Communications: directive radio wave systems and devices (e.g. – Determining distance – With frequency modulation
Reexamination Certificate
2002-10-10
2003-10-28
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
With frequency modulation
C342S135000, C342S137000
Reexamination Certificate
active
06639546
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to radar systems, and more particularly to pulsed radar systems used for surveillance.
BACKGROUND OF THE INVENTION
Radar systems are in widespread use for military, commercial, and private purposes. Radar systems have well-known characteristics, in that long-range detection of small targets is known to require transmission of more power, higher-gain antennas, and/or more sensitive receivers than that or those required for short-range detection of large targets. Among the characteristics of radar systems used for detecting targets at long range are those relating to range ambiguity, which has to do with reception of signals returned from a target lying beyond the range defined by the pulse repetition interval, which may make the distant target appear to be near the radar system. Another such characteristic of radar is that of range eclipsing, which has to do with the inability of a radar receiver to receive return signals during the pulse transmission interval.
A conventional solution to range eclipsing is to vary the pulse repetition interval, so that the transmitted pulses are staggered over time, thereby allowing the receiver to periodically “see” returned signals at times which would otherwise be lost or eclipsed. The eclipsing still occurs for each individual pulse train, but the totality of the radar returns over time includes information which fills in the gaps attributable to the individual transmitted pulse trains. The tradeoff is that a longer time is required to produce all the information required for an uneclipsed view of the region. Another possible solution to range eclipsing is to reduce the duty cycle of the radar by reducing the transmitted pulse duration, to thereby reduce the duration of the eclipsing. The reduction of the pulse duration, however, tends to reduce the transmitted energy, which reduces the range sensitivity, which again requires a longer period of integration in order to obtain the same effective range.
Another possible solution to range eclipsing is to reduce the duty cycle of the radar by increasing the pulse repetition interval to thereby move the increased range interval to a distant range not of interest. The reduction of the duty cycle and increase in the pulse repetition interval, however, tends to consume additional radar resources resulting in a greater overall time required for completion of a surveillance scan.
Conventional range ambiguity resolution techniques require transmission of additional signals with additional dwells for resolving the range interval of the ambiguous target. The additional dwells or transmissions consume additional radar resources, resulting in a greater overall time required for completion of a surveillance scan.
Improved radar systems that provide unambiguous and/or uneclipsed range coverage are desired.
SUMMARY OF THE INVENTION
A method for transmitting and receiving radar signals according to an aspect of the invention includes transmitting time-sequential pulses including first and second pulses at first and second mutually different frequencies, where the first and second pulses being separated by a first time duration, and receiving radar return signals at the first frequency during times corresponding to the first time duration and during a second time duration that begins after transmission of the second pulse, which second time duration is different from the first time duration, and also receiving the second frequency during a third time duration. The method according to this aspect of the invention also includes processing the radar return signals to provide signals representative of targets lying in an uneclipsed range nominally corresponding to the sums of (a) the sum of the first and second time durations and (b) the duration of the second pulse. In one mode of the method according to this aspect of the invention, the second time duration is less than the first time duration. In another mode of the method according to this aspect of the invention, the third time duration follows the first and second time durations.
According to a further aspect of a method according to an aspect of the invention, the additional step of transmitting additional pulses at additional frequencies is performed, where the additional frequencies are different from each other and from the first and second frequencies. These additional pulses are transmitted following the second pulse, and at least some of the additional pulses are transmitted later than the immediately preceding pulse by alternating ones of the first and second time durations. In this further aspect of a method according to an aspect of the invention, the further step is performed of receiving radar return signals at a frequency corresponding to one of the additional frequencies during a time interval which begins at a time following that pulse on which the one of the additional frequencies was transmitted by the sum of (a) one of the first and second time durations and (b) a pulse duration.
In yet another mode of a method according to an aspect of the invention, the further step is performed of processing the radar return signals at the additional frequencies together with at least some of the radar return signals at the first and second frequencies to produce the signals representative of targets. Another mode of the method according to an aspect of the invenition further comprises, with a particular antenna pointing, repeating a particular number of times the steps of transmitting time-sequential pulses, receiving radar return signals, transmitting additional pulses at additional frequencies, receiving radar return signals at the additional frequencies, and processing the radar return signals.
Another method according to an aspect of the invention is for transmitting and receiving radar signals. This method comprises, with the radar antenna pointed in a particular direction, of generating a timing signal train including at least one sub-timing signal train, where each of the sub-timing signal trains includes at least first, second and third alternating and sequential pulse repetition intervals characterized by one of a &bgr;:1 and a 1:&bgr; ratio of (a) the duration from the start time of a given first pulse repetition interval to the start time of a second pulse repetition interval, where the second pulse repetition interval next follows the first pulse repetition interval, divided by (b) the duration from the start time of the second pulse repetition interval to the start time of a third pulse repetition interval next following the second pulse repetition interval, where &bgr; is given by 1+(&agr;D), where &agr; is equal to or greater than unity, and D is the duty cycle of the radar. In this method, in response to each sub-timing signal train, electromagnetic radar pulses are transmitted from the antenna such that the frequency of transmission in response to the first pulse of the sub-timing signal train is at a first radio frequency and the frequency of transmission in response to the second pulse of the sub-timing signal train is at a second radio frequency, different from the first radio frequency by an amount which allows subsequent separation of signals at the first and second frequencies. In addition, this method includes the reception, during that interpulse period immediately following the first pulse, of electromagnetic radar return signals originating in response to the first pulse, to thereby generate first received signals, reception, during that interpulse period immediately following the second pulse, of electromagnetic radar return signals originating in response to the first pulse, to thereby generate second received signals, and reception, during the third pulse repetition interval, of electromagnetic radar return signals originating in response to the second pulse, to thereby generate third received signals. The first and second received signals are concatenated, with a delay therebetween no less than the duration of the second pulse, to thereby produce a concatenated return sign
Koubiadis Fotis
Ott David Ronald
Duane Morris LLP
Lockheed Martin Corporation
Sotomayor John B.
LandOfFree
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