Micromaps host Satellite Design Proposal



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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 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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