E. Space Data Evaluation and Validation
Since the goal of this work is to "space evaluate" the Tunneling Infrared Sensor (TIS) and its sensing technique through the operation of the Tunneling Horizon Detector (THD) payload, several ground based data processing techniques to assess the performance of, and verify and validate the THD data are discussed. Initial (first 2 weeks of the SAPPHIRE mission), short term (< 3 months) and long term (> 3 months) THD operation will be analyzed to determine the cumulative effects of the space environment on the payload. In addition, a section on the effects of several potential sources of error is included. Note: the following sections assume nominal spacecraft operations including functional power, communication, and CPU including A/D converter, subsystems. If the spacecraft or spacecraft subsystems failing to operate nominally affect THD data collection and recovery, only the data prior to the failure will be used in evaluating the performance of the sensor.
Data Verification and Validation
The THD payload data will be sampled and stored onboard the SAPPHIRE spacecraft until it is downlinked to the ground. On the ground, this raw data will be processed against the expected results based upon the calibration and simulation testing, and the data verification and validation techniques explained in the next few sections.
Basic THD Data Processing and Validation
The primary THD output is an analog signal sampled at 100 Hz nominally. This signal consists of a 3.0 V signal + 1/400 of the tunneling high voltage + the infrared detected signal. Given the nominal SAPPHIRE spacecraft spin rate of 1 rpm and nominal orbit altitude of 500 km, the Earth will appear approximately 120 deg. wide, corresponding to a 22 second Earth transition between horizon crossings. Based upon the calibration tests, the space/Earth and Earth/space crossings should cause the signal to transition approximately minus and plus 40 mV respectively. Due to the constant 1 rpm SAPPHIRE spacecraft spin rate, the sensor data should be periodic repeating approximately every 60 seconds. Space sampled data from the THD sensor output voltage levels will be plotted as a function of time and verified as explained above. Some electronic data filtering may be utilized.
The high voltage level telemetry point will be utilized to verify operation of the high voltage module. A 0 V reading indicates the payload is not in operation or the high voltage module is not functioning, while a 1.8 - 2.4 V level indicates nominal high voltage module operation. THD payload temperature data might be utilized to correlate drift in the tunneling high voltage level of the TIS.
Data Comparison Between the 2 Infrared Sensing Elements Within Each TIS
Since each THD payload contains one TIS that consists of 2 infrared sensing elements with the same FOV and environmental conditions, the infrared output signals of each element will be compared. The Earth horizon detection of these signals should be perfectly correlated in time. However, due to slight sensing element variations (performance, sensitivity, and tunneling high voltage level) as explained in section 5.2, horizon crossing magnitude changes may be slightly different for each sensing element.
Data Comparison Between the 2 THD Payloads
The SAPPHIRE spacecraft contains 2 independent THD payload packages configured on opposite sides of the payload tray. This configuration corresponds to each THD payload seeing the same sensor FOV, but 180 degrees out of phase of each other. Given the nominal 1 rpm SAPPHIRE spacecraft spin rate, this 180 degree phase lag corresponds to a 30 second time difference between Earth horizon detections. The Figure below illustrates the expected time correlation between THD packages.
Figure 6-1 THD Payload Time Correlation of Data
Data Correlation with Solar Panel Telemetry Data
The Gallium Arsenide solar cells mounted on the exterior panels of the SAPPHIRE spacecraft are photovoltaics and thus detect photons reflected off the Earth's surface. Therefore, the solar panel telemetry data (when looking at a sunlit Earth (local daytime)) can be correlated with the THD output data to verify Earth horizon crossings. However, it should be cautioned that the Earth's albedo (or reflected photon energy from the sun) varies considerably more than the infrared radiation emitted by the Earth due to different soil conditions, ice, clouds, day/night terminator, and also seasonal effects. In addition, solar panel telemetry data will be utilized to verify the SAPPHIRE spacecraft spin rate.
Several factors that were not included in the Earth horizon crossing simulation and calibration testing performed for the THD payload have been categorized as "Error Sources". These Error sources include: the Earth's Atmosphere, clouds, diurnal and seasonal variations, Sun and Moon effects, SAPPHIRE spacecraft pitch, and payload temperature variations. Each of these "Error Sources" will affect the THD output data, however these errors should be small in relation to the Earth horizon signal, and can be qualified and quantified through ground based data processing and error source correlation. The expected effects of each source of potential error is detailed in the thesis. Effects of Earth surface (water and soil) variations are expected to be very small and is not currently included.
Back to THD Summary