2.3.4 Hardware Modifications Plan
Since the THD payload was completely designed for a space application - the payload hardware needs minimal modifications. Assuming the THD payload structure survives and passes all integrated SAPPHIRE spacecraft system testing (i.e. shake, vib.), no hardware modifications are planned at this time.
Work that needs to be done
If determined necessary by the analysis of the spacecraft thermal model by the Thermal subsystem team, copper wiring between the DC to DC converter (which is the component that consumes the most power of any component in the THD circuitry) and the aluminum payload box structure should be added to provide a thermal pathway and minimize temperature buildup.
To eliminate any possible vibration during launch, a space compatible RTV should be used to secure loose connector wiring and components to the THD PCB.
2.3.5 Electronic Modification Plan Since the THD payload was completely designed for a space application - the payload electronics needs minimal modifications. For example, tantalum capacitors were selected instead of electrolytic capacitors because electrolytics contain an internal liquid that would vaporize since these components are not hermetically sealed. Also, due to the vacuum environment, the heat energy transfer mechanism is limited to radiation and conduction. This limited heat transfer can create large temperature variations. To minimize the effect of these temperature variations on circuit and component performance, high reliability components were selected. For example, ceramic instead of plastic op-amp packaging and 1% precision metal film resistors were utilized to minimize voltage drifts due to temperature changes. A second important modification was performed due to the harsh acceleration, shock, acoustic, and vibration loads that will be applied to the payload during launch. To ensure signal I/O continuity, all wires from the payload DB-9 connectors to the PCB were double wire wrapped and soldered. In addition, all solder joints on the PCB were carefully inspected visually.
Work that needs to be done
Assuming no electronic component problems occur during THD/SAPPHIRE spacecraft integrated system testing, no electronic modifications are currently planned.
2.3.6 Software Modification Plan
The software to run the THD must be written, incorporated into the SAPPHIRE mission software, and thoroughly tested. Preliminary software has been written to verify the SAPPHIRE CPU had the capability to activate the THD, sample and store THD output analog data, and deactivate the payload. However, more detailed software must be written and tested by the CPU team.
Work that needs to be done
write more detailed software code to operate the THD payload. Specifically, this code should active the THD, sample and store data (at 100 Hz), and deactivate the payload. Data should then be examined to verify THD payload operation. In addition, this code should be incorporated into the overall SAPPHIRE mission software and thoroughly tested.
2.3.7 Additional Future Work
At the THD payload component level, the only significant issue currently unresolved is the required hermetic (vacuum) seal of the TIS. Since the vacuum seal is critical to the successful space based THD payload operation, continued vacuum testing of the TIS will be required until a reliable solution is found. Currently it is believed that a standard commercial vacuum sealant, such as TORR seal, applied on top of the epoxy that currently secures the TIS package together will solve this problem. Once this vacuum seal issue is resolved, final THD payload system sensitivity testing will be performed to select the 2 "best" TIS's for the spaceflight THD payloads.
At the time of writing of this report, the final SAPPHIRE spaceflight hardware and components are being constructed and tested. Consequently, the SAPPHIRE spacecraft has not been built yet and the 2 THD payloads have not been integrated with the actual spacecraft. However, the THD payload has been integrated (electrical, mechanical, and software integration) into the prototype spacecraft system, Al Wood. Because of this prototype integration, the THD to SAPPHIRE integration process should consist of duplicating or "copying" the integration steps and procedures off of the integrated Al Wood spacecraft system. Some THD payload software modifications will be required as the final SAPPHIRE mission software is written and verified, but no electrical component or hardware modifications should be required. Continued THD payload testing as part of the integrated SAPPHIRE spacecraft system will be required to verify successful payload operation and noise rejection. This integrated THD payload testing will primarily consist of using the SAPPHIRE spacecraft CPU to activate the THD payload, sample the THD output data, and finally deactivate the payload. THD payload telemetry points will be utilized to verify payload operation. THD operational verification should include: verification of the high voltage module operation, verification of TIS tunneling, and verification that the THD payload responds to changes in input infrared radiation (by waving an object warmer than room temperature such as a hand in the THD sensor FOV and monitoring payload output telemetry data). Similar to the required testing of the other SAPPHIRE payloads and subsystems, this THD payload testing will need to be performed before and after each integrated SAPPHIRE spacecraft test, before and after ground transportation of the SAPPHIRE spacecraft to the launch site, before and after mating of the SAPPHIRE spacecraft to the launch vehicle, and during final pre-launch SAPPHIRE spacecraft checkout.
In addition to pre-launch integrated SAPPHIRE spacecraft system testing, THD payload operational support will be required after launch and throughout the SAPPHIRE mission. Initially, this operational support will consist of test and checkout of the THD and SAPPHIRE spacecraft (approximately the first 2 weeks of the SAPPHIRE mission). Also, THD payload data validation and performance verification, as explained in Chapter 6, will be required throughout the SAPPHIRE mission (nominally 1 year) to ensure successful "space evaluation" and to characterize the long term space exposure effects on the THD payload.
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