Earth Station Maintenance in Satellite Communications

Slide Number 1Rev -, July 2001

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

4.7: Earth Station Maintenance ConsiderationsVol 4: Earth Stations

Principle Test EquipmentPart 1



Vol 4: Earth Stations

Principle Test Equipment

4.7: Earth Station Maintenance Considerations

Part 1



Contents

4.7.1.1 The Spectrum Analyzer4.7.1.2 The Scaler Analyzer4.7.1.3 The Power Meter4.7.1.4 The Bit Error Rate Test Set (BERT)4.7.1.5 The Oscilloscope4.7.1.6 The Frequency Counter4.7.1.7 Peripheral Equipment


Sec 7: Earth Station Maintenance Considerations

4.7.1: Principle Test Equipment



The Spectrum AnalyzerSpectrum Analyzers (SPA) are the instrument of choice for making frequency-selective measurements during satellite system alignment and troubleshooting.

Vol 4: Earth Stations, Sec 7: Earth Station Maintenance Considerations

Part 1: Principle Test Equipment

4.7.1.1: The Spectrum Analyzer

Typical UseThe SPA is typically used for peaking antennas, setting specific carrier levels and frequencies, checking intermodulation, looking for spurious signals, determining noise levels, verification of signals, and troubleshooting.

SPA’s display amplitude characteristics in the frequency domain. Some are optioned with demodulators for working with complex signals. These instruments are single channel devices with receive only capability and cannot measure phase information.

Figure 4.7.1.1a The Spectrum Analyzer

Photo Courtesy of Telesat Canada



The Spectrum AnalyzerMeasurement Accuracy Proper understanding and use of SPA’s is essential to making meaningful measurements.

The accuracy of a SPA could be as high as ±3dB over a broad frequency band, as it is a very wideband device. Narrowband measurements are more accurate.

Power level results may have to be corrected to true RMS values depending on the nature of the signal (modulation type, signal bandwidth etc.)

Power level corrections may have to be considered due to coaxial cable losses and frequency response characteristics introduced by using a connecting cable for measurement.






The Spectrum AnalyzerCalibrationThere are two kinds of calibration: field and factory.

Always perform the required field calibration procedure on the SPA before use, as detailed by the manufacturer. Some require the use of an external cable to patch a calibrated output signal to the SPA input port, while others automatically calibrate on the power up sequence.

In addition to the field calculation, the manufacturer will specify a schedule for a more extensive calibration routine to be performed at a qualified laboratory. Such a calibration is usually called for annually. The calibration of the equipment used at the laboratory must be traceable back to universal standards.






The Spectrum AnalyzerAvoiding Measurement ErrorSignificant errors in measurements can be avoided if the following points are noted:

• Ensure correct calibration, both field and factory• Use the correct cable for the proper RF frequency • Check cable integrity by looking for loose connectors• Optimize the SPA so the front end is not overdriven (compression)• To maintain power level accuracy, avoid impedance mismatch,

distortion products, amplitudes below the Log amplifier range, signals near noise, noise causing amplitude variations, two signals not being completely resolved

• Always use an appropriate sweep speed, as the SPA trace display becomes distorted with certain settings (whenever possible, allow the SPA to automatically assign a sweep speed setting)






The Spectrum AnalyzerAvoiding Measurement Error

• Use a front-end attenuator setting sufficient to prevent overloading

• Use the lowest appropriate I/P attenuator setting to minimize impact of SPA input noise

• On low signal level measurements always confirm the impact of SPA input noise on the measurement results

• Where practical, use sweep averaging to average out carrier and noise fluctuations

• If using Max Hold be aware of noise integration effects on measurement results






The Spectrum AnalyzerDevice ProtectionMost SPA’s have zero tolerance for DC voltage on their input port.

Most L-Band signal distribution systems feed 18 or 24 VDC on the center coax conductor to power the LNB. If a SPA is connected to such equipment without the use of a DC Block, front end damage will occur (typical repair costs could be $3K to $5K US).

Some SPA’s can be ordered with optional DC Block protection but this may compromise RF accuracy.Best solution may be to use a external DC block whenever working on L-Band equipment with DC voltage on the center coax.

Maximum RF input is approximately +30 dBm (1watt). Higher levels must be padded down before applying these signals to the SPA. Vol 4: Earth Stations, Sec 7: Earth Station Maintenance Considerations





The Spectrum AnalyzerResolution Bandwidth

What is the Bandwidth of a CW Carrier?

The 3 SPA measurements below of the same CW carrier were made using same SPA setup with the same settings except that different resolution bandwidth settings were selected.




3 Khz RSB 10 Khz RSB 30 Khz RSB

Figure 4.7.1.1b Resolution BandwidthImages Courtesy of Telesat Canada



The Spectrum AnalyzerResolution Bandwidth




Carrier Hidden Carrier Resolved

Figure 4.7.1.1c Resolution Bandwidth

Images Courtesy of Telesat Canada



The Spectrum AnalyzerCW C/N Measurements




Carrier level = -54.5 dBmNoise level = -78 dBm/10Khz

C/N = 24 dB/10Khz

Marker noise on = -116.3 dBm/Hz

C/No = 61.8 dB/Hz

Figure 4.7.1.1d CW C/N Measurements




The Spectrum AnalyzerCalculating SPA No

Calculation of No from the marker value (-78 dBm from previous slide) can also be done based on the following:

No = NSPA - 10 Log (RSB x 1.2) + 2.5dB

Where NSPA = SPA marker value of the noise floor

RSB = SPA resolution bandwidth

No = -78 - 10 Log (10000 x 1.2) + 2.5dB

= -78 - 40.8 + 2.5

= -116.3 dB/Hz






The Spectrum AnalyzerCW C/N Measurements




Apparent C/N = 28.5 dB

Apparent C/N = 18.5 dB

3 Khz RSB 10 Khz RSB

30 Khz RSB

Apparent C/N = 24 dB

Figure 4.7.1.1e CW C/N Measurements




The Spectrum AnalyzerCW Carrier MeasurementsUse standard SPA setup techniques such as using a Video Bandwidth (VBW) of < 0.01 x RBW to provide proper trace smoothing. The operator would normally use the Sweep Average function set to approximately 5 sweeps to further stabilize the level of the measurement result. Max Hold may be used if the operator wants to record the carrier maximum value over the measurement period.Use the marker set to the peak of the desired carrier to measure signal level and frequency.The SPA carrier level is a combination of C+N. If possible, make SPA adjustments to achieve a C/N of 15 dB or better so that the noise to the carrier level is insignificant.









The Spectrum AnalyzerDigitally Modulated Carrier Measurement Using a SPA to directly measure the signal power of a digitally modulated carrier can normally not be done, as there are several SPA bandwidth and signal response issues that lead to errors in results.

As shown on the next slide, the displayed carrier power for RBW reductions reduces the displayed carrier power on a 10 Log BW basis.

The SPA Envelope Detector and Log Amp also respond differently to digitally modulated (noise like) signals compared to their response to a CW signal.

The SPA displays these signals 2.5 dB lower than actual signal power.



The Spectrum AnalyzerCW Carrier vs Digital Modulated Measurements




56 Ks carrier SPA RBW

30 KHz

56 Ks carrier SPA RBW 1.0 kHz

56 Ks carrier SPA RBW 10 kHz

CW Level Reference

Note variations in displayed carrier level

Figure 4.7.1.1f CW Carrier vs. Digital Modulated Measurements.




The Spectrum AnalyzerManual Digital Carrier Power Measurement Technique

• Set the SPA RBW to a value which is < 20% of the carrier bandwidth.

• Average out the variations inherent in the noise-like signals by using a SPA VBW setting that is less than 0.01 x RBW. Sweep averaging may also be employed for the same purpose.

• Place the SPA marker on the flat portion on the top of the digital carrier and select the Marker Noise On function. The displayed marker value will be the noise power at the marker frequency expressed in dBm/Hz.

• Switch off the Marker Noise function.






The Spectrum Analyzer• Using the delta marker function record the 3 to 6 dB

bandwidth of the carrier being measured and calculate carrier power as follows:

Carrier Power = Marker Noise Power + 10 Log10 (Carrier Bandwidth in Hz)






The Spectrum Analyzer

Max Hold versus Video Averaging




In both casesC/N = 24 dB

Observed C/N = 22.5 dB

Video Averaging Off Video Averaging On

Max hold for 15 sec

Figure 4.7.1.1g Max Hold vs. Video Averaging





Measuring Other Types of CarriersMeasuring FM video modulated carriers, VSAT TDMA carriers, and DAMA voice activated carriers, all require different measurement techniques.

The SPA Max Hold function is good for measuring power levels for FM carriers but not good for C/N measurements.

Measuring TDMA carriers is difficult because multiple sites burst in the same time slots. Specific measurements require timing information from a NCS Hub. The Hub is usually designed to measure this information much more easily.

DAMA Carriers require frequency information from a NCS, as DAMA carriers are Demand Assigned to frequencies with a given range specified within the NCS. The NCS for a DAMA system is designed to measure DAMA carriers.







Measuring other types of carriersMeasurement of voice activated carrier power has its own issues, as the SPA measurement device normally does not know when the carrier is active or inactive.

Due to sweep speed and measurement characteristics, the traditional SPA does not respond well to varying or intermittently active levels. Capturing the maximum carrier envelope using Max Hold can be done, however this has an impact on level measurement accuracy as only signal (and noise) peaks are recorded.






The Spectrum AnalyzerSensitivity & SPA Input NoiseModern SPA’s have an Input Attenuator which is normally selectable in 10 dB steps between 0 and 70 dB.

The normal minimum setting for this attenuator is 10 dB. Selecting the 0 dB position will improve SPA sensitivity by 10 dB, however the SPA input VSWR will be degraded significantly resulting in possible frequency response degradations that can compromise measurement results. The possibility of SPA input stage overload is also increased.

Using the normal 10 dB setting is suggested for most satellite system receive signal measurements.






The Scaler AnalyzerNetwork analyzers are instruments that measure transfer and/or impedance functions of linear networks through sine-wave testing.

A Network Analyzer measures components, devices, circuits and subassemblies, either active or passive.



4.7.1.2: The Scaler Analyzer

Figure 4.7.1.2 Scaler Analyzer




The Scaler AnalyzerA Scaler Network Analyzer measures insertion loss, gain, return loss, SWR and power with the aid of high performance detectors and directional bridges.

With multiple inputs and two-channel displays, simultaneous ratioed and non-ratioed measurements of amplitude and phase can be displayed through frequency or power sweeping.

These instruments measure known signals, as opposed to spectrum analyzers that measure unknown signals.



4.7.1.2: The Scaler Analyzer



HP Power meters use external power sensors. Each power sensor is rated for frequency coverage, 50 or 75 ohm Impeadance, Power maximum & range. They employ a sensing element (thermistor, diode, thermal converter or thermocouple) that converts the RF power to a measurable DC or low frequency voltage.



4.7.1.3: The Power Meter

The Power MeterTypical Use A power meter can measure composite power or CW at a HPA output sample port, or any composite level required on a TX or RX chain.

Power meters cannot read individual frequencies (they are not frequency selective).

Figure 4.7.1.3a The Power Meter




The Power MeterCare must be taken not to exceed the power capability of the sensor head or damage will occur.

AccuracyMeasurement accuracy is much better than a SPA (typically ±0.25dB).

Power sensors must be calibrated, some with a supplied 30 dB reference attenuator.

Proper power sensor calibration factor must be applied.

Note that when using sensitive power heads in the range of -70 to -20 dBm and making measurements below -50 dBm, extreme care must be taken in zeroing the power sensor. If not zeroed properly, inaccuracies can easily be introduced, creating errors in measurement results.






The Power MeterPower Sensor Heads

It is important to select the correct sensor when measuring power levels.




CDMA, TDMA, GSM, and Radar may require the use of specialized sensor heads due to their complex modulation schemes.Other sensor heads are designed to measure CW signals.The correct head must be used for the proper signal type, correct power region and frequency, or incorrect measurement readings may result.The correct head must be used with the proper meter as well.Backwards and Forwards compatibility between power sensor heads and power meters is not always assured.

Figure 4.7.1.3b

Power Sensor Heads




The Bit Error Rate Test SetPurpose & Typical Use

A Bit Error Rate test set is a test instrument for analyzing, evaluating and troubleshooting digital communications systems and equipment.

BER test sets can operate as low as 50 bps to 56 Mbps.

Most sets offer various interfaces, such as RS-232/V.24, EIA-530, V.35/306, X.21, RS-449/422/423, MIL-188C, T1/FT1, DDS, E3 and T3 services, ATM, Frame Relay.

Most sets support standard loopback, clock recovery, clock/data inversion tests, balanced and unbalanced modes.

Most sets feature software-selectable DTE and DCE emulation.



4.7.1.4: The Bit Error Rate Test Set



Carrier Size Pseudo-Random Pattern(information rate bits) Length (bits)

64 k 2047(211-1)192 k 2047(211-1)384 k 2047(211-1)512 k 2047(211-1)

1024 k 2047(211-1)1544 k 215-12048 k 215-16312 k 215-18448 k 215-132064 k 215-134368 k 223-144736 k 215-1

Test Patterns According to O.151 & O.152 Recommenadtions

The Bit Error Rate Test SetA BER set allows complete flow-control troubleshooting, with user-controllable signaling leads (CTS, RTS, DTR, and DSR), LED status indicators, and CTS-RTS delay measurement.

To measure the error rate accurately, a sequence of bits simulating real data is transmitted at a rate equal to the transmission rate.

This pattern from the BER Test set is a Pseudo-Random Bit Sequence (PRBS).






The Bit Error Rate Test SetThe PRBS is then compared with the one generated at the receiver and the ratio of detected mismatched bits to the total number of bits is calculated as the Bit Error Rate.

The pseudo-random bit sequence must adhere to ITU-T recommendations O.151 and O.152 to ensure compatibility between equipment.

The length of test patterns is selected according to the recommended ITU-T test pattern for different transmission rates.




Figure 4.7.1.4a Acterna FIREBERD ® 6000

Image Used by Permission



The Bit Error Rate Test SetTo configure a BERT you will need to know a few parameters to set up the instrument properly

1) Clocking mode – internal or external2) Frequency Synthesizer if clock set to internal clock mode3) Emulate DTE or DCE4) Interface type – set up for sync or async and correct interface5) Test Mode or pattern – selects the type of test such as:

Mark, Space, 1:1, 63, 511, 2047, 215-1, 220-1, 223-1, QRSS, PRGM, FOX






The Bit Error Rate Test SetReceiver functions can monitor bit error, sync losses, clock slips, blocks, block errors, time delay, receiver and transmitter frequency, timing jitter, T carrier alarms like Bipolar violations, frame errors, CRC errors, alarms such as power loss, signal loss yellow alarm, excess zero seconds, AIS seconds, and so on.

Round trip delay measurements should only be made using 63, 511, 2047, 215-1, 220-1, 223-1, pseudorandom patterns. Using other patterns will result in erroneous results, and using the QRSS pattern will yield results that are 20.5 bits periods longer than the actual delay.






The Bit Error Rate Test SetThe pattern must also be of sufficient length to ensure that the longest possible delay will be measured. But on the other hand, a pattern that is too long may result in the user waiting longer than may be required. The proper block length is necessary when measuring delay on satellite circuits.

The BER test set is generally used to confirm the satellite link is good, equipment is functioning properly, and the end user has reliable communications.

A BER test can identify low carrier levels, converters with phase noise or synthesizer problems, digital modem problems, and sometimes power problems, jitter, or level variations.






The Bit Error Rate Test SetBER testing is also used to ensure that the BER QOS specification for a given circuit is being met.

When a specific BER figure is to be confirmed, the length of the BER test becomes important. Specifically, the test must be run long enough for a sufficient number of bits to be passed.

For example, suppose a BER QOS threshold of 1 X 10-7 is to be demonstrated by a BER test. To properly validate the service, the test must run long enough to ensure that 107 bits have passed through, giving sufficient opportunity for 1 bit to arrive in error.

For a 56 kbps circuit, the test must run 1 x 107 / 56,000 = 178.57 seconds, or just about 3 minutes. A shorter test would not properly confirm that the circuit in question actually met its BER performance criteria.






The OscilloscopeOscilloscopes have become one of the most important and most used test equipment tools.

Questions now arise: Analog or Digital or both, how much bandwidth, how much sampling speed, how many channels, how much memory, what kind of triggering, what types of probes?

Based on the user’s requirement the appropriate scope can be purchased.



4.7.1.5: The Oscilloscope

Figure 4.7.1.5

Oscilloscope




The OscilloscopeOscilloscopes are used for tracing and debugging all kinds of problems from logic, to power supplies, to high frequency switching, to noise, to solving leakage and hum problems.

With the correct equipment an oscilloscope will let you see problems you could otherwise never understand.

In satellite communications they are used for repairing equipment, looking at eye patterns of digital modulators, looking at power supply rails for excess ripple effects, to troubleshooting clocking and data problems on digital interfaces.

Oscilloscopes can be seen as complementary with spectrum analyzers in that they present amplitude information in the time, rather than frequency, domain.



4.7.1.5: The Oscilloscope



The Frequency CounterFrequency Counters are primarily used to measure frequency, while others include the measurement of phase, time interval signal characteristics, and some even power levels.

Frequency counters become useful in measuring the frequency offset caused by the multiplication factor of a local oscillator used in an up- or down-converter. A 5 MHz signal off by 1 Hz represents a 1200 Hz offset at 6 GHz. A frequency counter could be used to correct this offset providing the frequency counter’s own internal oscillators are calibrated correctly.



4.7.1.6: The Frequency Counter

Often frequency counters are connected externally to GPS receivers or to very stable external cesium controlled clocks to ensure no errors occur in frequency measurements.

Figure 4.7.1.6 Frequency Counter




Peripheral EquipmentDigital Volt Meter (DVM)Digital Voltmeters have become inexpensive and a must for the toolbox.DVMs offer measurements for:• AC Volts - for checking for prime input power or other

voltages• DC Volts - for checking DC power supplies and test

points• AC Amps • DC Amps• Ohms for checking resistances in cables or

components in non energized states• Diode tester for checking diodes or continuity checks

for opens and shorts • Other options such as scopes, frequency counters,

capacitance etc are available.



4.7.1.7: Peripheral Equipment

Figure 4.7.1.7a Digital V

olt Meter (D

VM

)-Fluke MeterPhoto Courtesy of

Telesat Canada



They come in a combination of connectors such as TNC, BNC, SMA, N-Type etc with 50 or 75 ohm impedances.

Typical fixed pads come in 1, 2, 3, 6, 10, 20, 30 dB values.

Variable pads are available, but they are more expensive and not as popular. Variables typically come in 0 to 1 dB in 0.1dB steps, 0 to 9 dB in 1 dB steps, and 0 to 100 dB in 10 dB steps.




Peripheral EquipmentPAD Set

Pads are sometimes very useful in setting up proper levels.

Typical Pads are IF or RF type.

Figure 4.7.1.7b

PAD Set




Peripheral EquipmentMost Pads have frequency rolloff characteristics. This is more of an issue at RF frequencies as the attenuator value can change for the upper or lower frequency band.

Pads are typically used for multiple level matching IF/RF inputs/outputs or when onboard equipment attenuation has limited adjustment.

DC Blocks

DC Blocks are used to block DC voltages on the center coax conductor.

DC blocks may look like a small Pad with no terminals for connecting a DC power source. These are true DC blocks.






Peripheral EquipmentBias Insertion units are DC blocks that are capable of inserting or removing DC voltages from the center conductor of a coaxial cable.

DC blocks or Bias Insertion Units are frequency sensitive.

A DC block good for C-Band may not be used for Ku-band

Figure 4.7.1.7a DC Block

DC B lock




Image Courtesy of Telesat Canada



Peripheral EquipmentSome DC Blocks may cover both bands

BIU/DC blocks are required to remove DC voltages from the coaxial line when equipment such as spectrum analyzers, splitters, and power meters are to be connected. This equipment is not designed to handle DC voltages on the center coax.

+15 VDC

To LNB withDC voltage

To Receiver ortest equipm entNo DC voltage

BIU/DC B lock

Figure 4.7.1.7b BIU/DC Block







Peripheral EquipmentTest CablesTest cables are specifically manufactured for RF testing and are designed to provide low loss and very good frequency response.

A good test cable comes equipped with a data sheet outlining the frequency response characteristics and its associated insertion loss and its VSWR.

Specific uses for these cables are for frequency tuning Klystrons or other microwave components, critical gain measurements of microwave components and other frequency/gain type measurements.

Other specialty cables exist for Data patch panels, Audio patch panels, Video patch panels, BIX interfaces, and so on. These cables are equipped with unique connectors so that access may be gained to a variety of signals for testing purposes.






Peripheral EquipmentBreakout Boxes

Breakout Boxes are specialty boxes used to assist in troubleshooting electrical signals on a data interface.

Available in a variety of formats for RS232, V.35, RS422 etc.

Figure 4.7.1.7c RS232 Breakout Box




Photo Used by Permission of IDS



Peripheral EquipmentThese boxes break out, access, and monitor all leads in the data cable type they are designed for. They are used to test cable connections between equipment, find cable shorts or opens in a cable, or check pinouts to see what type of cable you need to connect between two devices (DCE/DTE).

Breakout boxes provide test points on each side of On/Off dip switches to allow signal isolation, monitoring, and cross-pinning.

LEDs connected to many of the common signals provide visual display of activity.

These boxes can be line-powered, battery powered, or supply powered.






Peripheral EquipmentVideo Analyzers

Two types available, Analog version and now the newest Digital version.




Figure 4.7.1.7d Digital Version

Phot

os C

ourte

sy o

f Tel

esat

Can

ada

Figure 4.7.1.7e Analog Version



Peripheral EquipmentVideo Analyzers

Analog versions accept an NTSC, PAL, or Secam type video input and display the 1 volt peak to peak IRE signal for evaluation.

The digital version demodulates a 16, 64 or 256 QAM signal carried through a DVB system.

A typical analyzer can measure average channel power, adjacent channel power, constellation display, vector magnitude error, modulation error ratio, and equalizer response. A typical analyzer can also perform data measurements to verify that the MPEG data has been correctly decoded from the DVB RF signal and check transport stream content.

Digital analyzers are all GUI controlled and make video measurements extremely easy.






Peripheral EquipmentTransmission Measuring Set (TIMS)A TIMS is a Transmission Impairment Measuring Set. These sets are capable of measuring Frequency, Level, Noise, Noise with Tone, Signal with Tone, Impulse Noise, P/AR (Peak to average Ratio) and Noise to Ground. They have TX and RX capability simultaneously.




Figure 4.7.1.7f TIMSPhoto Courtesy of Telesat Canada



Peripheral EquipmentTransmission Measuring Set (TIMS)Most measurements are made on leased or dial up lines with impedances of 135, 150, 600, 900 or 1200 ohms.

Frequency range is typically 20 Hz to 100 kHz.

Various tests are performed to determine the quality of the line such as frequency response, noise etc.

Various filters can be introduced to measure background noise with no tone applied such as C-Message, 3kHz flat, 15 kHz flat, Program and 50 kbit.






Peripheral EquipmentNoise Generator

Noise sources are used for several reasons:

1) To simulate noise as encountered in a satellite link in a non satellite simulation (less satellite delay).

2) To stress the demodulator circuits in a modem to determine if the modem is working satisfactory.

The idea is to inject noise at the IF level on the receive side of the modems via a IF combiner.







A variable attenuator on the noise source is then adjusted to vary the noise to a specific level.

Usually the noise would be set to a value of zero to several dB above the threshold Eb/No of the modem.

Figure 4.7.1.7g Noise Generator








M odem Upconverter H P A TestTra ns la to r

LN AM odem Downconverter Pad

NoiseSource

Tx

Rx

Combiner

Figure 4.7.1.7h Typical Test Scenario with a Noise Source







Peripheral EquipmentTest TranslatorA Test Translator is a frequency converter that performs the same frequency translation as a satellite transponder.

Typical Test Translators offer:• High frequency stability• Low phase noise contribution• 30 dB level control• Low intermodulation distortion• Minimum amplitude & delay distortion• Typical use for a test translator is to simulate a satellite link without

the access to an Earth Station for uplink and downlink capability• A Noise source as shown in Figure 4.7.1.7.1 complements the

testing of a test translator as it inserts typical noise as would be expected on a satellite link




Figure 4.7.1.7i Test Translator




A Calibration ProgramA calibration program is very important part of keeping test equipment fit and within specifications.

ISO9001 projects require equipment to be calibrated.

Uncalibrated equipment or damaged equipment should be sent out for repair immediately. Otherwise this equipment could be taken on site and be expected to perform within specifications.

Uncalibrated or out of date calibrated equipment can lead to higher project costs as tests and measurements will need to be duplicated.



4.7.1.8: A Calibration Program




Maintenance OverviewPart 2




Sec 7: Earth Station Maintenance


4.7.2: Maintenance Overview

The purpose of maintenance is to ensure that the network or service in question continues to meet its Quality of Service (QOS) requirements.

Maintenance is an operations function: the initial installation and testing of a new service is not considered maintenance.

Data gathered during the turnover testing of a new service, however, is integrated into the maintenance function as benchmark data (see 4.7.3).

Field maintenance can be loosely divided into two categories: preventive and corrective. The distinction between these two categories lies in whether a QOS parameter has been breached. If so, this is considered a failure and maintenance becomes corrective.

Maintenance Overview






The most important aspect of preventive maintenance is record keeping. The preventive maintenance process is the periodic gathering of relevant data, the analysis of that data, and any adjustment of equipment operating parameters that may be suggested by the results of the analysis.

The type of analysis involved in preventive maintenance is usually trend analysis, in which periodically-acquired data is assessed for change vectors that may lead to future service failure.

The data from routine preventive maintenance is also very important when a service has failed and we enter the realm of corrective maintenance. When a formerly working service fails to work, something has changed; past data helps us determine what has changed.







Every good maintenance program should include the regular performance of Preventive Maintenance Routines (PMR). The records resulting from these routines should be kept on site for a duration of not less than five years.

In addition to PMR records, a site Maintenance Log should be kept, either electronically or on paper. The Maintenance Log is intended as a peer-to-peer communication of maintenance activities, observations or ideas. Any anomaly found during a PMR, any corrective action taken to resolve a problem, and any change to equipment programming, configuration or operating point should be noted in this log. Each entry in the log should bear a date and time stamp.

Keeping this log electronically is recommended because the ability to do keyword or subject searches will make the log much easier to use.







Finally, a separate Equipment Replacement Log should be kept. Keeping records of equipment and system performance is a powerful tracking tool allowing field personnel to discover long term, drifting changes in equipment performance.

If, however, a piece of equipment is replaced by another, the historical continuity of a tracked parameter may be lost. Keeping track of every equipment replacement helps in the interpretation of historical data.





Benchmarking


Part 3



Contents

4.7.3.1 Block & Levels4.7.3.2 Fade Margins4.7.3.3 Bit Error Rate (BER) Tests



4.7.3: Benchmarking



Part 3: Benchmarking

Vol 4: Earth Stations, Sec 7: Earth Station Maintenance

4.7.3.1: Block & Levels

As indicated earlier, a Block & Level diagram should accompany any new equipment lineup.

This document of record contains signal level readings at numerous significant points through the uplink and downlink signal paths of satellite link equipment.

After all testing and equipment setup, and before the turnover of the servcie from installation engineering to operations personnel, this document should be completed with all applicable levels entered.

This document should remain on site for the life of the service, and updated as required.

When kept current, this document provides an essential reference for further site operations.

Block & Levels





4.7.3.2: Fade Margins

As we shall see in Volume 5, every satellite link is overdesigned to accommodate some unavoidable signal strength fading. This overdesign is called a “margin”.

Fade margin amounts are calculated based on the type and QOS parameters of the link and remain theoretical until the link is established. At that time, the link must be tested to ensure that the required margin actually exists.

Typically, fade margin testing is part of acceptance testing prior to turning the link over to the customer. Maintenance personnel must keep a record of the actual fade margin value as determined by this initial test.

It is advised that this value be re-tested each year of the life of the service, with the results compared back to the original value.

Fade Margins





4.7.3.3: Bite Error Rate (BER) Tests

Another significant QOS parameter of all digital satellite links is the Bit Error Rate (BER) figure.

Once again, this value will be tested during link acceptance tests, and the value actually obtained—not just the fact that the test passed—should be recorded.

As with the fade margin parameter, BER testing should be repeated annually, with the resultant values compared back to all previous values. A reduction in BER may indicate a failing piece of equipment.

It is common for links to exceed the required BER value by a significant amount. By recording and comparing actual values, link problems can often be identified and corrected before the BER QOS value is reached.

BER Tests




Preventive Maintenance (PM)


Part 4



Contents

4.7.4.1 Daily4.7.4.2 Weekly4.7.4.3 Monthly4.7.4.4 Semiannual4.7.4.5 Annual



4.7.4: Preventive Maintenance (PM)






A Preventive Maintenance Plan (PMP) must always be tailored to a specific facility and situation. It is not possible to determine the scope and depth of a PMP until details concerning staffing, manpower, facility capacity, equipment specification, and contractural obligations are known.

This section of the course suggests a number of very useful tests and checks that may be included in such a plan.

Generally, employing extensive PM testing results in better link performance. However, PM activitities must be balanced against other requirements.

Chiefly, satellite links exist to provide end-to-end service to the user. When a link is down for scheduled maintenance, it is not providing that service. Thus, PM downtime must be minimized.

Introductory Remarks






A good PMP should be designed to closely monitor and track equipment performance while minimizing service disruption. There are three main goals:

• The prompt discovery of sudden events• The timely discovery of long-term trends and slow degradations• The continued testing and maintenance of equipment as recommended

by most manufacturers

To accomplish these three tasks without undue service impact, most preventive maintenance performance items are equipment checks which do not involve or risk loss of service.

PMP Design






Even where “live” link or facility behaviour is monitored, system design usually provides numerous test points which may be accessed without interrupting circuit paths.

A very effective structure for a PMP is to organize tasks according to their frequency: Daily, Weekly, Monthly, Semiannual and Annual.

The Daily checks, to be completed each morning, provide close tracking of equipment and facility conditions. This provides a mechanism for the prompt discovery of any faults, degradation or damage that may have arisen during non-staffed hours.

The other maintenance periods provide a more detailed examination of equipment status and link performance. They also include the periodic exercise and testing of standby facilities.

PMP Design






Once each year link quality will be tested to confirm that contract specifications are continuously met.

Finally, PM’s generate a lot of data, and this data, in order to be useful, must be maintained in an organized fashion.

It is recommended that a log sheet be created for each PM routine, and that detailed instructions be written for the completion of the log sheet. These instructions should include equipment setup parameters and other information that will help to ensure that data is collected in a standardized fashion by different operators.

Completed log sheets should be held for at least 5 years. If feasible, it is recommended that the data be held electronically as well, facilitating graphing and trend analysis.

PMP Design






In completing any Preventive Maintenance Procedure, the technologist should take care to check each log entry against the entry for the previous maintenance period, noting any significant change.

What constitutes significance in each case depends upon equipment specifications, programmed parameters, and the magnitude of the difference. Recognizing significance is often a matter of experience alone.

All log sheets should include a “Remarks” section. If any significant changes are observed, the “Remarks” section can be used to describe them. The technologist will then begin an investigation into the change.

Performance Notes






In addition to checking each log entry against the previous maintenance period, the technologist will look back over the readings taken over numerous preceding maintenance periods. Any significant trends or irregularities will be observed.

Again, the “Remarks” section of the log can be used to describe the long-term irregularities, and the technologist should begin an investigation into the change.

Log sheets will typically be designed with some fields requiring numerical entries, while many others will require only a check mark to show that the task—often just an observation—has been completed.

Performance Notes






Corrective maintenance has a higher priority than preventive maintenance. Should a fault or abnormality be discovered during the completion of any preventive maintenance procedure, the technologist must decide whether to take remedial action before continuing with the preventive routine.

The technologist will normally choose to correct the fault.

Performance Notes



Part 4: Preventive Maintenance (PM)


4.7.4.1: Daily

Any PM routine that is to be run daily must not include service downtime, must not involve the risk of service downtime, and must not be invasive. Daily routines involve simply alarm and meter readings designed to catch any obvious changes in equipment operating parameters and alarm status.

A typical daily routine is also suitable as a shift change routine: a set of observations made by the on-coming maintenance shift. Facilities in which 2 or 3 shifts operate around the clock may wish to adopt such a routine at the beginning of each shift. These routines must be quick and easy to perform.

The following recommended inclusions assume that the technical staff is responsible for maintaining the facility as a whole, including building and support services. This is ofetn the case in Earth Station work.

Daily




4.7.4.1: Daily

Recommended inclusions are as follows:

1. TemperatureEquipment performance is often related to ambient temperature. Equipment working normally at one temperature may fail at another. It is therefore a very good idea to hang thermomenters in all equipment rooms and record the temperature in each room on a daily basis. If this data is kept, it can be determined whether observed equipment or service problems are related to ambient temperature.This is also a good way to determine if buliding HVAC systems are functioning correctly.

Daily





4.7.4.1: Daily

2. HumidityAs with temperature, and depending on where the Earth Station is located, ambient humidity may also pose problems for eqipment and link performance.It is therefore a good idea to keep track of ambient humidity in each equipmnet room.

3. HVACIt is recognized that communications technicians are not qualified to maintain HVAC systems. However, most HVAC systems provide simple alarm and operating lamps, and sometimes intake and outlet operating temperature guages, that can alert communications staff to impending HVAC system trouble. The daily PM should include a visual check of these HVAC system indications.

Daily





4.7.4.1: Daily

4. Fire PanelAll Earth Station buildings should be equipped with a fire alarm system properly scaled to the size and nature of the Earth Station building(s). This could be just a few smoke or heat detector units, to a full scale detection and supression system operated from a fire alarm panel.Whatever the case, communications personnel should make a visual inspection of sensor head or alarm panel indications to ensure that the system is operating normally. This action should be checked off on the daily log sheet.

Daily





4.7.4.1: Daily

5. Communications Equipment AlarmsAlmost all communications equipment provides a front-panel indication of alarm conditions. This could be in the form of red lamps or LED’s, or front-panel screen readouts.For the purpose of the daily PM routine, a walkthough should be conducted, with the front panel of each piece of equipment carefully observed for alarm status indications.

6. Online ChainsIn cases where equipmnet redundancy is employed, one or the other of two equipment pairs, or equipment “chains” will be online serving traffic. During each days PM routine, it should be noted which chain is online.

Daily





4.7.4.1: Daily

7. Pressurized WaveguideMost satellite systems pressurize waveguide with dry air to prevent humidity damage to the inside of the waveguide runs. These systems are important. The pressure and humidity of these systems, where metered, should be noted on a daily basis.

8. Remote/Auto StatusMany redundancy systems employ Monitor and Control (MAC) panels. These pannels should always remain in “Remote” and “Automatic” so that they can perform their designed function in the absense of operator intervention. However, it is very common for routine maintenance activies to require that these devices be switched to “manual”. It is a good idea to ensure that these devices remain set as required for unmanned operation.

Daily





4.7.4.1: Daily

9. Eb/NoMost satellite modems provide Eb/No indications by way of front-panel readouts. Should this be the case, since it is very easy, it is recommended that the reported Eb/No value be recoreded every day.The Eb/No value of offline modems should also be recorded as a validation of the online modem’s reading, as well as to ensure that the offline modem remains ready to assume online duty.

10. Transmit PowerWhere the online HPA provides a readout of transmit power, this value can be recorded daily. If this value is not readily available, it need not be measured. It is not feasible to require test equipment connection and measurement on a daily basis.

Daily





4.7.4.1: Daily

11. UPSShould the Earth Station employ a UPS with a front panel screen, UPS operating values might be included in the daily collection of data. Temperatures, phase voltage and currents, power factors and battery bank-related parameters are all good candidates for daily monitoring.Again, it may not be the responsibility of the communications technologist to reapair or maintain the UPS equipment, but significant changes in these values should be brought to the attention of the qualified people.

Daily





4.7.4.2: Weekly

Occasionally, equipment manufacturers may recommend performance checks on a weekly basis. A proceedure for performing such checks can be written into a weekly PM routine.

Weekly PM routines can also capture non-invasive checks of equipment alarm and status conditions which must be pulled from the equipment via a computer interface.

This type of check is usually done to draw statistics from multiplexers, routers, and similar equipment.

Older equipment might require checks of local oscillator frequencies or other equipment parameters.

Weekly





4.7.4.3: Monthly

MonthlyRecommended inclusions are as follows:

1. FansElectronic equipment is often cooled by fans. At least once a month, the cleanliness and function of all cooling fans should be confirmed. All fans should be moving and unobstructed.

2. Internal TestsModern electronic equipment sometimes offers non-disruptive, internal tests that can be executed via the front panel or by serial connected computer. Such tests should be run monthly.

3. Lamp TestsEquipment lamp tests should be executed monthly to ensure that all front panel indications are available.





4.7.4.3: Monthly

Monthly4. Equipment Parameters

It was recommended earlier that, because of their importance, Eb/No and transmit power be checked every day. Other parameters, while less important, are still useful, and these can be checked monthly. Such checks might include equipment front panel displays of C/N, BER, frequency, power supply voltages, AGC, buffer fill levels, traffic statistics, or any other parameter the equipment manufacturer has provided that is deemed useful.If the manufacturer has provided test points at which power supply voltages can be measured, this should also be performed monthly.





4.7.4.3: Monthly

Monthly5. Carrier Level Checks by Satellite Operator

It is a good idea to have the satellite operator measure and confirm the correct transmit carrier levels on a monthly basis.The level of a carriers transmitted from the Earth Station should be checked, but so should the level of all carriers being received by the Earth Station.This information will assist maintenance personnel to determine if internal system level variations are the result of local equipment changes, or changes occurring on the satellite or at the distant transmit location.





4.7.4.3: Monthly

Monthly6. Internal Carrier Level Checks

Carrier level measurements should be made at available test ports. These can be measurements of individual carriers made with a spectrum analyzer, or aggregate power level measurements made with a power meter, whichever is deemed appropriate.Note that these are non-disruptive measurements made a test or coupling ports where no disruption of cabling is required.





4.7.4.3: Monthly

Monthly7. Spectrum Check

Satellite link performance can be affected by other carrier activity on the transponder. It is a good idea for maintenance personnel to be familiar with the appearance of the spectrum on the transponders used. Each month, a spectrum analyzer should be used to check applicable portions of the spectrum for recent changes. Of special interest are new carriers, carriers that may have been increased, and the level of the noise floor.

8. C/N MeasurementsEach month, C/N measurements should be performed on all carriers of interest.





4.7.4.3: Monthly

Monthly9. HPA’s

HPA’s, especially TWTA’s, should be closely monitored. Each month, current, voltage, and other significant parameters, as made available by the manufacturer. Increases in the helix current of TWTA’s is especially noteworthy.

10. Equipment Fault LogsSome pieces of equipment maintain their own fault logs. Each month, or more often if necessary, these logs should be reviewed and cleared. Any logged faults should be investigated.





4.7.4.4: Semiannual

SemiannualSemmiannual maintenance will prove necessary only when equipment manufacturers call for equipment routines or checks twice yearly, or where tracking antennas are used.

In the case of tracking antennas, greasing of gears and pinions is recommended twice yearly, unless specified otherwise by the manufacturer.

If a semiannual PM is to be used, it will be found advantageous to check and correct the internal date and time stamps of all equipment so possessed. Unless all equipment is locked to an external timing source, these clocks will drift. If personnel depend upon these clocks to accurately timestamp recorded events and alarms, they must be checked.





4.7.4.5: Annual

Annual


Annual maintenance is typically the only maintenance period in which service outage time is required. Recommended inclusions are as follows:

1. Antenna InspectionThoroughly inspect all Earth Station antennas. Look for rust on the antenna, on the antenna mount, and on any fixtures which may be attached. Check the antenna surface for accumulations of dirt, or excessive pitting. Check that all electrical boxes and other fixtures mounted to the antenna are securely fastened and function properly. Oil hinges on electrical box doors. Check that all cabling is routed properly, intact, and tightly connected. On larger antennas, include the interior of the hub in this inspection. Clean, treat rust, and repair any problems found.




4.7.4.5: Annual

Annual


2. Antenna GreasingFor larger antennas having gear-driven azimuth and elevation adjustments, brush a coating of stiff grease onto all disused portions of gear travel. This coating will melt away over the year and must be reapplied. The coating helps prevent rust and ensures that the entire range of gear travel remains available, even if it has not been used in years.

3. Antenna DeiceFor antennas so equipped, the function of all deice equipment must be confirmed, according to the manufacturer’s specifications, once each year. If possible, this should be done in the fall.




4.7.4.5: Annual

Annual


4. Confirmation of Alarm SettingsEquipment that provides alarms often incorporates programmable or settable alarm thresholds. The accuracy and applicability of these thresholds should be checked annually.

5. Remote Monitor and ControlWhen Earth Stations are monitored and/or from a remote location, the function of all monitor and control points should be checked yearly. This involves system downtime as the remote operator must test his control over Earth Station HPA inhibits, and local faults must be induced or mimicked so that the remote operator can confirm the reception of the correct information.




4.7.4.5: Annual

Annual


6. Fade Margin ChecksAs mentioned earlier, fade margin testing is done at the beginning of service life. It is recommended that these margins be confirmed annually. This requires link down time.The basic method for conducting a fade margin test is as follows:• Connect appropriate BER test sets to both ends of the link to be tested.• When both test sets have synchronized, conduct a 1 hour test run. This

test must run better than the BER QOS figure specified for the link. Record the test results.




4.7.4.5: Annual

Annual


6. Fade Margin Checks (Cont.)The basic method for conducting a fade margin test (cont.):• After the 1 hour test, drop carrier power at both ends by 5 dB. Resynchronize the BER sets

and watch them for a few minutes. They should be running clean. • If so, drop carrier power at both ends by another 2 dB and again watch the test sets.

Continue to drop carrier power at both ends until the BER sets start counting errors. • Now, increase carrier power slightly and attempt to perform a 10 minute error-free BER run.

Continue to increase carrier power at both ends very slightly until it is just possible to run a 10 minute BER test with no errors.

• At this point, have the satellite operator measure both carriers in dB below nominal. The smallest of these readings (they should be almost the same) represents the fade margin for the link. It should not have changed significantly since the last time the test was performed,




4.7.4.5: Annual

Annual


7. Raditaion SurveyA radiation survey should be conducted in accordance with local law or practice. A good way to ensure that such a survey is conducted often enough is to add it to the annual PM. This is a recommended safety practice.

8. UPS MaintenanceIt is very likely that UPS maintenance will be required annually. Chiefly, this will mean battery checks. Develop a PM that incorporates the manufacturer’s instructions concerning UPS maintenance.

Engineering

Earth Station Maintenance in Satellite Communications