Date:
Meeting:
22nd July 2001
RSSPWG 2nd August 2001
Document Filename:
Paper Number:
prRSSP 0301 024 0
RSSP (03-01)/24
18 July 2001
Report on the Fixed Service Task Group – 11th July, Wyndham
House
Ian Flood, James Richardson and I attended the above meeting at Wyndham
House. The meeting was attended by research academics (e.g. Bath
University, RAL Essex University etc), a few operators and RA staff.
In summary the three key points for FTSLU are regarding:
 year to year variation in rain statistics, causing higher outages in individual
years than the long term mean would suggest;
 inadequate characterisation of rainfall statistics in specific areas (Rainfall
measurements more difficult in Scotland due to the mountainous terrain);
and
 lower air temperatures in the higher latitudes, and on higher ground which
increases the likelihood of extra attenuation from either (a) melting snow
(sleet) on the link path or (b) icing on the antenna (Ice itself is not a
problem, as the dielectric properties of ice do not result in significant
attenuation, but rather ice melting could allow significant amounts of
water/ice mixture to adhere to the antenna/radome.
1.
Where we are
There are three main campaigns of relevance being undertaken within the
FSTG, which are
Campaign 1: The Bath University rain rate study - has now concluded and
will be presented to the RS-SPWG on the 2 August 2001 (PM). This was
conducted in three phases:
 Phase 1 – 0.01% intensity maps converted from hourly data maps via
British Atmospheric Climatic Data (BACD). BADC data is collected hourly
and provided by the Met. Office. The critical part here was deriving a
scaling factor to derive 1-minute data from the BADC 1 hour data;
 Phase 2 – Following phase 1 it was decided to investigate the scaling
factor further using UK Met Office data collected from Solid State Event
Recorders (SSERs) (providing 1-minute integration time rain rate intensity)
to check with the Precision Encoded Pattern Relations (PEPR) stations.
For SSER there are 33 sites with 8 years/site data collected between 1986
to 2000. Equipment is ‘tipping bucket’ with 1-minute sampling. For PEPR
data there were 20 sites with 25 years/site collected between 1949 and
1970. The data was optically scanned from siphon-float chamber gauges
using 1-minute sampling (NB: safer to integrate data to 5-minutes and
scale back to 1-minute). Therefore 60/1 minute conversion of BADC was
also required to correlate PEPR and SSER data.
 Phase 3 – to produce the 0.01% rain intensity grids from best available
data (either SSER or SSER plus scaled BADC) and to evaluate (a) the
worst in five year rain intensity values and (b) the longest record data for
evidence of climatic change.
Key results
 The ITU-R maps of 1-minute rain intensity equalled or exceeded for 0.01%
time are not representative in the UK.
 The rain rates used in FiLSM’s frequency assignment criteria show over
emphasis orographic rainfall (e.g. typical UK rain and drizzle excluding
thunder and torrential rain) in the West by 5 to 12 mm/hr, while under
emphasis convective rains falling in the South–East by roughly the same
amounts.
 There is no substantive evidence to support any trend, particularly in
Scotland, for increasing rainfall intensity.
Campaign 2: The sleet study has yet to start but is likely to commence this
winter in Scotland which is a key area for unexplained link outages thought to
be related to sleet. This is where operators are mainly concerned and
reporting severe outage events. The equipment is likely to be installed in one
of two locations; St. Andrews or Cragiowl. RA has purchased a sleet meter to
distinguish sleet events from rain and snow. A distrometer – a 2-way video
mounted close to one of the antennas to record size and distribution of sleet
and rain – will also be installed somewhere near one of the link ends. Link
data is expected to be on stream by the time of the next meeting, 21
November 2001. The campaign will last throughout the winter.
Campaign 3: The Chilbolton Radar campaign is a monitoring exercise making
measurements on One2One and C&W links in southern England. At present
the links are seeing more attenuation than the radar would suggest. In
quantitative terms it is difficult to say what is actually happening since there is
no indication of any melting layers in the sight of the radar. The suspicion is
that melting is occurring closer to the ground and the radar is being obstructed
by ground clutter. This is being investigated!
2.
Where we want to be
RS-SPWG wants to be in a situation to recommend FLCC about the
necessary changes needed to bring the current rain intensity model up-todate. Bath University has now consolidated the rain intensity data into contour
maps and OS grid square maps, the latter provides values which could
replace those in FiLSM. However, this approach is thought rather antiquated
and is questioned by the FSTG. Rather than just substituting old values for
new, a better approach is to have an algorithm for determining average rain
rates for links crossing several grid squares.
3.
Getting where we want to be
It was agreed that I would now co-ordinate a new FSTG campaign to deliver
models for (a) the rain intensity interpolation program for links across 2/3 grid
squares (also taking account of near surrounding values) and (b) the sleet
model which will at least characterise the precipitation affecting operational
links in Scotland. No changes to current assignment processes will be
proposed until (a) and (b) become available. RS-SPWG will initiate this work
through the FSTG. Scoping of the new work item will start shortly so that a
proposal can be tabled for the next FSTG meeting. Time scales are not
quantified at the moment but (b) will be the critical factor, item (a) is said to be
relatively straightforward.
4.
Other points of interest
The following points were noted:
 Rutherford Appleton Laboratory agreed to check Rec. 530 clear air
changes between versions 7 and 8;
 Rec. 530 had been changed in respect to (a) tilted antennas to avoid
multipath effects and (b) the Australians had made changes to increase
attenuation for 0.1% of time. Therefore there was an action to look into the
implications for UK Planning. (A criticism was made about Rec. 530 in that
it was becoming more of a manual than a recommendation!)
 COST 280 – was inaugurated on 18/19 June to look into fade mitigation
techniques using adaptive power control (covers both Earth stations and
fixed links).