IEEE Aerospace and Electronic Systems Magazine - July 2020 - 17

Davarian et al.
volume and mass. Because of this, it has been suggested
in the literature that combining these FPGAs into one can
decrease or eliminate the added overhead [10], [14]. With
large-capacity radiation-hardened FPGAs now available,
utilizing one FPGA board addressing the combined processing needs of command and data handling (C&DH)
unit, radio, instruments, etc., allows space and power overhead of the circuit board to be diminished, which is especially critical for small satellites.
The use of one radio for two or more applications such
as DTE and relay capabilities may also be beneficial to
reduce SWaP. Furthermore, investments in component
technologies could result in lower power, less volume,
and lower mass for a radio.

RISK POSTURE AND SMALLSAT RADIOS

Figure 5.
Iris version 2.1 is a software-defined radio that consists of four slices: Processer, power supply, transmit, and receive.

Due to the amount of power Iris draws during transmission, some of Artemis 1 users of Iris have to limit their
transmission period to preserve battery power and/or
avoid overheating the satellite. Therefore, based on the
Artemis 1 experience, it is felt that the priority for further
development should be radio power usage reduction. For
CubeSats, lowering mass is another desired goal because
at 1.2 kg, the weight of the radio is about 10% of a 6U
CubeSat's overall mass-note that 1.2 kg does not include
the low-noise amplifier or the high-power amplifier.
The Frontier Radio [4], [10] is another possible option
for use in SmallSats. This radio offers X-band communications, coherent two-way range and Doppler, and DOR tones.
Options exist for S-band receive and transmit and Ka-band
transmit slices to be added. Data rates up to 75 Mb/s are possible at X-band. When this radio is connected to a 30% efficient solid-state power amplifier (SSPA) with 4W RF
output, it consumes 23W of power. The radio mass and size
are 1.9 kg and 1.7U, respectively, with a form factor of
$11.7 Â 16.0 Â 9.4 cm3 (X-band receive and transmit). The
smaller form factor Frontier Lite [11] is another option, but
would need some additional work for flight readiness.
For LEO applications, Vulcan Wireless offers a software-defined radio transponder that operates at S-band, and it
has been flown on multiple LEO missions. Its size is less than
1/2 U [12]. Similarly, IQ wireless offers a high data rate XBand software defined radio for LEO applications. This radio
can operate at deep space X-band frequencies [13].
An approach that could potentially reduce mass and
power requirements of SmallSats may be sought through
combining spacecraft digital processing with the radio's
digital segment on a single-board computer. In most current spacecraft designs, each subsystem has its own FPGA
board. Each separate FPGA has a certain overhead of quiescent power and also supporting circuitry that takes up
JULY 2020

In order to save cost, SmallSat missions typically assume a
higher risk profile than primary missions. However,
because the radio must last for years when cruising to a
deep space destination and must provide significant telemetry, tracking, and command (TT&C) during those years,
it is advisable to design the radio to a lower risk level. The
radio needs to be more dependable than most other parts
of the satellite because it provides the link to Earth and
success of the mission critically depends on it.

CHOICE OF FREQUENCY BAND AND SMALLSAT RADIOS
Most DTE links of deep space vehicles in the recent past
have used X-band for data transmission to Earth. Due to
investments made by the DSN during the last two decades
[15], deep space missions have recently begun to use Kaband and take advantage of its benefits [16]. For SmallSats, Ka-band downlinks offer two advantages. a) X-band
is already very crowded and there are likely to be many
more small missions than large ones in the future, so the
use of Ka-band eases the ongoing channel allocation problem. b) Ka-band links provide more gain than X-band
links for the same amplifier power and antenna size.
Hence, if SmallSats baseline their downlink at Ka-band,
they would enjoy the above benefits.

RANGE AND DELTA DIFFERENTIAL ONE-WAY RANGE
Two important measurements that navigators use to navigate
a deep space satellite are range (distance) and the angular
direction of a spacecraft (position on the sphere). The technique used to estimate the angular direction is called Delta
Differential One-Way Range (DDOR). In this section,
improvements on these two observables will be discussed.
Spacecraft range is measured by sending an uplink signal
(range signal) to a spacecraft and having the spacecraft return
that signal to the ground. The phase difference between the
received signal and the transmitted signal can be used to estimate range [17]. The standard practice at the DSN has been

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