January 16, 2012

Usage Justification File - PA-12

Justify a specific definition concern related to EEE components

A. Scope

As specified in Chapter E-7 "Design Justification File (DJF)", the DJF includes all design justification documents.

A Usage Justification File (DJU) is one of the justification documents. It covers a specific detail of the design and gives an overview of the reliability of the design with regards to this detail.

The DJU must not be longer than two pages.

With specific regards to the choice of EEE components, this notion of DJU corresponds to the Justification Document abbreviation (document including information gathered, the risk analysis, risk reduction measures and results) indicated on form PAF-4 "List of EEE components".

With regards to the use of EEE components, a DJU may cover subjects such as resistance to radiation (SEL, SEU, SET, TID), compliance with derating, and resistance to ON/OFF cycle stresses.

With regards to the use of materials, a DJU may cover resistance to expected environments such as resistance to a monatomic oxygen ATOX environment.

In addition to the Product Assurance domains (dependability, component QA, materials and processes QA, software QA, security), the DJU format can be used to add details about a specific point related to the satellite or system design.

B. Fundamentals

The DJU is drawn up in phase B based on the product design. It must be updated if the design or when the justification documents are called into question.

C. Typical content

1. Scope

Information defining the product and problems concerned by the DJU.

2. Documentation

Bibliography used to support the justification, including documents applicable to the product design when relevant to problematic DJU issues.

3. Justification

The justification results from the analysis and/or tests explaining how the design of the product and its use satisfy the needs and requirements expressed.

The justification validity dates must be specified (uncertainties, tasks open, hypotheses, etc.).

Examples

Following pages:

  • TAR-DJU-S-7-MB-6593-LATMOS concerning ON/OFF
  • TAR-DJU-S-7-MB-6560-LATMOS effect of atomic oxygen
  • TAR-DJU-S-7-MB-6594-LATMOS concerning SEL

 

 

DJU ON/OFF NUMBER OF ON/OFF

IME-BF will be switched ON/OFF many times (10,000/year). This document justifies the reason for the number of ON/OFF in accordance with the usual ON/OFF rules.

  • Demeter reference: Demeter Satellite launched in May 2004
    • EEE parts: COTS except FPGAs and connectors
    • Temperature min / max for night / day: -10°C to +55°C
    • Radiation TID: 5 to 10 Krads for 4 years
    • CETP instruments: IAP (Ion analyzers) and ICE (Electric field measurement)
    • Other instruments: Langmuir probe, high energy electrons, magnetic instrument
    • All instruments are working perfectly after 4 years and 1 month
    • Number of orbits per day: 14
    • Number of ON/OFF for all instruments: 2 per orbit (OFF above poles)
    • Number of ON/OFF per year: 10,000
    • Number of ON/OFF today: 55,000
  • CNES reference
    • Article "Accounting for the ON/OFF problem on PLEIADES" in CCT no. 27.
  • IME-BF sensors and analyser
    • EEE components: COTS and High Rel
    • Specific components: see comment below
  • Justification
    • Taking into account Demeter feedback, the CNES CCT item and the types of components used both in the sensors (completely identical to those of Demeter) and in the analyser, the number of ON/OFF estimated for IME-BF (10,000/year) is not critical.
  • Remark
    • There is no component expert in the scientific labs. Therefore, we are only able to conduct an analysis as indicated above.
    • If a particular type of component may be potentially critical, only a CNES expert can notify us of this, given that CNES has our list of EEE components.

 

DJU example: TAR-DJU-S-7-MB-6560-LATMOS effect of atomic oxygen

Atomic oxygen atoms O that strike IME sensors can be recombined with carbon atoms contained in the layer of DAG213 resin that covers the sensors. The reactivity of atomic oxygen and the affinity of carbon to oxygen are such that the effectiveness of the reaction is certainly significant with the potential to cause significant deterioration of the layer of resin. The purpose of the calculations that follow is to assess the time it would take the layer of DAG213 to completely disappear.

1. Density n(O) of atomic oxygen on the TARANIS orbit

n(O) largely depends on solar activity. The predicted solar activity during a cycle is F10.7. According to the prediction of the NOAA Space Weather Prediction Center, in the period 2014-2016, F10.7 will decrease from ~140 to ~ 70 (fig. 1), i.e. an average of 110 (identical to Demeter).

AP12

 

Fig. 1 Observed and predicted (by NOAA Space Weather Prediction Center) f10.7 radio flux value

2. Variation of n(O) and T(O) on TARANIS orbit as a function of F10.7

The altitude specified for the TARANIS orbit is 700km. According to the model used (J. Lilenstein, private communication), we obtain the following densities and temperatures for O:

AP12

 

The average aspect values are used for the variation laws:

Log n(O) = 5.146 + (6.255-5.146) * (F10.7-80)/70
T = 850 + (1100-850) * (F10.7-80)/70

For F10.7 = 100, we therefore obtain at 700km: n(O) = 2.9 105 cm-3 and T(O) = 920°K
For F10.7 = 140, we therefore obtain at 700km: n(O) = 1.2 106 cm-3 and T(O) = 1060°K
For F10.7 = 70, we therefore obtain at 700km: n(O) = 105 cm-3 and T(O) = 810°K

3. Calculation of O atom flows over 1cm2

As the thermal velocity of O is low in relation to the satellite speed, we can write:

F = n(O) Vsat (where Vsat = 7.5 km/s)
F = 2.2 1011 cm-2 s-1

4. Characteristic of the DGA213 layer

In an uncured state, density is 0.84 and the carbon grain content is 30%. The equivalent carbon density is therefore: 0.84 * 0.3 = 0.25 g cm-3.

The thickness of the layer applied is between 1 and 3 mil (ref. GSFC) i.e. 25 to 75 ?m. The minimum thickness specified is 25 ?m.

The weight of carbon over 1cm2 is therefore: 0.25 g cm-3 *1 cm2* 25 10-4 cm= 6.25 10-4 g

The number of carbon atoms per cm2 is therefore: (6 1023 * 6.25 10-4) / 12 = 3 1019

5. Calculation of deterioration time

We are not taking into account the interaction of O atoms with the resin, but making the hypothesis that each O atom incident interacts with a C and deletes it. This results in far quicker deterioration, due to the fact there is probably at least as much resin as carbon. The time required to remove all atoms from 1cm2 is therefore:

3 1019 / 2.2 1011 s-1 = 1.36 108 s = 1570 days = 4.3 years

The time required for the layer to erode is therefore approximately 4.3 years. Taking into account the approximations made, this time is certainly several times longer.

Activities / documentation

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