SEMINAR

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SpaceOps 2002 SPECTRUM USE IN THE XXI CENTURY
Benito O. Gutiérrez-Luaces Houston, Texas,
October 9 to 12, 2002 T5-48
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AGENDA

• Basic parameters to be considered to achieve
  optimum use of the electromagnetic spectrum (EM)
  among all its potential users
• Emphasizing the importance of passive
  observations of the Earth and the Universe
• Suggestions of how to optimally assign the S-band
  space-to-Earth link of near-Earth satellite
  constellations randomly distributed in space
• Results for the down-link studies of this
  presentation are verified by the use of actual
  satellite constellations models. These models to
  be used in future studies on the optimization of
  the Earth-to-space link

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INTRODUCTION

• The use of the electromagnetic spectrum in
  free-space has been increasing since the
  demonstration of the EM energy transmission by
  Hertz (Germany) in 1887
• Today it is a very scarce, appreciated and
  therefore expensive global asset
• Optimization of its use is therefore a priority
  to be pursued by all of its users

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Receiver Sensitivity Limitations

• The evolution of radio-transmissions started at
  low frequencies where the high transmitted power
  was more important than the receiver sensitivity
• Receiver sensitivity becomes important at
  frequencies in the Earth based window (0.5-10
  GHz) because the antenna temperature may be as
  low as 3K
• Another potential limitation that must always be
  taken into account is the natural radiation of
  the Sun. Antenna pattern is clearly an important
  factor when all these natural limitations are
  considered

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The Radio ReceiverNOISE TEMPERATURE of
AMPLIFIERS(from Cryogenic,HEMT, ...
S.Weinreb...,1988 IEEE MTT-S Digest)
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NATURAL LIMITS (summary)
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Typical Spectral Power Flux Density (SPFD)Sun,
Radio Astronomy, Deep Space
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Passive Observations

• Through the observation of the EM energy present
  in the Universe, Radio Astronomy has contributed,
  contributes today and will contribute in the
  future to increase our knowledge of the Universe
• Radio Astronomy observations that may last
  several hours, are realized through the
  measurement of the variations in temperature of a
  usually very directive antenna
• Same approach will allow the probing of a limited
  area of the Earth atmosphere
• Also through the interaction of the EM waves with
  the Earth, surface parameters can be measured

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Passive Space Earth Exploration(the importance
of the band 18.6-18.8 GHz)
SALINITY
WIND SPEED
LIQUID CLOUDS
WATER VAPOR
Frequency (GHz)
SEA SURFACETEMPERATURE

• Antenna temperature relative sensitivity to
  different geophysical parameters (oceanic) v.s.
  observing frequency

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Transmission of messages

• The most popular use of the antenna/receiver
  combination is for the reception of messages from
  a distant transmitter
• EM power from those transmitters are a man-made
  source of noise that should be limited as much as
  possible to allow an optimum use of the spectrum
• It appears that digital transmissions because of
  the substantial power reduction over analog ones
  should take precedence in future (see next
  viewgraph)

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Analog or digital transmissions?
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Optimizing the space-to-Earth transmissions

• As a result of the narrower bandwidth usually
  required in the Earth-to-space direction (uplink)
  in most of the Space Science applications, the
  space-to-Earth(downlink) studies of a two
  way-link for near-Earth satellites took
  precedence
• Given a limited spectrum bandwidth the maximum
  number of satellites that may be allowed to
  operate above the Earth station horizon for a
  given link degradation is of importance
• Results for the band 2200-2290 GHz (S-band) given
  in Fig. 6. have helped to propose the assignment
  of common bands for systems with similar
  characteristics, instead of the first-come
  first-serve approach

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Optimizing the Earth-to-space transmissions

• Nowadays, the Space Science uplink S-band
  spectrum (2025-2110 GHz) is increasingly shared
  with new incoming near-Earth applications
• Uplink bandwidth requirements for these new
  systems will most likely be as large as those of
  the downlink, therefore this will be the subject
  of further studies
• Computer programs have been developed for
  simulations of these scenarios
• Results of down-link simulations for near-Earth
  constellations have been included into Fig. 6 in
  the next viewgraph showing a very close agreement
  with previous models

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Near-Earth Total Number of Space Craftin-view at
S-Band (2,3 GHz)
70
ssnr
(dB)
(4)
60
(9)
50
40
Constellation
(1400km 88deg.
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Max. Number of s/c in view for plt0.001
I/No-6dBp0.001)
48x8
20
(14)
24x8
10
12x8
0
6x8
0
5
10
15
20
25
30
Antenna Diameter (m)
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CONCLUSIONS

• Some of the limitations imposed by Nature in the
  use of free-space propagation of the EM energy
  have been introduced
• Active users of free-space propagation should pay
  attention to limitations imposed by passive
  observations
• Satellite-constellation computer models have been
  completed and verified with previous results for
  the space-to-Earth data transmissions
• These models will be used to define requirements
  for the Earth-to-space scenario optimization