Measuring and testing – Fluid flow direction – With velocity determination
Reexamination Certificate
1999-11-09
2002-08-06
Oen, William (Department: 2855)
Measuring and testing
Fluid flow direction
With velocity determination
Reexamination Certificate
active
06427531
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to a remote wind sensing instrument for measurements of atmospheric wind. More specifically, the present invention relates to an active acoustic phased array antenna system that measures all three orthogonal components of a three dimensional wind field on a single transmission pulse utilizing simultaneous beams of acoustic waves.
2. Background Art
It is commonly known that changes in the frequency of transmitted electromagnetic or acoustic waves propagating the atmosphere are due to the movements of the atmospheric media. This Doppler effect enables remote measurements of atmospheric wind by transmitting a pulse of signal to illuminate a volume of the atmosphere and then measuring the changes in frequency, called Doppler shifts, of the reflected waves scattered by air in the illuminated volume. In the early development of remote wind sensing instrument, radio waves were first exploited for Doppler shift measurements. Subsequently, after the field of Doppler radar had been well established, acoustic waves were employed for remote wind measurement applications. Many of the principal operations used in acoustic remote wind sensing today had been adopted from the field of Doppler radar.
The term SODAR is an acronym for “SOund Detection And Ranging”, and SODAR or Doppler sodar is the term used for a remote wind measurement system utilizing acoustic waves for Doppler shift detection. Measurements that provide the information of atmospheric wind speed and direction as a function of height above the ground are called vertical wind profiles. Doppler radars and sodars are commonly used for vertical wind profiling. Hence, they are often referred to as radar and sodar wind profilers. Typically, a wind profiler is arranged into either a monostatic or bistatic configuration as discussed by Neff, et at., Probing the Atmospheric Boundary Layer, D. H. Lenschow, Editor,
American Meteorological Society
, Boston, Mass., pp. 201-239, September 1984. A monostatic wind profiling system such as those described in U.S. Pat. No. 4,558,594, to Balser, et al., entitled “Phased Array Acoustic Antenna,” U.S. Pat. No. 4,647,933, to Hogg, entitled “Phased Antenna Array for Wind Profiling Applications,” and U.S. Pat. No. 5,509,304, to Petermann, et al., entitled “Phased Array Acoustic Antenna System,” concerns only with backscatter signals, and uses a common antenna for both the transmission and reception of signals that propagate along the same path. A bistatic wind profiling system such as those described in U.S. Pat. No. 3,889,533, to Balser, entitled “Acoustic Wind Sensor,” and U.S. Pat. No. 4,219,887, to MacCready, Jr., entitled “Bistatic Acoustic Wind Monitor System,” and in J. Appl. Meteo. vol. 15, pp. 50-58, 1976, on the other hand, has different transmission and reception propagation paths, and hence, uses different antennas for the transmission and reception.
The atmosphere absorbs and scatters acoustic waves much more strongly than it does electromagnetic waves. Strong absorption limits the maximum height range of SODAR systems to about 1 km. Strong scattering, on the other hand, provides the advantage of well defined scattering signals contributing to good spatial resolution, a favorable circumstance to employ SODARs for remote wind measurements in a 1-km height range.
Basically, a minimum of three orthogonal components is required for a three dimensional wind vector measurement. Therefore, in a typical monostatic configuration, a wind profiler employs three fixed beams: two tilted beams that are slightly off the vertical to the east-west and north-south directions, and a vertical beam. Two additional tilted beams found on some wind profiling systems are used for consistency check. Doppler shifts in the backscatter signals received on the axis of each beam are interpreted as wind components in the radial direction. The measured components along different axes are transformed into components in the east-west, north-south, and vertical directions resulting in a three dimensional wind vector profile. Prior to the development of a phased array antenna technology, individual antenna was required for each radial wind component measurements. With the advancement in the phased array antenna technology, many wind profilers nowadays employ a single phased array antenna capable of beam steering for the various beams requirement.
The utilization of phased array antenna technology has notably reduced the size and improved the mobility of wind profilers. For example, a single phased array antenna can be employed in place of three separate antennas in a monostatic wind profiling system. Despite this development, however, the technique involved in Doppler wind measurements remains unchanged. For a three-dimensional wind measurement, a monostatic wind profiler obtains radial wind components along a minimum of three fixed beams sequentially on a pulse-by-pulse basis. Following a transmission of a pulse, backscatter signals are received for Doppler wind processing. Backscatter signals from lower height ranges arrive before those from upper height ranges. The time delay for receiving backscatter signal from the highest height range is referred to as a pulse repetition period. A sequence of pulsing is typically arranged in a cyclic order of the number of beams. For example, the pulsing sequence of a wind profiling system employing three fixed beams is: (1, 2, 3), (1, 2, 3), . . . (1, 2, 3). A pulse repetition period of one beam must be completed prior to an initiation of the next pulse repetition period in the sequence. If the pulse repetition periods are overlapped, signals from the current pulse will start to be received while signals from the previous pulse are still arriving. Unless some other information is available, it is not possible to interpret these signals.
For wind profilers employing radio waves that propagate the atmosphere at a speed of light, approximately 3×10
8
m/s, a pulse repetition period is of an order of 10 &mgr;s and is considered insignificant. However, for wind profilers employing acoustic waves that propagate the atmosphere at a relatively slow sonic speed, approximately 340 m/s, the pulse repetition period is large and becomes a significant performance load factor. For example, it takes 2 full seconds to retrieve backscatter signals from a height range of 340 m (i.e., a round trip distance of 680 m). To complicate the matter, because the individual radial wind components are temporally separated from one another by at least one pulse repetition period, averaging of these wind components over many consecutive pulsing sequences is required in order to obtain meaningful wind measurements. Thus, the propagation delay associated with the retrieval of backscatter signals in the sequential pulsing operation becomes a significant problem for SODARs to achieve a high temporal resolution.
In a conventional spectral processing, spectral estimation is implemented by a discrete frequency analysis using a Fast Fourier Transform (FFT). The detectability of a signal peak is enhanced by an incoherent spectral averaging process that averages a number of consecutive power spectra to smooth out the noise floor and better define the signal peak for a greater measurement resolution. Because the incoherent spectral averaging process does not increase the signal-to-noise ratio (S/N), a large number of spectra is required for average processing. Typically, for spectral processing of SODAR signals, a minimum of 20 spectra is required for each radial wind component. This translates to a minimum of 60 pulse repetition periods for measurements of a three dimensional wind profile. Thus, in the conventional spectral processing, a greater spatial resolution is rendered at an expense of a lower time resolution resulting from the many pulse repetition periods required in the spectral averaging process.
It is seen from the discussions presented that the signal retrieval technique originally developed for Doppler ra
Myers Jeffrey D.
Oen William
Ownbey Nancy E.
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