A.

Operating Conditions

During the mission the detector will be used in TDI (Time Delay Integration) mode. The detector will be read out in 5 blocks of 20 rows each, with TDI applied to each odd numbered block such that blocks 1, 3 and 5 represent the optical scene bands from the dichroic filters; band 7 (λ_centre = 8.8μm, BW = 0.9 μm), band 8(λ_centre = 10.8μm, BW = 0.9 μm) and band 9 (λ_centre = 12μm, BW = 0.9 μm) of the MSI TIR. The TDI processing is performed in the front end electronics (FEE) by adding the first line of a block for the first ground line interval to the second line of the block during the second ground line interval etc. to the 20^th line (i.e. In TDI mode signals from N rows in each image area are co-added from N successive frames). During each ground line interval the detector is readout multiple times (nominally 5) and the pixel data added to get the data for a ground line interval ready for TDI addition.

A.

Functional tests

For all environmental tests apart from those of a destructive nature all devices were functionally tested before and after the specific environmental test sequence. The devices subjected to particularly long tests were functionally tested at intermediate steps to ensure the integrity of the devices or determine premature failure.

B.

SSTL’s electro-optical test facility.

The general schematic diagram for the tests setup is shown in Fig 3. The detector was mounted in a dedicated ‘flex’ board (with sprung loaded sockets to avoid soldering of the detectors and allow easy mounting) connected to the FEE board. The flex board was mounted on a specifically designed jig with pipe connections for temperature control. Heat was removed from the hot side of the detector by a re-circulated liquid coolant.

Fig. 3:

Schematic diagram of microbolometer test set up

The detector clocks and biases were supplied by the FEE board powered by the Electrical Ground Support Equipment (EGSE) board programmed from the host PC via a 1G Ethernet interface. The optical stimulation was provided by a wide area black body mounted inside the temperature enclosure (see Fig. 3) to allow for an f/1 signal to the system. The accuracy of the measurement system depends on the resolution of the black body control unit, water bath and front end electronics hence minor changes on the test conditions were expected during the measurements.

A.

Vibration Testing

A dedicated test cube with tapped holes on the sides and top was used for these tests. The cube was bolted onto an intermediate plate which bolts directly onto the shaker. The cube allowed for testing all axes without the need for a slip table. The detector was bolted directly onto the cube where a facet had been modified to accommodate the detector ensuring that the device plus pins sat flat on the surface of the cube keeping all pins connected to ground by means of a nylon clamp fixed onto the cube.

Fig. 4

Detector test mount with ULIS 03041 device mounted on (left), test mount (right)

The test article was required to be tested in all three axes. The order of testing in each axis was as follows:

B.

Shock Testing

Same test cube used during the vibration campaign was used for these tests. The cube allowed for measuring on all axes simultaneously. The detector was bolted onto the side of the cube ensuring that the device plus pins sat flat on the surface and all pins were connected to ground. The pins were held in position by means of a nylon clamp fixed onto the plate. The Test Levels for the detector shock qualification testing were derived from the shock measurements performed on the Engineering Confidence Model [ECM] of the MSI.Table 3 shows a summary of the change in electro-optical parameters after shock testing

Table 3:

Summary of functionality results after shock testing

A.

Total Ionizing Dose

Cobalt60 irradiations were carried out during June 25-26 2013 at the irradiation facility at ESTEC. Two devices were irradiated biased (#10024 and #09047) and two irradiated unbiased (#09024 and #10025). All devices were irradiated up to 10krad(Si). The devices were measured before and after the irradiations and for the biased devices the bias currents monitored during the exposure. A monitor device (#09037) was also measured before and after the irradiations. Functionality post rad tests were started immediately on return to SSTL, followed by annealing tests after 1 week storage at RT (Room temperature) and after 168 hours at 100°C. Fig. 5 shows a photograph of the irradiation set up

Fig. 5.

Irradiation setup showing the lead blocks used for shielding the drive electronics

Table 6 summarises the post-irradiation performance showing the bias currents that changed during Co60 irradiation and the change in percentage of the electro-optical parameters. All parameters were also measured after 24hrs, 1 week anneal at RT and 100°C showing a small change of less than 5% from the values shown

Table 6

Percentage of change of electro-optical parameters after Co60 irradiation

B.

Heavy Ion Testing

Heavy Ion tests were performed on 18 November 2013 at the heavy ion facility (HIF) at the Université Catholique de Louvain (UCL), Louvain la Neuve, Belgium. This facility gives a beam homogeneous to ±10% over a 25 mm diameter. Ion cocktail number 1 (High LET, M/Q = 5) was used (Xe, Kr Ar and Ne ions). For these tests only Kr and Ar ions were used giving a maximum of 40.4 MeV/mg/cm² at normal incidence.

Two devices were irradiated and a total of 5 hours beam time was used for these tests. The ion flux at the device was adjusted between 2000 and 12000 ions/cm²/s. Power supply currents to the device were monitored using a tcl program that scanned the outputs of two digital voltmeters. Each supply had a current limit which, if exceeded (due to a latch-up event), triggered an event flag and automatic power off. Following such event the device had to manually be turned on and the monitoring software re-initiated. During exposure all bias currents were monitored for SELs and multiple images taken to analyse for image quality and potential degradation due to overheating a line of microbolometers. Figure 6 shows photographs of the test board inside the vacuum chamber

Fig. 6.

Photographs of the test board inside the HIF chamber. Drive electronics positioned behind the DUT (left). The device at normal incidence to the beam (right)

The devices were tested at 3 different energies as shown in Table 7. During exposure all bias currents were monitored for SELs and SEUs and multiple images taken to analyse for SEFIs or any other effects that could occur. Due to mechanical constraints both devices could not be tested at the same LET values; a higher than 42 degrees and 40 degrees for devices 10016 and 11022 of tilt was not practicably achieved, hence maximum LET values of 20.76 and 21.4 MeV/mg/cm² were used during the tests. Table 7 summarizes the irradiations

Table 7

Summary of Heavy Ion irradiation

Fig. 7 shows the SEL cross section measured and the weibull fit for the raw data for both devices irradiated. Both devices showed a very similar behaviour with latch ups down to 15.9 MeV/mg/cm².

Fig. 7.

Measured cross section for SEL events for devices 10016 and 11022. Error bars are 1σ (left) and weibull fit of raw data (right)

Due to practical issues a higher than 40 degrees of tilt was not practicably achieved hence LET values of greater than 9 MeV/mg/cm² were not feasible using lower LET ions so given the low SEL count the minimum LET the devices were tested at was 15.9 MeV/mg/cm². The SEL rate and threshold estimated using OMERE analysis software. Table 8 shows mission daily SEL rate predictions for nominal background and solar flare conditions.

Table 8

SEL rate predictions for both UL03041 devices tested

(*

Assuming 1 large anomalous solar flare lasting 3 days)

The rates were calculated using the cross section plots provided in Figure 7 and OMERE analysis software. The Weibull parameters used were: W=25, S=2.4 and the saturated cross-section was taken at the 40MeV test point.

Table 9 summarises the SEUs measured for both devices at all the LETs tested. Any event during beam exposure that did not trigger an automatic power off but corrupted the image capture was recorded. The images were taken at a rate of approximately 1 in 13 ms and analysed back at SSTL. The images were analysed using Image J and the data correlated to the latch up log files created during the exposure at each run.

Table 9

Number of SEUs measured for each of the two detectors tested at different LET values