Chapter 1
Introdu
tion to W Band
"The only way to get rid of a
temptation is to yield to it".
Da il "Ritratto di Dorian Gray"
Os
ar Wilde
1.1 W Band
W band is an innovative frequen
y range,
overing the millimeter part of the
ele
tromagneti
spe
trum (75-110 GHz, respe
tively 4-2.7 mm). In the last
years an in
reasing interest for this range has been demonstrated in some
elds due to its advantageous
hara
teristi
s su
h as high bandwidth avail-
ability, short wavelength, redu
ed interferen
e, redu
ed antenna size. So far,
main appli
ation whi
h has demonstrated great interest in W band is the
radar domain. A
tually, the frequen
y range around 94-95 GHz is at present
employed both for terrestrial and spa
e appli
ations. Rheinmetall Italy (ex
Oerlikon Contraves Italy) developed a W band operative radar (95 GHz)
devoted to monitor the airport surfa
e tra
(surveillan
e duties) through
high resolution imaging, exploiting the short wavelength intrinsi
to the new
frequen
y. Moreover, re
ently (2006),NASA laun
hed a new mission, Cloud-
Sat, to study
louds and
limate using a weather W band radar (94 GHz); it
is the rst spa
e mission employing W band even if for radar appli
ations.
2
CHAPTER 1. INTRODUCTION TO W BAND 3
1.2 Feasibility studies for W Band tele
ommu-
ni
ations: WAVE and DAVID Proje
t
Con
erning tele
ommuni
ation appli
ations, at present no operative mission
has been developed but only feasibility studies. Italy, through the Italian
Spa
e Agen
y (ASI), is one of the rst
ountries that have made an eort to-
wards the exploitation of W band for tele
ommuni
ations. Present, the more
advan
ed studies have been
arried out by ASI with DAVID (DAta and Video
Intera
tive Distribution), devoted to pioneer the use of the W band
hannel
for tele
ommuni
ations purposes (C/D phase proposal), and later WAVE (W
band Analysis and VEri
ation), a feasibility study for designing a tele
om-
muni
ation payload to be deployed in geostationary orbit.However, main
question around the use of W band for tele
ommuni
ations is to understand
the propagation
hannel behavior, at present unknown and derivable only
through the appli
ation of attenuation models validated at lower frequen
ies
(su
h as ITU models). It is known that to in
rease the frequen
y from Ka
to W ae
ts more the signal through dierent ee
ts (attenuation, depolari-
sation, s
intillations, et
.), mainly due to rain, gases absorption (oxygen and
water vapor) and
louds, but physi
al models of propagation do not exist at
these high frequen
ies and espe
ially experimental measurements to be used
to validate these models are not available. Any design of a spa
e mission
using W band for tele
ommuni
ations needs to know the
hannel behavior
in order to assign the suited margin in the link. WAVE phase A [1℄ has been
arried out in 2004 to evaluate the feasibility of a W band tele
ommuni
a-
tions payload for a geostationary orbit mission in order to provide both an
experimental study on the propagation in W band and a preliminary exper-
imentation towards the
ommer
ial suitability of the
hannel for data relay
servi
es. A se
ond phase of WAVE,
alled phase A2, started in Mar
h 2007
and
urrently on-going, aims at
arrying out at the same time two main re-
sear
h lines due to the
omplexity of the GEO mission. The rst resear
h
line is a demonstrative study
on
erning the development of an experimental
payload on-board of a High Altitude Platform (HAP), referred to as Aero-
WAVE, as well as the development of a nano-satellite platform
arrying a
small W band payload referred to as IKNOW [2℄. The se
ond resear
h line is
a pre-operative study on the GEO-orbit outlined in the previous phase, and
a payload in LEO-orbit on a dedi
ated mi
ro-satellite platform.
CHAPTER 1. INTRODUCTION TO W BAND 4
1.2.1 Aero-WAVE
The Aero-WAVE mission is, within the WAVE A2 phase, one of the two
demonstrative studies, devoted to the exploitation of a High Altitude Plat-
form (HAP)
on
ept for W band propagation
hannel and data transmission
hara
terisation [1℄. Even if the full
hannel
hara
terization will be
ar-
ried out through the LEO mission (IKNOW), preliminary signi
ant results
are expe
ted by the shorter-term HAP mission. In fa
t, sin
e the main
phenomenon that
ould limit the exploitation of this frequen
y band is the
troposphere attenuation due to rain,
louds, fog, gaseous absorption and
s
intillation, measurements
olle
ted by the HAP experiment
ould already
give important highlights to the future work on W band propagation. Time
onstraints on the development of the payload to be embarked on a HAP,
limits on the ying time of the HAP, and hen
e on the period of time avail-
able for the measurements, poses many
hallenges to the mission de nition
and payload design. The a
tivities within the Aero-WAVE program fo
us
on the development of a payload
arried on-board a High Altitude Platform.
This rst preliminary mission aims at demonstrating the ee
tiveness of W
band te
hnology in Earth-to-Spa
e
ommuni
ations. The heritage that will
be gained through Aero-WAVE mission will be useful for the se
ond in-orbit
demonstrative mission (IKNOW) [3℄. The payload designing and developing
time will be very tight in order to gather in a little time the rst results
obtained on
hannel
hara
teristi
s. In this frame no ad-ho
te
hnologi
al
development are foreseen and all the hardware used for P/L development
will be COTS (Commer
ial O The Shelf).
The Aero-WAVE Ar
hite
ture
The payload will perform experimental measurements on W band
hannel
embarked on-board a HAP. The stratospheri
platform is a high altitude air-
raft known as the M-55 Geophysi
a as in Figure 1.1, a Russian air
raft well
known in the atmospheri
resear
h environment and the European s
ienti
ommunity sin
e 1996. The M-55 Geophysi
a will be equipped with the pay-
load instrumentation to
arry out the measurements. These measurements
and the resulting data provided throughout the Aero-WAVE mission will give
important indi
ations of the
hannel behavior at dierent weather
onditions
and at dierent lo
ations.
This mission has two important goals:
1. In- ight test ofW band te
hnology
2. W band satellite
hannel measurements
CHAPTER 1. INTRODUCTION TO W BAND 5
Figure 1.1: M-55 Geophysi
a
For what
on
erns the in- ight test at present, most of the
ommer
ial
W band hardware available has been developed mainly for radar and radio
astronomy appli
ations; it should be noted that these are both terrestrial ap-
pli
ations. Hen
e, Aero-WAVE represents a
han
e to test for the rst time
some COTS hardware in- ight. This rst test is also helpful for the IKNOW
mission and for the two preoperative missions, identifying the
riti
al te
h-
nologies that need to be developed to be suitable for spa
e appli
ations. In
fa
t, at least for the IKNOW mission, also the use of not spa
e quali ed
hardware is foreseen. Con
erning the latter obje
tive, at present the fa
-
tor that
ould potentially limit the exploitation of this frequen
y band is
the troposphere attenuation due to rain,
louds, fog, gaseous absorption and
s
intillation. At present, the impa
t of these phenomena on W band signal
propagation is still unknown (from an empiri
al point of view) be
ause of the
pioneering state of W band
ommuni
ations. Spe
i
ally, the proje
t aims
at analyzing the W band
hannel through two dierent experiments:
1. Channel propagation experiment, in
luding power measurements in or-
der to
reate a rst troposphere attenuation model (on both uplink and
downlink)
2. Data transmission experiment, in
luding BER measurements in order
to evaluate the W band
hannel quality (on downlink)
From the point of view of the time evolution before mentioned, the Aero-
WAVE demonstrator
onstitutes a preliminary step to the development and
deployment of IKNOW. At present, ASI requested the ling of a frequen
y
range in W band for the DAVID mission and the publi
ation will expire in
2009. ASI Li
ense to use these frequen
ies is subje
t to the realization of
CHAPTER 1. INTRODUCTION TO W BAND 6
a
oordination a
tivity with other
ountries and is subje
t to the mainte-
nan
e of some
onstraints (orbits, name of missions, ground stations, et
).
Aero-WAVE will employ other frequen
y in the W band range, thus it will
not be useful for the DAVID frequen
ies issue, but this mission should be
planned before IKNOW. Moreover, the empiri
al knowledge of the W band
spa
e
hannel represents a strategi
interest be
ause of the la
k of any past
experiment.Thus, the development time has been identi ed as a strategi
el-
ement of Aero-WAVE mission. To fa
e this obje
tive requires to
oordinate
the phase A2 eorts in order to realise with hardware already developed or
dedi
ated to other appli
ations a small payload to be embarked on-board the
HAP platform. The mission shall be
omposed by two ight
ampaigns;
1. One around Rome
2. Another One over Spino d'Adda
Ea
h
ampaign shall have a duration of 2 weeks-2 months depending on
the number of M-55 ights and on logisti
issues (air
raft basing manage-
ment, man-power
osts). As above mentioned, the mission is
omposed of two
dierent experiments, based on a W band payload with two RF transmitting
hannels on-board the Geophysi
a M-55 air
raft (see [1℄):
• A bea
on transmission and re
eption se
tion operating at frequen
ies in
the band of 92-96 GHz, used to provide the ne
essary signal re
eption
experiments in W band at the xed ground station in Spino d'Adda,
and the transportable station around Rome (
hannel propagation ex-
periment); the ground segment is the same identi ed in the phase A.
The re
eived signals will provide the ne
essary elements to analyze
the
hannel behavior at dierent weather
onditions. The operative
frequen
y
omes as an output of a preliminary te
hnologi
al market
analysis
• A data transmission se
tion operating at a frequen
y around 94 GHz,
used to transmit a known bit stream saved in the on-board memory.
Ground stations will be able to perform BER measurements by a simple
omparison between the transmitted bit sequen
e (known a-priori) and
the re
eived one, in order to evaluate the W band
hannel quality
One of the obje
tives of Aero-WAVE is to evaluate the troposphere at-
tenuation and to use the a
quired knowledge for the development of a W
band satellite
ommuni
ation payload. ITU allo
ated the 71-76 / 81-86 fre-
quen
y bands for satellite
ommuni
ations in W band, respe
tively for down-
link and uplink. One of the requirement of the Aero-WAVE mission is the
CHAPTER 1. INTRODUCTION TO W BAND 7
use of COTS hardware, so the re
ommended operating frequen
y range is
92-95 GHz (where most of the COTS hardware
omponents developed for
radar and radio astronomy systems operates). Using a re
ently developed
mm-wave radiative model,
alled SNEM (Sky Noise Eddington Model), it
an be noted a high
orrelation between path attenuation in W band, even
though beam lling problems and horizontal atmospheri
inhomogeneity are
not here
onsidered. These results suggest that an instantaneous frequen
y
s
aling between link path attenuation at 75/85 GHz (satellite foreseen opera-
tive frequen
ies) and 92/95GHz (Aero-WAVE foreseen operative frequen
ies)
may be theoreti
ally foreseen as a power law regression. The probability of
W band slant path attenuation for the Rome site at 92 GHz is reported as
an example in Figure 1.2, (data and pi
ture taken from [1℄).
Figure 1.2: Slanth Path Attenuation at 92 Ghz at Rome
These results are based on the ITU-R models whi
h may be questionable
at W band, but they still represent a valuable ben
hmark. If a probability
of 1 % is
hosen, the expe
ted slant-path attenuation is about 18 dB whi
h
represents a
hallenge for the established link. This probability
orresponds
to about 87 hours/year, that is less than 4 days per year. If a
ampaign has
to be planned, its timing should be driven by high-resolution meteorologi
al
fore
ast. As a matter of fa
t, a Con
erning the RF re
eiver on-board the
M-55, after the antenna subsystem, the in
oming signal goes to an RF de-
te
tor that
ompares the re
eived power with a threshold value. This value
will be pre-
al
ulated on the basis of the link budget analysis to maximise
the dete
tion probability and at the same time minimising the false alarm
probability. If the re
eived signal power is greater than the threshold, then
it is assumed that the payload on-board M-55 is re
eiving the bea
on from
the ground station and informs the Data A
quisition Centre or store this
CHAPTER 1. INTRODUCTION TO W BAND 8
information (together with a time stamp) in an on-board memory. The two
proposed sites for the airborne
ampaigns are:
1. Over the area around Rome where a transportable ground station
ould
be designed and used
2. Over the area around Spino D'Adda, near Milano, where the xed
ground-station of the Olympus and Italsat
ommuni
ation and propa-
gation experiments is lo
ated
The outage probability at W band is signi
antly ae
ted by the site and
its
limatology. In our examples we
onsidered data from both Milan and
Rome sites whi
h are Alps-dominated and Coastline-dominated meteorolog-
i
al regions, respe
tively. This re e
ts into a dierent
limatology whi
h is
mid-
ontinental for Milan and typi
ally Mediterranean for Rome with very
dierent
loudy and pre
ipitation regimes. With respe
t to this, it is
onsid-
ered essential to a
quire data from both sites to
hara
terize the site diversity
from a radio -meteorologi
al point of view at W band. The air
raft altitude
for both
ampaigns will be around 17− 20km. Figure 3 shows a s
heme of a
typi
al ight mission. Indeed, for a given onboard antenna a higher altitude
would provide a larger footprint at ground whi
h is preferable for tra
king
purposes. The ight route of all
ampaigns will have a diameter of about
10− 20km with an air
raft roll angle variable from 6 to 260, due to the ma-
neuvers that have to be performed by the pilot in order to
ountera
t winds
upheaval. The roll angle should be kept as
onstant as possible in order to
maintain the
overage over the ground station and to a
quire propagation
data in a stable link for a long time; a fast and a
urate antenna tra
king
system
ould be deployed, as dis
ussed later on. Experiments at various el-
evation angles
ould also be foreseen in order to a
quire a large database of
propagation measurements in order to nd out the variation of the attenua-
tion at dierent elevation angles, and verify the validity of theoreti
al models
(i.e. se
ant law). However, the baseline is to
ondu
t some ight with the
same path, be
ause the IKNOW mission s
enario is more suitable to meet
this s
ope due to the intrinsi
variation of the elevation angle during the
satellite pass.
1.2.2 Aero-WAVE payload
Design of Antenna and Pointing System
The fun
tional payload ar
hite
ture should be
arried out
onsidering the use
of existing proved hardware to minimise failure risks and easy integration and
CHAPTER 1. INTRODUCTION TO W BAND 9
test pro
edure. Figure 1.3 shows the fun
tional blo
k s
heme for Aero-WAVE
payload (Graphi
s from [1℄).
Figure 1.3: Fun
tional Blo
k Diagram of Aero-WAVE payload
blo
k
In this payload, it
ould be possible to use solid-state power ampli ers
(SSPA) due to the fa
t that the transmission distan
e to the HAP is rela-
tively small with respe
t to geostationary orbits. In fa
t, if the altitude is
about 20km and the antenna pointing
ould be −300 o-nadir (as in Fig-
ure 1.4) , the free spa
e losses are only about −159dB at 92 GHz. This
may relax the
onstraints on the on-board and ground EIRP.A 100mW (i.e.,
−10dBWor20dBm) devi
e would be suitable both for the data transmission
se
tion and radio bea
on. Anyway, during the proje
t a trade-o analysis will
be performed in order to
hoose between a HPA (tube ampli er) or a SSPA
on the basis of the power requirements and exhaustive link budget analysis.
A unidire
tional antenna is proposed for the bea
on in order to
arry out
the propagation experiment even in poor M-55 stability
onditions. A single
re e
tor antenna with a
ir
ular-horn feeder of about 5mm diameter might
be su
ient to ensure the ful lment of the link requirements. The antenna
pointing losses should be
arefully taken into a
ount due to the instability
of the HAP.
In fa
t one of the most
riti
al design issue
on
erning Aero-WAVE pay-
load is the antenna and the pointing system. During design a
tivities, an
antenna design trade o has been performed
onsidering three dierent so-
lutions:
1. Narrow beam high gain lens antenna with me
hani
al pointing system
2. Beam-shaped se
toral horn antenna
3. Multibeam horn antenna system
CHAPTER 1. INTRODUCTION TO W BAND 10
Figure 1.4: S
heme of Aero-WAVE mission
The rst solution
ould be
onsidered the best in terms of performan
e,
allowing a tra
king of the ground station depending on the air
raft attitude.
In this
on guration, the antenna
ould have narrow beam and therefore
an high gain and thanks to the tra
king
apability the system
ould avoid
pointing losses. The me
hani
al pointing system
ould be
ontrolled via two
dierent approa
hes:
1. Tra
king the ground station by
al
ulating the
orre
t pointing dire
-
tion through an algorithm based on the a-priori knowledge of the sta-
tion position, of the air
raft GPS positioning data and attitude data
(roll, pit
h and heading angles) provided by M-55 navigation system
2. Maintaining a xed pointing on the elevation plane through a Gyro-
stabilization using a two-axis gimbal
ompensating roll and pit
h osets
through rate sensors in ea
h axis
Due to the expe
ted attitude dynami
of the M-55 during the ight, a 2-
axis gimbal
ould be employed. The major problem of the rst solution is the
need for developing a real time software that is able to provide the pointing
dire
tion as the air
raft moves and
hanges its attitude. This problem is also
present for the se
ond solution, but it is less
riti
al sin
e the stabilization
shall be performed on the basis of the air
raft attitude, and not
onsidering
the position. The se
ond solution is based on a xed antenna, with a beam
CHAPTER 1. INTRODUCTION TO W BAND 11
shaped on the basis of the expe
ted attitude variations during the ight.
On the basis of su
h a
onsideration, a se
toral horn
an be
onsidered as a
suitable solution. The third option for the antenna subsystem
onsists in N
standard gain horns aligned along the elevation plane. This
on guration al-
lows to a
hieve higher gain, be
ause ea
h beam will be narrower with respe
t
to previous
on gurations but implies the use of an additional 1XN swit
h
with additive losses and linear growth of
osts as N in
reases. In this phase
the rst solution has been
hosen, be
ause of the availability of
ommer
ial
gimbals gyro-stabilised and at the same time the sustainable
ost of su
h a
platform.
Ground Station Antenna Pointing Loss 1.5 dB
Ground Station Antenna Gain 38 dBi
Re
eiver NF 11 dB
Ground Station Tx Line Temp 290 K
G/T 7 dB/K
Table 1.1: Ground Station Chara
teristi
s
table1
Moreover, the approa
h based on su
h a pointing system allows both
the employment of a high gain antenna, relaxing payload features like EIRP
and G/T, and the a
hievement of a good link budget margin (about 3dB),
taking into a
ount the
hara
teristi
s of the ground station transmission
experiments, and in
ludes also identi ed in Table ??.
Design of tele
ommuni
ation and measurement payload
If both the propagation and data transmission experiments will be performed,
the payload will be
omposed of two se
tions:
1. An on-board memory
ontaining the bit stream to be transmitted fol-
lowed by the modulation se
tion in whi
h the signal
arrier is mod-
ulated by the signal; the modulated signal is then ampli ed using a
solid-state or a tube power ampli er, ltered and nally sent through
the antenna
2. The bea
on whi
h generates a sinusoidal signal to be transmitted for
power measurements. To measure the power of the sinusoidal signals,
CHAPTER 1. INTRODUCTION TO W BAND 12
the bea
on must send su
iently stable signal both in terms of fre-
quen
y and instantaneous power. A reasonable re
eiver bandwidth
derived from the extension of the ITALSAT bea
on experiments and
Spino d'Adda station
on guration, might be of the order of 1 kHz
It must be outlined that the payload ar
hite
ture determines if the propa-
gation and data transmission experiments
an be
arried out simultaneously
or not. It would be
learly better to perform both measurements in parallel
at the same time in order to exploit all the ight time for ea
h experiment,
but it would require the redundan
y of some hardware (SSPA and others).
On the basis of previous
onsiderations, three payload ar
hite
tures are pro-
posed. Ea
h of them addresses dierent experiments:
• Con guration 1 : performing only the propagation experiment
• Con guration 2 : performing propagation and data trasmission experi-
ments
• Con guration 3 : performing propagation and data trasmission exper-
iments, and in
ludes also a transparent (bent pipe)
hain in order to
test a data relay experiment
The rst
on guration, hereafter
alled
on guration 1 , is shown in Fig-
ure 1.5 . It is the simplest of the three s
hemes, transparent link (Figure 1.5),
addressing only the propagation experiment-
hannel measurements (Experi-
ment 1) on both uplink and downlink.
Figure 1.5: Aero-WAVE Payload Con guration 1
The horn antenna is shared for both the link dire
tions (up and down).
The uplink signal
entred at 96GHz is ltered and then ampli ed by an LNA.
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