Report D36-5: Winchester and Thornhill links

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PROJECT REPORT D36-5
REPORT
on
Link Reliability Project:
Winchester and Thorne Hill
K. S. Paulson and J. W. F. Goddard
Radio Communications Research Unit
Rutherford Appleton Laboratory
Chilton, Didcot
Oxfordshire OX11 0QX
July 2002
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1.
Introduction
Radio signals can be badly affected by the weather, fading significantly in the rain or sleet.
This is a problem for telecommunications operators who must be able to guarantee a reliable
service in all weathers. In particular, some operator claim to have experienced higher than
expected outage time for links in the 23 GHz and 38 GHz bands.
The aim of this project is to investigate why these particular frequencies are so badly affected.
The RCRU has monitored the signal levels of a number of links in Winchester and Thorne
Hill, near Southampton. During severe weather, when the links experience high attenuation,
meteorological radar data is collected from the Chilbolton Observatory to try and determine
what may have caused the problem. The aim of the experiment is to use this data to determine
whether the fading is a link hardware problem, if a greater power margin needs to be built into
the system to compensate for the unexpectedly strong effect of the weather, or if sleet and
snow is to blame.
When radar images over the hubs are available, link rain attenuation time series are calculated
from the radar derived rainrate maps. If the attenuation experienced by the links agrees well
with the radar-derived predictions, then the attenuator is rain and the processes are well
understood. Where the agreement is poor, the conditions have been investigated further to
determine the processes involved.
This report examines the results of this monitoring exercise from February 1999 to March
2002.
2.
The Links
T-Mobile allowed access to links from two hubs at Winchester and Thorne Hill near
Southampton. Up to eight links were monitored at each hub. A 13 GHz bi-directional link
connects the two hubs.
Figure 1: Link paths relative to Chilbolton; Winchester Links are in orange and Thorne Hill links in green.
The horizontal and vertical scales are in kilometres north and east of Chilbolton Observatory.
Tables 1 and 2 list the length, frequency, polarisation and 0.01% exceeded rain attenuation of
each link. The graphs in Section 4 have the reference level set to the long-term average
attenuation. Assuming these links are engineered to 99.99% availability, the 0.01% exceeded
rain attenuation level can be taken as the beginning of possible outages.
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In the course of the experiment several links have failed and one link was found to be too
unstable for comparison with radar predicted attenuations. Several other restrictions effect the
comparison of link attenuation and that predicted from the radar data. In order to increase the
rate at which the links were scanned by the radar, the scan field was reduced until one of the
links, Winchester link 0, extended outside the imaged area. Winchester link 3 has a
transmitter at Chilbolton which is inside the 5 Km near-field of the radar and so part of its
length could not be observed. The radar predicted rain attenuation on these links should be
less than that actually experienced due to the attenuation caused by rain outside the scanned
area. Finally, several links lie along directions where radar measurements are partially
obscured by ground clutter. These links should also experience higher attenuation than that
predicted by the radar.
Table 1. Thorne Hill Links
Link
Number
0
1
2
3
4
5
6
7
Length
km
8.33
26.52
9.69
25.5
8.66
16.86
6.61
2.73
Frequency
GHz
23
14
23
14
38
13
38
38
Polarity
V
H
V
H
V
V
V
V
RainFade
0.01%
14.8 dB
14.5 dB
16.5 dB
14.2 dB
32.9 dB
8.2 dB
26.7 dB
12.6 dB
Comments
Erratic
Partially obscured by ground clutter.
Failed on 6/5/01.
Partially obscured by ground clutter.
Link failed early in experiment.
Failed on 7/7/00.
Table 2. Winchester Links
Link
Length
Frequency Polarity
RainFade
Comments
Number
km
GHz
0.01%
0
30.41
14
V
13.0 dB
Only partially covered by radar scan.
1
16.86
13
V
8.2 dB
Partially obscured by ground clutter.
2
5.5
38
V
23.1 dB
3
8.87
38
V
33.5 dB
Only partially covered by radar scan.
4
4.25
38
V
18.6 dB
5
14.54
23
V
21.8 dB
6
9.05
38
V
34.0 dB
Failed on 9/6/01.
7
24.91
14
V
11.9 dB
Never connected.
Table 3. The parameters of links monitored at the Winchester hub.
3.
Comparison of Measured and Radar Predicted Link Attenuation
In Section 4, time-series of monitored and radar predicted link rain attenuation are compared.
The monitored data is derived from records of AGC levels in the receive equipment located at
the hubs. This is compared with predicted link rain attenuation derived from near-horizontal
PPI radar scans, e.g. Figure 4.1a. To determine the attenuation experienced by the links,
calibration curves are necessary to translate AGC levels into receive power levels. T-Mobile
has not supplied this information. To estimate these curves, linear calibration relationships
have been assumed and link attenuation exceedance statistics have been fitted to the ITU-R
Rec. P.530 prediction of average annual attenuation exceedance. Calibration factors were
calculated from AGC records spanning the period 11/8/99-30/8/00. Figure 2 compares the
Winchester link attenuation exceedance statistics for the period 5/2/01-5/2/02, calculated
using these calibration factors, with the Rec. P.530 prediction. This method relies on the
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training period approximating an average year. For most links this appears to be valid but
other links e.g. Winchester Link 0, have experienced unusual clear-air events that have
skewed the statistics. For most of the links, the calculated link calibration appear to be
reliable with an accuracy of 10-20%
Winchester Link 0
Winchester Link 1
Winchester Link 2
Winchester Link 3
Winchester Link 4
Winchester Link 5
Figure 2: Comparison of link attenuation exceedance statistics (purple) for the period 5/2/01-5/2/02,
calculated using the earlier derived calibration factors, with the ITU-R Rec. P.530-10 predicted average
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annual attenuation exceedance statsitics (black dashed).
The radar acquires data by scanning over a sector of 80o with a low elevation angle, typically
1.5o. The radar needs to be elevated slightly or the return data is contaminated by reflections
off the ground. However, at an elevation angle of 1.5o the radar is looking at a point at an
altitude of 650 m over the Thorne Hill hub, 25 Km from Chilbolton. The conditions at this
altitude may not be the same as those near the ground.
The PPI scan yields radar reflectivities on a polar grid centred on Chilbolton. These data can
be converted to volume averaged rain rate by inverting the relation (Jun Tan, 1998):
zhdB  280R1.48
For a given frequency and polarisation, these rainrates can be converted into specific
attenuations using the power law given in ITU-R Rec. P.838. Finally, by summing the
specific attenuations along the link paths, instantaneous link rain attenuation can be estimated.
Radar predicted link rain attenuation time series are derived by repeating this process for a
number of PPI scans.
There are a number of reasons why the radar derived link attenuation may be different from
the measured attenuation:
1. The process described above is sensitive to the drop size distribution when radar
reflectivity is converted to rainrate and when rainrate is converted to specific attenuation.
2. The radar and the link sample different space-time volumes. The link can be thought of as
instantaneously sampling the rain in the first Fresnel zone. Link attenuation was averaged
over one minute. The radar-derived attenuation depends upon instantaneous rainrates
averaged over voxels with sides of approximately 300 m.
3. The radar samples a volume at an altitude above the link depending upon the range.
4. Other attenuating mechanism may be important such as gaseous attenuation, multi-path
and the effects of water or wet ice on the antennas.
It is difficult to estimate the combined effects of 1&2. Just the -R relationship, evaluated at
the 0.01% exceeded rainrate of 22 mm/hr, yields results that vary by a factor of two at 38
GHz with different, common used, drop size distributions. To some extent the errors in the
Z-R and -R transformations may cancel as the combination is effectively a frequency scaling
of rain scatter. Due to the different sample volumes the radar derived attenuations should
exhibit smoother temporal variation than the links. The slower temporal sampling of the
radar data obscures this effect. The over-all effect is that the radar-derived rain-attenuations
can be expected to predict link attenuations with an error of approximately 20%. When nonrain fading mechanisms are important, arbitrarily large errors can result.
4.
Events
There was no real-time monitoring of link attenuations so the Chilbolton radar was used to
image the atmosphere above the link networks when:
A) likely weather systems were active in the area,
B) inside normal working hours,
C) the radar was operational, and
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D) the radar was available for this project.
Due to these restrictions, only a small number of events leading to outage have simultaneous
link and radar information. This sample is so small and its selection criteria so complicated,
that it is impossible to extrapolate these results into annual statistics. However, by
examination of specific events where high attenuation was measured on the links, it is
possible to build up a picture of weather events that lead to outage.
Table 3 provides the number of events in an average year where the attenuation is higher
than that exceeded 0.01% of the time for durations longer than 1,2,5 and 10 minutes. These
are predictions from the RAL Fade Duration Model. The results are independent of link
length and frequency. If the constraints on radar operation lead to only 10% of outage events
being imaged, we would expect to have data from only one event of longer than 2 minutes
duration each year.
Duration
Number of Events
1 min
20
2 min
9.2
5 min
2.0
10 min
0.4
Table 3. The number of events in an average year where the attenuation is higher than the attenuation
exceeded 0.01% of the time for a duration longer than specified.
Table 4 lists the events that are examined in detail in the remainder of Section 4. A series of
figures is given for each event. Four radar images illustrate the radar reflectivity on a near
horizontal plane (PPI) and three vertical slices (RHI) illustrating radar reflectivity (dBz),
differential reflectivity (Zdr) and linear depolarisation ratio (Ldr). The latter two images help
distinguish irregularly shaped, semi-frozen hydrometeors from rain. Also, for each event, the
rain attenuation time series is given for each monitored link: the measured attenuations are the
solid green line and the black symbols are the predictions derived from the radar PPI's.
Date
Radar File
11/4/00
6395
6/12/00
6500
25/1/01
6510
6/2/01
6515
20/3/01
6529
1/5/01
6544
Comment
Low melting layer leads to erroneously high attenuation predictions.
Stratiform band of rain aligned with some links
Localised sleet event.
Small but very intense convective cells.
Snow event causing long outages.
Heavy convective rain.
Table 4. Events examined in Section 4.
When the radar is used to predict link attennuation it can either predict too high attenuation,
too low or just right. Which of these outcomes occurs depends upon the conditions at the
altitude of radar measurement. Figure 4.0 illustrates the radar reflectivity in a vertical slice
through a stratiform event. The vertical structure is due to the changing nature of
hydrometeors as the temperature drops with altitude. Typically, the temperature drops by
approximately 6oC/Km of altitude. For example, if the ground temperature is 12oC then at an
altitude of approximately 2 Km the temperature will be 0oC. Above this level the
hydrometeors will be frozen and, due to the dielectric properties of ice, will produce very low
radar reflectivities. As the frozen hydrometeors fall through the 0 oC level they begin to melt.
This melting layer, typically about 500m thick, contains wet accumulations of ice particles.
These particles produce very large radar reflectivities. Below the melting layer the ice has
melted and raindrops produce radar reflectivites which follow the assumed Z-R relation. If
the melting layer is well above the radar measurement altitude then radar predicted link
attenuation is quite accurate. When the radar measurement altitude is in the melting layer,
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anomalously high reflectivities and derived rain rates are produced. If the radar measurement
altitude is above the melting layer then very low reflectivities and derived rain rates are
produced even when it is raining heavily at ground level. All three of these conditions occur
in the events examined.
Figure 4.0 RHI radar reflectivity, differential reflectivity and linear depolarisation ratio in a vertical slice for
event 6500.
In Figure 4.0 the strong, horizontal, differential reflectivity and linear depolarisation ratio
feature at approximately 2 Km is caused by large, flat accumulations of wet ice in the melting
layer. The clarity of the melting layer in these plots makes them useful for determining the
phase of the attenuating hydrometeor.
Events 6500, 6515 and 6544 are illustrative of conditions where some links experience high
attenuation and the measured and radar predicted attenuations are in good agreement. In these
events, rain is the dominant attenuator and the atmosphere between the altitude of radar
measurement and the ground is relatively uniform. Typically the ground temperature is above
5o C and the melting layer is above 1 Km. Event 6395 illustrates conditions where the radar
predicts much higher attenuation than the links experience. For this event the melting layer is
at 500 m and within the radar PPI plane at the range of Thorne Hill. Over Thorne Hill the
radar falsely interprets high reflectivity as heavy rain and hence predicts high attenuation.
During this event the links experience insignificant attenuation. Events 6510 and 6529 are
cases where wet snow or hail leads to high attenuation on the links. For event 6529 the radar
detects little or no rain as the melting layer is at ground level and so the radar is measuring the
relatively low reflectivity of dry ice.
4.1 6395 11/4/00
April 11 2000 was cold and wet with two fronts covering the country for most of the
day. Temperatures in the South were between 0oC to 4oC. The showers, some heavy, turned
to snow in the afternoon. The RHI radar images indicate a clear melting layer below 500m.
The PPI gives the impression of an intense rain event but the strong radar return at a range of
20 to 40 km is an artefact caused by imaging of the melting layer. This artefact has been
interpreted as erroneously high rain rates and hence high predictions of link attenuation. This
is apparent from the poor agreement between radar predicted link attenuation and measured
attenuation for the Thorne Hill links. The Winchester hub is closer to Chilbolton and so the
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radar artefact does not effect these predictions. Although the melting layer is low, the
attenuating hydrometeors are light, stratiform rain.
Figure 4.1a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
Figure 4.1b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
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Figure 4.1c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
4.2 6500 6/12/00
December 6th 2000 was unseasonably warm with temperatures around 12oC all day.
In the southeast the afternoon brought patchy rain and some heavy showers. The strong fade
event at 16:40 was due to an intense band of stratiform rain aligned with several of the links.
A clear melting layer was present at 15 km indicating that the attenuators were raindrops.
Figure 4.2a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
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Figure 4.2b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
Figure 4.2c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
4.3 6510 25/1/01
January 25 2001 was a mild day with temperatures in the South rising from 2oC to
9 C. The day was generally sunny but with periods of intense showers and thunderstorms,
o
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some including hail. The radar images of the storm that occurred between 1 o'clock and 2
o'clock indicate a strong convective event with no clear melting layer. The Zh return is
consistent with areas of rain rate up to 20 mm/hr. Some melting does appear very close to the
ground indicating that the attenuation experienced by the links was probably due to a mixture
of wet hail and rain. The low Zdr and Ldr returns do not indicate the presence of snow or
graupel. The Thorne Hill links experience more attenuation than the radar predicts suggesting
a localised sleet/hail event passing close to the Thorne Hill hub.
Figure 4.3a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
Figure 4.3b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
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Figure 4.3c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
4.4 6515 6/2/01
On February 6th 2001 a deep depression passed over the country bringing snow to
Scotland for much of the day. Although temperatures in the south were very mild, between
5oC and 12oC, a succession of thundery troughs passed across the south-west. Between 2
o'clock and 6 o'clock in the afternoon a series of strong convective events passed over the
hubs leading to short periods of high attenuation. The radar images indicate a clear melting
layer at an altitude of 1 km. Small cells of very intense rain are included in the event with
rain rates up to 60-70 mm/hr. Most links experienced at least one short period of extreme
attenuation, generally lasting just a few minutes. Differences between the measured and radar
predicted attenuations around these extreme events are probably due to small-scale spatial
variations in rain rate and drop size distribution. The Zdr data is not reliable in this dataset.
Figure 4.4a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
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Figure 4.4b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
Figure 4.4c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
4.5 6529 20/3/01
March 20th 2001 was a cold day, between 0oC and 4oC, with strong easterly winds
reaching gale force at times. Southern regions experienced rain, snow and sleet. Between
8:30 and 12 a wide spread snow event covered the hubs. The Zh data indicates snow
everywhere in the scan region. No melting layer is present in the RHI scans and the strong
Ldr signals indicate large snowflakes. The attenuating hydrometeors were wet snowflakes.
Several links experienced an intense fade lasting several hours. The Ldr data in this dataset is
dominated by noise.
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Figure 4.5a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
Figure 4.5b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
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Figure 4.5c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
4.6 6544 1/5/01
May 1st 2001 started dry and mild. The north-easterly winds swung during the
afternoon bringing westerly winds carrying some heavy pulses of rain along the south coast.
Temperatures remained stable 6oC and 10oC. The links experiences a series of intense fades
as convective cells passed over or near the hubs. A clear melting layer is evident at 2 km.
The attenuating particles are raindrops and there is very good agreement between the link
measurements and the radar predictions.
Figure 4.6a: radar reflectivity PPI and RHI radar reflectivity, differential reflectivity and linear
depolarisation ratio.
Figure 4.6b: rain attenuation time series for Thorne Hill Links. Measured attenuations (green) and radar
predictions (black).
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Figure 4.6c: rain attenuation time series for Winchester Links. Measured attenuations (green) and radar
predictions (black).
5.
Discussion
Event 6529 is of particular significance. Two Thorne Hill links experience protracted
periods of sufficiently high attenuation to cause outage. Link 1 experiences more than 1.5
hours with attenuation above the 0.01% exceeded level. This single event could account for
twice the average annual outage. T-Mobile confirmed that the actual availability of links 1
and 4 are 99.997% and 99.982% respectively. Figure 5 displays the attenuation time series
with the 99.99% availability and actual availability attenuation levels indicated. Both links
should have experienced an extended period of outage due to this wet snow event.
Figure 5. Attenuation time series (green) for Thorne Hill links 1 and 4 on 20/3/01 compared to 0.01% exceeded
rain fade (dashed) and availability provided by T-Mobile (dotted).
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T-Mobile has no record of problems on this day although this could be a problem with their
outage warning system. Both links appear to have maintained lock through the event.
Generally when a link fails there is a plateau in the time series. There is no evidence of
plateaux in the data for this day. It is possible that the links maintained operation but at a
greatly increased BER.
The link monitoring experiment can expect to image only 10% of outage events. This
suggests that events as significant as 6529 probably occur as frequently as yearly. The
attenuator for this event was wet snow. Under these conditions, operators can expect links in
these frequency ranges to suffer sever and extended outages. In Southern England snow
occurs most winters. Counties such as Kent can experience heavy snowfalls. Thorne Hill
links 1 and 4 also experienced high attenuation for a much shorter duration during the
wet/hail event 6510.
6.
Conclusions
This experiment has verified that sleet/hail/snow leads to significant periods of outage
(or reduced capability) on links in the Southern UK. Attenuation by these semi-liquid
hydrometeors is not currently accounted for in ITU-R or Radiocommunications Agency
models. This will lead to protracted periods of outage occurring at unexpected high
frequency. Operators will experience lower availability than expected. Furthermore, the wet
snow event 6529 illustrates that these outages may have very long durations; much longer
than those predicted from models of rain variation.
The experimental methodology has shown problems when monitoring links greater
than 15 Km from the radar. The events of interest have melting layers close to the ground.
When this is the case, the Chilbolton radar is often measuring radar reflectivity in the dry ice
zone above the melting layer and so providing little information on the conditions
experienced by the links. Although conditions close to the radar can be extrapolated to
greater ranges, this situation is less than satisfactory.
This experiment suggests that a correction to current ITU-R/RA models is necessary
to account for attenuation by semi-liquid hydrometeors. Further work is necessary to estimate
the relative occurrence and attenuation caused by sleet, snow and hail.
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