ANDE Risk Reduction Mission - Design and fabrication of satellites

5/12/2005 - 22-05s
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The primary goal of the risk reduction mission is to test and verify the capabilities of the orbit insertion mechanism. Secondary goals for the mission include spin axis and orientation measurements, space qualification of the backup communications system, and space qualification of copper-indium-gallium-diselenide (CIGS) photovoltaic arrays.

Mock ANDE Active satellite -

The MAA spherical 50 kilogram (kg) satellite is 19 inches in diameter and constructed from two anodized aluminum hemispheres. The hemispheres were formed by spin-casting aluminum. They were then rough machined, heat treated, machine finished, and finally, anodized. The equator of this sphere was constructed from an engineered polymer material that is a durable, lightweight, low-wear, low-friction plastic that acts as a non-conductive separator between the hemispheres. This allows the satellite's shell to perform as a dipole antenna for the communications system developed at the U.S. Naval Academy. The communications system is powered by four battery boxes, which gives the satellite an estimated lifetime of about 1.5 years. The payload is split into the two hemispheres, with two battery banks, a communications box, and the laser driver box stacked in a vertical configuration.

The MAA sphere is painted with a pattern of four 90 longitudinal segments, alternating bare black anodized aluminum and gloss white paint. The purpose of this paint scheme is two-fold; to provide an easy visual pattern for observing the initial spin rate and orientation, and to provide means to determine spin rate and orientation from the polarization return to be observed from the Air Force Maui Optical and Supercomputing (AMOS) facility.

Onboard instrumentation for the MAA satellite consists of a set of six CIGS photovoltaic cells that are mounted flush with the surface of the sphere. These light sensors are located at the endpoints of three nearly orthogonal axes and are used for attitude and spin rate determination. Thermistors (thermally-sensitive resistors whose resistance changes with temperature) are placed at several points within the satellite to monitor the temperature of the various components of the satellite. The temperature and photovoltaic voltage values are telemetered to the ground by a "heartbeat" communications system that activates for 2 seconds out of every 20 seconds. If a ground station is detected, the data are transmitted; if not, the system returns to a sleep cycle for another 20 seconds. A set of six laser diodes, also located at the endpoints of three nearly orthogonal axes, are programmed to turn "on" during passes over Maui. These diodes emit light at 810 nanometers (nm), which will be observed from the AMOS facility.

Fence Calibration satellite -

The ANDE risk reduction FCal satellite is a 17.5-inch sphere with a mass of 75 kg. Its name is derived from its intended use as a calibration target for the radar fence, a space surveillance system recently transitioned from the Naval Network and Space Operations Command to the Air Force 20th Space Control Squadron. The size requirement of FCal satellite was determined by the resonant frequency of the Navy radar fence. The two hemispheres are fabricated by spin-casting brass, followed by machine finishing. The exterior of each hemisphere is nickel coated for durability in the harsh space environment. The equator consists of an anodized aluminum deck that incorporates an antennae deployment system, and mounting locations for the FCal payload. A cubesat ballast mass is included for center of gravity symmetry.

The FCal satellite will have the same geometry as the MAA satellite for its for the retro reflector array. It will also contain temperature sensors and sun sensor modules distributed similarly. The FCal satellite is different from the other ANDE satellites in size, mass, finish, and payload. The size and mass differences will cause the satellites to separate over time into lead-trail orbit as the atmospheric drag causes a greater retardation of the larger and less massive MAA sphere. Consequently, the greater mass of the FCal sphere will enable it to have a longer lifetime before reentry, two years as compared to one year. By having two co-planar satellites, scientists can gain a better understanding of the geographic and temporal variations of the local atmospheric density at these altitudes. Attaining the necessary mass of the FCal satellite was one of the main challenges of designing this particular satellite.

The FCal satellite contains another microsat as a payload: a cubesat, a cube-shaped microsatellite that stores its experiments like shelves in a cabinet. The cubesat is a four-inch pico-satellite originally designed as an educational platform. It is a fully functional satellite with solar power (not used in FCal), battery power, a computer, and a communications subsystem. It can accommodate small payloads in the internally available space. The cubesat in Fcal was developed by Ivan Galysh, a member of the StenSat group working in the Space Applications Branch of NRL's Naval Center for Space Technology.

When the FCal is deployed, the processor boots and initializes all the satellite components. It then waits one minute and deploys the antennas one at a time. There is a one-second delay between the deployment of each antenna. Once deployed, the satellite starts monitoring all the sensors and internal parameters and transmits the telemetry once every two minutes. The satellite can accept commands during this period. The telemetry transmission interval can be set and the receiver audio can be routed to the transmitter to turn the satellite into a transponder. Telemetry data includes voltage and temperature from each of the six phototransistors, battery and power data, and internal temperature data. The purpose of the telemetry is to determine the spin rate and orientation of the FCal spacecraft.

The arrangement of the MAA and FCal satellites in a lead-trail orbit provides an exceptional set of targets to study drag modeling and its effect on satellite drag. This mission will demonstrate the effectiveness of flying low-cost calibration targets to support DoD and NASA requirements for precision orbit determination and collision avoidance. In addition, data from this mission will be used to improve the scientific understanding of the interaction between a spacecraft and its environment.

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