IEEE - Aerospace and Electronic Systems - September 2019 - 42

The Iris Deep-Space Transponder for the SLS EM-1 Secondary Payloads

Figure 15.
Iris V2.1 measured carrier tracking threshold at 70-Hz LBW
setting for various FM units across temperatures.

Figure 13.
Iris V2.1 flight unit in the thermal chamber.

to run, the ease of testing is diminished. It should also
be noted that although Cassy is sufficient for testing
general performance and modes of flight radios, it does
not provide a pure high-fidelity replica of a proper DSN
ground receive system. As such, early compatibility
testing with the true high-fidelity ground station platform is necessary to reveal any idiosyncrasies that may
have been missed.

MEASURED TEST DATA
A summary of tests performed on the Iris transponder at
JPL and the Space Dynamics Laboratory are described
in this section. Most of the measurements have been
taken on the Iris Engineering Development Unit (EDU);
recent flight unit test data has been included for further
comparison.
For all ambient and thermal tests, the Iris transponder,
SSPA, and LNA are mounted to a baseplate as shown in
Figure 13. Thermal tests at both ambient and vacuum
pressures were performed at þ25 C, -20 C and þ50 C.

Additional test data have also been presented in the paper
by Kobayashi [9].

Carrier Tracking Threshold
Figure 14 shows the measured receiver carrier tracking
threshold at X-band on the EDU at ambient temperature
as a function of LBW settings. The theory line is the 6-dB
loop signal-to-noise ratio point including the measured
LNA NF of 3.4 dB and an additional 3 dB of implementation loss in the carrier-tracking loop (CTL).
Figure 15 is a summary of the measured carrier tracking threshold of several flight model (FM) units at 70-Hz
LBW setting across temperature as a function of LNA NF.
The measured values average to À150 dBm with a standard deviation of 3.2 dB. All measurements are well
below the requirement of -136 dBm.

Bit Error Rate (BER) Measurements
A comprehensive set of BER measurements were taken at
ambient on the EDU, and the results are presented in
Figure 16. The theory line is for an uncoded binary phase
shift key (BPSK) modulation, and the predict line includes
the additional 3-dB implementation loss from the CTL.
Rates above 500 bps are in well agreement with the predict
line (< 1 dB variation), but lower rates start to exhibit more
loss from the symbol synchronizer (up to 4.0-dB additional
loss at 62.5 bps). All measurements were taken using a predefined randomized sequence generated by the Cassy GSE
and uplinked on a 16-kHz sinewave subcarrier.

Downlink Carrier Performance

Figure 14.
Iris V2.1 EDU measured carrier tracking threshold at ambient.

The one-way phase noise of the X-band downlink carrier
shown in Figure 17 equates to 1.5 ps of rms jitter, integrated from 10 Hz to 50 MHz. Three distinct phase-noise
contributors can be seen in the measurement: reference

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2019

IEEE - Aerospace and Electronic Systems - September 2019

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