Communications: directive radio wave systems and devices (e.g. – With particular circuit – For pulse modulation
Patent
1997-11-20
1999-07-06
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
For pulse modulation
342 26, 342159, 342162, G01S 7282, G01S 13522, G01S 1395
Patent
active
059202823
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates generally to pulsed radar systems, and more specifically relates to methods and apparatus for shaping the radar pulses to reduce the spectral sidelobe energy being transmitted.
BACKGROUND OF THE INVENTION
Wind profilers are radars that are able to detect scattering of their electromagnetic energy by the atmosphere when there are no precipitation particles or hydrometers present. These radars are often referred to as "clear-air" radars to distinguish them from weather radars. The targets that cause the scattering for clear-air radars are the atmospheric inhomogeneities along the path of the transmitted radar beam. The present invention is particularly applicable to these types of radar, but more generally is applicable to any pulsed radar system in which a region of space is examined by transmitting normally rectangular radar pulses thereto and analyzing the resulting scattered and/or reflected energy.
The necessary bandwidth of a pulsed radar transmitter, such as used in a wind profiler, is defined as the frequency range between the first two zeroes in the Fourier spectrum of the transmitted pulse. For a perfect rectangular pulse envelope this is equal to two divided by the pulse duration. Any energy falling outside of this frequency range contributes nothing to the performance of the radar, but can cause problems with interference to other nearby radio services.
The occupied bandwidth of any pulsed radar, is defined as the frequency range containing ninety-nine percent of the transmitted power. This includes the useful power contained within the necessary bandwidth, and the power outside of the necessary bandwidth. The necessary bandwidth and the occupied bandwidth are shown graphically in FIG. 1. It is desirable to increase the proportion of the transmitted power contained within the necessary bandwidth in relation to that of the occupied bandwidth. A common practice is to compare the occupied bandwidth to the necessary bandwidth, expressed as the ratio of the occupied bandwidth to necessary bandwidth. A lower occupied bandwidth to necessary bandwidth ratio is indicative of a more efficient use of the RF spectrum.
Pulsed radars and wind profilers typically operate with transmit pulses with nearly rectangular leading and trailing edges. This rectangular pulse tends to have a rather large occupied bandwidth to necessary bandwidth ratio. One way to reduce the ratio of occupied to necessary bandwidth is by shaping the transmit pulse into something other than a rectangular pulse.
SUMMARY OF INVENTION
Pursuant to the present invention a digitally-controlled pulse shaper (DCPS) is provided which precisely controls the shape of the leading and trailing edges of the wind profiler, or pulsed-radar transmit pulse. In the case of pulse coding, where phase transitions are used to segment an otherwise longer pulse, the intra-pulse phase transitions are also shaped.
The use of the DCPS in pulsed radar and wind profilers can reduce the occupied to necessary bandwidth ratio from approximately 10.0 in the non-shaped case, to under 3.0 when shaped. FIG. 2 shows the spectrum of a typical prior art rectangular pulse. FIG. 3 shows the spectrum of a radar pulse shaped with the DCPS using a cosine squared transfer function, although an almost limitless number of pulse shapes is possible with this invention. The cosine squared transfer function is used as an example since the spectra content of a pulse with leading and trailing edges exhibiting this shape is well known.
FIGS. 1, 2 and 3 were displayed and captured on an HP4396A Network Analyzer/Spectrum Analyzer. The x-axis shows the spectra centered at a frequency of 60 MHZ. The spectra are displayed for a total span of 15 MHZ. The y-axis is an indication of the power of the spectral content. The y-axis is referenced to 0 dBm with 10 dB per division.
The DCPS is placed in the transmit chain after the existing intra-pulse phase shifter and before the existing pulse RF switch. In the case illustrated, the shaped pulse was forme
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Neuschaefer John William
Peterman K. Russell
Vik David A.
Radian International LLC
Sotomayor John B.
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