failure modes and troubleshooting techniques in satellite communications

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Slide Number 1Rev -, July 2001



Contents

4.8.1.1 Information Gathering4.8.1.2 Test Equipment Required

Vol 4: Earth Stations

Sec 8: Failure Modes & Troubleshooting Techniques

4.8.1: Getting Started

There are two scenarios when considering troubleshooting: problems encountered with in-service systems and those encountered when commissioning new equipment.

Generally new equipment problems will be minimal because the contractor installing the new equipment will have provided an operational system. The following information is therefore directed more towards the in-service type of problem, however the same techniques apply to both.



Information GatheringGathering information is like looking before leaping.

When troubleshooting complex problems, extra time spent investigating information details can save hours of time in the troubleshooting process. Some questions to be asked are:

• What time did it happen? When & Where & How!• Has it happened before?• Is it equipment related, temperature related or operator error

(finger trouble) related? Is it intermittent or constant?• Are you dealing with the person who knows the most about a

problem? Details passed from person to person always get distorted.

• Are you spending time on the most important aspects of the problem, or on unimportant symptoms?

4.8.1.1: Information Gathering

Vol 4: Earth Stations, Sec 8: Failure Modes & Troubleshooting Techniques

Part 1: Getting Started



Test Equipment RequiredA basic troubleshooting kit for any technician or engineer comprises the following:

• A Spectrum Analyzer, a tool kit and a DVM. RF adapters (SMA, N-type, etc).

From there, the basic kit plus specialized equipment requiring servicing, dictates what equipment is required such as:

• Data Problems - Breakout Box, BERT set, maybe an Oscilloscope, various test data cables, data adapters

• Voice Problems - TIMS set, BERT set, maybe a Telephone or Oscilloscope, 310, Bantum, RJ11, RJ12, RJ45 test cables

4.8.1.2: Test Equipment Required





Test Equipment Required• Video Problems - BERT set, Video Analyzer, video cables

• HPA Problems - RF Sweep Generator, Scalar Analyzer, Power Meter, high voltage probes, special high frequency low loss RF test cables

• Frequency Converter Problems - Frequency Counter, Power Meter, special high frequency low loss RF test cables

• LNA - Basic kit, special high frequency low loss RF test cables

As can be seen from above, a collection of specialized test equipment is required to correct various equipment problems.

4.8.1.2: Test Equipment Required





4.8: Failure Modes & Troubleshooting TechniquesVol 4: Earth Stations

No Receive SignalPart 2



No Receive CarrierAlthough this section assumes no carrier is being received, similar troubleshooting techniques should be employed if there is a degraded carrier exhibiting characteristics such as a high BER or noisy video signal.

For problems with data, such as synchronization loss etc., see section 4.8.4 Data Problems.

A no Rx Carrier indication can be caused by items such as:• Transmit Carrier down from the Transmit Site

• Heavy rain or thunderstorm activity (especially with Ku-Band)

• Modem, Downconverter or LNA turned off

• No AC power or blown fuse within the Modem, Downconverter or LNA

4.8.2.1: No Receive Carrier


Part 2: No Receive Signal



No Receive Carrier• RX frequency incorrectly set on Modem/Downconverter

• RX IF output disconnected from the Modem

• Downconverter has a Phase Lock Alarm

• IFL cables not securely connected

• Heliax, waveguide or Flex not properly secured at the antenna

• Water in any of the outdoor connections. Are all the outdoor connections properly weatherized and used with proper gaskets?

4.8.2.1: No Receive Carrier





Equipment FailuresVerify that each piece of equipment is working normally, with all displays and equipment alarms normal. It is possible that there will be performance alarms e.g. low Eb/No, which indicate a fault in the system prior to it. Some equipment provides a history of alarmed events (can be useful).

Power Supply Failures

• The most common way to view a power supply failure is monitor for the presence of LED summary alarms or Digital readouts, if so equipped.

• If neither of these test points are available, a DVM should be used to check all Power supplies.

• Don’t forget to check for ripple on DC power supplies.

4.8.2.2: Equipment Failures





Equipment FailuresPower Supply Failures

• Check AC input - Is Fuse blown?

• Is AC breaker tripped?

• Check DC power supply outputs?

• Are all DC voltages present?

These simple tests should determine if a power supply has failed.

Apply proper corrective action to resolve problem.

4.8.2.2: Equipment Failures





LNA Failures and All Receive FaultsAn all receive fault occurs when the complete receive spectrum is lost.

Typically, if the LNA fails, an all receive fault is activated.

In some Earth Stations LNA’s have no alarm points, so other equipment summary faults are combined to generate the all receive fault indication. Should all the equipment summary faults indicate simultaneously, an All Receive Fault is active.

4.8.2.3: LNA Failures and All Receive Faults





LNA Failures and All Receive FaultsPossible sources of an “All RX Fault” are:

• LNA defective

• LNA power supply defective

• LNA power supply fuse blown or AC power not present

• RF cable/connector problem

• Receive splitter problem

• Antenna misaligned or damaged

When troubleshooting an “All Rx Fault” always verify the spectrum with an analyzer first. A transponder failure could cause a “All Rx Fault” if the Earth Station is only operating on one specific Transponder. In this case the LNA may still be operating but a fault condition of course does exist.

4.8.2.3: LNA Failures and All Receive Faults





RF/IF LevelsCompare with Block & Level Diagram

The best way to ensure proper levels is to verify the signals against a Block & Level Diagram.

A Block & Level diagram will detail levels and impedances. Sometimes spectrum analyzer values may be detailed as well.

If levels are not known, it is always worthwhile to check with the equipment manual to determine optimal manufacturer’s level settings.

IF and RF Cable losses can be significant and must be considered when using block and level diagrams. For instance, when a level is shown at one end of a cable, this can change significantly if the other end is measured.

4.8.2.4: RF/IF Levels





RF/IF LevelsToo High/Too Low

High RF/IF levels can cause various problems such as:

• Compression

• Intermodulation

• Equipment Malfunction

Compression and intermodulation caused by high RF/IF levels cause equipment degradation and sometimes equipment damage. Typically, if the high level is in the transmit chain, these problems affect the communications system and cause problems to other satellite customers.






RF/IF LevelsLow RF/IF levels can cause equipment to malfunction or simply operate in a degraded mode. Low RF/IF levels in the transmit chain can cause low C/N in the receive Earth Station which in turn causes high Bit Errors/Sync problems and higher susceptibility to noise problems.

Low RF/IF levels in the receive chain can cause marginal equipment performance and signal dropouts. Typically the demodulator is outside the range of its operational specifications or beyond its threshold and either works in a degraded fashion or stops working altogether.






Cable & Connector Problems

Cable & connector problems appear to be simple problems. In fact they can often create hours of troubleshooting, as these problems are usually intermittent.

Remember, never assume anything as simple as a cable or connector as being above suspicion in the troubleshooting process.

A quick way to check for cable problems is to monitor the signal at each end of a suspect cable.


4.8.2.5: Cable & Connector Problems




Cable & Connector Problems

Vigorously wiggle all cables and connectors. This will identify if an intermittent connection is present. Intermittent connectors can be identified as large signal variations during the wiggling process.

Chances are the cable is good but the connector is not. Verify that the correct pins have been used in the faulty connector.

Is the proper cable type used for the signal you are investigating? Trying to insert an RF signal through an IF cable doesn’t work!


4.8.2.5: Cable & Connector Problems




Antenna ProblemsAntenna not peaked

Antenna problems are most likely to arise during the commissioning of a new system.

Typically, problems affect the output signal strength of the antenna, such as:

• Antenna off peak or misaligned

• Subreflector misaligned

• OMT incorrectly tuned or damaged or the TX & RX ports may be reversed

• Polarization not correctly set

• Antenna parabola warped or damaged


4.8.2.6: Antenna Problems




Antenna ProblemsAll of these problems interact with each other and affect sidelobe performance, antenna gain and cross polarization.

Simply repeaking an antenna and resetting crosspole will eliminate most antenna problems.

Subreflector alignment requires specific manufactures measured position information. With this information it can be determined if the subreflector is out of position.

Antenna misalignment is possible with multi-piece antenna construction. A theodolite or string test can correct these situations, depending on the type of antenna.






Antenna ProblemsString Test

S trings

A ntennaS eam s

A ntennaFace

Bad: S tring sco llid e a t cen te r

Bad: S p ace be tw eens trin gs a t ce n te r

Good: S tring sju s t to uch a tcen te r

Vertical S tring

Horizontal S tring(cross sectional view)

S tring D eta il

O p tionalS trings

Image Courtesy of Telesat Canada

Figure 4.8.2.6 String Test






Sun TransitsSun transits occur when the sun crosses the Earth's equatorial plane during the spring and fall equinoxes (late February or early March; late September or October). At these times, the sun aligns directly behind the satellite for a few minutes of the day.

When the sun moves directly behind the satellite, the satellite signal can be overwhelmed by the enormous amount of thermally generated radio frequency (RF) noise radiated by the sun. This can cause reception interference for a few minutes every day during this occurrence.

Typically the voice, data or video services become degraded and at times can be completely unusable. Being unaware of Sun Transits could instigate trying to troubleshoot a problem that does not exist.

Do not be fooled by Sun Transits. They are a natural phenomenon that occur twice a year.


4.8.2.7: Sun Transits




Satellite Problemsa) Satellite Spin Up

b) Transponder Trip Off

c) Transponder Compression

d) Adjacent Satellite Interference or Unwanted Carriers (DI)

Note: although it is of course possible that a satellite problem can develop, this is an extremely rare occurrence and should not be suspected until other possible sources of a problem have been eliminated.


4.8.2.8: Satellite Problems




Satellite ProblemsSatellite Spin UpA Satellite spin up occurs when a problem on the satellite has occurred with respect to the Attitude Control System and Earth/Sun sensors.

When the satellite looses lock for some unknown reason the satellite begins to rotate in a 360 degree motion. As this occurs, all Earth Stations pointing to the satellite have their signals fade away until nothing but noise is left. Eventually, after some period of time from several minutes to hours as the satellite remains in a spin mode, its antenna comes back into alignment and the signal reappears. Just as the Signal has normalized, it begins to fade again only to repeat this process until the satellite control system reacquires proper lock.

This type of signal fade is the result of a satellite spin up. This is a very rare occurrence on today's satellites.






Satellite ProblemsTransponder Trip Off

Occasionally a transponder trips off, meaning that the satellite transponder has stopped transmitting.

This problem may be a result of several on board parameters reaching trip points. When a trip point is activated, the transponder is inhibited to prevent a catastrophic type transponder failure. These are safety measures to ensure that the transponder will not destroy itself and fail.

These trip points may be activated as a result of satellite thermal conditions changing.






Satellite ProblemsA transponder trip off condition can make some of the Earth Station equipment look faulty.

If you suspect it’s a transponder problem, a quick check with the satellite operator can determine if this has occurred or a peak at the satellite spectrum if you know what to look for.

This type of fault is not a common occurrence but typically happens more frequently than a satellite spin up.






Satellite ProblemsTransponder Compression

Transponder compression occurs when all the available power within the satellite Transponder has been exceeded and is beyond its proper OPBO rating (over 100% utilization).

This problem may be caused by a single user transmitting more power to the satellite than normal.

It can be caused by too many users operating their carriers at too high a level.

A transponder in compression is operating in a non-linear mode and therefore intermodulation products will be generated which will interfere with the wanted carriers.






Satellite ProblemsThe noise floor on the transponder is thus higher, and Eb/No values of data modems are much lower. This results in degraded services on the transponder.

If this occurs, all transmit carriers must be checked for proper OPBO carrier level settings. The objective will be to reduce all carriers transmitting higher than their OPBO rating. If the transponder is still in compression mode after this check, some carriers will have to be transferred to another transponder.

Remember, in multicarrier mode, SSPAs and TWTAs cannot be operated beyond their 3 and 7 dB OPBO respectively, without causing problems.






Satellite ProblemsAdjacent Satellite Interference or Unwanted Carriers (DI)

Adjacent satellite Interference can be caused by:• Antenna not 2 degree compliant (sidelobes out of spec)• Earth Station antenna on adjacent satellite not 2 degree compliant

when transmitting signals• Earth Station antenna on adjacent satellite transmitting too high a

carrier• Adjacent satellite is outside its 0.1 degree box (unlikely)

DI represents Double Illumination or two carriers transmitting on the same frequency.

This scenario makes this frequency unusable.







No Transmit CarrierPart 3



No Transmit Carrier

A no TX Carrier indication can be caused by items such as:• Modem, Upconverter or HPA turned off• No AC power or blown fuse within the Modem, Upconverter or HPA• TX Frequency incorrectly set on Modem/Upconverter• TX Carrier inhibited within the Modem or HPA• TX IF output disconnected from the Modem• Upconverter has a Phase Lock Alarm• IFL cables not securely connected• Heliax, waveguide or Flex at the antenna not properly secured• Water in the waveguide/connectors. Are all the outdoor connections

properly weatherized and used with proper gaskets?

Note: Some data equipment may mute the carrier if certain signals from the data interface disappear, which in turn will drop the IF/RF carrier.

4.8.3: No Transmit Carrier





HPA ProblemsTypical HPA problems are:

• No RF input

• HPA inhibited due to summary alarm, external MAC or interlock

• Low/high voltage power supply problems

• Main AC supply not present, one or all phases






HPA ProblemsTypical Klystron problems are discussed in section 4.5.4.1 Klystron Faults which include: air flow alarms, arc detector, high temperature alarms, high current, low gain, low beam power supply voltage, or a “gassy” tube resulting in a high body current alarm.

Typical TWTA problems could be high voltage power supply problems, high helix current (beam needs to be focused better), low gain, TWTA oscillation (preventable by increasing attenuation in the helix support structure), or high intermodulation (TWTA gain too high for multicarrier operation)

Typical SSPA problems are usually related to DC power supplies or a GaAsFet Transistor failure, and either creates lower output or no output power.







Data ProblemsPart 4



Data ProblemsData problems are not always simple to solve, but usually it comes down to the physical interface having not been wired correctly.

It is important to note, DCE to DTE connections are straight through pin for pin connections. DCE to DCE or DTE to DTE may require special pinning or null modem type adapters.

Sometimes the equipment requires proper handshaking, such as RTS, CTS and DSR, DTR to send and receive data properly. If this is the case, experiment by connecting these pins high. Data should begin to flow.

If Data buffers are not installed sync hits may occur daily due to the Satellite motion or Doppler shift (correct buffer size is important).

4.8.4: Data Problems





2 TxD

3 RxD

4 RTS

5 CTS

6 DSR

8 DCD

20 DTR

Com puter Modem

2 TxD

3 RxD

4 RTS

5 CTS

6 DSR

8 DCD

20 DTR

Com puterModem

Note: This drawing refers to a RS232 DB25 type data interface


Figure 4.8.4a Handshaking






Data ProblemsClocking Issues

If data is flowing but high BER’s or sync hits are occurring, chances are it’s a clocking issue. Clocking must be sourced from only one location in normal telecommunications networks. Multiple clocks tend to cause sync problems.

Refer to the drawings on the next two slide for the different clocking options.

Note:

• These examples use a DB25 connector with RS232 signaling. Pins will differ for other type data interfaces.

• Options 3 & 4 are the most commonly used timing options.







Figure 4.8.4ba Timing Scenarios







Figure 4.8.4bb Timing Scenarios




failure modes and troubleshooting techniques in satellite communications

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