IEEE - Aerospace and Electronic Systems - September 2019 - 49

Angkasa et al.

Figure 1.

Figure 2.

Nonregenerative ranging and telemetry: required PT =N0 (solid
curve) as a function of Rb with fT (dashed curve) optimized.

Regenerative ranging and telemetry: required PT =N0 (solid
curve) as a function of Rb with fT (dashed curve) optimized.

the exception of rTR , the same set of parameter values (see
Tables 1 and 2) is used here for both ranging schemes.
Appendix summarizes the equations used to calculate the
results given here.
Figure 1 plots, as a function of the telemetry bit rate
Rb , the PT =N0 that is required to support both nonregenerative ranging and telemetry, where the performance
and link parameters are given in Tables 1 and 2. The
telemetry modulation index fT has been selected, at each
point on the curve, for minimum PT =N0 . A second curve
showing the optimized fT as a function of Rb also appears
in Figure 1, with the corresponding vertical scale on the
right-hand side of the figure.
Figure 2 plots, as a function of the telemetry bit rate
Rb , the PT =N0 that is required to support both regenerative ranging and telemetry, where the performance and
link parameters are given in Tables 1 and 2. The telemetry
modulation index fT has been selected, at each point on
the curve, for minimum PT =N0 . A second curve showing
the optimized fT as a function of Rb also appears in
Figure 2.
A comparison of Figures 1 and 2 shows that a
smaller PT =N0 is required by regenerative ranging
with telemetry than by nonregenerative ranging with
telemetry. The relative advantage of regenerative ranging is most pronounced for small telemetry bit rates.
For example, regenerative ranging needs 18 dB less
link power for Rb ¼ 1 kb=s with the parameters considered here. The performance gap narrows as Rb
increases. Regenerative ranging needs 3 dB less link
power for Rb ¼ 100 kb=s with the parameters considered here. As Rb increases, the constraint Eb =N0 ! 3.0
dB plays a more prominent role.
Regenerative ranging outperforms nonregenerative
ranging because regeneration of the range code within the
transponder, using a small bandwidth, precludes uplink
noise from having a significant effect on the range

measurement. With nonregenerative ranging, in comparison, the ranging-channel filter must be wideband in order
to accommodate the range clock, and such a wideband
ranging channel causes downlink power to be wasted in
sidebands that convey uplink noise.
In regenerative ranging, advantage can be taken of the
higher SNR to achieve a higher probability of acquisition
and/or a small measurement integration time than is possible with nonregenerative ranging. This means that regenerative ranging permits quicker acquisition and more
measurements per tracking pass. The higher probability
of acquisition leads to simplification in operations.

SEPTEMBER 2019

IMPLEMENTATION IN IRIS
The implementation of a regenerative ranging system
closely follows the algorithm outlined by Hamkins et al.
[12] The general Iris radio system, as well as the regenerative ranging subsystem, is illustrated in Figure 3. In Iris,
the incoming RF signal is digitized and the incoming signal is input into the carrier-tracking loop (CTL). The
CTL, once locked, tracks the uplink carrier's time-varying
phase. The output of this system is the carrier signal
(in-phase component) as well as the ranging signal that is
extracted from the carrier (quadrature component).

Figure 3.
Iris radio block diagram: The Iris radio carrier tracking system is
utilized to remove bulk Doppler and other frequency offsets from
the data and extract the uplink ranging code.

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