Saturday, March 9, 2019

Exploring Rain Fade in an Extreme Rain Fall Zone


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%


How bad are these storms?  Here are two of the worst, each dropping over one foot of rain in under one hour.








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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