The Akebono satellite observed the Australian Omega signals when it passed about 1000km over the Omega station. In this paper, we compare the observed Omega signal intensities with the values obtained using a full wave calculation and we discuss a mechanism of modulation of the signals. The relative spatial variations of the calculated Omega intensities are quite consistent with those observed, but the absolute calculated intensities themselves are several dB larger than the observed intensities. This difference in intensity may be due to the horizontal inhomogeneity of the D region, which is not modeled in the full wave calculation, or to an incorrect assumption about radiation characteristics of the Omega antenna. It is found that modulation of the observed signals is caused by the interference between the waves with different k vectors.

A new loop antenna system is proposed to detect the magnetic field of the natural electromagnetic wave in space in a very wide frequency band from 0.1 to 100 kHz. A single small loop antenna is followed by a network consisting of two transformers in series, one of which is optimized to pick up the signals at lower frequencies and the other, the signals at higher frequencies. Detailed analyses of the network are performed and the optimized parameters both on the loop and network are deduced for a small loop with an area of 0.50.5 m2. The theoretically obtained pick-up factor and the threshold levels for minimum detection in each frequency band are confirmed through the laboratory experiments and the field observations of the natural VLF waves such as the whistler as well as the transmitted carrier signals from the VLF and MF stations. The system proposed has a potential merit when applied to the space experiment where the payload is limited. The work in this paper gives the fundamental for the design of the loop antenna system onboard the Japanese satellite (EXOS-D) to be launched in 1989.

The Viterbi algorithm is a well-established technique for channel and source decoding in high performance digital communication systems. However, excessive time consumption makes it difficult to design an efficient high-speed decoder for practical application. This paper describes the implementation of parallel Viterbi algorithm by multi-microprocessors. Internal computations are performed in a parallel fashion. The use of microprocessors allows low-cost implementation with moderate complexity. The software and hardware implementations of the Viterbi algorithm on parallel multi-microprocessors for real-time decoding are presented. The implemented method is based on a combination of forming a set of tables and calculations. For efficient operation under fully parallel Viterbi decoding by microprocessors, we considered: (1) branch metrics processing, path metrics updating, path memory updating and decoding output for microprocessor, (2) efficient decomposition of the sequential Viterbi algorithm into parallel algorithms, (3) minimization of the communication among the microprocessors. The practical solutions for the problems of synchronization among the miroprocessors, interconnection network for communication among the microprocessors and memory management are discussed. Furthermore the performance and the speed of the parallel Viterbi decoding are given. For a fixed processing speed of given hardwares, parallel Viterbi decoding allows a linear speed up in the throughput rate with a linear increase in hardware complexity.

Radar imaging technique is one of the most powerful tool for underground detection. However, performance of conventional methods is not sufficiently high when the observational direction or the aperture size is restricted. In the present paper, an image reconstruction method based on a model fitting with nonlinear least-squares has been developed, which is applicable to arbitrarily arranged arrays. Reconstruction is executed on the assumption that targets consist of discrete point scatterers embedded in a homogeneous medium. Model fitting is iterated as the number of point target in the assumed model is increased, until the residual in fitting becomes unchanged or small enough. A penalty function is used in nonlinear least-squares to make the algorithm stable. Fundamental characteristics of the method revealed with computer simulation are described. This method focuses a much sharper image than that obtained by the conventional aperture synthesis technique.

This paper presents an adaptive rate error control scheme for digital communication over time-varying channels. The cyclic code with majority-logic decoding is used in a cascaded way as an inner code to create a simple and powerful hybrid-ARQ error control scheme. Inner code is used only for error correction and the outer code is used for both error correction and error detection. When an error is detected, retransmission is required. The unsuccessful packets are not discarded as with conventional schemes, but are combined with their retransmitted copies. Approximations for the throughput efficiency and the undetectable error probability are given. A high reliability coupled with a simple high-speed implementation makes it suitable for high data rate error control over both stationary and nonstationary channels. Adaptive error control scheme becomes the best solution for time-varying channels when the optimum code is selected according to the actual channel conditions to enhance the system performance. The main feature of this system is that the basic structure of the encoder and decoder need not be modified while the error-correction capability of the code increases. Results of a comparative analysis show that the proposed scheme outperforms other similar ARQ protocols.

The electron density profile in the lower ionosphere was determined from the right and left handed circular polarized wave fields of the 40 KHz ground based signal observed by means of a rocket borne receiver. The determination of the electron density from 70 Km up to 100 Km was made under the condition for the values calculated by the full wave analysis to agree with the observed values.

The authors implemented a directionally-constrained adaptive array system in an audio-frequency range. This system has a good prospect of applications since it does not require a rigorous a priori knowledge of the desired signal. Its algorithm contains matrix calculation, for which some technique has to be devised in implementation. For this, data are sampled and A-D converted so that the necessary processing for feedback control may be done by a microcomputer in digital form. The signal path remains in analog form, which will allow us to extend the system to higher frequency range. The performance of the system is demonstrated through the experiments. Effects of hardware errors are found: (1) Nulling behavior against the interference is not affected at all by virtue of the feedback control. (2) A little deviation from the constrained response to the desired signal is perceived. The behavior against a wideband interference is investigated analytically and experimentally. It was found that the final SIR becomes lower with wider frequency bandwidth.

We developed an automatic data processing algorithm for a ground-probing radar which is essential in analyzing a large amount of data by a non-expert. Its aim is to obtain an optimum result that the conventional technique can give, without the assistance of an experienced operator. The algorithm is general except that it postulates the existence of at least one isolated target in the radar image. The raw images of underground objects are compressed in the vertical and the horizontal directions by using a pulse-compression filter and the aperture synthesis technique, respectively. The test function needed to configure the compression filter is automatically selected from the given image. The sensitivity of the compression filter is adjusted to minimize the magnitude of spurious responses. The propagation velocity needed to perform the aperture synthesis is determined by fitting a hyperbola to the selected echo trace. We verified the algorithm by applying it to the data obtained at two test sites with different magnitude of clutter echoes.

This paper presents new go-back-N ARQ protocols for point-to-multipoint communications over broadcast channels such as satellite or broadcast radio channels. In the conventional go-back-N ARQ protocols for multidestination communications, usually only error detection codes are used for error detection and m copies of a frame are transmitted at a time. In one of our protocols, a bit-by-bit majority-voting decoder based on all of the m copies of a frame is used to recover the transmitted frame. In another protocol, a hybrid-ARQ protocol, which is an error detection code concatenated with a rate repetition convolutional code with the Viterbi decoding, is used. In these protocols, a dynamic programming technique is used to select the optimal number of copies of a frame to be transmitted at a time. The optimal number is determined by round trip propagation delay of the channel, the error probability, and the number of receivers that have not yet received the message. Analytic expressions are derived for the throughput efficiency of the proposed protocols. The proposed point-to-multipoint protocols provide satisfactory throughput efficiency and perform considerably better than the conventional protocols under high error rate conditions, especially in environments with a large number of receivers and large link round trips. In this paper we analyze the performances of the proposed protocols upon the random error channel conditions.

The MU radar of Japan is one of important candidates for providing accurate ground truth for the TRMM precipitation radar. It can provide the dropsize distribution data together with the background atmospheric wind data with high accuracy and high spatial resolution. Special observation scheme developed for TRMM validation using the MU radar is described, and preliminary results from its test experiment are shown. The high-resolution MU radar data are also used in numerical simulations to validate the rain retrieval algorithm for the TRMM PR data analysis. Among known sources of errors in the rain retrieval, the vertical variability of the dropsize distribution and the partial beam-filling effect are examined in terms of their significance with numerical simulations based on the MU radar data. It is shown that these factors may seriously affect the accuracy of the TRMM rain retrieval, and that it is necessary to establish statistical means for compensation. However, suggested means to improve the conventional α-adjustment method require careful treatment so that they do not introduce new sources of errors.