Micromaps host Satellite Design Proposal

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2.4.1 Power Requirements
2.4.2 Power Generation

2.4 Power

The power needs of every component on the satellite are supplied by the power system. The power system is comprised of the power generation system and the energy storage system. The power generation system collects and converts solar energy into electrical power. The energy storage system provides power to the satellite components during periods of eclipse. The power requirements of the satellite are used to design the power generation and energy storage systems.

2.4.1 Power Requirements

The components on the VASCAT require conditioned electrical power to function. Each component has a specific power and voltage requirement. The power system must meet the needs of all the satellite components. Table 18 is a list of all the satellite components and their power requirements.
Table 18: Component power requirements

System	Device	Average power(W)	Peak power(W)	Voltages (V)
Comm:	Uplink	1.5	3	28
	Downlink	3	5	28
	AMSAT	1.8	2	28
	Dual single board computer	3.5	4	28
ADCS:	Momentum Wheel	9	25	28
	Magnet Torque Bars	4.2	5.4	28
	Magnetometer	0.0008	0.001	28
	Magnetometer Board	0.036	0.037	28
	Earth Sensors	5	8	28
	Earth Sensor board	0.12	0.6	28
	Sun Sensor	2	2.5	28
	Rate Gyro	1.5	2.1	28
	MT and RG Board	0.5	0.8	28
UMAPS:	Calibrate	27.2	27.2	28
	Normal Operation	16.2	16.2	28
GN&C:	GPS	3	5	28
Camera:	Camera	3	3.5	28

The power system is modeled from the peak power requirements to ensure that it supplies enough power to the satellite. A power budget quantifies the amount of power required for a single orbit, and it shows which components operate during daylight and eclipse. If the power system provides enough power to fulfill the budget for peak power needs, it is sufficient for all other cases. The VASCAT runs off of a 28 V bus voltage.

Table 19: Daylight and eclipse power budget

		Daylight		Eclipse
Operation:	Power (W)	Duration (s)	Energy (W-s)	Duration (s)	Energy (W-s)
Uplink	3	600	1,800	0	0
Downlink	5	600	3,000	0	0
AMSAT	2	3,500	7,000	2,150	4,300
Dual single board computer	4	3,500	14,000	2,150	8,600
Momentum Wheel	25	3,500	225	2,150	19,350
Magnet Torque Bars	5.4	3,500	6,480	0	0
Magnetometer	0.001	3,500	3.5	2,150	2
Magnetometer Board	0.037	3,500	128	2,150	79
Earth Sensors	8	3,500	28,000	2,150	17,200
Earth Sensor board	0.6	3,500	2,100	2,150	1,290
Sun Sensor	2.5	3,500	8,740	2,150	5,370
Rate Gyro	2.1	3,500	7,340	2,150	4,510
MT and RG Board	0.5	3,500	1,750	2,150	1,075
Calibrate	27.2	120	3,260	0	0
Normal Operation	16.2	3,500	56,600	2,150	34,820
GPS	5	3,500	17,500	2,150	10,750
Camera	3	3,500	10,500	0	0

Table 19 is a power budget of all the components showing the peak power, duration of operation, and energy during daylight and eclipse. These power requirements are used to design power generation and energy storage systems.

2.4.2 Power Generation

The power generation components fulfill the daylight power budget and adequately charge the energy storage system. Power generation system options include fuel cells, radio-isotope thermoelectric generators (RTG), and solar arrays. Fuel cells are not used for the VASCAT because of their relatively short lifetimes. Radio-isotope thermoelectric generators are too large and politically impractical because of their radioactive contents. VASCAT uses arrays of solar cells that convert solar energy from the sun into electrical energy. This method of power generation has extensive space heritage. Solar-to-electric energy conversion is a beneficial method of power generation for the VASCAT because solar energy is an inexhaustible resource and is easily harnessed. The amount of energy the VASCAT receives from the sun remains relatively constant over the 3 year lifetime.

The efficiency of solar cells degrades over time due to prolonged exposure to solar radiation. The severity of the lifetime degradation is different for each type of solar cell. These effects range from two to four percent of the power produced each year. The VASCAT uses a Gallium Arsenide, single-junction solar cell made by Spectrolab with low lifetime degradation of three percent. They convert solar energy into electrical energy at an electric potential of around 0.9 V.¹⁶

The VASCAT’s power requirements make body-mounted solar cells a feasible option. The solar cells are connected in series to produce the required 28 V bus voltage. There are 11 strings total on the side and top panels connected in parallel to provide the necessary power for the bus. The power generated by a solar cell, P, depends on the incident angle of the sun to the cell, :

2-15

In this equation P_o is the maximum power generated by a solar cell. Because the MAPS instrument requires the satellite to point at the earth which defines the variation in , we model the power generated over one orbit is created. Figure 11 is a plot of the power from each of the faces of VASCAT over one orbit.

Figure 11: VASCAT power model

In this model, the satellite is in a circular orbit with altitude 500 km, and the body frame of the satellite is always aligned with the orbital frame. This assumption is accurate with the satellite pointing requirements. The model starts as the satellite is eclipsed and no power is generated. Ninety percent of the side panel area and 10% of the zenith and nadir area is covered with solar cells. This configuration accounts for the minimum power generated halfway through the orbit as the satellite eclipses Earth, and the zenith surface is facing the sun. The model runs for the maximum off-nadir attitude for the instrument to ensure that the power system meets the needs of the satellite in any normal operation orbit.

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