C. Tunneling Horizon Detector Payload Design

For many space missions, such as communication, Earth observation, and space exploration, the spacecraft must determine its inertial or relative position and attitude. Typically, an Earth sensor is one of the primary sensors utilized in attitude determination by the spacecraft. For low Earth orbit (LEO) missions, the Earth encompasses 35% of the celestial sphere and is the second brightest object (exceeded only by the sun). However, unlike the sun or other stars which are generally modeled as point sources of radiation, merely detecting the presence of the Earth in the sensor field of view (FOV) is insufficient for attitude determination. Therefore, nearly all LEO Earth sensors are horizon detectors that utilize the Earth/space horizon as the principal means for determining the attitude of the spacecraft with respect to the Earth. Most Earth horizon detectors consist of 4 basic elements: a radiance sensor, a scanning technique, a radiance collector, and electronics. The following sections briefly explains the chosen Tunneling Horizon Detector payload design including design tradeoffs and selection rationale.

Radiance Sensor

The element of the horizon detector that detects or responds to incident radiation is called the radiance sensor. Specifically, the radiance sensor can be defined as the transducing elements that produce an electrical signal proportional to the radiation incident upon the device. Since the goal of this work was to design a horizon detector utilizing the Tunneling Infrared Sensor as the radiance sensor, no radiance sensor tradeoff analyses were performed. However, it should be noted that the TIS's characteristics demonstrate it is an excellent choice for this application (high performance and sensitivity, small, low cost, and robust).

Radiance Collector

The radiance collector limits the FOV of the sensor, as well as, focusing and possibly filtering the incident radiation onto the radiance sensor. Since low cost and simplicity are the primary design drivers, a slit aperture was chosen as the radiance collector for the THD payload. The width of the aperture was sized to limit the FOV of the detector to 6 degree in the transverse spin plane. The rationale for a 6 degree transverse FOV was based upon the TIS's characteristics and the selected body fixed spacecraft configuration (scanning technique described in the next section). The boundaries of the aperture are sized to define the desired sensor FOV and ensure sufficient detectable incident radiation. The slit is simple, very inexpensive, highly reliable, and enables better characterization of the performance of the sensor element since the incident radiation is not filtered, focused or modified.

Scanning Technique and THD Payload Configuration

The scanning technique refers to the method or mechanisms utilized by the horizon detector system to search the celestial sphere. To ensure low cost, reliability, and simplicity, and due to the fact the SAPPHIRE is to be a spinning spacecraft, the natural selection was to fix the THD payload to the body of the spacecraft and utilize the spin of the spacecraft as the scanning mechanism. Therefore, it was determined that the THD payload would be housed inside the payload tray with its FOV(field of view) looking out a small slit aperture in the side solar panel of the spacecraft (as shown in the Figure below).

Electronic Circuitry

The two THD payloads function completely independently, each requiring 4 electrical inputs (12 V, gnd, 5 V, and a digital high voltage on/off control line). These inputs are connected to each THD payload from the SAPPHIRE spacecraft wiring bus using a DB-9 connector. The pinout for the payload input connector pins 1-9 respectively is as follows: 12 V, 12 V, gnd, gnd, 5 V, 5 V, gnd, gnd, HIV on/off.

Each THD payload has 4 output signals. Each output is an analog (0-5 V) signal with a corresponding ground line. These outputs are connected to the SAPPHIRE spacecraft wiring bus using a DB-9 connector. The pinout for the payload output connector pins 1-9 respectively is as follows: Analog1, gnd, Analog2, gnd, Analog3, gnd, Analog4, gnd, no connection. Note that between each analog signal line a ground wire was placed to minimize noise induced errors. Analog1 and Analog2 are the THD tunneling element output voltages. Each tunneling output voltage level consists of a 3.0 V signal + 1/400 of the tunneling high voltage + the tunneling signal disturbance. Analog3 is a measure (.008 * high voltage supply ) of the high voltage supply and is used to verify the operation of the high voltage DC to DC converter module. Finally Analog4 is a temperature reading (10 mV/K) for the payload near the TIS.

For final circuit testing a printed circuit board (PCB) was made. Since the spaceflight THD payload needed to be a small, robust electronic package, a PCB was required for the final payload circuitry. Also, PCB fabrication was needed since at least 5 separate THD payloads were required (2 for spacecraft, 1-2 for the ground satellite (Al Wood), and 1-2 for testing purposes). The final THD payload circuitry consists of 4 parts: a high voltage supply module, a voltage reference resistor network and 2 feedback modules to independently control the 2 sensing elements within each Tunneling Infrared Sensor. Each of these circuit elements is described in more detail in the thesis.

THD Payload Summary

The THD payload is a small, low power Earth horizon detector designed to be mounted to the spinning SAPPHIRE microspacecraft. The assembly contains a TIS with 2 independent sensing elements and corresponding electronic circuitry. The THD is sensitive to the entire infrared spectrum with an average sensitivity of 1,500 V/W. The assembly is fastened to the payload tray of the SAPPHIRE spacecraft and looks out a slit aperture in the side of the spinning spacecraft, perpendicular to the spin axis. The slit aperture limits the FOV to 6 degrees in the transverse spin plane and 22 degrees in the vertical plane. The slit aperture was sized according to the nominal 1 rpm spin rate of the SAPPHIRE spacecraft, the payload configuration and sensor characteristics (see section 4.2). The payload requires 4 inputs (12V, gnd, 5V and a digital high voltage on/off control line) and produces 4 analog (0-5V) outputs (tunneling element1, tunneling element2, high voltage level, and payload temperature). The tunneling element outputs are proportional to the level of variation in incident infrared radiation on the TIS and consist of a 3.0 V signal + 1/400 of the tunneling high voltage + the tunneling signal disturbance. The specifications are listed below in Table 4-1.

Mechanical and Electrical Hardware Design and Payload Shelf Configuration

The THD electronics is housed in a 0.040 in. thick bent Aluminum sheet metal box 1.8 " tall x 1.25 " deep x 7.2 " wide. To provide easy access to the internal electronics the Aluminum box enclosure consists of 2 pieces which fasten together using 4 #4 locking screw fasteners. Prior to launch epoxy should be applied to these screws to further secure the box. The payload box is secured to the payload shelf by 2 #6 aluminum screws which fasten the into inserts in the honeycomb payload tray. The payload electronics consists of a single double sided printed circuit board (PCB) 1.7" x 6.5" with a ground plane. All payload electronics including the Tunneling Infrared Sensor are soldered to the top surface of the PCB. The PCB is mounted on six 1/4" #4 standoffs and secured with locking nuts within the payload box enclosure. All electronic interface signals are transmitted through 2 DB-9 connectors that are located on one side of the payload. One connector is for the input signals (12V, 5V, gnd, and high voltage on/off). The other connector is for the 4 output signals (tunneling element1, tunneling element2, high voltage level, payload temperature and associated gnd wires).

Table THD Payload Specifications

mass 6 oz
size 1.8" x 7.2" x 1.25"
power 0.2 W (quiescent)
0.9 W (operation)
inputs 5V, 12V, Gnd, and a digital high voltage on/off line
outputs 4 analog signals (0-5V) and 4 gnd lines
(tunneling element1, tunneling element2, high voltage level, and payload temperature)
FOV aperture 6 deg. FOV in the spin plane
22 deg. FOV in the vertical plane
external slit aperture 0.17" x 1.7"
spectral band multi spectral (infrared)
Temperature constraints -10 - 60 deg. C (storage) and
0 - 50 deg. C (operation)
detector responsively 1,500 V/W
sample frequency 100 Hz nominal
(10 Hz minimum)
electrical connections 1 input signal DB-9 connector and
1 output signal DB-9 connector

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