Digital Automatic Gain Control Research Articles

Comment on “Seasonal variation in wind speed and sea state from global satellite measurements” by D. Sandwell and R. Agreen

In a recent paper, Sandwell and Agreen [1984; hereafter SA] presented figures of global seasonal wind speed and sea slate as measured by the GEOS 3 satellite altimeter. Since that time, Chelton and McCabe f 1985; hereafter CMI have found that problems exist in the algorithms used to retrieve wind speed from altimeter measurements of radar backscatter. These problems were discovered too late to be of use in the analysis of SA. however, because they have a significant impact on the accuracy of wind speed estimation from altimeters, it is important that they he pointed out now so that the results of SA are not misused. Although the results presented here do not alter many of the conclusions of SA in a qualitative sense, they do become important for any quantitative interpretation of the seasonal winds presented by SA. En addition, the data distribution maps presented here (Figures 3a-3() are useful For pointing out limitations in other applications of GEOS 3 data (e.g., use of the altimeter sea level measurements to study surface geostrophic currents). A detailed description of sea surface wind speed estimation from radar altimeters is given in CM. Briefly, wind speed retrieval is a two-step procedure. In the first step, the power of microwave radiation backscattered from the sea surface is deterrnined From parameters measured by the altimeter receiver. To account for variations in transmitted power, the return power is normalized by the transmitted power. This normalized radar cross section is usually referred to as o°. The o measurements must be corrected for variations in the attitude angle of the satellite and variations in the height of the satellite above the sea surface. The second step in wind speed retrieval is to estimate wind speed from c°. This estimation is based on the principle that the roughness of the sea surface increases with increasing wind speed. The backscattered power measured by the altimeter receiver consists of microwave radiation reflected specularly from the sea surface over an approximate 10-km footprint directly beneath the satellite. As the sea surface roughens, much of the transmitted radiation is specularly scattered away from the radar antenna. Thus wind speed is inversely related to . The algorithms used to estimate wind speed from c are purely empirical, based on comparisons with coincident measurements from buoys. In a detailed investigation of the performance of wind speed estimation from the Seasat altimeter, CM identified problems with both steps of the procedure for wind speed retrieval. For Seasat, the power received by the altimeter antenna was converted to a constant output level for other receiver stages using a digital step attenuator automatic gain control (AGC). Thus o° can be computed directly from AGC (with the aforementioned satellite attitude angle and height corrections). CM found that the tables used to compute a resulted in a dis-

The ADF, an experimental self-adaptive digital filter

This paper describes a special-purpose digital processor for use in performing operations on sampled data. The processor is organized as multiple digital filters which can be grouped in cascade or canonic form. In addition, the weighting coefficients are modifiable in a self-adaptive mode during filter operations to form nonrecursive structures based upon the degree of coherency in two signal input channels. The adaptable system executes an LMS algorithm on a set of filter coefficients to perform, in real-time noise tracking and cancellation. An array with digital AGC, multiplier, interleaved memory and fixed-point processor are described.

Digital AGC for Multichannel Signals

Analog Multi Channel Recorders have a limited paper width to reproduce the data of each of the channels and in view of the finite line width, the signal range that can be recorded is 100:1 for most commercial recorders. Seismic signals vary over a range of 1:20,000 and hence it is preferred to use an AGC circuitry to avoid overloading of the recorder. It is more convenient to use a digital AGC on a multiplexed digital data and the paper describes a digital AGC circuit developed. The circuit described is a very low-speed one, typical of applications for seismic signals.

Dynamics of a digital automatic gain control for Gaussian signals

A simple digital automatic gain control loop, suitable for stabilizing the level of a slowly varying Gaussian signal in single-channel or multichannel applications is described. The probabilities of the excursion levels of the feedback control signal are derived and presented in normalized charts. These enable tradeoff between the signal level accuracy and the magnitude of the gain change steps to be readily accomplished and the corresponding increase in signal rms deviation due to the control signal to be determined.

Improved digital automatic gain control for p.c.m. signals

A digital automatic gain control is described which uses an open-loop instead of a closed-loop configuration. This change has been made to eliminate oscillations in gain which occurred with sinusoidal input signals in the closed-loop system described previously. Results of a computer simulation of the new system are presented.

New digital automatic gain control for p.c.m. signals

A new design for digital automatic gain control for p.c.m. telephony is presented. This new design is capable of processing 30 speech channels. It avoids putting highest gain where there is no speech. It has manual and automatic facilities.