Aerospace and Electronic Systems - April 2019 - 24

The CubeSat Mission to Study Solar Particles

Figure 6.
(A) VHM sensor is a compact version of previous sensors, optimized for use with a laser pumping source. (B) VHM electronics, including
the diode laser, are contained in a compact 3-D printed aluminum chassis.

23S1 level, depending on the sense of the circular polarization, but when reemitted can return to either state. Therefore, in equilibrium there are fewer atoms in mJ ¼ þ1
state; this process is referred to as optical pumping. The
VHM works by sensing the change in optical pumping
efficiency of this population of metastable helium atoms,
as a function of the angle of an applied magnetic field to
the sensor's optical axis. The optical pumping efficiency
depends on the angle between the magnetic field and the
optical axis of the sensor. A rotating magnetic field at
frequency fo , applied via Helmholtz coils surrounding the
cell, produces a variation in the optical pumping efficiency
as the angle between the rotating field and the optical axis
of the cell changes; therefore the light intensity transmitted through the helium is modulated at a frequency 2fo . In
a nonzero external field, the superposition of the external
field with the rotating field causes additional modulation
of the intensity at fo . The VHM works by applying an
additional nulling magnetic field via the Helmholtz coils
to zero out the fo modulation. This nulling field is equivalent to the external magnetic field, and a measurement
of the current needed to generate it is proportional to this
field. Table 2 summarizes the VHM performance and
resource requirements for CuSP.
Recent developments have focused on reducing the
instrument mass and volume of the JPL-developed magnetometers, resulting in an instrument with a sensor mass
of 155 g, and an electronics box and cables mass of
660 g. Developments include upgrading the instrument
electronics to be largely FPGA based [see Figure 6(B)],
and replacing the pumping lamp with a diode laser. The
latter has enabled a significant reduction in sensor size
[see Figure 6(A)], and together these changes have produced an order of magnitude decrease in instrument
mass. This instrument was developed and delivered for
the Interplanetary Nano-Spacecraft Pathfinder in Relevant
Environment (INSPIRE) CubeSat mission and the CuSP
VHM is derived from this instrument.
The VHM has a measurement range of Æ1000 nT for
each magnetic field vector component and a sensitivity of
<20 pT, which is more than adequate to achieve the
24

magnetic field measurement requirements needed to
address CuSP science objectives. The s/c magnetic field
detected by VHM is removed using an approach similar to
that commonly employed for spaceflight magnetometers,
i.e., the "Davis-Smith technique" described by [19],
which takes advantage of the fact that most of the spectral
power in IP magnetic field fluctuations is in direction and
not magnitude. An offset in any magnetic field component
will cause a correlation between that component and the
field magnitude-this correlation is a minimum when the
appropriate offset is subtracted from the magnetic field
component. It has one operating mode, with a single
20 bit data range, and can produce data at several cadences
(from 0.1 s). Instrument control and data handling are
provided by an FPGA-based state machine.

BUS AND ANCILLARY SYSTEMS
CuSP is a 6U CubeSat developed and based on the SwRI
Southwest LEO EXplorer (SLX-6) [20] with accommodations for deep-space operation. It consists of five systems,
shown in Figure 7(A) and described below, necessary
to support s/c and payload operations: Command and
Data Handling System (C&DH), Electrical Power System
(EPS), Attitude Determination and Control System
(ADCS), Telecommunication System (RADIO), and
Mechanical, Structural, and Thermal System (MST).

COMMAND AND DATA HANDLING SYSTEM
The spacecraft and its highly autonomous mission operations are controlled by the SATYR [see Figure 7(B)], a
single board computer (SBC) designed and developed by
SwRI. SATYR is a heritage system based on the SwRI
CENTAUR SBC [21] and has been reformatted for the
CubeSat form factor. CENTAUR is currently flying on the
eight microsatellites of the Cyclone Global Navigation
Satellite System (CYGNSS) Mission. CuSP's SATYR is
an input/output (I/O) reduced version of the CENTAUR.
The SATYR consists of SwRI's space-qualified heritage

IEEE A&E SYSTEMS MAGAZINE

APRIL 2019

Aerospace and Electronic Systems - April 2019

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - April 2019