Exercise Satellite Payloads and · PDF filepayload should receive as little interference as...

© Festo Didactic 86311-00 99

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.

The Discussion of this exercise covers the following points:

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:

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.

Satellite Payloads and Telemetry

Exercise 1-3

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Ex. 1-3 – Satellite Payloads and Telemetry Discussion

100 © Festo Didactic 86311-00

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:

High power gain

Low noise (low effective input noise temperature)

High power output

Large bandwidth

High availability, high reliability and adequate lifetime

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.


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.



Amplifier Variable attenuator

Mixer Band-pass filter

Local oscillator 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 low-noise 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 low-noise 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

Wideband receiver

HPA

Uplink antenna

Transponder (channel)

IMUX OMUX

Downlink antenna

HPA

HPA

HPA

Linearizer

Linearizer

Linearizer

Linearizer

LNA



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.

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:

center frequency

bandwidth

threshold level

saturation point

operating point

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 solid-state 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

Dividing the repeater band-

width into sub-bands, one

for each transponder, is

called channelization.



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.

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

V

H

T1 5945

T3 5985

T5 6025

T7 6065

T9 6105

T11 6145

T13 6185

T15 6225

T17 6265

T19 6305

T21 6345

T23 6385

T2 5965

T4 6005

T6 6045

T8 6085

T10 6125

T12 6165

T14 6205

T16 6245

T18 6285

T20 6325

T22 6365

T24 6405

Uplink Frequencies (MHz)

Downlink Frequencies (MHz)

V

H

T1 3720

T3 3760

T5 3800

T7 3840

T9 3880

T11 3920

T13 3960

T15 4000

T17 4040

T19 4080

T21 4120

T23 4160

T24 4180

T22 4140

T20 4100

T18 4060

T16 4020

T14 3980

T12 3940

T10 3900

T8 3860

T6 3820

T4 3780

T2 3740



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.

Figure 1-65. Dual frequency conversion.

Regenerative repeater

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.

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.

Uplink antenna

Downlink antenna

One of several transponders

LO 1 LO 2

RF

Sw

itch

es

RF

Sw

itch

es

Ba

se

ba

nd

Pro

ce

ssin

g a

nd

Sw

itchin

g

Receiver

Receiver

Receiver

Receiver

Demod.

Demod.

Demod.

Demod.

Mod.

Mod.

Mod.

Mod.

HPA

HPA

HPA

HPA

Control

Uplink Antenas

Downlink Antenas



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.

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

Uplink Antenna

LNA

LNA

IMUX To

Transponders



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.

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).

HPA

IMUX OMUX

HPA

HPA



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)

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.

With the Satellite Communi-

cations Training System,

there is no tracking. For

brevity, all telemetry and

command functions are

referred to using the term

“telemetry.”

C-band omnidirectional antenna

Subreflector

20 GHz downlink antenna

Solar panel array

Steerable phased-array antenna

30 GHz uplink antenna

Ka-band TTC antennas

Ex. 1-3 – Satellite Payloads and Telemetry Procedure Outline


Health and status information may include information such as:

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:

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.

The Procedure is divided into the following sections:

System startup


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 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.

PROCEDURE OUTLINE

PROCEDURE

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:

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 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.


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?



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.

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.

Earth Station Transmitter

Satellite Repeater

Up Converter 2

Up Converter1

Wideband FM Modulator

Spectrum Analyzer

RF OUTPUT



B. Alternatively, you can exchange the Satellite Repeater and the Earth Station Transmitter, as shown in Figure 1-71, using the following steps:

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.

a 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.



Figure 1-71. Setup to observe the spectrum at the input and output of the Satellite Repeater using the Telemetry and Instrumentation Add-On.

Satellite Repeater

Earth Station Receiver

Add-On

Uplink




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.

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 columns of Table 1-15. (Refer to the uplink and downlink frequencies you recorded in Table 1-13.)


Satellite Repeater

Up Converter 2

Up Converter1


Spectrum Analyzer

RF OUTPUT

RF OUTPUT RF INPUT



Table 1-15. Repeater characteristics.

Channel (GHz) (GHz) (dBm) (dBm) (dB)

A

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 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.

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.


Satellite Repeater

Up Converter 2

Up Converter1


Spectrum Analyzer

RF OUTPUT

SMA-SMA adapter



13. Calculate the gain of the Satellite Repeater for each channel and enter your results in Table 1-15.

Plot the gain of the Satellite Repeater versus the input frequency (uplink frequency) in Figure 1-74.

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.

0

10

20

30

40

10.7 10.8 10.9 11.0 11.1 11.2

Input frequency (GHz)

Gain

(dB

)



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 Selection

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.

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.

Satellite Repeaters

Available: Lists the available repeaters (repeaters detected but not owned).

Owned: List the repeaters under control of the earth station.

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 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.



16. Examine the tab for the owned repeater. This tab has three zones:

Zone Description

Power Sensor

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.

Status & Redundancy

Shows the Status and the Redundancy Unit currently in use for each redundant component in the Satellite Repeater.

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?

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.

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.

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?

CONCLUSION

REVIEW QUESTIONS

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.

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