A satellite uplink test was successfully conducted. The DA-TDMA Satellite Loopback Network
consists of the following: Outdoor units
1 Two 2.4m VSAT dishes. 2 Two Radio Frequency Transceivers
RFTs. 3 Two Solid-state Power Amplifiers
SSPAs. 4 Two feedhorns.
Indoor units 1 Five satellite modems.
2 A Network Management System. 3 A Cisco switch.
4 A video conferencing server and video conferencing clients.
6. Phase 2 – Network Performance Measurement Tests on performance measurement will be carried
out on the following: 1 Impact of Bandwidth on Demand BoD on QoS.
2 Impact of Satellite Propagation Delays on QoS. 3 Performance Evaluation for IPv4 and IPv6
Networks. 4 QoS for several applications
over the DA-TDMA satellite network. They will be done by adjusting different values of setting at
different layers of the OSI Layer. a QoS at Physical Layer Layer 1 Wireless Media
In order to prevent loss of connection between the Hub VSAT and the remote VSATs, all transmitting
satellite modems have to ensure a sufficient transmit power level so that receiving modems can decode
the bits correctly. Receiving system must monitor the Bit Error Rate BER and EbNo values at the
receiving modems in order to alert the transmitting party if the required QoS is being compromised.
b QoS at Link Layer Layer 2 DA-TDMA, SCPC In order to prevent under utilising of satellite
transponder bandwidth, allocation of the inbound remote VSATs to Hub VSAT transmission rate
has to be chosen wisely. Transmission of short messages over high inbound transmission link will
result in poor channel utilisation while transmission of long messages over low inbound transmission
link will result in longer queuing delay at the transmitter. Therefore, a suitable inbound
transmission rate has to be chosen to provide the required QoS for transmission of short and long
messages. c QoS at network layer Layer 3
Tests under IPv4 and IPv6 platform will be performed. Only the necessary routes will be added
to the routing table. d QoS at Transport Layer Layer 4 TCP
For best performance, the buffer sizes on routers and switches have to be adjusted.
e QoS at Application Layer Layer 7 FTP, Video conferencing
For best performance, OS window size has to be adjusted.
7. Schedule
Work accomplished 1. ODU and IDU set-ups at TP
2. Completed satellite loopback test at TP 3. Collected BER and EbNo data at TP
Work in progress 1. Set up modems at USM and UPM
2. Establish satellite links with USM and UPM
3. Set up IPv6 network 4. Measure network performance
5. Video conferencing 6. Set up Mobile IP system
8. Conclusion
The infrastructure construction at Temasek Polytechnic was successfully set up. Currently, the
project team is liaising with University Science of Malaysia USM and University Putra Malaysia
UPM to establish satellite links. The BER and the EbNo values were ascertained c.f. Table 1 using
the DA-TDMA Satellite Loopback Network at Temasek Polytechnic. The method on how the BER
and EbNo values were ascertained is shown in the following flow chart:
Figure 4 : Flow Chart On Obtaining BER and EbNo
EbNo BER
2.5 dB 1.0 E-02
3.4 dB 1.2 E-03
4.3 dB 4.5 E-05
5.3 dB 1.5 E-06
6.4 dB 2.5 E-08
7.2 dB 4.5 E-10
8.1 dB 3.5 E-12
9.0 dB 1 E-12
Table 1: EbNo and BER readings at 768 kbps, Viterbi ½
The performance measurement on providing QoS over demand-assigned TDMA-based satellite
networks and the impact of channel errors on the performance of a demand-assigned TDMA satellite
network will be carried out within the above EbNo ranges. FTP and Video Conferencing applications
will be tested under IPv4 and IPv6 platforms.
References
1. Hadjitheodosiou, M.H. Coakley, F.p., Evans, B.G., “Next Generation
Multiservice VSAT Networks”, July 1997. http:www.isr.umd.edu~michalisEl_Co
mms.pdf 2. Snyder, M., Yu, V., Heissler, J., “Two
New Media Access Control Schemes For Networked Satellite Communications”.
http:www.mitre.orgsupportpaperstech _papers_01snyder_communicationssnyd
er_communications.pdf
IMPEDANCE OF WIRE ANTENNA ONBOARD SPACECRAFT
Minoru Tsutsui Department of Information and Communication Sciences, Kyoto Sangyo University,
Kamigamo-Motoyama, Kita-ku, Kyoto 603-8555, Japan
tsutsuicc.kyoto-su.ac.jp
ABSTRACT
Measurements of impedance of wire antenna aboard the GEOTAIL spacecraft were carried out in the earth’s magnetosphere. The measured values of the antenna resistance were of the order from 10 M
Ω
to a few G
Ω
depending on the different magnetosphere regions. On the other hand, antenna resistances measured by the WIND spacecraft showed an appearance of antenna-base resistance. Since the antenna-base resistance has a serious effect on
determining the field intensity of low frequency waves detected by the wire antenna, I here propose a structure of wire antennas which is able to avoid such the effect.
MEASUREMENTS OF IMPEDANCE OF WIRE ANTENNA
In the analysis of wave electric fields detected by wire antenna aboard spacecraft, it is important to take into account the antenna impedance, because the detected signal amplitude varies with the antenna impedance which
varies with the density of plasma around the antenna. To make clear the relation between the antenna impedance and the plasma density in the earth’s magnetosphere, we conducted its quantitative measurements using the GEOTAIL
spacecraft [1]. GEOTAIL has two types of wire dipole antennas of 100 m tip-to-tip; one is a simple wire dipole WANT which is dedicated to measuring the AC electric field, and another is also wire dipole called PANT but with
a sphere 10 cm in diameter on each tip. Since the purpose of the PANT was to measure both the AC
and the DC electric fields, the wire was covered by an electrically insulated sheet Polyimide except
for a part of 1m from the tip. Since the antenna impedance was derived as a complex function of
frequency, a resistance and a capacitance values can be obtained from its real and imaginary parts,
respectively. Figure 1 shows relations between resistances and capacitances measured in different
regions of the earth’s magnetosphere [1]. Values of the precisely obtained resistances are in a range
from 10 M
Ω
to a few G
Ω
, and capacitances are from 130 to 180 pF depending on the different
plasma density regions of the magnetosphere. Using Fig. 1 from Tsutsui et al., Radio Science, 1997 these data, précised analysis of plasma wave behavior
was performed by Kojima et al. [2]. On the other hand, Kellogg and Bale [3] attempted to derive
the antenna-base resistance from measurements of antenna resistance by the WIND spacecraft. The result showed that the
base resistance would be formed in an area between the foot part of the wire antenna and the spacecraft body as shown in Fig. 2.
The result also showed that the antenna-base resistance reduced to about few tens of M
Ω
when the antenna is in the sunlight although the value of the antenna-plasma resistances are the
consistent with the values obtained by GEOTAIL. They pointed Fig. 2 from Kellogg and Bale, JGR, 2001
out the necessity of taking into account of both the base resistance and the antenna-plasma resistance in determining the field intensity of low frequency waves especially when the spacecraft is in a rarefied plasma such as in the solar
wind.
PROPOSISION FOR WIRE ANTENNA STRACTURE
Kellogg and Bale [3] show that the value of the base resistance became small when the wire antenna was in the sunlight and that the base resistance formed for a short antenna has more serious effect in determining intensity of
low frequency waves when the spacecraft is in a rarefied plasma. As seen in Fig. 1, the variance range of the antenna resistance in the magnetosphere robe region is wider than those in other regions, in contrast that the variance range of
capacitance is extremely small. This result also supports the easy reduction of the antenna-base resistance. In the derivation of the antenna-base resistance given by Kellogg and Bale [3], they used a condition on a current
balance between the escaping photoelectrons and the ambient plasma electrons. It is true that this situation is extremely effective in formation of the base resistance. Since a part of 10 m from the tip of WANT wire was
exposed to the plasma and other part was covered by an electrically insulating sheet whereas most of PANT wire was covered by it except for a part of 1m from each tip, we checked the relations between resistance and capacitance
values of PANT for the same events given in Fig. 1, in order to examine the effect of insulation of electron currents fromto the wire element, and to find out any differences of antenna resistance between the WANT and PANT. We
obtained the similar feature as seen in Fig. 1 and that its trend was rather deviated to higher resistance value than those in Fig. 1.
Judging from the observational situations in the GEOTAIL and WIND spacecraft, I here propose a new structure of wire antennas aboard the spacecraft planned from now on. It is that wire antennas for observing wave electric field in
space should be covered with a insulating sheet, because the key point is that the situation of DC electron current flows fromto the surface of the wire antenna should be avoid.
REFERENCES
[1] M. Tsutsui et al., Measurements and analysis of antenna impedance aboard the Geotail spacecraft, Radio Science,Vol. 32, No. 3, 1101-1126, 1997.
[2] H. Kojima, et al., GEOTAIL Waveform Observations of BroadbandNarrowband Electrostatic Noise in the
Distant Tail, J. Geophys. Res., 102, 14439-14455, 1997. [3] P. J. Kellogg and S. D. Bale, Antenna-plasma and antenna-spacecraft resistance on the Wind spacecraft, J.
Geophys. Res., Vol. 106, No. A9, 18721-18727, 2001.
Page 1
Lecture Lecture
6 6
: Telescopes and Spacecraft : Telescopes and Spacecraft
Claire Max April 16th, 2009
Astro 18: Planets and Planetary Systems UC Santa Cruz
Jupiter as seen by Cassini spacecraft
Page 2
Outline of this lecture Outline of this lecture
• •
Telescopes and spacecraft: how we learn about Telescopes and spacecraft: how we learn about
the planets the planets
– –
Lenses Lenses
– –
Cameras and the eye Cameras and the eye
– –
Telescope basics optical, x-ray, radio telescopes Telescope basics optical, x-ray, radio telescopes
– –
Blurring due to atmospheric turbulence; adaptive Blurring due to atmospheric turbulence; adaptive
optics optics
– –
Airborne telescopes Airborne telescopes
– –
Spacecraft Spacecraft
Please remind me to take Please remind me to take
a break at a break at
2:45 pm 2:45 pm
Page 3
The Main Points The Main Points
• •
Telescopes gather light and focus it Telescopes gather light and focus it
– –
Larger telescopes gather more light Larger telescopes gather more light
– –
Telescopes can gather Telescopes can gather
“ “
light light
” ”
at radio, infrared, visible, at radio, infrared, visible,
ultraviolet, x-ray, ultraviolet, x-ray,
γ γ
-ray wavelengths -ray wavelengths
• •
Telescopes can be on ground, on planes, in space Telescopes can be on ground, on planes, in space
• •
If Earth If Earth
’ ’
s atmosphere weren s atmosphere weren
’ ’
t turbulent, larger ground- t turbulent, larger ground-
based telescopes would give higher spatial resolution based telescopes would give higher spatial resolution
– –
Adaptive optics can correct for blurring due to turbulence Adaptive optics can correct for blurring due to turbulence
Every new telescope technology has resulted in Every new telescope technology has resulted in
major new discoveries and surprises major new discoveries and surprises
Page 4
What are the two most important What are the two most important
properties of a telescope? properties of a telescope?
1. 1.
Light-collecting area: Light-collecting area:
Telescopes with a larger Telescopes with a larger
collecting area can gather a greater amount collecting area can gather a greater amount
of light in a shorter time. of light in a shorter time.
2. 2.
Angular resolution: Angular resolution:
Telescopes that are larger Telescopes that are larger
are capable of taking images with greater are capable of taking images with greater
detail. detail.
Page 5
Telescopes gather light and focus it Telescopes gather light and focus it
• •
Telescope as a Telescope as a
“ “
giant eye giant eye
” ”
– –
You can gather more light with a telescope, hence see fainter You can gather more light with a telescope, hence see fainter
objects objects
Refracting telescope
Page 6
Amount of light gathered is Amount of light gathered is
proportional to proportional to
area area
of lens of lens
• •
Why area? Why area?
• •
“ “
Size Size
” ”
of telescope is usually described by of telescope is usually described by
diameter diameter
d d
of its primary lens or mirror of its primary lens or mirror
• •
Collecting area of lens or mirror = Collecting area of lens or mirror =
π π
r r
2 2
= =
π π
d2 d2
2 2
versus
Page 7
Light-gathering power Light-gathering power
• •
Light-gathering power Light-gathering power
∝ ∝
area = area =
π π
d2 d2
2 2
• •
Eye: Eye:
– –
At night, pupil diameter ~ 7 mm, Area ~ 0.4 cm At night, pupil diameter ~ 7 mm, Area ~ 0.4 cm
2 2
• •
Backyard telescope: Backyard telescope:
– –
d d
= 5 = 5
” ”
= 12.7 cm, Area = 127 cm = 12.7 cm, Area = 127 cm
2 2
• •
Keck Telescope: Keck Telescope:
– –
d d
= 10 meters = 1000 cm, Area = 7.85 x 10 = 10 meters = 1000 cm, Area = 7.85 x 10
5 5
cm cm
2 2
– –
Light gathering power is 1.96 million times that of the eye Light gathering power is 1.96 million times that of the eye
Page 8
Refracting telescopes focus light Refracting telescopes focus light
using using
“ “
refraction refraction
” ”
• •
Speed of light is constant in a vacuum Speed of light is constant in a vacuum
• •
But when light interacts with matter, it usually But when light interacts with matter, it usually
slows down a tiny bit slows down a tiny bit
• •
This makes This makes
“ “
rays rays
” ”
of light bend at interfaces of light bend at interfaces
Page 9
Refraction animation Refraction animation
• http:www.launc.tased.edu.auonlinesciencesphysicsrefrac.html
Page 10
Example: Refraction at Sunset Example: Refraction at Sunset
• •
Sun appears distorted at sunset because of how light bends Sun appears distorted at sunset because of how light bends
in Earth in Earth
’ ’
s atmosphere s atmosphere
Page 11
A lens takes advantage of the A lens takes advantage of the
bending of light to focus rays bending of light to focus rays
Focus Focus
– –
to bend all light waves coming from to bend all light waves coming from
the same direction to a single point the same direction to a single point
Page 12
Parts of the Human Eye Parts of the Human Eye
• •
pupil pupil
– –
allows light to allows light to
enter the eye enter the eye
• •
lens lens
– –
focuses light to focuses light to
create an image create an image
• •
retina retina
– –
detects the detects the
light and generates light and generates
signals which are sent signals which are sent
to the brain to the brain
Camera works the same way: the Camera works the same way: the
shutter shutter
acts acts
like the like the
pupil pupil
and the and the
film film
acts like the acts like the
retina retina
Page 13
The lens in our eyes focuses light The lens in our eyes focuses light
on the retina on the retina
Note that images are upside down
Our brains compensate
Page 14
Camera lens focuses light on film Camera lens focuses light on film
or CCD detector or CCD detector
Upside down
Page 15
What have we learned? What have we learned?
• •
How does your eye form an image? How does your eye form an image?
– –
It uses refraction to bend parallel light rays so It uses refraction to bend parallel light rays so
that they form an image. that they form an image.
– –
The image is in focus if the focal plane is at The image is in focus if the focal plane is at
the retina. the retina.
• •
How do we record images? How do we record images?
– –
Cameras focus light like your eye and record Cameras focus light like your eye and record
the image with a detector. the image with a detector.
– –
The detectors The detectors
CCDs CCDs
in digital cameras are in digital cameras are
like those used on modern telescopes like those used on modern telescopes
Page 16
What are the two basic designs of What are the two basic designs of
telescopes? telescopes?
• •
Refracting telescope: Refracting telescope:
Focuses light with lenses Focuses light with lenses
• •
Reflecting telescope: Reflecting telescope:
Focuses light with mirrors Focuses light with mirrors
Page 17
Cartoon of refracting telescope Cartoon of refracting telescope
Page 18
Telescopes can use mirrors instead Telescopes can use mirrors instead
of lenses to gather and focus light of lenses to gather and focus light
• •
For practical reasons, For practical reasons,
can can
’ ’
t make lenses t make lenses
bigger than ~ 1 meter bigger than ~ 1 meter
• •
Can make mirrors Can make mirrors
much larger than this much larger than this
– –
Largest single Largest single
telescope mirrors telescope mirrors
today are about 8.5 m today are about 8.5 m
• •
Old-fashioned Old-fashioned
reflecting telescope: reflecting telescope:
– –
Observer actually sat in Observer actually sat in
“ “
cage cage
” ”
and looked and looked
downward downward
Page 19
Mount Palomar near San Diego: Mount Palomar near San Diego:
Prime focus cage and an inhabitant Prime focus cage and an inhabitant
• •
NOTE: NOTE:
Smoking and Smoking and
drinking are not drinking are not
permitted in the prime permitted in the prime
focus cage On web focus cage On web
page of Anglo page of Anglo
Australian Telescope Australian Telescope
• •
Until the 1970 Until the 1970
’ ’
s, women s, women
weren weren
’ ’
t permitted t permitted
either either
Page 20
Today Today
’ ’
s reflecting telescopes s reflecting telescopes
• •
Cassegrain Cassegrain
focus: focus:
– –
Light enters from top Light enters from top
– –
Bounces off primary Bounces off primary
mirror mirror
– –
Bounces off Bounces off
secondary mirror secondary mirror
– –
Goes through hole in Goes through hole in
primary mirror to primary mirror to
focus focus
Page 21
Examples of real telescopes Examples of real telescopes
• •
Backyard telescope: Backyard telescope:
– –
3.8 3.8
” ”
diameter refracting lens diameter refracting lens
– –
Costs ~ 300 at Amazon.com Costs ~ 300 at Amazon.com
– –
Completely computerized: it will Completely computerized: it will
find the planets and galaxies for find the planets and galaxies for
you you
Page 22
Largest optical telescopes in world Largest optical telescopes in world
• •
Twin Keck Telescopes on top of Mauna Kea Twin Keck Telescopes on top of Mauna Kea
volcano in Hawaii volcano in Hawaii
Page 23
36 hexagonal segments make up 36 hexagonal segments make up
the full Keck mirror the full Keck mirror
Page 24
Keck Keck
’ ’
s 10-meter diameter mirror is s 10-meter diameter mirror is
made of 36 segments made of 36 segments
Page 25
One Keck segment in storage One Keck segment in storage
Page 26
Future plans are even more Future plans are even more
ambitious ambitious
Thirty Meter Telescope Keck Telescope
Page 27
Future plans are even more Future plans are even more
ambitious ambitious
People
Page 28
Concept of angular resolution Concept of angular resolution
Car Lights Car Lights
Angular resolution Angular resolution
• •
The ability to separate two objects. The ability to separate two objects.
• •
The angle between two objects decreases as your The angle between two objects decreases as your
distance to them increases. distance to them increases.
• •
The smallest angle at which you can distinguish two The smallest angle at which you can distinguish two
objects is your objects is your
angular resolution angular resolution
. .
Page 29
How big is one arc second of How big is one arc second of
angular separation? angular separation?
• •
A full circle on the sky contains 360 degrees A full circle on the sky contains 360 degrees
or 2 or 2
π π
radians radians
– –
Each degree is 60 arc minutes Each degree is 60 arc minutes
– –
Each arc minute is 60 arc seconds Each arc minute is 60 arc seconds
1 arc sec 1 arc min
60 arc sec 1 degree
60 arc min 2 radians
360 degrees
❂
❂
2 60 60 360
radians = 4.8 10
-6
radian = 4.8 µrad 5 µrad
or 1 µrad 0.2 arc sec
Page 30
What does it mean for an object to What does it mean for an object to
“ “
subtend an angle subtend an angle
θ
” ”
? ?
θ θ
is the apparent angular size of the object is the apparent angular size of the object
Your eye A distant
object angle θ
Page 31
“ “
Small angle formula Small angle formula
” ”
• •
sin sin
θ θ
~ ~
θ θ
if if
θ θ
is is
1 1
radian radian
• •
s = d sin s = d sin
θ θ
~ d ~ d
θ θ
• •
Example: how many ar Example: how many ar
c sec does a nickel subtend if it c sec does a nickel subtend if it
is located 2 km away? is located 2 km away?
d s
θ
A dime is about 1 cm across, so s
d 1 cm
2 km 1 km
1000 m 1 m
100 cm =
1 2
10
5
radians 1
➭
rad 10
-6
rad = 5
➭
rad = 1 arc sec
Page 32
Concept Question Concept Question
From Earth, planet A subtends an angle of 5 arc sec, and From Earth, planet A subtends an angle of 5 arc sec, and
planet B subtends an angle of 10 arc sec. If the radius planet B subtends an angle of 10 arc sec. If the radius
of planet A equals the radius of planet B, then of planet A equals the radius of planet B, then
a planet A is twice as big as planet B. a planet A is twice as big as planet B.
b planet A is twice as far as planet B. b planet A is twice as far as planet B.
c planet A is half as far as planet B. c planet A is half as far as planet B.
d planet A and planet B are the same distance. d planet A and planet B are the same distance.
e planet A is five times as far as planet B. e planet A is five times as far as planet B.
Page 33
What do astronomers do with What do astronomers do with
telescopes? telescopes?
• •
Imaging: Imaging:
Taking digital pictures of the sky Taking digital pictures of the sky
• •
Spectroscopy: Spectroscopy:
Breaking light into spectra Breaking light into spectra
• •
Timing: Timing:
Measuring how light output varies with Measuring how light output varies with
time time
Page 34
Imaging Imaging
• •
Filters Filters
are placed in are placed in
front of a camera to front of a camera to
allow only certain allow only certain
colors to be imaged colors to be imaged
• •
Single color images Single color images
are then are then
superimposed to superimposed to
form true color form true color
images. images.
Page 35
How can we see images of How can we see images of
nonvisible nonvisible
light? light?
• •
Electronic detectors such as Electronic detectors such as
CCDs CCDs
can can
record light our eyes cant see record light our eyes cant see
• •
We can then represent the recorded light We can then represent the recorded light
with some kind of color coding, to reveal with some kind of color coding, to reveal
details that would otherwise be invisible to details that would otherwise be invisible to
our eyes our eyes
Page 36
Crab Nebula - supernova remnant Crab Nebula - supernova remnant
where a star blew up 1000 yrs ago where a star blew up 1000 yrs ago
Infra-red light Infra-red light
Visible light Visible light
X-rays X-rays
From above the
atmosphere
Page 37
In principle, larger telescopes In principle, larger telescopes
should give should give
sharper sharper
images images
• •
Concept of Concept of
“ “
diffraction limit diffraction limit
” ”
– –
Smallest angle on sky that a telescope can resolve Smallest angle on sky that a telescope can resolve
– –
Numerically: Numerically:
d
= D
radians
where = wavelength of light, D = telescope diameter in the same units as
diffraction limit = 2.5 10
5
wavelength of light diam of telescope
arc seconds
In same units In same units
Page 38
Image of a point source seen Image of a point source seen
through a circular telescope mirror through a circular telescope mirror
• •
Size of central spot ~ Size of central spot ~
λ λ
D D
Diffraction limit animation Diffraction limit animation
Page 39
Example of diffraction limit Example of diffraction limit
• •
Keck Telescope, visible light Keck Telescope, visible light
• •
BUT BUT
: Turbulence in the Earth : Turbulence in the Earth
’ ’
s atmosphere blurs s atmosphere blurs
images, so even the largest telescopes can images, so even the largest telescopes can
’ ’
t t
“ “
see see
” ”
better than about 1 arc second better than about 1 arc second
– –
A decrease of a factor of 1 0.0125 = 80 in resolution A decrease of a factor of 1 0.0125 = 80 in resolution
telescope diameter D = 10 meters
wavelength of light = 5000 Angstroms = 5
10
-7
meter
diffraction limit = 2.5 10
5
✭ ✮
5 10
-7
10 arc seconds = 0.0125 arc second
Page 40
Images of a bright star, Images of a bright star,
Arcturus Arcturus
Lick Observatory, 1 m telescope
Long exposure image
Short exposure image
Diffraction limit of telescope
Page 41
Snapshots of turbulence, Snapshots of turbulence,
Lick Observatory Lick Observatory
These are all images of a star, taken with very short exposure times 100 milliseconds
Page 42
How to correct for atmospheric blurring How to correct for atmospheric blurring
Measure details Measure details
of blurring from of blurring from
“ “
guide star guide star
” ”
near the object near the object
you want to you want to
observe observe
Calculate on a Calculate on a
computer the computer the
shape to apply shape to apply
to deformable to deformable
mirror to correct mirror to correct
blurring blurring
Light from both guide Light from both guide
star and astronomical star and astronomical
object is reflected object is reflected
from deformable from deformable
mirror; distortions mirror; distortions
are removed are removed
Page 43
Infra-red images of a star, from Lick Infra-red images of a star, from Lick
Observatory adaptive optics system Observatory adaptive optics system
With adaptive optics No adaptive optics
Page 44
Adaptive optics increases peak intensity Adaptive optics increases peak intensity
of a point source of a point source
Lick Observatory,
Near infrared images of a
star
No AO With AO
No AO With AO
Intensity
Page 45
Deformable mirror is small mirror Deformable mirror is small mirror
behind main mirror of telescope behind main mirror of telescope
Page 46
Mirror changes its shape because Mirror changes its shape because
actuators push and pull on it actuators push and pull on it
• •
Actuators are glued to back of thin Actuators are glued to back of thin
glass mirror glass mirror
• •
When you apply a voltage to an When you apply a voltage to an
actuator, it expands or contracts in actuator, it expands or contracts in
length, pushing or pulling on the length, pushing or pulling on the
mirror mirror
Page 47
Neptune in infra-red light, Neptune in infra-red light,
Keck Telescope adaptive optics Keck Telescope adaptive optics
Without adaptive optics With adaptive
optics
2.3 arc sec
Page 48
Telescopes can Telescopes can
“ “
see see
” ”
infrared light as infrared light as
well as visible light well as visible light
• •
Infra-red image shows new stars forming Infra-red image shows new stars forming
• •
Not visible in Not visible in
“ “
visible light visible light
” ”
image because they are image because they are
deeply embedded in clouds of dust deeply embedded in clouds of dust
Page 49
Movie of volcanoes on Jupiter Movie of volcanoes on Jupiter
’ ’
s moon Io, s moon Io,
from Keck Telescope adaptive optics from Keck Telescope adaptive optics
Page 50
Concept Question Concept Question
• •
The Keck Telescope in Hawaii has a diameter The Keck Telescope in Hawaii has a diameter
of 10 m, compared with 5 m for the Palomar of 10 m, compared with 5 m for the Palomar
Telescope in California. The light gathering Telescope in California. The light gathering
power of Keck is larger by a factor of power of Keck is larger by a factor of
a a
2 b 4 c 15 d 50 2 b 4 c 15 d 50
• •
By what factor is Keck By what factor is Keck
’ ’
s angular resolution s angular resolution
better than that of Palomar, assuming that better than that of Palomar, assuming that
both are using their adaptive optics systems? both are using their adaptive optics systems?
a a
2 b 4 c 15 d 50 2 b 4 c 15 d 50
Page 51
Reflecting telescopes work fine at Reflecting telescopes work fine at
radio wavelengths radio wavelengths
• •
The radio telescope at Green Bank, NC The radio telescope at Green Bank, NC
Page 52
Largest radio telescope fills a whole Largest radio telescope fills a whole
valley in Puerto Rico valley in Puerto Rico
Page 53
Interferometry Interferometry
is a method to is a method to
improve spatial resolution improve spatial resolution
Page 54
The The
“ “
Very Large Array Very Large Array
” ”
radio radio
interferometer in New Mexico interferometer in New Mexico
Page 55
Spectroscopy Spectroscopy
• •
A spectrograph A spectrograph
separates the separates the
different different
wavelengths of wavelengths of
light before they light before they
hit the detector hit the detector
Diffraction grating breaks
light into spectrum
Detector records
spectrum Light from
only one star enters
Page 56
Spectroscopy Spectroscopy
• •
Graphing Graphing
relative relative
brightness of brightness of
light at each light at each
wavelength wavelength
shows the shows the
details in a details in a
spectrum spectrum
Page 57
Timing Timing
• •
A light curve represents a series of brightness A light curve represents a series of brightness
measurements made over a period of time measurements made over a period of time
Page 58
Timing: Dust devils on Mars seen Timing: Dust devils on Mars seen
from Spirit Rover from Spirit Rover
Page 59
Want to buy your own telescope? Want to buy your own telescope?
• •
Buy binoculars first e.g. 7x35 - you get much Buy binoculars first e.g. 7x35 - you get much
more for the same money. more for the same money.
• •
Ignore magnification sales pitch Ignore magnification sales pitch
• •
Notice: aperture size, optical quality, weight Notice: aperture size, optical quality, weight
and portability. and portability.
• •
Product reviews: Astronomy, Sky Telescope, Product reviews: Astronomy, Sky Telescope,
Mercury Magazines. Also amateur astronomy Mercury Magazines. Also amateur astronomy
clubs. clubs.
Page 60
Why do we need telescopes in Why do we need telescopes in
space? space?
Page 61
Why do we need telescopes in Why do we need telescopes in
space? space?
a a
Some wavelengths of light don Some wavelengths of light don
’ ’
t get through t get through
the Earth the Earth
’ ’
s atmosphere s atmosphere
• •
Gamma-rays, x-rays, far ultraviolet, long infrared Gamma-rays, x-rays, far ultraviolet, long infrared
wavelengths wavelengths
• •
The only way to see them is from space The only way to see them is from space
b b
Going to space is a way to overcome blurring Going to space is a way to overcome blurring
due to turbulence in Earth due to turbulence in Earth
’ ’
s atmosphere s atmosphere
c c
Planetary exploration: spacecraft can actually Planetary exploration: spacecraft can actually
go to the planets, get close-up information go to the planets, get close-up information
Page 62
Depth of light penetration into Depth of light penetration into
atmosphere at different wavelengths atmosphere at different wavelengths
Page 63
X-ray mirrors also concentrate light X-ray mirrors also concentrate light
and bring it to a focus and bring it to a focus
• •
X-ray mirrors X-ray mirrors
Page 64
Chandra spacecraft: x-rays Chandra spacecraft: x-rays
Page 65
Hubble Space Telescope: clearer vision Hubble Space Telescope: clearer vision
above atmospheric turbulence above atmospheric turbulence
Hubble can see UV light that doesnʼt penetrate through atmosphere
Page 66
Example of robotic planet exploration: Example of robotic planet exploration:
Galileo mission to Jupiter Galileo mission to Jupiter
Artists conception Artists conception
Page 67
Types of space missions Types of space missions
• •
Earth orbiters Earth orbiters
• •
Planetary fly-bys Planetary fly-bys
– –
Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune so Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune so
far far
– –
New Horizons flyby of Pluto arrives there July 14 2015 New Horizons flyby of Pluto arrives there July 14 2015
• •
Planetary orbiters Planetary orbiters
– –
Venus, Mars, Jupiter, Saturn so far Venus, Mars, Jupiter, Saturn so far
• •
Probes and Probes and
landers landers
– –
Mars rovers: Spirit and Mars rovers: Spirit and
Opportunity Opportunity
– –
Mars landers: e.g. Phoenix Mars landers: e.g. Phoenix
– –
Probes Probes
sent from orbiters of Venus, Mars, Jupiter sent from orbiters of Venus, Mars, Jupiter
– –
Titan Titan
lander lander
Huygens probe from Huygens probe from
Cassini Cassini
spacecraft spacecraft
Page 68
Space missions carry telescopes, Space missions carry telescopes,
other instruments as well other instruments as well
• •
Typically planetary fly-bys and orbiters carry Typically planetary fly-bys and orbiters carry
small telescopes small telescopes
– –
If you are close, you don If you are close, you don
’ ’
t need super-good angular t need super-good angular
resolution resolution
• •
Other instruments: Other instruments:
– –
Particle collectors and analyzers, radio antennae, Particle collectors and analyzers, radio antennae,
spectrographs, laser altimeters, dust detectors, ..... spectrographs, laser altimeters, dust detectors, .....
– –
Mars rovers: probes to get rock samples and Mars rovers: probes to get rock samples and
analyze them analyze them
Page 69
Spirit Rover on Mars Spirit Rover on Mars
Page 70
Concept Question Concept Question
• •
You are trying to decide whether to observe a new You are trying to decide whether to observe a new
comet from a 10m telescope on the ground without comet from a 10m telescope on the ground without
adaptive optics, or from the Hubble Space Telescope adaptive optics, or from the Hubble Space Telescope
diameter 2.4m. diameter 2.4m.
• •
Which of the following would be better from the Which of the following would be better from the
ground, and which from space ground, and which from space
a a
Ability to make images in ultraviolet light Ability to make images in ultraviolet light
b b
Spatial resolution of images in infrared light Spatial resolution of images in infrared light
c c
Ability to record images of a Ability to record images of a
very very
faint distant comet faint distant comet
Page 71
The Main Points The Main Points
• •
Telescopes gather light and focus it Telescopes gather light and focus it
– –
Larger telescopes gather more light Larger telescopes gather more light
– –
Telescopes can gather Telescopes can gather
“ “
light light
” ”
at radio, infrared, visible, ultraviolet, x- at radio, infrared, visible, ultraviolet, x-
ray wavelengths ray wavelengths
• •
Telescopes can be on ground, on planes, in space Telescopes can be on ground, on planes, in space
• •
If Earth If Earth
’ ’
s atmosphere weren s atmosphere weren
’ ’
t turbulent, larger telescopes would t turbulent, larger telescopes would
give higher spatial resolution give higher spatial resolution
– –
Adaptive optics can correct for blurring due to turbulence Adaptive optics can correct for blurring due to turbulence
• •
Every new telescope technology has resulted in major new Every new telescope technology has resulted in major new
discoveries and surprises discoveries and surprises
A Brief History of Satellite Laser
Ranging: 1964 – present
Jan McGarry Tom Zagwodzki
NASA GSFC 694
With special thanks to John Degnan for additional background information and slides.
From the GSFC perspective
3292005
• Unambiguous time-of-flight
measurement
• 1 to 2 mm normal point
precision
• Passive space segment
reflector
• Simple refraction model
• Night Day Operation
• Near real-time global data
availability
• Satellite altitudes from 400 km
to 20,000 km GPS, GLONASS and the Moon
• Centimeter accuracy satellite
orbits
~ 1-2 cm LAGEOS
~
2-3 cm GPS
SLR generates unambiguous centimeter accuracy orbits SLR generates unambiguous centimeter accuracy orbits
Observable: The precise measurement of the roundtrip time-of-flight of an ultrashort 500 psec
laser pulse between an SLR ground station and a retroreflector- equipped satellite which is then corrected for atmospheric refraction using ground-based meteorological sensors.
3292005
Satellite and Lunar Laser Ranging
• Terrestrial Reference Frame SLR
– Geocenter motion
– Scale GM
– 3-D station positions and velocities 50
• Solar System Reference Frame LLR
– Dynamic equinox
– Obliquity of the Ecliptic
– Precession constant
• Earth Orientation Parameters EOP
– Polar motion
– Length of Day LOD
– High frequency UT1
• Centimeter Accuracy Orbits
– Testcalibrate microwave navigation
techniques e.g., GPS, GLONASS, DORIS, PRARE
– Support microwave and laser altimetry
missions e.g., TOPEXPoseidon, ERS 12, GFO-1, JASON, GLAS, VCL
– Support gravity missions e.g. CHAMP,
GRACE, Gravity Probe B •
Geodynamics
– Tectonic plate motion
– Regional crustal deformation
• Earth Gravity Field
– Static medium to long wavelength components
– Time variation in long wavelength components
– Mass motions within the solid Earth, oceans, and
atmosphere
• Lunar Physics LLR
– Centimeter accuracy lunar ephemerides
– Lunar librations variations from uniform rotation
– Lunar tidal displacements
– Lunar mass distribution
– Secular deceleration due to tidal dissipation in Earth’s
oceans –
Measurement of GM
E
+ M
M
• General Relativity
– Testevaluate competing theories
– Support atomic clock experiments in aircraft and
spacecraft –
Verify Equivalence Principle
– Constrain β parameter in the Robertson-Walker Metric
– Constrain time rate of change in G
• Future Applications
– Global time transfer to 50 psec to support science, high
data rate link synchronization, etc French L2T2 Experiment
– Two-way interplanetary ranging and time transfer for
Solar System Science and improved General Relativity Experiments Asynchronous Laser Transponders
3292005
~60 satellites tracked since 1964
3292005
SLR Retro-Reflector Array RRA: GPS 35,36
32 cubes – each 28mm Aluminum coated reflective surfaces
Array shape: planar square Array size: 239 x 194 x 37 mm
Array mass: 1.27 kg
3292005
9 cubes – each 32 mm - 1 nadir pointing and 8 on sides Research grade radiation resistant suprasil quartz, silver coated
Arrray shape: hemispherical Array size: 16cm diameter
Array mass: 731 gm
3292005
• There are 5 retro-reflectors arrays: 3 Apollo and 2 Luna.
• Apollo RRA’s have 3.8 cm cubes. Apollo 11 14 have 100, Apollo 15 has 300.
• Regularly tracked by only a few stations. NASA funded University of Texas MLRS
has successfully ranged continuously since 1970s. Ranges are accurate to a few
centimeters.
GSFC invented and developed Satellite Laser Ranging and continues to advance SLR RD, however, there have
been many contributors to SLR over the years, including SAO U.Texas in U.S. and many international groups.
1960s 1970s
1980s 1990s
GODLAS STALAS
MOBLAS 4-8 SLR2000
524 GSFC
Codes
810 OPS 723 RD
901 CDP 920
453 OPS 694 RD
48 INCH
SLR at GSFC
TLRS
3292005
• 1964:
First successful demonstration of SLR to Beacon Explorer 22-B satellite at GSFC 3 m ranging
• 1968-1976:
NASA, CNES, and SAO SLR systems carried out first meter level global geodetic and gravity field measurements using reflectors on remote sensing satellites
• 1969:
NASA Apollo 11 mission places first retroreflector array on Moon to begin international Lunar Laser Ranging LLR effort.
• 1975-1976:
CNES and NASA launched first passive satellites dedicated to SLR Starlette and LAGEOS to begin modern space geodetic era
• 1975-1979:
NASA builds up SLR network for POD support of GEOSAT and SEASAT ocean altimetric missions 10 - 20 cm ranging.
• 1979-1993:
NASA Crustal Dynamics Project CDP provides focus for further technology development 1 cm ranging and international cooperation in defining contemporary tectonic plate motions, regional crustal
deformation, Earth Orientation Parameters, Earth gravity field etc.
GSFC Firsts a 40+ year history
• 1992-present:
Various US and European remote sensing missions e.g. ERS-1 2, TOPEXPoseidon,
GFO rely heavily on centimeter orbits provided by SLR. NASA provides the world’s data most precise data and
until budget cuts in 2003, provided half of the global SLR data.
• 1998-present:
GSFC selected as the Central Bureau CB for the new International Laser Ranging Service
ILRS. The CB is responsible for overseeing global operations of 40 international stations providing cm-
accuracy orbits for 20 artificial satellites and the Moon and ensuring that all ILRS stations, operations, data,
and analysis centers adhere to ILRS standards.
formerly Goddard Optical Research Facility GORF
Located ~ 3 miles from GSFC in middle
of BARC on Springfield Road.
Home to MOBLAS-7, 48” telescope, VLBI
MV3, GPS, and numerous other
facilities and experiments.
GGAO has been the site of all NASA SLR
system development, testing and
collocations. The Italian MLRO system,
the Saudi SALRO, and other ILRS
systems have also been developed and
tested at site.
GODLAS
GSFC records first SLR returns ever on Oct 31, 1964 GSFC team lead by Henry Plotkin
BE-B: first satellite with retro-reflectors
- Developed in early 1970s as a “stationary laser” system at GORF. - X-Y mount with 61cm 24” telescope.
- Initial system had 1 Hz ruby 694nm laser.
MOBLAS - 7: ~1980 with Jack Waller
-Systems built in 1978 by Contraves
telescope mount, and BFEC
electronics.
- 76 cm 30” diameter telescope.
- Laser now: 5Hz, 532nm, 100mJ.
-5 systems, all still operating, now
located in:
California Mon.Peak Australia
South Africa Maryland GGAO
Tahita.
- Built in 1973-74 by Kollmorgen Corporation as multi-user facility - Arcsecond precision tracking
- RD Facility used by many groups:
• Field testing of bread board for optical heterodyne spectrometers in 1970s 1980s M.Mumma colleagues
• Automated guiding and two-color refractometry D. CurrieUMd, D. WellnitzUMd: 1970s. • Lunar laser ranging test facility C. AlleyUMd: 1980s.
• Comparison of one way propagation times of laser pulses East-West vs
West-East by C. Alley and R. Nelson in 1983. • Single and two-color satellite laser ranging test bed Zagwodzki, Degnan, McGarry:
1980s 1990s.
MLA Earthlink Calibration Experiment: May 2005.
and many others…
48” Telescope Facility aka 1.2 m Telescope Located at GGAO
Code 723: ~1985
Who are these people and why do they look so young?
DEGNAN COYLE
RALL ZAGWODZKI
MCGARRY PACINI
UNGER ABSHIRE
Prototype at GGAO: ~2002
• SLR2000 was concept developed by John Degnan in the 1990s, with many innovative ideas requiring RD efforts.
• System designed in the mid 1990s, development began in late 1990s. Technical development is now in code 694 SLR OPS is in code 453.
• Currently tracking LEO satellites. Nearing the completion of major technical challenges. Expect to have a working semi-automated system by end of this year.
NASA plans to build 12 new SLR systems contracted out
and will replace all of its existing SLR Network with
systems using technology developed on this prototype.
OMC ranging plot
Satellite returns
SLR Precision has improved from meter to sub-cm level. NASA operating costs have decreased reductions in manpower.
And global data volume has been increasing.
NASA SLR is part of the
International Laser Ranging Service ILRS
The International Laser Ranging Service ILRS began in 1998 and provides global satellite and lunar laser ranging data and their related products to
support geodetic and geophysical research activities as well as IERS products important to the maintenance of an accurate International
Terrestrial Reference Frame ITRF. The service develops the necessary global standardsspecifications and encourages international adherence to its
conventions. The ILRS is one of the space geodetic services of the
International Association of Geodesy IAG.
GSFC has been chosen to run the Central Bureau of the ILRS. The Central Bureau is responsible for overseeing global operations of 40 international
stations
3292005
The Future of Laser Ranging: Asynchronous Transponders
SLR Ground Station Spacecraft Transponder
532nm uplink 1064nm downlink
Ground System Analysis
computes Ranges
Fire and Return Event Times sent via Internet
Fire and Return Event Times sent via Comm Link
1R2 signal loss for transponders, 1R4 signal loss for RRA ranging.
Transponders are the only viable option for ranging to planetary distances.
3292005
For further information on the History of Satellite Laser Ranging, see:
“Thirty Years of Satellite Laser Ranging”, John Degnan, Keynote Speech, Proceedings of the Ninth International
Workshop on Laser Ranging Instrumentation, Canberra, Australia, November 7-11, 1994.
Direct Broadcast Satellite:
Architecture and Evaluation
Venkata N. Padmanabhan
padmanabcs.berkeley.edu
Daedalus Retreat, June 1996
Overview
• Geostationary satellite broadcasts directly to user premises
– 24 inch dish antenna, ISA adaptor card
• Asymmetric Internet access
– users typically receive more data than they send – 12 Mbps satellite downlink; target rate of 400
Kbps per user – slow uplink: SLIPPPP over a modem line
• Easy to deploy at short notice even in remote locations
User Subnet SLIPPPP
Internet
Uplink Site
Internet Server
Asymmetric Routing
• Outgoing traffic over the SLIPPPP line; Incoming traffic over the satellite hop
• Option 1: Encapsulation
– outgoing packets use DBS source address
– packets sent encapsulated over the SLIP line – works for a single host but not for a subnet
• Option 2: Home agent-based routing
– outgoing packets use home source address
– home agent tunnels incoming packets over DBS – a more general solution
Transport Issues
• Large bandwidth-delay product
– TCP sender and receiver need to maintain large windows to keep the “data pipe” full
– 500-1000 Kbps times 1 second = 50-100 KB
• Asymmetric bandwidth
– Uplink has much smaller bandwidth than the downlink
– TCP acknowledgements stream might throttle the flow of data packets
UDP Throughput
– Throughput tapers off beyond an offered load of about 1.4 Mbps
200 400
600 800
1000 1200
1400 1600
200 400
600 800
1000 1200
1400 1600
1800 2000
Offered Load Kbps
Throughput Kbps
Day Night
UDP Loss Rate
0.05 0.1
0.15 0.2
0.25 0.3
0.35
200 400
600 800
1000 1200
1400 1600
1800 2000
Night Day
Offered Load Kbps
Loss Rate
– High loss rate due to Internet during the day
– Sharp upswing for offered load beyond 1.4 Mbps
TCP Throughput
– Poor throughput for the 8-32 KB buffers used by most Internet servers
– Comparable to UDP when loss rate is low
TCP Buffer Size bytes
100 200
300 400
500 600
700
20000 40000
60000 80000
100000 120000
140000
Night
Day
Throughput Kbps
TCP Dynamics
5000 10000
15000 20000
25000 30000
20 40
60 80
100 120
140
Time seconds
Congestion Window KB
Fast retransmissions
– 2 MB transfer with 130 KB buffers – Poor throughput due to fluctuation in congestion
window size though not timeouts at source
TCP Throughput
Internet only
– Best throughput for 16-32 KB buffers – Deterioration for large buffers presumably due to
increased burstiness of the source
TCP Buffer Size bytes
200 400
600 800
1000 1200
20000 40000
60000 80000
100000 120000
140000
Throughput Kbps
Day Night
Conclusions
• Performance of DBS system is often limited by the Internet
• Large TCP windows needed to keep data pipe full
– buffer sizes typically used by servers on the Internet are too small
– but large buffer sizes could increase source burstiness and lead to Internet losses.
Future Work
• Improve performance of data transport
– install host close to uplink to evaluate the satellite hop in isolation
– split-connection approach for instance, in conjunction with a Web proxy cache
– reduce burstiness of TCP source
• Evaluate application-specific solutions
– plentiful downlink bandwidth, large latency
– suitable for predictive prefetching of Web data [PM96]
Status of the UCB Testbed
• One DirecPC host fully functional
– BSDOS driver for the ISA adaptor card Keith Sklower
• Home agent-based routing
• Web browsing and video dissemination demonstrated
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A comprehensive, quantitative tutorial designed for satellite professionals
Course Outline
1. Mission Analysis. Kepler’s laws. Circular and
elliptical satellite orbits. Altitude regimes. Period of revolution. Geostationary Orbit. Orbital elements. Ground
trace.
2. Earth-Satellite Geometry. Azimuth and elevation.
Slant range. Coverage area.
3. Signals and Spectra. Properties of a sinusoidal
wave. Synthesis and analysis of an arbitrary waveform. Fourier Principle. Harmonics. Fourier series and Fourier
transform. Frequency spectrum.
4. Methods of Modulation. Overview of modulation.
Carrier. Sidebands. Analog and digital modulation. Need for RF frequencies.
5. Analog Modulation. Amplitude Modulation AM.
Frequency Modulation FM.
6. Digital Modulation. Analog to digital conversion.
BPSK, QPSK, 8PSK FSK, QAM. Coherent detection and carrier recovery. NRZ and RZ pulse shapes. Power spectral
density. ISI. Nyquist pulse shaping. Raised cosine filtering.
7. Bit Error Rate. Performance objectives. EbNo.
Relationship between BER and EbNo. Constellation diagrams. Why do BPSK and QPSK require the same
power?
8. Coding. Shannon’s theorem. Code rate. Coding gain.
Methods of FEC coding. Hamming, BCH, and Reed- Solomon block codes. Convolutional codes. Viterbi and
sequential decoding. Hard and soft decisions. Concatenated coding. Turbo coding. Trellis coding.
9. Bandwidth. Equivalent noise bandwidth. Occupied
bandwidth. Allocated bandwidth. Relationship between bandwidth and data rate. Dependence of bandwidth on
methods of modulation and coding. Tradeoff between bandwidth and power. Emerging trends for bandwidth
efficient modulation.
10. The Electromagnetic Spectrum. Frequency bands
used for satellite communication. ITU regulations. Fixed Satellite Service. Direct Broadcast Service. Digital Audio
Radio Service. Mobile Satellite Service.
11. Earth Stations. Facility layout. RF components.
Network Operations Center. Data displays.
12. Antennas. Antenna patterns. Gain. Half power
beamwidth. Efficiency. Sidelobes.
13. System Temperature. Antenna temperature. LNA.
Noise figure. Total system noise temperature.
14. Satellite Transponders. Satellite communications
payload architecture. Frequency plan. Transponder gain. TWTA and SSPA. Amplifier characteristics. Nonlinearity.
Intermodulation products. SFD. Backoff.
15. The RF Link. Decibel dB notation. Equivalent
isotropic radiated power EIRP. Figure of Merit GT. Free space loss. WhyPower flux density. Carrier to noise ratio.
The RF link equation.
16. Link Budgets. Communications link calculations.
Uplink, downlink, and composite performance. Link budgets for single carrier and multiple carrier operation. Detailed
worked examples.
17. Performance Measurements. Satellite modem.
Use of a spectrum analyzer to measure bandwidth, CN, and EbNo. Comparison of actual measurements with
theory using a mobile antenna and a geostationary satellite.
18. Multiple Access Techniques. Frequency division
multiple access FDMA. Time division multiple access TDMA. Code division multiple access CDMA or spread
spectrum. Capacity estimates.
19. Polarization. Linear and circular polarization.
Misalignment angle.
20. Rain Loss. Rain attenuation. Crane rain model.
Effect on GT.
IIn nssttrru
ucctto orr
Dr. Robert A. Nelson is president of Satellite Engineering Research Corporation, a consulting firm in
Bethesda, Maryland, with clients in both commercial industry and government.
Dr. Nelson holds the degree of Ph.D. in physics from the University of Maryland
and is a licensed Professional Engineer. He is coauthor of the textbook Satellite
Communication Systems Engineering,
2nd ed. Prentice Hall, 1993 and is Technical Editor of Via Satellite magazine. He is a member of IEEE, AIAA,
APS, AAPT, AAS, IAU, and ION.
March 16-18, 2009
Boulder, Colorado
June 15-17, 2009
Beltsville, Maryland
1740
8:30am - 4:30pm
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“Fantastic It couldn’t have been more relevant to my work.”
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Additional Materials
In addition to the course notes, each participant will receive a book of collected tutorial articles written by
the instructor and soft copies of the link budgets discussed in the course.
1
Via Satellite, October, 1998
Earth Station High Power Amplifiers
KPA, TWTA, or SSPA?
by Robert A. Nelson
The high power amplifier HPA in an earth station facility provides the RF
carrier power to the input terminals of the antenna that, when combined with the
antenna gain, yields the equivalent isotropic radiated power EIRP required
for the uplink to the satellite. The waveguide loss between the HPA and the
antenna must be accounted for in the calculation of the EIRP.
The output power typically may be a few watts for a single data channel, around
a hundred watts or less for a low capacity system, or several kilowatts for high
capacity traffic.
The choice of amplifier is highly dependent on its application, the cost of
installation and long term operation, and many other factors. This article will
summarize the technologies, describe their important characteristics, and identify
some issues important to understanding their differences and relative merits.
TYPES OF AMPLIFIERS
Earth station terminals for satellite communication use high power amplifiers
designed primarily for operation in the Fixed Satellite Service FSS at C-band
6
GHz, military and scientific communications at X-band 8 GHz, fixed
and mobile services at Ku-band 14 GHz, the Direct Broadcast Service DBS in the
DBS portion of Ku-band 18 GHz, and military applications in the EHFQ-band
45 GHz. Other frequency bands include those allocated for the emerging
broadband satellite services in Ka-band 30
GHz and V-band 50 GHz.
Generally, the frequency used for the earth-to-space uplink is higher than the
frequency for the space-to-earth downlink within a given band.
An earth station HPA can be one of three types: a klystron power amplifier
KPA, a traveling wave tube amplifier TWTA, or a solid state power amplifier
SSPA. The KPA and TWTA achieve amplification by modulating the flow of
electrons through a vacuum tube. Solid state power amplifiers use gallium arsenide
GaAs field effect transistors FETs that are configured using power combining
techniques. The klystron is a narrowband, high power device, while TWTAs and
SSPAs have wide bandwidths and operate over a range of low, medium, and high
powers.
The principal technical parameters characterizing an amplifier are its
frequency, bandwidth, output power, gain, linearity, efficiency, and reliability. Size,
weight, cabinet design, ease of maintenance, and safety are additional
considerations. Cost factors include the cost of installation and the long term cost
of ownership.
KPAs are normally used for high power narrowband transmission to specific
satellite transponders, typically for television program transmission and
distribution. TWTAs and SSPAs are used for wideband applications or where
frequency agility is required.
Originally, TWTAs provided high power but with poor efficiency and
reliability. Compared to a KPA, these disadvantages were regarded as necessary
penalties for wide bandwidth. SSPAs first became available about 20 years ago.
They were restricted to low power systems requiring only a few watts, such as small
earth stations transmitting a few telephone channels.
Within the past decade, however, TWTA and SSPA technologies have both
advanced considerably. Today there is vigorous competition between these two
technologies for wideband systems.
KPA
The klystron power amplifier KPA is a narrowband device capable of providing
high power and high gain with relatively high efficiency and stability. The
bandwidth is about 45 MHz at C-band and about 80 MHz at Ku-band. Thus a
separate KPA is usually required for each satellite transponder.
In a klystron tube an electron beam is formed by accelerating electrons emitted
from a heated cathode through a positive potential difference. The electrons enter a
series of cavities, typically five in number, which are tuned around the operating
frequency and are connected by cylindrical drift tubes.
In the input cavity the electrons are velocity-modulated by a time-varying
electromagnetic field produced by the input radio frequency RF signal. The
distribution in velocities results in a density modulation further down the tube as the
electrons are bunched into clusters when higher velocity electrons catch up with
slower electrons in the drift tubes.
Optimum bunching of electrons occurs in the output cavity. Large RF currents are
generated in the cavity wall by the density- modulated beam, thereby generating an
amplified RF output signal. The energy of the spent electron beam is dissipated as
heat in the collector.
The intermediate cavities are used to optimize the saturated power, gain, and
bandwidth characteristics. Additional bunching of electrons is provided, yielding
higher gain.
The gain is typically 15 dB per cavity, so that a five-cavity klystron can provide a
total gain of about 75 dB if synchronously tuned. However, by stagger tuning the
individual cavities to slightly different frequencies, the bandwidth can be
increased with a reduction in gain. A typical gain is on the order of 45 dB.
For a cavity device like a klystron, the bandwidth is a fixed percentage of the
frequency of operation. The bandwidth is proportional to the frequency and inversely
proportional to the Q quality factor, which is defined as 2
π times the ratio of
the energy stored and the average energy lost in one cycle. Thus at C-band 6 GHz,
a typical bandwidth is 45 MHz. But at Ku- band 14 GHz the bandwidth is about 80
MHz. These bandwidths are well suited for C-band and Ku-band satellite
transponders. By adding a sixth, filter cavity the KPA bandwidth can be doubled.
Thus 80 MHz KPAs are also available at C-band.
2 Klystrons can be made with extended
interaction circuits in one or more cavities that increase the bandwidth substantially.
This technology can provide a bandwidth of 400 MHz at 30 GHz. Output powers up
to 1 kW can also be achieved at different bandwidths.
Although the bandwidth is relatively small, a conventional klystron can be
mechanically tuned over a wide frequency range. A klystron can be capacitively or
inductively tuned. All satcom klystrons are inductively tuned because of better
efficiency and repeatability. The inductance is varied by moving a wall in
the cavity sliding short.
The output power of a KPA is about 3 kW at C-band and 2 kW at Ku-band.
The lowest power KPA offered for commercial satellite communications is
around 1 kW, although for certain applications powers under 1 kW are
available.
TWTA
The traveling wave tube amplifier TWTA consists of the traveling wave
tube TWT itself and the power supply. The TWT can have either a helix or
coupled-cavity design.
The TWT is a broadband device with a bandwidth capability of about an octave,
which easily covers the 500 MHz
bandwidth typical of satellites in the FSS. It also covers the typical 800 MHz DBS
bandwidth requirement, as well as even broader bandwidths in Ka-band and higher
bands.
The TWT, like the klystron, is an example of a device based on modulating
the flow of electrons in a linear beam, but differs from the klystron by the continuous
interaction of the electrons with the RF field over the full length of the tube instead
of within the gaps of a few resonant cavities.
The TWT has a heritage of over half a century. The original concept was
proposed in 1944 by Rudolf Kompfner, who investigated experimental laboratory
microwave tubes while working for the British Admiralty during World War II.
The first practical TWT was developed at the Bell Telephone Laboratories in 1945
by John Pierce and L.M. Field. Bell Labs was interested in the technology for its
possible application to communication. By the early 1960s, the TWT was adapted
for use in satellite power amplifiers in the Telstar program.
In a TWT, amplification is attained by causing a high density electron beam to
interact with an electromagnetic wave that travels along a slow-wave structure,
which usually takes the form of a helical coil. A helix is the widest bandwidth
structure available. The electrons are emitted from a heated cathode and are
accelerated by a positive voltage applied to an aperture that forms the anode. The
electrons are absorbed in a collector at the end of the tube.
The RF signal is applied to the helix. Although the signal travels at nearly the
speed of light, its phase velocity along the axis of the tube is much slower because of
the longer path in the helix, as determined by the pitch and diameter of the coil, and is
nearly equal to the velocity of the electrons. For example, if the electrons are
accelerated by a 3,000 volt potential difference on the anode, the speed of the
electrons is about one tenth the speed of light. Thus the length of the helix wire
should be about ten times the axial length of the tube to bring about synchronism
between the RF traveling wave and the electron beam.
The electrons interact with the traveling wave and form clusters that replicate the
RF waveform. Midway down the tube, an attenuator, called a sever, absorbs the RF
signal and prevents feedback, which would result in self-oscillation. On the other side
of the attenuator, the electromagnetic field of the electron clusters induces a waveform
in the helix having the same time- dependence as the original signal but with
much higher energy, resulting in amplification. The gain is typically on the
order of 40 to 60 dB.
The beam-forming optics are critical parts of the tube. To minimize heat
dissipation caused by electrons striking the helix, the beam must be highly focused and
the transmission from one end of the tube to the other must be close to 100 percent.
When the electrons reach the end of the tube, they impact with the walls of the
collector, where most of the heat is generated.
The efficiency of the tube can be improved by applying a negative potential
to the collector, which retards the electron beam as the electrons enter it. A collector
designed to operate in this way is called a depressed collector. Less energy is
converted to heat as the electron beam impinges on the collector, and
consequently less energy is lost as thermal waste.
However, the distribution of electron energies is not uniform. In a multi-stage
depressed collector, high energy electrons are directed to stages with high retarding
fields and low energy electrons are directed to stages with low retarding fields.
This configuration improves the efficiency further, but with greater complexity.
Another means of achieving greater efficiency is through improving beam
synchronization. As the electrons travel along the tube and interact with the RF
signal, they give up energy and lose velocity. Thus with an ordinary helix, they
tend to fall behind the signal. This problem can be mitigated by brute force by
increasing the accelerating potential but at the expense of degrading the TWT
linearity.
A more elegant method is through the use of a tapered helix, in which the pitch of
the helix decreases along the tube. The signal velocity is thus retarded to
compensate for beam slowing. The selection of optimum helix configurations
has been made possible through advanced computer modeling techniques.
Another type of TWT is a coupled- cavity device, used for high power
applications. In this case a series of cavity sections are connected to form the slow-
wave structure and is similar to the klystron in this respect. However, in the
klystron the cavities are independent, while in the TWT the cavities are coupled by a
slot in the wall of each cavity.
The output power of a helix TWTA at C-band ranges from a few watts to about
3 kW, while power levels of 10 kW can be attained with coupled-cavity TWTAs.
Helix TWTAs at Ku-band have less power, with a maximum power of around
700 W.
Higher frequency TWTAs are also