9.3 Validation

a. Inspection of the spacecraft (including structure and cable harnesses) shall be performed to verify that there are no ungrounded metal components.

b. Resistance testing shall be carried out on grounded metal components to ensure that their grounding meet the requirements in 9.2.2.

c. Testing of circuits shall be performed at equipment level by the application of voltage spikes on input, output and power lines.

NOTE This determines whether filtering is adequate.

d. Bulk conductivity measurements used in calculations of internal charging electric fields shall be obtained for the lowest temperature to be used and through a measurement technique agreed with the customer.

NOTE 1 The objective is to use a technique appropriate to internal charging. Annex E.3 provides some examples of such measurement techniques.

NOTE 2 This requirement implies that the use of bulk conductivity measurements in open literature and company specifications is inappropriate in calculations of internal charging electric fields. Such measurements are usually made at room temperature and before time-dependent polarization effects have died away. This usually gives an apparent conductivity that is far higher than the longer-term conductivity which is relevant for internal charging.

e. Computer simulation of electric fields or irradiation testing within a dielectric may be omitted if one of the following three criteria is satisfied:

1. The charging current is very low, i.e. the charging current density j deposited in the material, after radiation shielding, is lower than the maximum electric field, multiplied by a worst case conductivity σ_min , i.e. j < E_max σ_min.

2. The part is very well shielded, i.e. the part is shielded such that currents under worst-case natural environments are too low to cause hazardous levels of charging, based on the current density described in 9.3e.1.

3. The material conductivity is very high, i.e. its bulk conductivity, at the lowest temperature at which it is intended to be used, is equivalent to or higher than the following ones for a radiation shielding of 0,2 g cm^-3:

o For GEO, 2.5 × 10^-15W^-1 m^-1 .

o In any other case, 2,5 × 10^-14W^-1 m^-1 .

NOTE 1 These conductivities prevent the creation of high electrical fields, and are calculated using j from the FLUMIC environment model given in Annex D.3.1.

NOTE 2 If one of the above criteria is met, it is an indication that problems with high electric fields are very unlikely. These criteria are based on the formula linking electric field E with current density j and conductivity σ, E = j/σ. This formula arises from Ohm’s law.

f. In satisfying 9.3e, the minimum value of dielectric conductivity σ_min, shall be:

1. 10^-16W^-1 m^-1 when mean daily temperatures are in excess of 25ºC,

2. 2 × 10^-17W^-1 m^-1 for other temperatures, or

3. a value agreed with the customer.

NOTE 1 When determining the value agreed with the customer, it is important to take into account the degradation of the material.

NOTE 2 Hence the threshold value of charging current is 2 × 10¹⁰ A m^-1 based on the default values of E_max and σ_min (1 × 10^-9 A m^-2 for materials in excess of 25º C).

NOTE 3 The current density of 2 × 10^-10 A m^-2 above, is not exceeded for a radiation shielding thickness of 5,1 mm aluminium for all terrestrial orbits and 2,6 mm for GEO according to the FLUMIC version 2 flux model and DICTAT shielding calculations (see Annexes D.2.1, D.3.1 and E.3.3).

NOTE 4 A conductivity value of 2 × 10^-17 W^-1 m^-1 lies below the normal range of bulk dielectric conductivity at room temperature. Although bulk conductivity can be even lower than this at lower temperatures, the addition of radiation-induced conductivity makes lower values unlikely.

g. Detailed assessment of the surface voltages on internal surfaces need not be performed if the following has been established.

1. The thickness of the dielectric is less than 0,1 mm.

NOTE In this case meeting the electric field requirements also meets the internal surface voltage requirement.

2. The product of charging current density and thickness is very low, i.e. less than the product of V_max and σ_min (as described in 9.3f).

NOTE For V_max=1 kVand σ_min=2 × 10^-17W^-1m^-1, this product is 2 × 10 ¹⁴ A m.

h. For materials other than those covered by 9.3e and 9.3f, a validation process shall be carried out to establish that the maximum electric field and internal absolute surface voltage remain below E_max (10⁷V/m) and V_max respectively, when irradiated with the worst-case environment, in the orbit and position on the spacecraft and at the lowest temperature, in which it is intended to be used.

NOTE 1 This involves either

• experimental validation using a worst-case spectrum in a laboratory chamber, or

• computer simulation.

NOTE 2 For simple structures a 1-d analytical charge deposition and conductivity code such as given in annex D.2 can be suitable. This calculation uses information on the environment and the density of the shield and dielectric and its electrical properties. For complicated structures, 3-d Monte Carlo simulations can be used to calculate current deposition. However, after this the field is calculated using the electrical properties of the structure.

i. For systems sensitive to absolute levels of internal electric fields and comprising insulators or isolated conductors, an analysis of the susceptibility to internal charging shall be made.

NOTE For devices like triaxial accelerometers, this is most likely to be achieved using particle transport codes. For MEMS devices, where the trapping of charge in dielectric layers is not easily calculated, this is most likely to be achieved by laboratory simulation. In all cases, it cannot be done without identification of suitable environment models. This can include energetic proton and ion models where these are significant sources of deposited charge.