Principles of Satellite Communications, 1-3 Satellite - Lab-Volt

Exercise
1-3
Satellite Payloads and Telemetry
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the payload of a
communications satellite and with the principles of TTC (telemetry, tracking and
command) between the earth station and the satellite.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:
ƒ
ƒ
ƒ
ƒ
DISCUSSION
Functions and characteristics of the payload
Repeater organization
Antennas
Telemetry, tracking and command (TTC)
Functions and characteristics of the payload
The payload of the communications satellite consists of all the components that
provide communications services, that is, which receive, process, amplify and
retransmit information. The payload can be divided into two distinct parts: the
antennas and the repeater. The antennas serve to capture the uplink signal from
the earth station and to radiate the downlink signal to other earth station. The
other components in the payload make up the repeater. This includes all the
components that process and amplify the uplink signal obtained from the
receiving antenna before delivering the downlink signal to the transmitting
antenna.
The main functions of all communications payload are as follows:
x
x
x
x
To receive the desired uplink carriers transmitted by the earth stations in
the desired frequency bands and with the desired polarization, and from
the desired region on the surface of the earth (service zone). The
payload should receive as little interference as possible from other
frequency bands, polarizations, and regions.
To convert the frequencies of all received uplink carriers to the
appropriate downlink frequencies. Frequency conversion is required to
prevent the high power downlink transmission from interfering with the
weak signals received on the uplink.
To amplify the received carriers to a level suitable for retransmission to
earth while limiting noise and distortion as much as possible.
To transmit the downlink carriers with the desired polarization to the
appropriate service zone on the earth’s surface.
A payload, or more precisely, the repeater, can be either of the transparent or
regenerative type. A transparent repeater may carry out only those functions
listed above. A regenerative repeater will have additional functions such as
demodulation, baseband signal processing and switching, and remodulation.
© Festo Didactic 86311-00
99
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
Payloads with multi-beam antennas will also perform routing of the carriers from
any given uplink beam to the desired downlink beam.
Regardless of the type of payload, the following characteristics are desirable:
x
x
High power gain
x
High power output
x
High availability, high reliability and adequate lifetime
x
Low noise (low effective input noise temperature)
x
Large bandwidth
Adequate linearity
Because of the high cost of designing building and launching the satellite,
satellites must be designed to operate dependably throughout their lifetime. This
is accomplished through stringent quality control and rigorous testing. In addition,
redundancy is used so that a spare component can be substituted for a failed
one.
Nonlinearity in the repeater arises when the output power is not proportional to
the input power. In order to amplify the signals as much as possible, repeater
power amplifiers are operated near their saturation point, in a region where their
response is not perfectly linear. As a result, intermodulation distortion occurs
when more than one signal is present. Various techniques are used to keep
nonlinearity and intermodulation distortion within acceptable limits.
c
Payload characteristics are covered in detail in the manual Link Characteristics
and Performance.
Repeater organization
The organization of the different components in a repeater depends on the type
of repeater (transparent or regenerative) and on various technological
constraints.
Transparent repeater
Figure 1-63 shows a simplified block diagram of a single-frequency-conversion
transparent repeater. A transparent repeater, or non-regenerative repeater, is
sometimes called a bent pipe because it captures the signal from earth and
redirects it back to earth without demodulating it. Before retransmission however,
the received uplink signal is frequency converted to the downlink frequency,
amplified and filtered. Other operations may also be applied to the signal.
A bent pipe repeater is simply a type of relay. It will relay back to earth any radio
signal it receives within its bandwidth, providing the received power is above the
threshold level, regardless of what type of information the signal is carrying. The
uplinks and downlinks are codependent, which means that any noise and other
degradation present in the received uplink signal will also be present in the
transmitted downlink signal. As a result, the signal received by the earth station
contains degradation introduced during both the uplink and the downlink.
100
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
Wideband receiver
LNA
Uplink
antenna
Transponder (channel)
IMUX
OMUX
Linearizer
HPA
Linearizer
HPA
Linearizer
HPA
Downlink
antenna
Linearizer
Amplifier
Mixer
Local oscillator
HPA
Variable attenuator
Band-pass filter
Low-pass filter
Figure 1-63.Transparent or “bent pipe” repeater with single frequency conversion.
The first stage in the transparent repeater is the wideband receiver. The lownoise amplifier (LNA) at the input is designed to amplify the extremely weak
uplink signal (typically a few hundred picowatts) while minimizing its own
contribution to noise. This is important because the first component in a cascade
has the greatest effect on the noise of the entire system. The gain of the lownoise amplifier is typically 20 to 40 dB.
Frequency conversion ensures decoupling between RF input and the RF output
of the repeater. This is accomplished by the mixer and local oscillator (LO)
according to heterodyne principle. Multiplication of the uplink signal and the
sinusoidal local oscillator (LO) signal results in frequencies at both the sum and
difference frequencies of the two signals. The undesired frequencies are filtered
out at a later stage.
In most repeaters, the uplink frequency fu is higher than the downlink
frequency fd. This is desirable because the directivity of an antenna increases as
© Festo Didactic 86311-00
101
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
the frequency increases. High directivity is important at the uplink earth station in
order to direct as much power as possible toward the satellite. High directivity is
often not required on the downlink; in fact, a wide footprint is often desirable.
Dividing the repeater bandwidth into sub-bands, one
for each transponder, is
called channelization.
To overcome intermodulation distortion between carriers while providing the
maximum possible amplification, the overall bandwidth of the repeater is split into
several sub-bands by a set of band-pass filters that make up the input multiplexer
(IMUX). The equipment that operates on a single sub-band is known as a
transponder or a channel. The following characteristics are common to all
transponders. These transponder characteristics are determined when the
satellite is designed in order to ensure correct operation:
x
x
center frequency
x
threshold level
x
operating point
x
bandwidth
x
saturation point
x
power gain
factors that affect linearity
Each transponder has a different center frequency (see Figure 1-64). The other
characteristics may be the same for each transponder in the payload.
The frequency response of each transponder should be relatively flat, that is, with
very low gain variations across its passband. The filters should provide high
rejection of frequencies outside the transponders passband.
A transponder consists of a chain of components that provide a signal path
through the repeater. These components may include a variable gain component
(amplifier or attenuator) that is controllable from earth, an amplifier which may be
referred to as a driver amplifier or a channel amplifier (CAMP), filters to reduce
out-of-band frequency components, a limiter to prevent saturation, a linearizer
designed to minimize distortion, and a high-power amplifier (HPA).
The HPA is usually a traveling wave tube amplifier (TWT or TWTA) or a solidstate power amplifier (SSPA). In order to increase the power of the weak uplink
signal to roughly 10 to 100 W, the power gain of each transponder must be of the
order of 100 to 130 dB.
Satellites may have several dozen transponders or more than a hundred for
some high-capacity satellites. Because each channel only covers a relatively
narrow sub-band, and is therefore shared by a small number of carriers, noise
due to intermodulation distortion is much less than if the entire bandwidth of the
repeater (with all the carriers) was amplified using a single channel. The way that
the bandwidth of the repeater is divided among the different transponders and
antenna polarizations is called the frequency plan or the frequency and
polarization plan.
Figure 1-63 shows all transponders connected to one downlink antenna. Most
communications satellites have at least two uplink and two downlink antennas of
opposite polarizations (horizontal and vertical linear polarization or left-hand and
right-hand circular polarization). In this case, it is common practice to use one
102
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
polarization for odd-numbered transponders and the opposite polarization for
even-numbered transponders.
With single channel per carrier (SCPC), the bandwidth of a modulated carrier
may be less than the bandwidth of one transponder. In this case, other
modulated carriers of somewhat different frequencies can pass through the same
transponder, providing a guard band is left between each of the modulated
signals so that their frequency ranges do not overlap. Using different carrier
frequencies to give several signals simultaneous access to the same transponder
is called frequency division multiple access.
Figure 1-64 shows a typical C-band frequency and polarization plan for a satellite
payload using linear (vertical and horizontal) polarization. The uplink signal is in
the 6 GHz range and the downlink signal is in the 4 GHz range. The odd
numbered transponders receive and retransmit using vertical polarization; the
even numbered transponders using horizontal polarization. For each polarization,
the center frequencies are separated by 40 MHz and a guard band of 4 MHz
between adjacent transponders assures that they do not interact. This leaves a
passband of 36 MHz per transponder.
Since vertical and horizontal polarizations are orthogonal, the passbands of
transponders using vertical and horizontal polarizations can overlap without
causing crosstalk. This is an example of frequency reuse through polarization
diversity. The center frequencies of the vertical polarization transponders are
offset so that they fall within the guard bands of the horizontal polarization
transponders, and vice versa. This further reduces crosstalk.
Uplink Frequencies (MHz)
V
T1
5945
T12
6165
T10
6125
T8
6085
T13
6185
T11
6145
T9
6105
T7
6065
T6
6045
T4
6005
T2
5965
H
T5
6025
T3
5985
T17
6265
T15
6225
T14
6205
T23
6385
T22
6365
T20
6325
T18
6285
T16
6245
T21
6345
T19
6305
T24
6405
Downlink Frequencies (MHz)
V
H
T1
3720
T5
3800
T3
3760
T2
3740
T4
3780
T6
3820
T8
3860
T13
3960
T11
3920
T9
3880
T7
3840
T10
3900
T12
3940
T17
4040
T15
4000
T14
3980
T16
4020
T21
4120
T19
4080
T18
4060
T20
4100
T23
4160
T22
4140
T24
4180
Figure 1-64. 24-transponder C-band frequency and polarization plan (transponder T15 is highlighted).
The amplified carriers from a group of transponders are recombined in the output
multiplexer (OMUX). The combined signal is then sent to the downlink antenna
for retransmission. Some satellites have a band-pass filter at the input and at the
output to provide additional uplink-downlink isolation. These filters must be
designed to have the lowest possible insertion loss.
In some cases, it is difficult to obtain a sufficiently high power gain at the
downlink frequency. In this case, dual frequency conversion can be used (see
© Festo Didactic 86311-00
103
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
Figure 1-65). The uplink signal is first down-converted to an intermediate
frequency, usually a few gigahertz, amplified, and then up-converted to the
downlink frequency.
݂௨௣
݂ூி
݂ௗ௢௪௡
One of several
transponders
Uplink
antenna
LO 1
Downlink
antenna
LO 2
Figure 1-65. Dual frequency conversion.
Regenerative repeater
Demod.
Receiver
Demod.
Receiver
Demod.
Receiver
Demod.
Mod.
HPA
Mod.
HPA
Mod.
HPA
Mod.
HPA
Uplink
Antenas
RF Switches
Receiver
Baseband Processing and Switching
RF Switches
A regenerative repeater is also called an on-board processing repeater, a
demod/remod repeater or a smart satellite. Like a transparent repeater, a
regenerative repeater includes one or more uplink antennas, downlink antennas,
low-noise amplifiers, frequency converters and high power amplifiers. Unlike a
transparent repeater, however, a regenerative repeater demodulates the uplink
RF signal to recover the baseband signal and later re-modulates the baseband
signal to produce the downlink RF signal (see Figure 1-66). This allows onboard
processing (OBP) and switching in the baseband. The type of processing used
depends on the application. Isolation between the uplink and downlink signals is
accomplished by remodulation of the baseband signal at a different frequency
rather than by frequency conversion.
Downlink
Antenas
Control
Figure 1-66. Regenerative repeater with on-board processing.
Regenerative repeaters offer improved performance compared with transparent
repeaters because the degradation in the uplink signal is not retransmitted in the
downlink. However, they must this be designed to handle predetermined data
formats, making them less flexible than transparent repeaters which don’t “care”
what kind of information the RF signal is carrying. In addition, they are more
complex and costly and require more electrical power to operate.
104
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
Redundancy
In order to ensure that a satellite will continue to operate over an adequate
lifetime, redundancy is used. Redundancy is the duplication, or backing up, of
critical components of a system with the intention of increasing reliability of the
system. Any single component which, if it fails, will stop the entire system from
working is called a single point of failure (SPOF). With the satellite, it is
essential that there are as few single points of failure as possible.
Although, theoretically, any element in a satellite could fail, the degree of
redundancy used for any component or subsystem depends on the probability of
failure, the consequences of failure, and the cost and complexity of a backing up
the component or subsystems. Certain components have a very low probability
of failure. This is the case for the passive input and output multiplexers (IMUX
and OMUX) in a repeater. For this reason, and because it would be very difficult
to duplicate them, redundancy is seldom used for these components.
Some components in the repeater are duplicated using one identical backup unit.
This would generally be the case for the low-noise frequency converter as shown
in Figure 1-67. A switch would be used to select one or the other. This is an
example of “2-for-1” redundancy, or “1 / 2” redundancy.
Redundancy is almost always used for amplifying equipment as it has a relatively
high probability of failure. When the satellite has many transponders (channels),
simple 2-for-1 redundancy is rarely used. For example, providing one backup unit
for every high-power amplifier in a repeater, and a switch to select either the
main or the backup amplifier for each transponder, would be very costly if the
satellite has many transponders. In addition, such a configuration would not
provide adequate reliability. The probability that both the main and the backup
amplifier in any given transponder would fail is not negligible. It would be likely,
therefore, that after a certain time, some transponders would be out of
commission with faults in both their main and their backup amplifiers, and the
unused backup amplifiers in other transponders would be of no help.
LNA
IMUX
Uplink
Antenna
To
Transponders
LNA
Figure 1-67. 2-for-1 Redundancy.
When a repeater has many transponders, the number of backup amplifiers
provided may be only half of the number of transponders, and the switching
© Festo Didactic 86311-00
105
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
arrangement used would allow multiple transponders to share the same backup
amplifier. Figure 1-68 shows an example of 3-for-2 redundancy, where two
transponders share the same backup amplifier. A more complex switching
arrangement could allow, say, eight transponders to share 12 amplifiers and
allow any of the four spare amplifiers to be switched into any transponder in case
of failure. This 12-for-8 redundancy would greatly increase the reliability of the
repeater at a reasonable cost.
IMUX
OMUX
HPA
HPA
HPA
Figure 1-68. 3-for-2 Redundancy.
Antennas
The communications antennas on the satellites are part of the payload. The type
in the number of antennas depends on the type of satellite. If global coverage is
to be provided by a beam, a conical horn antenna may be used. For spot
coverage, parabolic dish antennas are used with beamwidths that vary from
roughly 1° to 10°. By equipping a single dish with multiple feeds, the same
reflector can be used for both uplink and downlink communications. A satellite
that operates on more than one frequency band usually has separate antennas
for each band (see Figure 1-69).
106
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Discussion
Subreflector
C-band omnidirectional
antenna
20 GHz downlink
antenna
Steerable phasedarray antenna
Ka-band TTC antennas
Solar panel array
30 GHz uplink antenna
Figure 1-69.ACTS satellite (NASA illustration).
Some satellites use phased array antennas. A phased array antenna is an array
of radiating elements whose radiation pattern is determined by the phase
relationships of the signals that excite the elements. With adjustable phase
shifters operating under computer control, the beam can be scanned in azimuth
or elevation without mechanical movements. This produces antenna beams that
are steerable.
Telemetry, tracking and command (TTC)
With the Satellite Communications Training System,
there is no tracking. For
brevity, all telemetry and
command functions are
referred to using the term
“telemetry.”
Satellites are controlled from the ground through communications functions
grouped under the name telemetry tracking and command (TTC). This is
sometimes called telemetry tracking and control. The abbreviation “TT&C” is
frequently encountered.
During normal operation, TTC communications with the satellite are often routed
through the satellite payload using the same directional antennas and the same
frequencies as the regular satellite service. Conditions may occur however where
this link is unavailable, for example, when a satellite is being maneuvered into
orbit or when an attitude control problem prevents the uplink and downlink
antennas from being pointed to the earth stations. During these conditions, a
dedicated TTC link using an omnidirectional antenna on the satellites and space
operations service (SOS) frequencies is used.
In some cases, different frequency bands and antenna are used for TTC and for
the uplink and downlink transmission. Although TTC involves many
communications functions, it is usually considered to be part of the platform,
rather than of the payload.
Telemetry is technology that allows remote monitoring and reporting of
information. Obviously, this is the only way to obtain information from an
unmanned satellite. Telemetry makes use of sensors installed in the payload and
the platform to obtain information on their health and status as well as data
concerning the operation of the payload.
© Festo Didactic 86311-00
107
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure Outline
Health and status information may include information such as:
x
x
x
the amount of fuel available for maneuvers
the state and output of solar panels
electrical bus voltages
The payload data may include information such as:
x
x
x
the power output of transponders
the orientation of antennas
transponder switch configuration
Tracking is continuously or periodically determining a satellite’s position, altitude
and other orbital parameters. On many satellites, a beacon transmits a signal to
help ground tracking receivers locate the satellite. Various onboard sensors such
as inertial navigation sensors and star trackers provide additional tracking data.
Tracking information is essential in order to accurately determine orbital
parameters and to predict where the satellite will be at any point in the future, in
order to make any necessary adjustments. Since large antennas are required to
track satellites accurately, tracking stations are normally fixed sites and maybe
separate from earth traffic stations.
Command is controlling a satellite payload and platform from the ground. This is
accomplished by sending signals to the satellite. Commands may be executed
immediately or stored for execution at a later time or when a predefined condition
exists. Commands may control the thrusters in order to modify the orbit, or may
control electronics circuits in order to reconfigure the payload to meet the needs
of various users. Commands are also used to switch in redundant components in
case of failure.
PROCEDURE OUTLINE
The Procedure is divided into the following sections:
ƒ
ƒ
ƒ
ƒ
PROCEDURE
System startup
Repeater organization
Repeater characteristics
Telemetry with the Satellite Repeater (optional)
System startup
1. If not already done, set up the system and align the antennas visually as
shown in Appendix B.
2. Make sure that no hardware faults have been activated in the Earth Station
Transmitter or the Earth Station Receiver.
b
108
Faults in these modules are activated for troubleshooting exercises using DIP
switches located behind a removable panel on the back of these modules. For
normal operation, all fault DIP switches should be in the “O” position.
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
3. Turn on each module that has a front panel Power switch (push the switch
into the I position). After a few seconds, the Power LED should light.
4. If you are using the optional Telemetry and Instrumentation Add-On:
x
x
Make sure there is a USB connection between the Data
Generation/Acquisition Interface, the Virtual Instrument, and the host
computer, as described in Appendix B.
Turn on the Virtual Instrument using the rear panel power switch.
b
x
If the TiePieSCOPE drivers need to be installed, this will be done
automatically in Windows 7 and 8. In Windows XP, the Found New
Hardware Wizard will appear (it may appear twice). In this case, do not
connect to Windows Update (select No, not this time and click Next). Then
select Install the software automatically and click Next.
Start the Telemetry and Instrumentation application. In the Application
Selector, do not select Work in stand-alone mode.
b
If the Telemetry and Instrumentation application is already running, exit
and restart it. This will ensure that no faults are active in the Satellite
Repeater.
Repeater organization
5. Examine the front panel of the Satellite Repeater. What is the purpose of the
low-noise amplifier?
What is the purpose of frequency conversion in the satellite repeater?
6. The Earth Station Transmitter transmits in a frequency band ranging from
approximately 10.7 GHz to 11.2 GHz (the carrier frequency depends on the
selected Channel). In what two frequency bands will the output of the mixer
on the Satellite Repeater fall?
Which of these frequency bands is passed by the band-pass filter?
© Festo Didactic 86311-00
109
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
Is the downlink signal higher or lower in frequency than the uplink signal?
Why is this usually the case with satellite repeaters?
7. The Satellite Repeater has one transponder. Which components in the
Satellite Repeater are part of the transponder?
Repeater characteristics
In this section, you will use the spectrum analyzer to measure the power at the
RF INPUT and the RF OUTPUT of the Satellite Repeater. This will allow you to
determine the gain of the Satellite Repeater.
8. Setup the Earth Station Transmitter, the Satellite Repeater and the spectrum
analyzer as shown in Figure 1-70.
Wideband FM
Modulator
Up Converter
1
Up Converter
2
RF
OUTPUT
Earth Station Transmitter
Satellite
Repeater
Spectrum
Analyzer
Figure 1-70. Setup for measuring repeater characteristics.
In order to measure the RF INPUT and RF OUTPUT power of the Satellite
Repeater using the spectrum analyzer, the Satellite Repeater and the
spectrum analyzer must be set up close to each other. There are two ways to
arrange this:
A. If you are using conventional instruments, you can simply set up your
spectrum analyzer near the Satellite Repeater.
110
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
B. Alternatively, you can exchange the Satellite Repeater and the Earth
Station Transmitter, as shown in Figure 1-71, using the following steps:
x
x
x
x
a
© Festo Didactic 86311-00
Disconnect the antenna from the Earth Station Transmitter and both
antennas from the Satellite Repeater.
Set up the Earth Station Transmitter in place of the Satellite
Repeater. Connect the Small-Aperture Horn Antenna to the RF
OUTPUT of the transmitter.
Setup the Satellite Repeater in place of the Earth Station
Transmitter. Connect the Large-Aperture Horn Antenna to the RF
INPUT of the repeater.
Turn on both the transmitter and the repeater. The transmitter will
transmit an unmodulated carrier to the repeater.
In this section, measurements with the spectrum analyzer should be made
without using an external attenuator. It may be necessary to move the tables
further apart in order to obtain the appropriate signal level.
111
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
Earth Station
Receiver
Add-On
Satellite
Repeater
Uplink
Earth Station
Transmitter
Figure 1-71. Setup to observe the spectrum at the input and output of the Satellite Repeater
using the Telemetry and Instrumentation Add-On.
112
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
9. Optimize the antenna alignment of the two uplink antennas using the Power
Sensor LEDs on the Satellite Repeater (refer to Optimizing antenna
alignment).
10. Connect the RF OUTPUT of the Satellite Repeater to the input of the
spectrum analyzer. You can disconnect another microwave cable not
presently used to make this connection.
Wideband FM
Modulator
Up Converter
1
Up Converter
2
RF
OUTPUT
Earth Station Transmitter
RF INPUT
Satellite
Repeater
RF OUTPUT
Spectrum
Analyzer
Figure 1-72. Measuring repeater RF OUTPUT level.
a
If you are using the Telemetry and Instrumentation Add-On, connect the RF
OUTPUT of the Satellite Repeater directly to the Frequency Converter INPUT
of the Data Generation/Acquisition Interface.
The maximum input level of the Frequency Converter is 10 dBm.
On the Earth Station Transmitter, select each Channel in turn and observe
the approximate power level at the RF OUTPUT of the Satellite Repeater.
Make sure that the power for each Channel is below the maximum input level
of the spectrum analyzer. If the power exceeds this maximum, increase the
distance between the two antennas (you may have to move one of the
tables). The objective is to be able to make all measurements without using
an external attenuator.
Fill in the ݂௜௡ and ݂௢௨௧ columns of Table 1-15. (Refer to the uplink and
downlink frequencies you recorded in Table 1-13.)
© Festo Didactic 86311-00
113
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
Table 1-15. Repeater characteristics.
Channel
A
ࢌ࢏࢔ (GHz)
ࢌ࢕࢛࢚ (GHz)
ࡼ࢏࢔ (dBm)
ࡼ࢕࢛࢚ (dBm)
ࡳ (dB)
B
C
D
E
F
11. For each row in Table 1-15, set the Channel on the Earth Station
Transmitter, then observe the spectrum of the RF OUTPUT signal of the
Satellite Repeater. Record the power ܲ௢௨௧ of the unmodulated carrier.
12. Taking care to avoid moving the antenna, disconnect the cables at the RF
INPUT and at the RF OUTPUT of the Satellite Repeater. Connect these two
cables together using an SMA-SMA adapter. This will allow you to observe
the spectrum normally present at the RF INPUT of the Satellite Repeater.
Wideband FM
Modulator
Up Converter
1
Up Converter
2
RF
OUTPUT
Earth Station Transmitter
Satellite
Repeater
SMA-SMA
adapter
Spectrum
Analyzer
Figure 1-73. Measuring the RF INPUT level.
a
Since the microwave cables attenuate the signal, it is important that the total
cable length be the same in Figure 1-72 and Figure 1-73.
If you accidently change the orientation of the antenna, you should repeat
Step 11 before continuing.
For each row in Table 1-15, set the Channel on the Earth Station
Transmitter, then observe the spectrum. Record the power ܲ௜௡ of the
unmodulated carrier.
114
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
13. Calculate the gain ‫ ܩ‬of the Satellite Repeater for each channel and enter
your results in Table 1-15.
‫ ܩ‬ൌ ܲ௢௨௧ െ ܲ௜௡
Plot the gain of the Satellite Repeater versus the input frequency (uplink
frequency) in Figure 1-74.
40
Gain (dB)
30
20
10
0
10.7
10.8
10.9
11.0
11.1
11.2
Input frequency (GHz)
Figure 1-74. Repeater gain.
Is the frequency response of the Satellite Repeater adequate for laboratory
operation over all available system channels?
Describe the ideal frequency response of a real satellite transponder.
What is the approximate gain of the Satellite Repeater (averaged over all
channels)?
14. If you exchanged the Earth Station Transmitter and the Satellite Repeater in
Step 8, replace them in their original positions and reconnect their antennas.
© Festo Didactic 86311-00
115
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
Telemetry with the Satellite Repeater (optional)
In this section, you will use the telemetry functions provided by the Satellite
Repeater. This section requires the optional Telemetry and Instrumentation
Add-On.
15. In the Telemetry and Instrumentation application, select the Telemetry tab.
The Telemetry tab has three zones:
Zone
Description
Channel: Allows selecting one of 16 telemetry channels.
Each Satellite Repeater has a Telemetry Channel selector on
its front panel. Only repeaters using the same Telemetry
Channel as the earth station will appear in the Telemetry tab.
Channel Selection
Update: Click once to continuously update the Channel
Congestion bar graph. Click again to stop.
The Channel Congestion bar graph provides a visual
indication of the current congestion in the selected telemetry
channel. This is useful because the frequency band used for
telemetry is also used by many other devices, such as Wi-Fi
routers. It is preferable to select a telemetry channel with little
congestion.
Available: Lists the available repeaters (repeaters detected
but not owned).
Owned: List the repeaters under control of the earth station.
Satellite Repeaters
Each repeater has a unique ID (the module serial number).
Telemetry between a Satellite Repeater and an earth station
is only possible when that repeater is owned by the earth
station. An earth station can own several repeaters. However,
each repeater can only be owned by one earth station at a
time.
Owned Repeaters
This zone has a tab for each repeater currently owned by the
earth station.
Make sure the Channel setting in the Telemetry tab of the software
corresponds to the selected Telemetry Channel on the Satellite Repeater you
wish to own.
The ID of all repeaters using the same Channel should appear in the
Available list. Select the ID of the repeater you wish to own and click
.
This repeater’s ID will be removed from the Available list and will appear
under Owned. A tab for this repeater will be created in the Owned Repeaters
zone.
a
116
If a repeater using the selected Telemetry Channel is not listed, turn the
repeater off, wait a few seconds and turn it back on. If a software problem has
occurred, exit and restart the Telemetry and Instrumentation application.
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Procedure
16. Examine the tab for the owned repeater. This tab has three zones:
Zone
Power Sensor
Description
Indicates the power level detected by the Power Sensor in
the Satellite Repeater. The green Level LEDs provide a
relative indication of the repeater output power—the same as
the Power Sensor LEDs on the Satellite Repeater. The
measured level in dBm is also shown.
This information is updated once per second. When the
telemetry link is operational, the Level LEDs in the software
and the Activity LED on the repeater flash on and off.
Atmospheric
Attenuation
Provides controls for simulated atmospheric attenuation.
These controls will be used to study atmospheric attenuation
in the manual Link Characteristics and Performance.
Shows the Status and the Redundancy Unit currently in use
for each redundant component in the Satellite Repeater.
Status &
Redundancy
The Status for each of these components is either Pass or
Fail, depending on the instructor-inserted faults in the
Satellite Repeater.
Each component in the table has two redundancy units
identified as “Main” and “Backup”.
The Manage Faults button at the bottom of the zone allows setting faults in
the Satellite Repeater. This function is password protected and allows the
instructor to insert faults and to show or hide the Status column for the
troubleshooting exercises.
The Telemetry Link Power indicator provides a relative indication of the
power level of the telemetry link. At least one bar in this indicator should be
darkened.
17. Slowly turn the antenna connected to the Earth Station Transmitter while
watching the Power Sensor Level displayed in the Telemetry tab. The Level
will change accordingly.
Reorient the antenna connected to the Earth Station Transmitter in order to
maximize the displayed Power Sensor Level.
Why is it important for the control segment to be able to remotely measure
the power output of each transponder in a satellite payload?
© Festo Didactic 86311-00
117
Ex. 1-3 – Satellite Payloads and Telemetry  Conclusion
18. Change the Redundancy Unit for any of the components listed and observe
the Main and Backup LEDs on the Satellite Repeater. Since there are no
faults presently inserted in the repeater, this will not change the displayed
Status.
Explain the purpose of redundancy in a satellite payload and how it is used.
19. When you have finished using the system, exit any software being used and
turn off the equipment.
CONCLUSION
In this exercise, you became familiar with a satellite repeater, which is the
payload of a communications satellite. You studied the functions, characteristics
and organization of repeaters. You measured the gain of the Satellite Repeater
at various frequencies. You also became familiar with satellite telemetry.
REVIEW QUESTIONS
1. What are the main functions of any satellite communications payload?
2. What characteristics are usually desirable in a satellite payload?
3. What is the main difference between a transparent and a regenerative
repeater?
118
© Festo Didactic 86311-00
Ex. 1-3 – Satellite Payloads and Telemetry  Review Questions
4. Explain the purpose of redundancy and how redundancy is generally
implemented in a satellite payload.
5. Explain why two different links are often made available for satellite TTC.
© Festo Didactic 86311-00
119
Download PDF