C. THD Payload Operation Overview
The Tunneling Horizon Detector (THD) is a fixed narrow FOV horizon detector. Its primary output is an analog signal that triggers on the Earth/space and space/Earth horizon crossings as illustrated in the Figure below. The SAPPHIRE spacecraft spin axis will be roughly aligned with the North/South magnetic Earth alignment at low latitudes (near the ascending and descending nodal crossings). As the latitude of the spacecraft increases, the spacecraft pitch angle will increase causing the SAPPHIRE spacecraft to "flip" over at each pole crossing (North and South poles of the Earth). At high latitudes (> 65 deg.), the spacecraft pitch angle will become large enough that the THD FOV will not be able to view the Earth. Consequently, primary operation and testing of the THD will be performed +/- 20 deg. latitude of equator. Although the THD occasionally will be operated for an entire orbit to help characterize the spacecraft pitch variation.
Since the Tunneling Horizon Detector is a payload and not an integral element to attitude control operations, the payload is not always activated. Instead, the THD payload is scheduled according to the SAPPHIRE mission operations plan. To operate the THD, a file will be uploaded to the SAPPHIRE spacecraft from the SAPPHIRE Mission Operations Control Center that contains the time to turn on the THD payload, the output sample frequency, and the time to turn off the payload. At the appropriate time, the SAPPHIRE on-board computer will activate the THD payload (THD-A, THD-B or both), sample and store the payload data, and then turn off the payload. The nominal sample frequency will be 100 Hz per sensor channel per Tunneling Horizon Detector. The exact mission operations and payload scheduling is contained in the SAPPHIRE Mission Operations Document, but a brief description of the proposed operation is included below.
Nominal operations are divided up into science, and normal mode. Earth sensor testing and data collection will primarily be performed during science mode. As mentioned above, the payload will primarily be operated 20 minutes per orbit within +/- 20 deg. of the equator. With a 90 minute nominal orbit, the THD will be activated for 10 minutes near the ascending nodal crossing (equator), deactivated for 35 minutes, activated for 10 minutes near the descending nodal crossing (equator), and deactivated for the remaining 35 minutes. Given the nominal SAPPHIRE spin rate of 1 rpm and assuming the payload is operated for 20 minutes per orbit, 20 Earth horizon crossings should be detected (by both sensing elements in both THD payloads). Nominal THD output sample frequency is 100 Hz. At 12 bits per sample and 20 minutes of operation for each orbit, 180 Kbytes of data per channel per THD per orbit will be generated. Because a maximum of 200 - 400 Kbytes of CPU memory is available for the THD data, payload data management will be required. The THD data will be downlinked at the first available ground station pass. At a downlink rate of 1200 baud and assuming a ground station pass of 20 minutes, all 180 Kbytes of payload data can be transmitted from the SAPPHIRE spacecraft to the ground in one pass. However, in practice transmission errors and other required SAPPHIRE communication information limit typical ground station passes to 70-90 Kbytes of data, thus requiring 2 ground station passes to transmit all 180 Kbytes of payload data.
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