Subsystem Top Level Requirements
-- The software shall be written in
the C programming language and PIC assembly language
-- The system shall record health,
status, and sensor data in a consistent manner in memory
-- The software shall be modularized
for easier modifications and on-orbit updating
Subsystem Functional Requirements
-- The software shall be compatible with
the PIC24HJ256 microcontroller
The software shall allow for the ground station to upload or reload
software modules to the PIC during flight
software shall be capable of disabling all subsystems except Command and
Data Handling (CDH) and Power (EPS)
software shall nominally read from each sensor every 30 seconds in an
organized and consistent manner and record the data to memory
entering any mode of operation, that mode shall be recorded to memory
along with a timestamp
Priority levels shall be assigned to the different modes of operation so
that more critical modes can interrupt less critical modes
-- The polling rate for sensors shall be adjustable from the
The Science subsystem, internally known as
High Speed Communications (HSCOM), is responsible for proving the
effectiveness and reliability of an S-band communications link for
A special note of thanks towards
Pumpkin, Inc. is necessitated here. Pumpkin donated the SalvoPRO Real
Time Operating System (RTOS) that is being used on the PIC
microcontroller to control the operations of the satellite.
Modes of Operation
The software subsystem is built upon a
modular design with several different modes of operation as listed
The beacon mode is the default mode for the satellite in the event
of a loss of contact for an extended period of time. In beacon mode, the
satellite enters a short transmission burst of 5 seconds, a slightly longer receive
time of 30 seconds, and then a wait period of 5 minutes for battery recharging. This loop repeats
until the ground station is able to contact the satellite and tell it to
enter normal mode.
Normal mode is the most frequently utilized mode of operation for
the satellite. In this mode, sensors will be polled, data recorded, and
other modes initialized.
In receive mode, the transceiver will be put in a receive state and
will be ready to receive commands from the ground station. The PIC will
decode the packets it receives from the modem and respond to the
During ground passes over Boulder, the satellite will enter transmit
mode. This is where it will power the transceiver and send data to the
ground. In order to prevent spurious transmissions, the satellite will
always have receive capabilities whenever it is in transmit mode. This
will allow for a command to cease transmission to be sent at any time
Power Save Mode
If the power system informs CDH that the batteries have dropped
below 40% of charge capacity, the satellite will enter power save mode.
In this mode, all non critical systems will be shut down and the
satellite will focus on returning the batteries to a safe charge level.
Once this level is achieved, normal operations will resume.
In the event that one of the thermal sensors detects a temperature
below the specified minimum, the satellite will enter heater mode. Since
the PCOM transceiver wastes over 5W of power as heat, it will be told to
transmit to generate heat within the satellite. Once the thermal sensors
verify that the internal temperature has risen back to operating range
or if the batteries become too drained, the satellite will leave heater
mode and resume normal operations or enter power save mode.
Error Handling Mode
If a sensor detects values that are outside the normal expected
range, the satellite will enter error handling mode and will focus its
attention on determining the cause of the anomaly. Timestamps and
actions will be recorded to the status data and can be sent to the
ground upon request. Once the anomaly is noted and/or precautionary
action is taken, the satellite will reenter normal operation mode.
In order to protect the satellite against radiation induced errors,
analog reset circuitry is built into the power subsystem. At a specified
interval of time, the power subsystem will reset all of the other
subsystems. A warning signal will be sent to indicate that reset is
approaching and the satellite will cease current operations and enter
reset mode. It will then save status of its current mode and operation,
as well as a time stamp for the reset. The system will then sit idle
until the reset occurs and will then reenter its previous mode of
Emergency Shutdown Mode
In the event that the Command and Data Handling (CDH) current
sensors detect excess current, such as that caused by a Single Event
Latch-up (SEL) in a transistor, the PIC will enter emergency shutdown
mode and immediately reset the CDH system. This will prevent the excess
current from destroying transistors and the microcontroller. During a
normal shutdown, a status flag will be set to indicate that the shutdown
is planned. During emergency shutdown, this flag will not be set and the
PIC will investigate the cause of the shutdown upon reboot.
Programming mode is entered when the PIC is preparing to reprogram
part of its internal flash memory.
Ground Support Mode
Ground support mode will be used prior to the launch for interacting
with the satellite via the Ground Support Equipment (GSE) connector.