Aerospace and Electronic Systems Magazine June 2017 - 37

Macˇ iulis and Buzas

Figure 4.

Solar array electrical circuit.

COMMUNICATION (COM) SUBSYSTEM
Figure 3.

Internal satellite hardware layout (solar panels and science unit removed).

The solar arrays are mounted on 4 sides of the spacecraft with
2 panels per side - one fixed panel (panel B) and one deployable
panel (panel A). The deployable panels ensure aerodynamically
stable spacecraft attitude that is explained later in the ADCS section. Pairs of panels A and B are connected in parallel. The electrical circuit of panel A and B solar arrays are shown in Figure 4. Each
panel has 9 solar cells connected in series with by-pass diodes for
every 3 cells in series and one for an unshaded cell in panel B. The
unshaded cell is the cell that would still be illuminated even when
the solar panels are not deployed and all remaining cells are shaded
by the solar panel structure. This ensures that batteries can be recharged in case the satellite is deployed with discharged batteries
and deployment does not occur in nominal scenario.
Each solar cell is made of monocrystalline silicon, rated to
19.4% efficiency. The total array efficiency is however not more
than 17% due to the losses caused by protection cover glass, bypass and blocking diodes. Blocking diodes ensure that solar arrays
do not draw current from the system when not illuminated by sun.
Solar cells are assembled on the panel substrate using NASA qualified conductive adhesive to reduce thermal gradients in the cells
and avoid additional wiring.
The PCDU ensures maximum solar power harvesting from the
arrays with the help of the maximum power point tracking (MPPT)
algorithm. The extra energy is stored for eclipse operations in two
series connected Lithium Ion 18650 batteries with 2700 mAh total
capacity.
JUNE 2017

The communication subsystem consists of a UHF radio transceiver, RF splitter, and a quad monopole antenna array.
The system block diagram of the UHF radio is shown in
Figure 5. The UHF radio utilizes a half-duplex architecture with
high performance low power consumption transceiver, which is
controlled by a dedicated Cortex™-M4 microcontroller. It uses the
UHF band that is programmable in the 433 to 440 MHz range. The
receiver has a sensitivity of -120 dBm at 9600 baud rate which is
the minimum mission requirement for the data rate. James Miller
G3RUH, a subtype of frequency shift keying modulation for radio amateur AX.25 protocol, is used to transmit data. A 435-438
MHz band pass filter was selected for the radio amateur satellite
frequency band.
The power amplifier and system logic are supplied by separate
3.3V power lines for electromagnetic compatibility reasons. The
radio has 3 UART and 1 CAN data interface for communication
with external subsystems. 64 MB of NOR flash memory is connected via SPI data bus and is used to store mission data. The power
amplifier is operating at constant gain with output power depending on the input signal level from the transceiver. The maximum
RF output power is limited to +33 dBm at about 33% efficiency.
The current consumption of the power amplifier is monitored by
the microcontroller and in case an overcurrent event is detected the
power amplifier is switched off automatically.
RF splitter consists of RF input connector, a matching circuit,
and 4 RF output connectors. The function it performs is to split one
RF signal into a phasing network in order to form a single circular
polarized antenna. This results in almost omnidirectional radiation
pattern with no blind spots, which can cause signal fading due to
tumbling satellite.

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine June 2017