GB2509786A

GB2509786A - Method of Monitoring Microwave Emissions from the Environment Using a Radar System

Info

Publication number: GB2509786A
Application number: GB1300745.5A
Authority: GB
Inventors: Timothy Darlington; Jacqueline Sugier; Michael Roger Andrew Edwards; Anthony Illingworth; John Nicol; Robert Thompson
Original assignee: MET OFFICE; University of Reading
Current assignee: MET OFFICE; University of Reading
Priority date: 2013-01-15
Filing date: 2013-01-15
Publication date: 2014-07-16
Also published as: GB201300745D0

Abstract

A radar system includes a transmitter 20 operable to transmit microwave signals within a given range of frequencies and a receiver 50 operable to receive reflected or backscattered signals within the said given range of frequencies. The system includes further means for effecting radiometric monitoring of signals received by the receiver at frequencies which extend beyond the said given range of frequencies transmitted by the transmitter. A second aspect of the invention provides a method of monitoring microwave emissions from the environment by signals received by a receiver of a radar system by acting to exclude the range of frequencies transmitted by the transmitter from the range of received frequencies propagated through the radar waveguide 25 and antenna 30 subsystems, and performing radiometric analysis thereon.

Description

A RADAR SYSTEM AND A METHOD OF MONITORING MICROWAVE EMISSIONS FROM THE

ENVIRONMENT USING A RADAR SYSTEM

The present invention relates to a radar system and a method of monitoring microwave emissions from the environment using a radar system.

In particular, the present invention relates to a system for employing radiometric techniques to conduct various forms of investigation yielding valuable and interesting data on the environment and/or on objects or artefacts within an environment under investigation.

Modern weather radar systems generally comprise a microwave transmitter, a digital receiver. waveguide and antenna sub-systems. The method of the invention facilitates accurate, quantitative, microwave radiometric measurements using the existing antenna, waveguide and receiver sub-systems of a modern radar system.

It is known to employ weather radar receivers to make radiometric observations in the same frequency band as that utilized to receive the back scattered radar transmissions.

This is achieved by making the radiometric observations within range gates (or aggregates of range gates) located at radar ranges beyond the range at which physical targets (those capable of generating back scattered signals) are likely to be located.

The present invention seeks to improve on known systems enabling radiomethc observations and analysis to be undertaken substantially-continuously over an extended frequency range utilising existing radar systems, in particular the C and X-band radar systems, aFlowing them to function additionally as passive microwave radiometers whilst continuing to perform their primary function. The present invention thus relates to a modified C or X-band radar system which is capable of providing -accurate supplementary radiometric data, in particular identifying brightness : temperatures at microwave frequencies. * .

According to a first aspect of the invention, therefore, there is provided a radar system having a transmitter operable to transmit microwave signals with a given range of -frequencies and a receiver operable to receive reflected or back scattered signals * within the said given range of frequencies, in which there are further provided means * . for effecting radiometric monitoring of signals received by the said receiver at frequencies which extend beyond the said given range of frequencies transmitted by the said transmitter.

Freterably the said radioruetric monitoring means are operable to detect received signals within a range which is non-coterminous with the said given range of transmJtted frequencies.

In a preferred embodiment the radiometric monitoring detects a range of received signal frequencies above or below the said range of transmitted frequencies.

In addition, it is preferred that the said radiomeiric rnonitoñng means includes a filter operable to exclude received signals within the said range of transmitted frequencies.

In a preferred embodiment the said radiometric monitoring means is operable to determine brightness temperatures from signals received by the said receiver.

A problem with existing radar systems used to detect atmospheric artefacts, which may be the target for the radiometric analysis, is the inability to quantify the degree of noise present, which is influenced by, among other things, the brightness temperature. It will be appreciated that variations in the brightness temperature are due to variations in the energy in the atmosphere and/or in the atmospheric ortefacts, which may be the target for the radiometric analysis. Although radiometric measurements of these emissions are possible by rador, such measurements are difficult to make since they may be obscured by theeffects of back scattering, especially from targets of the signal transmitted by the radar.

The radar system may include timing means operable to exclude signals received by the receiver during transmission of the said microwave signals by the transmitter. This idea may be extended by the provision of timing means for excluding signals received by the receiver for a time period after transmission of the microwave signals by the transmitter.

In the radar system of the invention the radiometric monitoring means may be responsive to received microwave signal strengths whereby to determine brightness temperatures therefrom.

This technique can be used, for example, in centimetre and millimetre wave radar systems to measure the brightness temperatures of targets caused by. forexample, geophysical activities such as volcanic action, as well as atmospheric brightness temperature measurements.

The radiometric measurements may be made at frequencies above the transmission frequency band and/or below the transmission frequency band of the radar system itself, It will be appreciated that the selection of the frequencies for making these measurements will largely be dependent upon the reception capabilities of the receiver. One or more notch filters may be incorporated in the system for determining the ranges ot frequencies examined and those supressed.

Providing a filter which removes the microwave signals trom frequencies which are within the transmission frequency band ensures that any signals which are likely to relate to transmissions made by the radar system itself are excluded since they may otherwise influence the radiometric analysis.

In one embodiment, the radar sysfem is intended for. detecting meteorological conditions and the brightness temperatures determined by the radiometric analysis are atmospheric brightness temperatures. The receiver may be operable to receive microwave signals reflected from the atmosphere and microwave transmissions are transmitted into the atmosphere. It will be appreciated that such radar systems may.

inter a/ia. obtain precipitation and wind speed information from measuring the scattering of microwaves on targets. This information is collated to provide data on the prevailing meteorological conditions.

According to a second aspect of the invention, there is provided a method of monitoring microwave emissions from the environment from signals received by a receiver of a radar system by acting to exclude the range at frequencies transmitted by the transmitter, from the range of received frequencies propagated through the radar waveguide and antenna sub-systems, and performing radiometric analysis thereon, * In use of the method of the invention, it is preferred that the range of transmitted frequencies is excluded from the range of signals subject to radiometric analysis by filtering the signals received by the receiver. Further, the range of transmitted frequencies may be excluded from the range of signals -subject to rcidiometric analysis by disabling, excluding, or isolating the receiver during transmissions. Preferably, the exclusipn of transmitted frequencies continues for a period after transmission. --* * The method of the invention may involve radiometric analysis performed on signals extending over a frequency range defined by analogue filters within the radar receiver.

In either case the radiometric analysis may be performed within a range ot frequencies above and/or below the radar transmission frequency. H 5

Thus, the said radiometric analysis determines at least one brightness temperature from the received signals, and this brightness temperatüre may be an atmospheric brightness temperature.

Further particular preferred aspects of the present invention are set out in the accompanying independent and dependant claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

Various embodiments of the present invention will now be more particularly described' by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a radar system tormed as a first embodiment of the invention; Figure 2 illustrates an example of a signal received by the receiver of the embodiment of Figure 1 following the transmission of a pulse into the atmosphere; Figure 3 illustrates the received signal provided by the receiver at a frequency other than the transmitted frequency; Figure 4 illustrates the magnitude of the received signal at different frequencies; and Figure 5 schematically illustrates the relationship between the frequency bands. *

* Embodiments of the invention provide a technique which enables a radar system to be adapted to make radiometric measurements of microwave radiation in the atmosphere. Such radiometric measurements can provide complementary data to that received by the radar system in its primary function. *-* e

As will be explained in more detail herein radiometric measurements may also be made at frequencies outside the radar transmission band, as long as the chosen * frequencies propagate efficiently through the radar waveguide and antenna sub-systems. The idea underlying the present invention is to perform the radiometric measurements over the broad frequency band defined by the existing analogue filters within the radar receiver, either within a band above or below the radar transmission frequency. Alternatively the radiometric measurements may be performed over the whole at the defined band but then incorporating a notch filter to exclude the frequency band occupied by the radar transmissions and back scattered signals. The relationship between these various frequencies is illustrated schematically in Figure 5.

In operation, therefore, embodiments of the present invention exclude the effects of reflections caused by transmissions made from the radar system itself, either by excluding the frequencies on which such transmissions occur or by making the radiometric analysis at a time after which the effects of any reflections occurring in response to the transmissions are minimised or at least reduced to negligible proportions. By averaging the radiometric determinations made, either over time or over frequency, or both, a more accurate assessment of the brightness temperature can be made.

In the following description an exemplary embodiment using a C or X-band weather radar to observe atmospheric brightness temperatures will be discussed; however, it will be appreciated that the same techniques Fan be applied to other radar systems to measure the brightness temperatures of not only atmospheric artefacts, for example, due to physical activities such as volcanic action, but also physical objects such as aircraft, buildings or ships.

Referring now to the drawings, Figure 1 illustrates a radar system, generally indicated 10.

comprising a microwave transmitter 20 which generates a microwave signal transmission pulse at a frequency The transmitter 20 is coupled by a waveguide 25 to a parabolic radar antenna 30. The microwave signal is transmitted by the radar antenna 30. typically with a beam width of around 1°. The elevation angle eE of the radar antenna 30 above the horizon may be as ow as 0.5° and may be as high as * abput 5°, although other elevation angles are pOssible. Typically, around 40 transmission pulses may be generated for each 1° azimuth rotation of the radar * antenna 30. A waveguide isolator or decoupler 40 is provided which isolates or decouples the receiver 50 from the waveguide 25 during transmission of the transmission pulse. This prevents damage of the receiver 50 during such transmissions.

The transmitted pulses propagate through the atmosphere and reflections occur due, for example, to the presence of particles such as water droplets or changes in the density of the atmosphere. The received signal delivered to the receiver 50 is forwarded to an analogue-to-digital converter 60, filtered by a first filter 70 and provided to a microprocessor 80 for whatever signal processing operations are to be done, such as logging and analysis. in this embodiment a by-pass path 65 is also provided, which prpvides the microprocessor 80 with an unfiltered signal from the analogue-to-digital converter 60.

Figure 2 illustrates an example of a signal received by the receiver 50 following the transmission of a pulse into the atmosphere. The start time for the transmission of the pulse by the transmitter 20 is indicated t and the cessation time of the transmission of the pulse by the transmitter 20 is indicated te.

Accordingly, between t and t no signal is received because the waveguide 25 is blocked by the decoupler 40. Of course, it will be appreciated that a small degree of noise may stiU be sensed by the receiver 50 during this time period, although this may be removed by grounding the output of the receiver 50 during the period when transmissions occur.

As can be seen in Figure 2. a signal of ow magnitude is initially received. Thereafter, the magnitude of fhe signal rapidly increases as the reflected signal is received, and thereafter the received signal decays. The time at which a reflected signal is received indicates the distance of the reflecting oblect from the transmitter. The magnitude of the signal indicates the reflectivity of the object.

In this example, it will be apparent that a weather-related ortefact is located at a particular distance from the radar system; however, it is not certain whether any further weather-related artefacts exist at further di5tarces from the radar transmitter since these are difficult to discern due to the build-up of noise in the signal at increased distances. This noise is monitored at frequencies other than the transmission frequency to determine the brightness temperature of the atmosphere being viewed. *0**'

* S Figure 3 shows the received signal provided by the receiver 50 at a frequency other than the transmitted frequency.FTX. Although the weather radar 10 is optimised to operate at the transmission frequency Fm it will be appreciated that the antenna 30. the waveguide 25 and the receiver 50 will generally support the propagation of signals at : frequencies outside this transmission band. Because of the isolation by isolator 40 substantially no signal is received between the time h and t. Thereafter a time-varying signal is received, which may be subject to radiometric analysis. . Figure 4 illustrates the magnitude of the received signal at different frequencies. The iiiustration shows the magnitude at different frequencies at a given time, and it will be appreciated that the magnitude of the received signal at different times may vary from this as the received signal at different frequencies will vary with time. As illustrated by the dotted trace 105, the magnitude of the signal received around the transmission frequency Fi wili inàrease at a certain time. In particular the dotted trace 105 indicates the magnitude of the received signal around frequency FTx at the time t shown in Figure 2.

To mitigate the effects of the contribution to the received signal caused by the back scattering in response to transmissions by the transmitter 20. the second filter 80 is provided. The second filter 80 utilises a number of mechanisms. A first mechanism is to exclude a range of frequencies around the transmission frequency Fr. In this regard the second filter 86 operates as a notch filter, attenuating signals between a lower frequency FL and an upper frequency F0. When operating in this manner, the second filter 80 only provides an indication of the magnitude of the reflected signal for a frequency band below FL and for a frequency band above F. Hence, the effect caused by the transmissions made by the transmitter 20 is effectively excluded.

Another mechanism employed by the second filter 80 is to time-gate the sampling of the received signal at the different frequencies. Accordingly, the second filter 80 only passes the received signal at the different frequencies after the time period td which is set to correspond with a distance from the transmitter 10 beyond that at which atmospheric weather etfects are likely to occur. In this embodiment the time td is determined by a relationship which is inversely proportional to the elevalion angle eE.

As can be seen in Figures 2 and 3, this also helps to mitigate the effects of the contribution to the magnitude of the reflective signal caused by the transmissions made by the transmitter 20.

Figure 5 schematically illustrates therelation between the frequency bands mentioned above. PBa denotes the pass band ot the analogue receiver components. PB denotes the pass band of the waveguide and antenna sub-systems and PBd denotes : the pass band of a digital filter fpr selective reception of radar backscatter signals.

Accordingly, it can be seen that typical C or X band radar systems can be modified to provide accurate supplementary radiometric observations at microwave frequencies.

This adaptation enables the radar system to function additionally as a passive * microwave radiometer. This approach enables accurate quantitative microwave radiometric measurements to be mode using existing antenna, waveguide and receiver sub-systems. Such radiometric measurements are made at frequencies outside the radar transmission band as long as those frequencies propagate sufficiently through the radar waveguide and antenna sub-systems. The measurements are performed over a broad frequency band which is supported by the antenna, waveguide and receiver sub-systems. Typically) this may be above. below or both -above and below the radar transmission trequency. It is possible to operate over the whole frequency band, but in this case a notch filter is required to exclude the frequency band utilised by the radar transmissions and backscattered signals. More than one notch filter may be provided to obtain an appropriate profile of signal reception. The receiver sub-systems monitor the radiometric measurement frequency bands whilst simultaneously performing their normal function of sampling the backscattered transmissions. This approach enables any potential interference from the radar transmitter to be avoided and the radar receiver is adapted to monitor microwave emissions from the environment almost continuously Although embodiments show processing occurring mostly in the digital domain, it will be appreciated that similar processing could be undertaken using analogue devices.

Such an approach advantageously enables radiometric observations to be undertaken almost continuously, only being interrupted to avoid the transmitter burst.

This has advantages over prior systems where the sampling only occurs after a particular time period which is ot significant extent. In addition, radiometric observations are made over a broader frequency band than in the prior art. This leads to an improvement in sensitivity for a particular receiver gain. In addition, the risk that the radiometric observations may be contaminated radar echoes from physical targets located at extreme range is substantially eliminated. Moreover, the effect of :. attenuation of the radar by rainwater on the radome con also be quantified. * *

Although illustrative embodiments of the invention have been disclosed in detail herein, * * with reference to the accompanying drawings, it is fo be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein by one skilled in the art without departing from : .. the scope of the invention as defined by the appended claims and their equivalents.

In so far as the embodiments of the invention described herein above are implemented, at least in pad, using software-controlled data processing apparatus, it will be appreciated that a computer programme providing such software control and a storage medium by which such a computer program is stored are envisaged as aspects of the present invention. S. *S * . S * .

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Claims

CLAIMS1. A radar system having a transmitter operable to transmit microwave signals within a given range of frequencies and a receiver operoble to receive reflected or backscattered signals within the said given range of frequencies, in which there are further provided means for effecting radiometric monitoring of signals received by the said receiver at frequencies which extend beyond the said given range of frequendes transmitted by the said transmitter.

ID
2. A radar system as claimed in Claim 1, in which the said radiometric monitoring means are operable to defect received signals within a range which is non-coterminous with the said given range of transmitted frequencies.
3. A radar system as daimed in Claim 1 or claim 2. in which the radiometric monitoring means detects a range of received signal frequencies above or below the said range of transmitted frequencies.
4. A radar system as claimed in any of the Claims 1 to 3. in which the said radiometric monitoring means includes a filter operable to exclude received signals within the said range of transmitted frequencies.
5. A radar system as claimed in any preceding claim, in which the said radiometric monitoring means is operable to determine brightness temperatures from signals received by the said receiver.
6. A radar system as claimed in any preceding claim, further including timing means operable to exclude signals received by the receiver during transmission of the said microwave signals by the transmitter.S
7. A radar system as claimed in any preceding Claim, in which there are provided tihiing means for excluding signals received by the receiver for a time period after transmission of the microwave signals by the transmitter.
8. A radar system as claimed in any preceding Claim, in which the radiometric :": 35 monitoring means is responsive to received microwave signal strengths whereby to determine brightness temperatures therefrom.
9. A radar system as claimed in Claim 8, in which the brightness temperatures are atmospheric brightness temperatures.
10. A method of monitoring microwave emissions from the environment by signals received by a receiver of a radar system by acting to exclude the range of frequencies transmitted by the transmitter from the range of received frequencies propagated through the radar waveguide and antenna sub-systems and performing radiometric analysis thereon.
11. A method as claimed in claim 10, in which the range ot transmitted frequencies is excluded from the range of signals subject to radiometric analysis by filtering the signals received by the receiver.
12. A method as claimed in Cairn 10. in which the range of fransmitted frequencies is excluded from the range of signals subject to radiometric analysis by disabling the receiver during transmissions.
13. A method as claimed in Claim 11 or 12. in which the exclusion of transmitted frequencies continues for a period after transmission.
14. A method as claimed in Claim 10, in which the said radiometric analysis is performed on signals extending over a frequency range defined by analogue filters within the radar receiver.
15. A method as claimed in Claim 10 or Claim 11, in which the radiometric analysis is performed within a range of frequencies above and/or below the radar transmission frequency. es * . * * *
16. A method as claimed in any of Claims 10 to 15, in which the said radiometric analysis determines at least one brightness temperature from the received *...* -signals. a a
17. A method as claimed in claims 16, in which the said radiomettic analysis determines at least one atmospheric brightness temperature from the received signals.
18. A method of monitoring microwave emissions from the environment substantially as hereinbefore described with reference to the accompanying drawings. 11.
19. A radar system having means for monitoring microwave emissions from the environment, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings. 4. .* 4 S * .I*Se*Ie * S a..... * SII qt. *i * *0IS..... t,