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 1 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. 2 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 3 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 4 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): zhdB 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 5 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, 6 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 7 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). 8 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. 9 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 10 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). 11 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. 12 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. 13 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). 14 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). 15 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). 16 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. 17