Power Systems

The power delivery system chosen for the Martian Global Positioning System is based on a solar powered design. Solar power was chosen for this application because it provides adequate lifespan while reducing fuel and weight requirements. The solar powered design also has a sufficient performance history allowing for predictable performance, which in turn enhances the effectiveness of the design. A diagram of the power delivery system is shown below.

The power delivery system is designed to allow the satellite to operate during times when solar power is available as well as when solar power is not available. The power is generated by solar panels during the time when the satellite is exposed to sunlight. The panels modules have an open ircuit voltage of 2.7V. Attaching the panel arrays in two parallel strings allows the voltage to double to 5.4V. This is sufficient overhead voltage to overcome the 0.7V blocking diode loss and still charge the 3.6V[1] batteries to a full charge (3.6V batteries require at least 4.14V to charge). The charge control regulator controls the rate of charge and prevents over-charge, thereby increasing the battery life. The blocking diode prevents the battery from discharging into the solar panels during periods of darkness when the solar panels would become an effective part of the load [2].

Power Requirements

The power required for operation of the satellite can be divided into two categories: the power required during periods when sunlight is available and the power required during periods when sunlight is not available. A breakdown of the power requirements can be found in table below.

Sunlight	Shadow
Station Keeping Power (Psk)	Station Keeping Power (Psk)
Broadbast Transmit Power (Pb)	Broadbast Transmit Power (Pb)
Satellite to Satellite Transmit Power (Psat)	Satellite to Satellite Transmit Power (Psat)
Battery Charging Power (Pbat)

The required transmit power levels were determined by the link budgets for the different radio links. The calculations of these power values can be found in the location systems section for the Satellite to Mars broadcast link and the Communications section for the Satellite to Satellite and Mars to Earth links.

The power requirements must also allow for the charging of the batteries during the time in the sunlight such that satellite will continue to operate in the time during the shadow. In order to ensure that sufficient power is available for the duration of the shadow period a safety factor, SF=1.5, is applied to the time the satellite is without sunlight. In order to determine the power requirements, it is first necessary to determine the amount of time the satellite spends in sunlight and the time spent in the shadow. The diagram below shows the geometric calculations that were used to determine the percentage of time spent in sunlight and shadow.  The calculations were done at perihelion which is 206x106 km with an orbital radius of 16,000 km [3] .

The calculations yield an angle of 24.512˚ during which the satellite is in the Martian shadow. The shadow angle is then used to calculate the amount of time in shadow. The computation of the shadow period is shown below.

Time spent in the shadow

Time spent in the sunlight

The peak power required occurs during the period of sunlight. The following power values were determined from the link budgets as previously state. The station keeping power includes the power required for the operation of the satellite electronics as well as the losses experienced throughout the power delivery process.

Solar Panel Requirements

The solar panels used in the power delivery system must be designed to meet the specifications set forth by the peak power requirements. The solar panels chosen for this application have a nominal efficiency, η=28.5%, at a voltage of 2.7 volts. The solar panel efficiency degrades over time at a rate of 2% every 2000 thermal cycles [4]. The degradation in efficiency of the solar panels is the limiting factor for the satellite power delivery system. In order to achieve the desired lifetime of 10 years an efficiency of 23% is used in the calculations instead of the nominal 28.5%.

To determine the area of the solar panels required to produce meet the peak power demand, the peak power demand is divided by the solar irradiance of Mars, 589.2 W/m2, times the efficiency of the solar panels [4].

This best-case solar panel area will provide sufficient power for the on-board systems to operate. In order to increase reliability, a cell area of 0.5m² (100% over-design) is used. This will ensure that degredation will not cripple the system. This also allows for the panels to be in a fixed orientation. Optimal solar absorption occurs when the panels are normal to the sun's rays. Angled light produces less power/m², however solar cells that have to be aimed increase complexity and weight. The 0.75m² larger panel will weigh less than the motors and assembly necessary to steer the solar array and the panels can be folded for the journey to Mars.

Battery Requirements

The batteries are the primary storage component of the power delivery system. The batteries chosen for this application are lithium-ion batteries that are space certified with a voltage of 3.6 V and a nominal capacity of 4.5Ah.[1]  The calculation for the number of batteries required to meet the storage requirements is shown below

The satellite requires one battery to meet the storage requirements for the power delivery system. The battery is designed to have a lifespan greater than 100,000 discharge cycles. This yields a projected lifespan of 210.7 years which is significantly longer than the lifespan of the solar cells. The batteries also must be heated to operate within the operating range of 0°C to 42°C [1].

Lander Power System

The lander that contains the synchronization clock, Mars to satellite diagnostic communications link and the Mars to Earth communications link. All of these systems require a power delivery system. The lander, which will be located at the North Pole, requires a much larger power delivery system because of the Mars to Earth communication link. The location of the lander as well as the amount of power required mandates a larger power delivery system. The Mars to Earth link requires 14.487 kW of transmission power. Additionally, the clock synchronization beacon requires 50 W of transmit power and the Mars to satellite diagnostic communications requires a transmission power on the order of approximately 0.5 W of transmit power. To accommodate these requirements a 20kW power delivery system with an Atmospheric Accumulator Fuel Processor is used. This system continually converts the Martian atmosphere (O₂ and CO) to electric energy using photolysis. This system requires that electrochemical reduction of O₂ and the oxidation of CO [5]. The excess available power beyond the Mars to Earth link, Mars to satellite diagnostic link and synchronization beacon is used for heating the lander, processing the data that is to be sent to the satellites as well as to and from earth and powering the Atmospheric Accumulator Fuel Processor. Heaters are required to remove the CO₂ ice which can form on surfaces.

[1] http://www.quallion.com/sub-sp-ql015ka.asp
[2] Class notes from ECE 6456 Solar Cells
[3] http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html
[4] http://www.emcore.com/assets/photovoltaics/BTJ_Web.pdf
[5] http://findarticles.com/p/articles/mi_qa3957/is_/ai_n9008214