Iridium Satellite Communication System, Tsunami Warning System, Buoy

1.
Iridium Satellite Communications System
As
Utilized
by
The
Deep-­‐Ocean
Assessment
&
Reporting
of
Tsunami
Project
Warning
System
May
3,
2015
David
Regan
EN.635.411.81.SP15
Principles
of
Network
Engineering
Professor
John
Romano
Johns
Hopkins
University

2. Iridium-­‐DART
Network
|
P a g e
1
Introduction
&
Approach
............................................................................................
2
Scope
Limits
................................................................................................................
2
Terrestrial
Components
Overview
...............................................................................
2
Space-­‐Based
Components
Overview
...........................................................................
4
Network
Links
.............................................................................................................
6
Tsunameter
<
-­‐-­‐
>
Buoy
...........................................................................................................................................
6
Narrative
&
Technical
Details
...............................................................................................................................
6
Buoy
<
-­‐-­‐
>
Iridium
Satellite
...............................................................................................................................
11
Narrative
&
Technical
Details
............................................................................................................................
11
Iridium
<
-­‐-­‐
>
Iridium
Inter-­‐Satellite
Links
(ISL)
......................................................................................
12
Narrative
.....................................................................................................................................................................
12
Iridium
<
-­‐-­‐
>
Ground
Station
............................................................................................................................
13
Discussion
....................................................................................................................................................................
13
Routing
......................................................................................................................
13
Latency
Calculations
...................................................................................................
15
Total
Transfer
Time
....................................................................................................
16
Conclusions
and
Further
Research
..............................................................................
16
References
.................................................................................................................
18
APPENDIX
A
–
X-­‐Modem
Protocol
Structure
...............................................................
19
APPENDIX
B
-­‐
Images
.................................................................................................
20

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Abstract
This
paper
analyzes
how
the
Iridium
Satellite
Communications
System
is
used
by
The
National
Oceanic
and
Atmospheric
Administration’s
(NOAA)
Tsunami
Warning
System.
Each
network
link
is
evaluated
providing
a
basis
for
understanding
the
overall
system
that
yields
a
robust
and
trustworthy
tsunami
warning
system.
Introduction
&
Approach
This
paper
is
organized
as
follows;
first,
a
general
overview
of
both
the
space
and
terrestrial
based
components
of
The
Deep-­‐Ocean
Assessment
and
Reporting
of
Tsunamis
(DART)
Version
II
project
will
been
made,
then
we
will
delve
into
individual
network
links,
including
those
provided
by
Iridium.
A
focus
on
the
acoustic
link
is
made,
since
it
is
relatively
unusual.
This
will
provide
a
clear
and
logical
context
from
which
to
understand
the
entire
networked
warning
system.
As
each
link
is
analyzed,
end-­‐to-­‐end
networking
achievements
are
revealed.
Scope
Limits
This
paper
is
focused
on
the
DART
and
Iridium
network
links
and
uses
publically
available
information
in
the
process.
Some
detail
related
to
the
Iridium
system
are
proprietary,
therefore
extrapolation
from
known
systems
is
used
to
make
the
best
approximation
of
missing
detail.
Terrestrial
Components
Overview
For
more
than
30
years,
NOAA
researched
the
causes
and
impacts
of
tsunamis
and
in
response
to
a
massive
tsunami
on
March
28,
1964
in
Alaska,
NOAA
began
development
of
the
first
Tsunami
Warning
Center.
The
foundation
of
the
warning
system
is
DART,
whose
buoys
and
bottom-­‐sensor
components
are
shown
in
(Figure
1).
Figure
1:
DART
System
Components

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By
2008,
there
were
36
buoys
installed
in
DART’s
Pacific
Ocean
zone
providing
detailed
sea
level,
temperature,
barometric
pressure,
GPS
coordinates,
timing
and
other
buoy-­‐specific
information
[1].
The
secret
of
DART’s
success
is
the
use
of
global
communication
links
provided
by
the
Iridium
Satellite
Constellation
(Iridium).
Each
buoy
is
installed
with
a
companion
tsunameter,
as
depicted
in
(Figure
2)
(also
known
as
a
bottom
pressure
recorder)[1],
a
that
collectively
constitutes
the
transmission
system
for
the
water-­‐based
link
for
DART.
Buoy’s
have
GPS
receivers
to
maintain
geo-­‐location
for
servicing,
and
for
tsunami
calculations.
Messaging
Path
DART
uses
Iridium
as
the
backbone
for
transmitting
tsunami
data
from
buoys
that
are
sited
in
the
open
ocean.
An
RS232C
interface
with
AT
commands
is
used
for
accessing
satellites
and
PPP
is
the
LLC
layer
protocol
used.
Messages
destined
for
the
tsunami-­‐warning
center
are
triggered
by
tsunami
waves
passing
over
a
buoy
a
The
tsunameter’s
data
storage:
“The
FLASH
memory
provides
four
years
continuous
backup
of
the
entire
raw
pressure
record,
at
a
15-­‐second
sample
period.
Preserving
the
entire
time
series
in
memory
allows
post-­‐deployment
engineering
review
of
the
instrument’s
performance,
as
well
as
scientific
analysis
of
the
entire
deployment
record”.
For
more
information,
refer
to:
Sea-­‐Bird
Electronics.
http://www.seabird.com/sbe54-­‐tsunami-­‐pressure-­‐sensor
Figure
2:
DART
System
Boundaries

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(or
from
seismograph-­‐driven
auto
generated
commands)
that
activates
a
real-­‐time
data
feed
from
the
tsunameter
to
the
surface
buoy.
Data
received
at
the
surface
buoy
from
the
tsunameter
and
is
then
forwarded
to
an
Iridium
Satellite,
which
transmits
the
data
either
to
a
ground
station
(GS),
or
to
an
adjacent
satellite
via
inter-­‐satellite
links
(ISLs),
which
is
then
forwarded
to
the
nearest
GS
for
appropriate
distribution.
A
typical
GS
is
shown
in
(Figure
3)
(gateway)
–
there
are
two
b
of
them
-­‐
with
the
primary
being
in
Tempe,
AZ,
and
each
has
uplinks/downlinks
using
the
Ka
band
over
ranges
29.1-­‐29.3
GHz
and
19.1-­‐19.6
GHz
respectively.
The
gateway
interfaces
with
the
PSTN
and
ISPs.
Space-­‐Based
Components
Overview
Iridium
consists
of
66
low
Earth
orbit
(LEO)
satellites
c
distributed
over
6
polar
orbital
planes
that
are
approximately
31.6
degrees
apart
longitudinally
at
86.4
degrees
inclination[2]
that
co-­‐rotate.
A
seam
develops
between
plane
1
&
6
where
the
satellites
are
counter-­‐rotating.
As
shown
in
(Figure
4),
each
Iridium
satellite
produces
a
footprint
that
overlaps
adjacent
satellites’
footprints
providing
seamless
ground
coverage.
Also
note
that
as
satellites
approach
the
poles,
they
overlap
progressively
–
this
is
important
due
to
an
obvious
contention
issue
that
will
be
discussed
later.
“In
space,
each
Iridium
satellite
is
linked
to
four
others
—
two
in
the
same
orbital
plane
and
one
in
each
adjacent
plane
—
creating
a
dynamic
network
that
routes
traffic
among
satellites
to
ensure
a
continuous
connection,
everywhere
[2]”.
Iridium’s
routing
algorithms
are
proprietary,
but
the
mostly
likely
approach
is
Ad-­‐
b
After
contacting
Iridium
Communications,
and
an
exhaustive
Internet
search,
it
appears
that
there
are
2
ground
stations
left
out
of
the
original
13.
c
Additional
in-­‐orbit
spares
are
held
at
a
lower
orbit
and
are
moved
up
to
operational
height
as
needed.
All
of
the
spares
have
been
used
presently,
but
the
new
Iridium
NEXT
is
to
be
launched
this
year
(2015)
in
October
on
the
current
timeline.
This
will
include
all
new
satellites
with
higher
capacity,
although
it
is
beyond
the
scope
of
this
paper
to
analyze
the
new
system.
Figure
3:
Iridium
Ground
Station
Figure
4:
Iridium
Satellite
Footprints

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hoc
On-­‐Demand
Distance
Vector
(AODV)[3]
routing,
which
considers
the
complexity,
transmission
overhead,
dynamic
update
convergence
and
infinite
loop
issues
related
to
not
only
a
routed
environment,
but
also
one
that
is
continually
in
flux.
Select
aspects
of
AODV
are
considered
and
described
later
in
the
routing
section.
Each
satellite
has
three
antennas
with
16
spot
beams
(48
spot
beams
per
satellite)
and
240
channels
for
user
communications
utilizing
L-­‐
band
(1-­‐2
GHz)
over
the
range
1616-­‐1626.5
MHz
for
a
bandwidth
of
10.5
MHz
[4]
.
Within
each
satellite’s
footprint
(Figure
5),
48
separate
spot-­‐beams
(cells)
are
identified
alphabetically
A-­‐L
in
a
pattern
that
repeats
four
times).
As
noted,
since
the
space
vehicles
converge
near
the
poles,
footprint-­‐overlap
becomes
an
issue
–
to
maintain
a
uniform
loading
on
the
SVs,
outer
cells
in
the
overall
footprint
(Figure
4)
are
selectively
turned
off
at
SV
convergence
latitudes.
Similar
to
cellular
systems,
Iridium
reuses
frequency
bands;
specifically
with
a
reuse
factor
of
12
as
reflected
in
(Figure
5).
Reuse
allows
limited
spectrum
to
be
repetitively
provisioned
provided
sufficient
spatial
isolation
between
duplicated
frequency
ranges.
The
ability
to
employ
this
reuse
capability
is
a
function
of
the
attenuation
characteristics
of
the
specific
frequencies
[5]
i.e.
as
the
power
density
of
a
given
frequency
attenuates,
it
becomes
so
weak
that
it
can
be
ignored,
and
the
frequency
can
be
“re-­‐amplified”
and
used
once
more
for
a
different
unique
channel.
The
240
available
channels
per
satellite
are
divided
among
the
spot
beams
yielding
20
channels
per
spot
beam.
Each
satellite’s
10.5
MHz
user-­‐bandwidth
is
evenly
distributed
over
240
channels
using
FDMA,
where
each
channel
is
provided
with
41.67
kHz
(minus
guard-­‐bands)
of
bandwidth
as
visualized
in
(Figure
6).
The
remaining
500
kHz
is
used
to
provide
approximately
2
kHz
of
guard
band
between
channels.
This
arrangement
facilitates
managing
many
unique
user
channels.
Figure
5:
Iridium
Spot
Beams
(Cells)

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TDMA
is
also
utilized,
but
specific
details
are
not
published
by
Iridium
except
that
the
frame
is
90
ms
long
as
depicted
in
(Figure
7)
and
contains
4
full-­‐duplex
user-­‐channels
with
a
frame
burst
rate
of
50
kbps.
Time
slots
effectively
multiply
the
20
frequency
channels
into
80
after
time
division
slotting
into
the
90-­‐millisecond
TDMA
frame.
A
maximum
bit
rate
for
a
single
user
channel
is
2.4
kbps,
although
if
multilink
point
to
point
protocol
is
used,
channels
may
be
bonded
for
higher
effective
bandwidth
and
capacity
[6].
The
link
between
the
tsunameter
and
buoy
is
not
as
sophisticated
as
Iridium’s,
yet
it
has
unique
characteristics
–
particularly
its
carrier
and
media.
Network
Links
Tsunameter
<
-­‐-­‐
>
Buoy
Narrative
&
Technical
Details
The
tsunameter
and
buoy
both
use
acoustic
transducers
to
transmit
digital
data
over
analog
carrier
up
to
a
distance
of
6,000
meters
in
unguided
media
(seawater).
Modulation
of
the
digital
data
is
accomplished
by
use
of
multilevel
frequency
shift
keying
(MFSK),
which
is
highly
amplified
at
the
transducer
(dB
193)
as
it
produces
the
acoustic
signal
in
waterd.
The
tsunami-­‐messaging
channel
opens
when
the
tsunameter
sends
a
transmission
to
the
buoy
(or
buoy
to
tsunameter)
via
acoustic
modem
every
six
hours
in
standard
d
There
is
an
entire
field
of
study
dedicated
to
underwater
acoustic
networking.
See
https://seagrant.mit.edu/publications/MITSG_08-­‐37J.pdf
“Underwater
Acoustic
Communications
and
Networking:
Recent
Advances
and
Future
Challenges”
Figure
7:
User
Channels
Figure
6:
TDMA
User
Slots

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mode.
The
tsunameter
always
records
readings
every
15
seconds
from
its
sensors
and
saves
the
data
to
flash
memory.
If
the
tsunameter
detects
a
pressure
variation
determined
to
be
a
tsunami,
(determined
by
detection
algorithm)[7],
or
if
an
automated
remote
command
is
received,
the
tsunameter
goes
into
an
event-­‐mode
where
a
file
with
120,
one-­‐minute
average
readings
are
sent
to
the
buoy
immediately
vs.
the
six
hour
interval
in
standard
mode.
Whether
standard
or
event
mode,
the
transmissions
are
forwarded
to
the
tsunami
warning
center
using
Iridium.
(Figure
8)
shows
basic
blocks
of
the
tsunameter’s
computer
system.
The
message
payload
is
a
small
(2
KB
+/-­‐)
xml
file
containing
key
data
including
water
pressure
(leads
to
wave
height),
date-­‐time,
system-­‐status,
and
temperature.
A
more
detailed
diagram
of
internal
flow
appears
in
(Figure
9).
Figure
8:
Tsunameter
Computer
Block
Diagram
Figure
9:
Tsunameter
Logic
Diagram

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The
tsunameter
<-­‐-­‐>
water
<-­‐-­‐>
buoy
communication
channel
is
a
clear
instance
of
the
basic
communications
system
model,
which
is
repeated
throughout
the
DART
system
with
varying
degrees
of
complexity.
The
communication
model
maps
to
the
tsunameter/transducer
(source
system),
seawater
(transmission
system)
and
the
buoy/transceiver
(destination
system)
assuming
we
are
sending
signal
from
the
bottom
up
as
depicted
in
(Figure
10).
“The
acoustic
modems
on
the
DART
II
systems
are
configured
to
operate
in
the
9-­‐
14kHz
frequency
band
at
600
baud,
using
MFSK
modulation
and
error-­‐correcting
coding
[8]”.
X-­‐modem
protocol
is
used
and
its
packet
structure
is
described
in
Appendix
A.
Maximum
throughput
is
controlled
by
the
maximum
receive
rate
of
the
acoustic
modems,
fade
and
noise,
which
is
theoretically
2400
bps
[9].
The
actual
throughput
from
NOAA
is
shown
at
600
baud
and
it
is
assumed
that
r
=
(1
data
element
/
1
signal
element)
for
600
bps.
A
complete
latency
chart
will
be
presented
after
all
the
links
are
defined.
To
clarify,
we
have
digital
data
stored
for
analog
transmission
on
the
tsunameter,
therefore,
we
need
to
modulate
the
data
for
analog
transmission,
which
is
done
with
a
multilevel
frequency
shift-­‐keying
(MFSK)
approach
at
the
physical
layer.
From
the
surface
buoy,
data
is
transmitted
to
the
switched
Iridium
satellite
network,
then
to
a
GS
that
forwards
to
the
Tsunami
Warning
Center
e.
It
takes
30.16
seconds
for
a
2KB
message
to
arrive
complete
at
the
buoy
as
will
be
shown.
For
analysis,
a
e
Ground
stations
have
9-­‐foot
diameter
dishes
and
link
users
to
Internet/PSTN.
Figure
10:
Tsunameter-­‐Buoy
Communication
Model

10. Iridium-­‐DART
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depth
of
3000
meters
is
chosen
(approximate
average)
and
speed
of
sound
in
water
is
defined
to
be
1500
m/s.
Further,
a
single
data
file
size
of
2KB
is
used
for
transmission
calculation.
First,
sound
propagation
in
water
over
distance
3000
meters.
𝑇 𝑝 =
𝑑
𝑟
=
3000𝑚
1500𝑚/𝑠
= 2 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
Secondly,
our
2
KB
file
is
processed
by
x-­‐modem
protocol,
which
has
a
128-­‐byte
payload
per
132-­‐byte
packet,
requires
this
many
packets:
16000 𝑏𝑖𝑡 𝑓𝑖𝑙𝑒
128 𝑏𝑦𝑡𝑒𝑠/𝑝𝑎𝑐𝑘𝑒𝑡 𝑥 (
8𝑏𝑖𝑡𝑠
𝑏𝑦𝑡𝑒𝑠)
= 15.625 𝑝𝑎𝑐𝑘𝑒𝑡𝑠 𝑟𝑜𝑢𝑛𝑑𝑒𝑑 𝑡𝑜 16
Total
transmit
size
for
16
packets:
16 𝑝𝑎𝑐𝑘𝑒𝑡𝑠 132 𝑏𝑦𝑡𝑒𝑠
𝑝𝑎𝑐𝑘𝑒𝑡
= 2112 𝑏𝑦𝑡𝑒𝑠
Transmission
Time
for
a
2KB
file
over
600
bps
channel:
2112 𝑏𝑦𝑡𝑒𝑠 8
𝑏𝑖𝑡𝑠
𝑏𝑦𝑡𝑒
1
𝑠𝑒𝑐𝑜𝑛𝑑
600 𝑏𝑖𝑡𝑠
= 28.16 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
Considering
that
data
is
being
transmitted
through
3000
meters
of
seawater,
an
efficiency
of
94.7%
is
impressive
(2KB/2.112KB).
In
summary,
it
takes
two
seconds
for
the
setup
notice
to
arrive
at
the
buoy.
Ignoring
actual
setup
processing
delay
and
queuing
at
buoy,
it
takes
28.16
seconds
to
complete
the
data
transfer
for
a
total
of
30.16
seconds.
Time
for
resend
of
bad
packets
not
factored.
Table
1
contains
summary
details
and
short
narratives
to
complete
the
review
of
the
tsunameter
to
buoy
link.
Table
1:
Communication
Links
Overview
–
Tsunameter
-­‐-­‐
>
Buoy

11. Iridium-­‐DART
Network
|
P a g e
10
Link
Features
@
given
OSI
Layer
Tsunameter
-­‐-­‐>
Buoy
Application
Data
collected
from
pressure,
temperature
and
other
sensors
and
written
to
a
space-­‐delimited
text
file
by
C
application
Presentation
XML
data
file
–
space
delimited
text
Session
Half
Duplex
operation
due
to
open
water
medium,
manages
link
Transport
Checksums
and
X-­‐modem
protocol
–
no
port
addressing
entire
packets
of
data
with
many
blocks
are
sent
without
requesting
an
acknowledgement
from
the
receiver
after
each
block.
When
blocks
are
missing
or
erroneous,
receiver
requests
resend
of
individual
blocks.
Network:
X-­‐modem
protocol
packetizes
data
file
bit-­‐stream
DLL:LLC/MAC
Physical
Sea
Water
Media:
Characteristics:
pressure,
temperature,
salinity,
density
Speed
of
Sound
in
water
taken
as
1500
m/s
Bi-­‐directional
Acoustic
Telemetry
in
Water
(Media);
Benthos
ATM-­‐880
Telesonar
modem/AT
Multiplexing
Multilevel
Frequency
Shift
Keying
approach
is
used
to
modulate
the
binary
stream
into
an
analog
for
transport
via
media.
The
source
level
is
at
193
dB
re
1ųPa
@
1
m
with
a
40
VDC
supply.
Bandwidth
9-­‐14kHz
=
5kHz
Throughput
Ranges
Trans
Rate
Tx
=
150
-­‐
15360
bps
Receive
Rate
Rx
=
150
–
2400
bps
(see
signaling
rate
for
actual)
Signaling
Rate
Actual
-­‐
600
baud
(signal
elements/second)
assumed
1/1=r
Propagation
T
Sound
in
water
–
taken
as
1500
m/s;
T
(propagation)
t=d/r
Latency
Sum
of
Propagation
times:
signal
propagation
and
message
transfer
time
included;
queuing
and
processing
times
at
each
node
ignored
for
this
analysis
Attenuation
Not
addressed
Distortion
(fading
from
multipath)
UW
sound
propagation
is
characterized
as
either
vertical
or
horizontal.
Horizontal
calculations
have
to
consider
seafloor
reflections,
while
not
in
vertical;
issues
with
multi-­‐path
reflections
confusing
the
receiver
–
handled
algorithmically

12. Iridium-­‐DART
Network
|
P a g e
11
Noise
(Whale
Song)
Straight
SNR=
Signal/Noise;
SNRdB
=10log10SNR
Not
evaluated
in
this
analysis
Data
Rate
Limits
–
Noiseless
Channel
Nyquist
Bit
Rate:
Not
evaluated
on
this
link
in
this
analysis.
2
x
bandwidth
x
log2
L,
where
L
is
number
of
signal
level
Data
Rate
Limits
-­‐
Noisy
Shannon
Capacity:
Not
evaluated
on
this
link
Bandwidth
x
log2
(1+SNR)
Bandwidth
Delay
Product
(600-­‐bit/s)(2
s)
=
1200
bits
Buoy
<
-­‐-­‐
>
Iridium
Satellite
Narrative
&
Technical
Details
As
shown
previously,
data
arrives
at
the
buoy
and
is
queued
and
or
stored.
Then,
the
Iridium
transceiver
is
activated
and
the
file
is
transmitted
via
the
uplink
to
Iridium
for
switching
to
the
Tsunami
Warning
Center.
As
will
be
shown
in
the
next
section,
propagation
time
is
2.60
ms
and
file
transmission
time
is
6.67
seconds
for
the
2
KB
data
file.
Iridium
uses
FDMA
and
TDMA,
both
at
the
media
access
sub-­‐layer
of
the
DLL
for
channelization,
and
time
division
duplexing
(TDD)
at
the
physical
layer.
Channels
are
comprised
of
a
frequency
band
and
time
slot.
Multilink
Point
to
Point
Protocol
(MPPP),
at
the
DLC
layer
[5]
controls
establishing,
maintaining,
configuring
and
terminating
endpoint
connections
as
well
as
data
transfer[5].
Table
2:
Communication
Link
Overview
–
Buoy
-­‐-­‐
>
Iridium
Link
Features
@
given
OSI
Layer
Buoy
-­‐-­‐>
Iridium
Application
Proprietary
Presentation
Not
defined
-­‐
proprietary
Session
Not
defined
-­‐
proprietary
Transport
Checksums
and
X-­‐modem
protocol
–
no
port
addressing
Network:
Defined
in
section
entitled
“routing”
DLL:DLC/MAC
Muiltilink
PPP
at
LLC
and
FDMA/TDMA
at
MAC
Physical
The
radio
frequency
transmission
in
the
1565
MHz
to
1626.5
MHz
range
and
the
data
transmission
rates
are
at
2.4
kilobits
per
second.
Bandwidth
5kHz

13. Iridium-­‐DART
Network
|
P a g e
12
Throughput
2.4
kbps
constrained
at
ground-­‐user
modem,
although
can
be
upgraded
to
higher
rate
based
on
channel
bonding
via
inverse
multiplexing
or
MLPPP
Signaling
Rate
See
section
titled
“Space-­‐based
components
overview”
Propagation
T
See
latency
calculations
above
Latency
See
latency
calculations
above
Attenuation(db)
Db=10log10(P2/
P1)
Not
addressed
–
data
not
published
Distortion
(fading
from
multipath)
Not
addressed
-­‐
data
not
published
Noise
Not
addressed
-­‐
data
not
published
Data
Rate
Limits
–
Noiseless
Channel
Nyquist
Bit
Rate:
Not
addressed
2
x
bandwidth
x
log2
L,
where
L
is
number
of
signal
level
Data
Rate
Limits
-­‐
Noisy
Shannon
Capacity:
Not
addressed
–
data
not
published
Bandwidth
x
log2
(1+SNR)
Delay
Product
Time
to
fill
channel
with
data/bits:
2400bps
x
1s/1000ms
x
2.05
ms
=
4.92
ms
Iridium
<
-­‐-­‐
>
Iridium
Inter-­‐Satellite
Links
(ISL)
Narrative
Data
delivered
to
Iridium
cross
its
nodes
over
inter-­‐satellite
links
(ISL)
that
operate
at
22.55
–
23.55
GHz
at
25
Mbps
using
the
slotted
TDMA,
which
is
transmitted
with
QPSK.
Maximum
user
throughput
is
not
known
due
to
absence
of
data
on
ISL
overhead,
data
compression
and
factors.
Frequency
conversion
of
received
signal
allows
the
Iridium
to
receive
and
transmit
without
interference,
since
frequencies
utilized
for
inter-­‐satellite,
uplink,
downlink
and
user
links
are
different.
Crosslink
delay
is
shown
to
be
13.33
ms
over
an
average
distance
between
satellites
of
4000
meters.
An
interesting
issue
arises
as
the
satellites
converge
at
the
poles
remembering
that
each
orbit
is
in
a
polar
plane.
The
48
spot
beams
begin
to
increasingly
overlap
creating
a
situation
where
the
spot
beams
need
to
be
managed
to
avoid
interference.

14. Iridium-­‐DART
Network
|
P a g e
13
It
is
assumed
that
this
is
done
dynamically
by
algorithm
where
selected
spot
beams
are
progressively
shutdown
at
the
periphery
of
the
main
footprint
[3].
As
defined
in
the
space-­‐based
components
section,
there
are
six
orbital
planes
with
11
satellites
each.
They
all
rotate
in
the
same
direction,
which
gives
rise
to
a
“seam”
orbit
-­‐-­‐orbits
that
are
in
counter
rotation
relative
to
each
other.
This
is
managed
by
blocking
communication
between
the
“seams”
[10],
since
Doppler
effects
would
create
unacceptable
delay
and
overhead
based
on
frequency-­‐shifting
transcriptions.
Iridium
<
-­‐-­‐
>
Ground
Station
Discussion
From
a
networking
perspective,
the
Iridium
nodes
are
in
two
planes:
the
orbital
and
terrestrial
[11].
The
Iridium
network
is
similar
to
a
cellular
network
in
that
static
base-­‐stations
communicate
with
moving
devices
and
orders
handoffs
as
signal
strength
drops;
in
our
case,
the
cells
are
moving.
Another
difference
is
that
cellular
base
stations
switch
directly
into
a
terrestrially
based
network,
not
to
other
cellular
base
stations
using
wireless
transmissions.
This
is
an
important
distinction
since
the
ground-­‐based
network
has
more
capacity,
while
the
space-­‐based
Iridium
is
constrained
by
available
RF
throughput
and
signaling
overhead.
There
appears
to
be
two
ground
station
gateways
(Hawaii
and
Arizona)
and
21
antennas
distributed
geographically,
yet
this
is
difficult
to
verify
as
Iridium
Communications
Inc.
does
not
respond
to
questions
related
to
details
of
its
system
(warnings
manifest
if
enough
research
is
done).
Gateways
ensure
space
to
ground
link
availability,
no
matter
whether
a
satellite
is
passing
immediately
overhead,
or
not
given
that
traffic
is
routed
to
the
closest
GS
by
the
constellation.
Routing
is
examined
in
the
next
section.
Routing
Iridium
does
not
advertise
its
routing
algorithm,
and
speculation
abounds
about
it
in
the
aerospace
industry,
yet
researchers’
models
reveal
that
a
modified
Bellman-­‐
Ford
(mBF)
algorithm
is
likely
[3],
which
is
no
mean
feat
given
such
a
highly

15. Iridium-­‐DART
Network
|
P a g e
14
dynamic
system,
especially
if
we
are
relying
on
distance
vector
updates
propagating
through
the
system.
Given
that
system
details
are
scarce,
the
next
section
covers
a
macro-­‐view
of
a
potential
configuration.
If
we
consider
the
Iridium
system
as
two
elements
acting
on
two
separate
planes,
that
of
the
Iridium
constellation,
and
that
of
ground
nodes,
updating
routing
tables
based
on
node-­‐to-­‐node-­‐propagated
updates
seems
unnecessary
(if
we
could
rapidly
and
autonomously
calculate
each
node’s
position).
From
that
perspective,
consider
the
following;
inter-­‐satellite
distances
can
be
determined
instantaneously
based
on
GPS
calculations
on
each
node,
and
while
the
speed
of
the
satellites
quickly
negates
coordinate
calculation,
the
routing
tables
should
be
able
to
update
themselves
faster
using
an
application
specific
integrated
circuit
(ASIC)
existing
as
a
separate
subsystem.
Node
awareness
of
instantaneous
relative
position
would
allow
the
mBF
algorithm
to
determine
least-­‐cost
distances
by
the
node
for
itself,
rather
than
having
rapid
and
ongoing
route
updates
broadcast
everywhere.
Said
another
way,
this
would
allow
distance
vectors,
for
the
constellation,
to
be
maintained
by
calculation
internally
on
each
satellite
using
an
ASIC.
This
would
eliminate
convergence
time
and
route
update
traffic.
Uplinking
user-­‐access
devices
always
connects
with
the
nearest
satellite
based
on
signal
strength
(and
in
consideration
of
the
seam),
and
if
the
constellation
already
knows
the
least-­‐cost
path
through
the
constellation,
then
knowing
the
correct
exit
node
will
complete
the
path.
Handoffs
could
be
made
on
the
same
basis
by
having
ground
stations
feeding
the
nearest
satellite
a
flag
saying
“you
are
closest
to
me”;
this
distance-­‐vector
(for
the
two
ground
stations)
would
propagate
through
the
constellation
every
few
seconds,
creating
system-­‐wide
awareness
of
exit
points
relative
to
the
constellation’s
nodes.
System-­‐wide
latency
is
considered
next
using
our
original
2KB
source
file
from
the
tsunameter.

16. Iridium-­‐DART
Network
|
P a g e
15
Latency
Calculations
Tfile(2
kilobyte)
=
T(tsunameter-­‐>buoy)
+
T
(transmission)
+T
(uplink)
+
(N-­‐1)T
(cross)
+
T(downlink)
Where,
T(tsunameter-­‐>buoy)
=
propagation
delay
and
transmission
time
for
file
on
the
tsunameter-­‐>buoy
link
T
(transmission)
=
transmission
time
for
the
file
(2
kilo-­‐byte)
T
(uplink)
=
propagation
delay
from
buoy
to
the
satellite
T
(cross)
=
propagation
delay
on
satellites
cross
links
T(downlink)
=
propagation
delay
satellite
to
the
ground;
(processing/queuing
delays
per
satellite
ignored
for
this
analysis)
N
=
number
of
satellites
in
overall
link
T
(uplink)
=
T(downlink)
=
!"#$%%&#$ !"#$#%&'
!"##$ !" !"#!!
=
!"#!"
!.!!"!!!"!!"
!"#
= 2.60 𝑚𝑠
[10]
Distance
between
satellites
averages
4000
km[12].
So,
T
(cross)
=
!"#$$%&'( !"#$%&'(
!"##$ !" !"#!!
= 4000
!"
!.!!"!!!"!!"
!"#
= 13.34 𝑚𝑠
[10]
T
(transmission)
=
!"#$ !"#$
!"#$%&'()'" !!!"#$!!"#
= 2
!"#$%&
!.! !"#$
= 6667 𝑚𝑠
[10]
Therefore,
the
Total
time
to
move
a
2KB
file
over
the
system
is:
T
(Total)
=
([30.15𝑠 𝑥
!"""!"
!
] + 6667 𝑚𝑠 + 2 2.60 𝑚𝑠 + 13.34 𝑚𝑠) = 36.85 𝑠
To
simplify
this
analysis,
overhead/padding
to
frames
not
included
in
calculations
except
in
the
case
of
the
tsunameter
to
buoy.

17. Iridium-­‐DART
Network
|
P a g e
16
Total
Transfer
Time
Figure
11:
End-­‐to-­‐End
Latency
Conclusions
and
Further
Research
The
Tsunami
Warning
System
is
a
masterwork
of
technology
and
it
has
been
proven
to
give
warning
to
those
in
the
path
of
incoming
tsunamis
saving
lives.
This
was
demonstrated
in
Japan
in
2011,
and
while
the
scale
of
a
tsunami’s
potential
devastation
is
terrifying,
systems
can
be
improved
to
provide
better
resolution
of
expected
wave-­‐height
saving
as
many
as
possible
[13].
The
Iridium
Constellation
is
integral
to
the
success
of
the
warning
system,
and
its
networking
is
a
major
factor
in
this
success.
Given
the
complexity
of
routing
in
a
dynamic
system
of
nodes,
such
as
Iridium’s,
more
research
should
be
undertaken
to
improve
dynamic
routing
in
rapidly

18. Iridium-­‐DART
Network
|
P a g e
17
changing
RF
networks.
This
has
implications
well
beyond
satellite
constellations
in
our
ever-­‐expanding
mobility-­‐oriented
world.

19. Iridium-­‐DART
Network
|
P a g e
18
References
[1]
C.
Meinig,
S.
E.
Stalin,
A.
I.
Nakamura,
and
H.
B.
Milburn.
(2005,
March
29).
Real-­‐Time
Deep-­‐Ocean
Tsunami
Measuring,
Monitoring,
and
Reporting
System:
The
NOAA
DART
II
Description
and
Disclosure
[Article].
Available:
http://www.ndbc.noaa.gov/dart/dart_ii_description_6_4_05.pdf
[2]
Iridium.
(2015,
March
30).
Ground
Infrastructure.
Available:
https://www.iridium.com//About/IridiumGlobalNetwork/GroundInfrastructure.as
px
[3]
L.
Xiangdong,
G.
Dilip,
M.
Tim,
and
S.
Peter,
"Analysis
of
IP
Routing
Approaches
for
LEO/MEO
Satellite
Networks,"
in
28th
AIAA
International
Communications
Satellite
Systems
Conference
(ICSSC-­‐2010),
ed:
American
Institute
of
Aeronautics
and
Astronautics,
2010.
[4]
S.
R.
Pratt,
R.
A.
Raines,
C.
E.
Fossa,
and
M.
A.
Temple,
"An
operational
and
performance
overview
of
the
IRIDIUM
low
earth
orbit
satellite
system,"
Communications
Surveys,
IEEE,
vol.
2,
pp.
2-­‐10,
1999.
[5]
B.
A.
Forouzan,
Data
Communications
AND
Networking,
5th
ed.
New
York,
NY:
McGraw
Hill,
2013.
[6]
A.
M.
Jabbar,
"Multi-­‐Link
Satellite
Data
Communication
System,"
Master
of
Science,
Engineering,
Electrical,
Osamia
University
University
of
Kanas,
2001.
[7]
NOAA.
(n.d.,
April
20).
Tsunami
Detection
Algorithm.
Available:
http://nctr.pmel.noaa.gov/tda_documentation.html
[8]
NOAA,
"DART
II
System,"
ed,
2015.
[9]
Benthos.
(2014,
April
30).
Benthos
Modems
[PDF
Files].
[10]
S.
R.
Pratt,
R.
A.
Raines,
C.
E.
Fossa,
and
M.
A.
Temple,
"An
Operational
and
Performance
Overview
of
the
Iridium
Low
Earth
Orbit
Satellite
System,"
ed,
1999.
[11]
Staff.
(2015,
March
30).
Available:
https://www.iridium.com/About/IridiumGlobalNetwork/SatelliteConstellation.asp
x
[12]
I.
C.
Inc.,
"Ground
Station,"
ed:
Iridium
Communications
Inc.,
2015.
[13]
D.
Demetriou.
(2013).
Tsunami
two
years
on:
Japan
finally
gets
warning
system
that
would
have
saved
hundreds
of
lives.
Available:
http://www.telegraph.co.uk/news/worldnews/asia/japan/9920042/Tsunami-­‐
two-­‐years-­‐on-­‐Japan-­‐finally-­‐gets-­‐warning-­‐system-­‐that-­‐would-­‐have-­‐saved-­‐hundreds-­‐
of-­‐lives.html
[14]
Wikipedia.
(2015).
XMODEM.
Available:
http://en.wikipedia.org/wiki/XMODEM

20. Iridium-­‐DART
Network
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19
APPENDIX
A
–
X-­‐Modem
Protocol
Structure
X-­‐Modem
uses
a
132-­‐byte
packet
structure
with
128
bytes
reserved
for
data.
A
3-­‐
byte
header
that
included
a
<SOH>
control
character,
a
block
number
from
0-­‐255,
and
the
inverse
of
the
block
number
(-­‐255)
minus
the
block
number
with
block
numbers
starting
at
1
and
incrementing
by
1
for
subsequent
blocks.
The
packet
trailer
is
a
checksum
of
1-­‐byte.
The
checksum
is
the
sum
of
all
bytes
in
the
packet
module
256.
Only
the
eight
least
significant
are
retained,
ignoring
overflow
keeping
the
continuity
of
the
1-­‐byte
check.
Once
a
file
transmission
was
complete,
a
special
<EOT>
character
was
sent,
which
was
not
part
of
the
block
[14].

21. Iridium-­‐DART
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20
APPENDIX
B
-­‐
Images
Figure
12:
DART
II
Image

22. Iridium-­‐DART
Network
|
P a g e
21
Figure
13:
DART
Buoy
-­‐
Open
Ocean
Figure
14:
Tsunameter
Awaits
Deployment

23. Iridium-­‐DART
Network
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P a g e
22
Figure
15:
Iridium
Satellite