitter to receiver as groundwaves. In practice, skywave signal components also reach the receiver, by means of ionospheric propagation. Because these skywave components travel via longer paths, they normally arrive at least 35 μs after the corresponding groundwave components. The receiver is thus presented with the sum of groundwave and skywaves (Fig. 1a). However, the standard zero-crossing (30 μs into the pulse) should always precede the arrival of the earliest skywave component. Thus, the receiver makes its time measurement on the groundwave pulse prior to the arrival of the first skywave component. Ideal receivers, therefore, ought not to suffer from skywave interference. In practice, Loran-C receivers are limited in their ability to identify the standard zero-crossing in the presence of strong skywave signals, especially those of short delay. The finite bandwidths of their frontend filters increase the rise-times of the received pulses (Fig. 1b). In consequence, the amplitude of the third cycle is so greatly reduced that the timing measurement have to be made on a later zero-crossing. By then it is possible that skywave components will be present. The narrower the filter, the greater this rise-time, and so the greater the susceptibility of the receiver to skywave interference [6]. In practice, therefore, receiver design is a compromise between filter bandwidth and skywave tolerance. Receiver performance could be optimised if the choice of zero-crossing, rather than being fixed, could be adjusted so that latest zero-crossing prior to the arrival of significant skywave energy were always used. Since the delay of the first skywave varies greatly from moment-to-moment and with time and season, this adjustment would have to be made dynamically. Thus the receiver would need to be able to analyse the incoming signal and measure the delay of the first skywave component with respect to the groundwave. This paper presents and analyses a technique for doing so. 0 20 40 60 80 100 120 140 160 180 200 -5 -4 -3 -2 -1 0 1 2 3 4 5 Time (μs) Si gn al A m pl itu de standard zero-crossing