Rain Fade is a factor with Ku-band and Ka-band satellite communications. Rainfall rate is measured in mm/hour. The occurrence of significant Rain Fade events is scaled by the rainfall rate and its duration. Generally these events are transient as the rain cloud moves through. The path of interest is only the line from the satcom terminal to the servicing satellite. Only while the rain cloud is in the way do problems occur.
Panama is in
a very severe rainfall rate region and will create frequent issues with Ka band
service operating below about 15,000 feet. Ku should operate through these
scenarios with less disruption. It is prominent in the region along with the Brazilian rain forests and significant as a hub airport.
A model of rainfall rate for the Americas is shown below, where each region
is shown by a letter that corresponds to rainfall rate. Panama City is in region "P".
Figure 1 - Rec. ITU-R PN.837-1 1
The categorical rainfall rates (mm/hour) shown below
reveals Region P is severely impacted.
Figure 2 - Rec. ITU-R PN.837-1 1
The loss (rain fade) is measure in dB. It is a function of the height of the rain
cloud (assumed homogenous) and the look angle (the elevation angle to the
satellite). Lower elevation angles extend the impacted path length, while
looking straight up has the least impact.
Figure 3 - http://www.philsrockets.org.uk/Rain%20Fades.pdf
The height of the rain “cloud” is predicted from ITU Recommendation
P.839-4.
Figure 4 - https://www.itu.int/rec/R-REC-P.839-4-201309-I/en
For PTY 9 deg N = rain height is about 4.6 km (~15,000
feet).
With a satellite elevation of 75 deg., the slant range is
about 4.8 km.
For the purpose of this exercise a 5 km slant range is
assumed.
The forward channel can accommodate rain fade by boosting
the uplink signal to push through the rain fade loss. Any spectral efficiency
degradation can be offset by increasing the spectral contribution. Rain fade is
a function of rainfall rate, slant range, and frequency. The forward channel
operates on a lower frequency than the return channel and suffers less rain
fade as a result.
The return channel is much more constrained when compared to
the forward channel. Typically, nearly maximum EIRP is already utilized to gain
the greatest data rate. The return channel operates under higher network
congestion than the forward channel. Network throughput suffers as the return
channel degrades. Rain fade directly impacts the spectral efficiency – as rain
fade increases the return channel will degrade, presuming less efficient
modcods used with the same symbol rate. It is possible for the return channel to
accommodate higher symbol rates to spread and boost the signal back up to clear-sky
throughput, but that is not presumed.
DVB-S2X modcods can be used to predict the effect of rain
fade loss. The following figure (https://www.dvb.org/resources/public/factsheets/dvb-s2x_factsheet.pdf)
is used for analysis.
Assuming a return channel clear sky C/N of about 7 dB yields
spectral efficiency of 2.0.
A 5 dB rain fade will drop spectral efficiency to 50% compared
to clear sky.
A 10 dB rain fade will drop spectral efficiency to about 15%
compared to clear sky.
A 20 dB rain fade will sever communications (NoComm).
The rainfall rate/km for 50% throughput is -1 dB/km (5 km = -5
dB).
The rainfall rate/km for 15% throughput is -2 dB/km (5 km = -10
dB).
The rainfall rate/km for NoComm is -4 dB/km (5 km = -20 dB).
Rainfall rate loss (dB/km) can be determined by ITU Recommendation P.838-3.
Polarization
|
Freq (GHz)
|
RR (mm/H)
|
dB/km
|
Horizontal
|
14.5
|
17.4
|
1.0
|
Horizontal
|
14.5
|
32.0
|
2.0
|
Horizontal
|
14.5
|
59.0
|
4.0
|
Vertical
|
14.5
|
18.8
|
1.0
|
Vertical
|
14.5
|
36.5
|
2.0
|
Vertical
|
14.5
|
70.3
|
4.0
|
Circular
|
30
|
4.7
|
1.0
|
Circular
|
30
|
10.0
|
2.0
|
Circular
|
30
|
21.0
|
4.0
|
Horizontal polarization (worst case) is used for Ku.
Ku rain fade is slightly less sensitive with using Vertical
polarization.
Carrier (GHz)
|
Carrier (GHz)
|
||
14.5
|
30
|
||
dB/km
|
mm/hr
|
mm/hr
|
Throughput
|
1
|
17
|
5
|
<50%
|
2
|
32
|
10
|
<15%
|
4
|
59
|
21
|
NoComm
|
Rain rate for Panama City airport was not available. Data
from Corozal from 2007-2015 was available (https://biogeodb.stri.si.edu/physical_monitoring/research/panamacanalauthority)
and is used for this analysis as representative to PTY. The
data shows rain rate events > 4mm/hour based on 15 minute observations.
Each rain rate “dot” plotted in the figure below represents
one event from the data set.
For perspective, the rain rate thresholds are plotted to
show the preponderance of events above each threshold.
With 9 years of data for each hour of the day, the events
are summarized below as percent of the time in each hour. Because of the granularity in the data, a 4 mm/hour threshold was used
for Ka band throughput less than 50% (instead of 4.7 mm/hour).
For both Ka and for Ku, the plots shown are additive. The
threshold line for 15% is plotted on top of the 50% line. The NoComm line is
plotted on top of the 15% line.
14H is the worst case for rain fade.
14:00 - 14:59
|
<50%
|
<15%
|
NoComm
|
total
|
Ku
|
0.75%
|
0.49%
|
0.13%
|
1.37%
|
Ka
|
4.84%
|
1.09%
|
1.37%
|
7.30%
|
Stay tuned!
Peter Lemme
peter @ satcom.guru
Follow me on twitter: @Satcom_Guru
Copyright 2019 satcom.guru All Rights Reserved
Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.
Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).
An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.
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