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SODAR PCS 2000/64 with RASS Antennae
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CONTACT DETAILS
Tel UK: 01275
847787
Tel Intl: + 44 1275 847787
Fax UK: 01275 847303
Fax Intl: + 44 1275 847303 |
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Biral
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RASS extension for use with PCS2000 SODARsGeneral system description
of RASS
The RASS extension, used in addition to the SODAR
instrumentation is a reliable and easy to use tool to get a quick information about the
atmospheric temperature stratification. Especially in stable conditions the vertical
stratification could have a significant influence on the observed ground concentrations of
pollutants. The SODAR/RASS allows the detection of inversion heights and quantitative
measurements of the inversion strength on the basis of the temperature gradients.
The RASS method uses the scattering of vertically emitted radio waves at vertically
emitted acoustic waves to determine the vertical profile of the virtual temperature (in
very good approximation). Please see further details in RASS
Measurement Principle
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RASS Components
The RASS extension consists two parabolic antennas, a receiver and a transmitter
(shown either side of the SODAR in the photo below) plus the electronic RASS components
consisting of signal source, power amplifier and receiver (shown below right).
(click images to expand in new window) |
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CONTACT DETAILS
Tel UK: 01275
847787
Tel Intl: + 44 1275 847787
Fax UK: 01275 847303
Fax Intl: + 44 1275 847303 |
|
Email
Biral
|
|
The effects of wind on the acoustic waves of the RASS
extension
As the acoustic waves, while propagating upwards through the atmosphere, can be shifted by
the horizontal wind the location of the reception spot at the ground surface moves with
the height range the signal originates from. So during strong wind situations the spot can
be shifted very rapidly out of the reception area. This displacement is the most limiting
factor in RASS technology. It can be compensated by using not only a single acoustic
source, but a number of acoustic sources distributed around the radar antennas and
selected for operation according to the observed wind direction. This will significantly
increase the price for the RASS extension, but is at least a way to improve the data
availability coverage, especially for heights above 300-400 m.
A further choice out of the dilemma with sound wave shift is the usage of
bigger radar antennas which will also increase the antenna aperture for both, emission and
reception. So, the usage of bigger radar antenna can be a more cost efficient way and the
frequency of the radar part of the RASS can be changed to 480 MHz. The lower frequency
offers one more advantage: the atmospheric absorption of acoustic energy decreases with
decreasing frequency, so the scattering cross section will be higher for the scattering
process.
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1290 MHz versus 480 MHz RASS
From the experiences of long term operation with both systems we can state that a 80%
height coverage for the RASS will be provided for 300 – 400 m in wind situation below
5 m/s for the 1290 MHz RASS and 600-800 m for the 480 MHz RASS. In combination
with high powered wind profiler the RASS range can be extended up to 1000 m or even 3000 m
but with much higher costs and poor height resolution (factor 5-10 in both, price and
resolution). As environmental monitoring of dispersion parameters acts on emissions near
the surface or at height up to 100-200 m the dominating parts in the dispersion of
pollution will always be the wind and temperature profile within the first hundred of
meters, i.e. the typical height range of a system should be about 300-500 m. However, a
consideration of the 1290 MHz RASS versus the 480 MHz RASS is recommended. For further
information on a 480 MHz RASS system please contact Biral.
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RASS Subsystem
characteristics
(this information is based on the 1290 MHz RASS please contact us for 480 MHz
specific details)
Antenna
| Type: |
Two parabolic reflectors, with Ø 1.8 m |
| Feed: |
One focal feed ea. |
| Beam Width: |
± 3.5 ° (two way, 3 dB) |
| Beam Direction: |
Vertical |
| Beam Control: |
None |
| Polarisation: |
Linear |
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Transmitter/Receiver
Characteristics
Transmitter
| Frequency: |
1290 MHz ± d MHz, d to be
specified by the customer |
| Modulation: |
Continuous wave |
| RF-power: |
20 W approx. |
| Bandwidth: |
Depends only to
oscillator stability, approx. 10^-5 |
Receiver
| Frequency: |
1290 MHz ± d MHz, d to be specified by the
customer |
| Type: |
Homodyne |
| Noise Figure: |
< 1 dB |
| RF-Band Width: |
< 5 MHz |
| IF-Centre Frequency: |
approx. 3 KHz |
Accuracies:
Virtual Temperature: 0.3 °K
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RASS Installation
The parabolic antennas (receiver and transmitter) must be aligned strictly vertical (check
this with a spirit level!). The distance between the parabolic antennas should be
typically 5 to 6 meters, where the acoustic phased array must be placed in the middle
between the transmitting and receiving RASS antenna. The electronic RASS-components
(signal source, power amplifier and receiver) are typically mounted in a separate outdoor
housing.
Photo of trailer mounted SODAR
and RASS
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E&OE.
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