satellite communications reliability considerations

Slide Number 1Rev -, July 2001

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

Satellite dis h



Spacecraft reliability is used to dictate the level of redundancy that will be required in the final design.

Reliability figures help identify items with high failure rates.

Reliability figures are used as a means of comparison between manufacturers.

Reliability is very important for insurance coverage, since it determines a level of risk to the mission over the life of the spacecraft.

Spacecraft Reliability

2.6.1: Spacecraft ReliabilitySec 6: Reliability Considerations

Vol 2: Communication Satellites



Spacecraft designers need to establish a pass or fail criteria over the life of the spacecraft. These criteria, taken together, define a “Mission Success” target.

Generally the mission of the spacecraft is to maintain all transponders fully operational until the end of its service life, 12 or 15 years.

If 36 Ku-Band transponders are operational at the beginning of life, the failure of only one that cannot be replaced by a redundant component constitutes a mission failure.

Components of the Bus and other payload components must remain operational in order to support the Mission.

Mission Reliability Requirements

2.6.1.1: Mission Reliability RequirementsPart 1: Spacecraft Reliability

Vol 2: Communication Satellites, Sec 6: Reliability Considerations



Reliability requirements are usually specified in more than one way:

• First, a probability of “Mission Success” over the life of the spacecraft must be provided: 0.80 for 12 years and 0.75 for 15 years are acceptable as a general rule.

• Redundancy of active components could be specified in percentile of active components, or directly, such as 30 TWTAs for 24 actives.

• All active components should have redundancy.

Mission Reliability Requirements

2.6.1.1: Mission Reliability RequirementsPart 1: Spacecraft Reliability




The building block of spacecraft reliability over its service life is the rate of Failure-In-Time (FIT) of each unit, referred to as “FIT Rate.”

Each unit is assigned a FIT rate based on the reliability of the components it is made of.

The FIT rate () is equal to the number of failures in 1,000,000,000 hours.

In the space environment community the FIT rate is assumed to be constant over the service life of the unit.

Constant Failure Rate

2.6.1.2: Constant Failure RatePart 1: Spacecraft Reliability




Based on the reliability of components inside a unit, a FIT rate is calculated, and from the FIT rate () a probability of failure over time can be calculated using the following equation:

)t(Failure eP

• t being the period of time in hours and is the FIT rate divided by 1,000,000,000 hours

Probability of Failure

2.6.1.2: Constant Failure RatePart 1: Spacecraft Reliability


EQ. 2.6.1.2 Probability of Failure



To meet the overall “Mission Success” target, a probability of success will be assigned to each sub-system of the spacecraft, i.e. the payload, the TT&C, ACS, power, etc.

Each sub-system will then be divided in sub-sub-systems, with probabilities of success assigned to each unit of the sub-sub-systems.

Reliability Apportionment

Te lem etry&

C om m and

A ttitudeC ontro l P ow erP ropu lsion Therm al

C ontro l P ayload

0.962 0.967 0.9980.9870.985 0.892

S pacecra ft

0.806

2.6.1.3: Reliability ApportionmentPart 1: Spacecraft Reliability


Figure 2.6.1.3 Satellite Reliability Block Diagram



In general all active components have a low reliability over life. The higher the power handled by the component the lower its reliability becomes.

To ensure that the overall probability of mission success is met, spare units have to be added to the design, along with switching, to ensure failed units are replaced during the service life.

The next slide is a simplified block diagram showing redundancy for active units.

Reliability Improvement/Redundancy

2.6.1.4: Reliability Improvement/RedundancyPart 1: Spacecraft Reliability




InputSeries -

Antennas

Ku-BandReceivers

EPCs

Imuxes &Passive

Components

TWT &DLA

OutputSeries

Ku-BandReceivers

4 for 2

EPCs

TWT &DLA

TWT &DLA

TWT &DLA

32 Sets

20 for 16

40 for 32

Figure 2.6.1.4 Simplified Payload Reliability Block Diagram

Spacecraft Reliability

2.6.1.4: Reliability Improvement/RedundancyPart 1: Spacecraft Reliability




At the beginning of a program, some units, which may be new designs, will not have any heritage information to draw upon for reliability figures.

In this case a preliminary design will be established and a FIT rate will be determined based on theoretical values.

A reliability prediction will be made for the unit based on the operating conditions, i.e. temperature, radiation environment.

As the program progress, reliability demonstration will take place using available live test data.

Reliability Predictions

2.6.1.5: Reliability PredictionsPart 1: Spacecraft Reliability




The reliability prediction will be refined as required.

Environmental Stress Screening will be performed:

• Burn-in to eliminate early and potential failure, known as “infant mortality”

At the end of the test period, a final FIT rate will be assigned to the unit.

Reliability Predictions

2.6.1.5: Reliability PredictionsPart 1: Spacecraft Reliability




Launch vehicle life duration is measured in minutes.

Reliability block diagrams for launch vehicles are useful to the manufacturer, not to the client.

Pass/Fail criteria is the only statistical data available to the users of launch services.

With most launch vehicles, the failure rate will be higher early in the program.

Reliability growth is expected with increasing numbers of launches.

Launch Vehicles

2.6.2: Reliability of Launch VehiclesSec 6: Reliability Considerations




Launch failures are made public and are easy to track.

Easiest reliability model is to take the number of successes over the total number of launches.

Accuracy of the probability of success figure thus achieved is directly proportional to the number of launches. A large database will provide more accurate numbers.

Launches ofNumber TotalSuccesses ofNumber PSuccess

Launch Vehicles

2.6.2: Reliability of Launch VehiclesSec 6: Reliability Considerations


EQ. 2.6.2 Probability of Success



It is difficult to quantify in-orbit reliability.

Detailed reliability models require detailed knowledge of sub-systems and component failure data that is not always available.

Even if available, the database would be unmanageable.

For in-orbit reliability we must rely on mean-time-to-failure figures based on in-orbit operation time and failure numbers.

In-Orbit Reliability

2.6.3: In-Orbit Reliability of SpacecraftSec 6: Reliability Considerations


satellite communications reliability considerations

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