IEEE - Aerospace and Electronic Systems - September 2019 - 48

Regenerative Ranging for JPL Software-Deﬁned Radios
Table 1.

Table 2.

Performance Requirements

Link Parameters

Range measurement
Pacq
sR

! 0.95
0.5 m

Telemetry
Eb =N0

! 3.0 dB

Carrier
rC

Regenerative
ranging þ
Telemetry

1 MHz

1 Hz

0.22 rad rms

rTR

À25 dB

þ37 dB

! 10 dB

performance, where a smaller required PT =N0 indicates
better performance. As discussed below, regenerative
ranging requires less PT =N0 than does nonregenerative
ranging.
The required PT =N0 depends on the telemetry bit rate.
In order to calculate the PT =N0 required to support both
ranging and telemetry of a given bit rate, it is first necessary to define acceptable performance. Table 1 lists the
performance parameters used in the calculated results
reported here for both regenerative and nonregenerative
ranging. The parameter values of Table 1 are typical for
downlinks to the DSN [9], [10].
For range measurement, the performance parameters
are the probability of acquisition Pacq and the standard
deviation s R (meters) of the measurement error due to
noise. A measurement is correctly acquired only if the
measurement ambiguity is resolved. (The ambiguity arises
in the periodicity of the range code.) For a set of range
measurements to be useful, Pacq must be close to 1 [9].
Many deep-space missions require range measurements
that are accurate to about 1 m [11]. Since there are other
sources of measurement error besides noise, such as calibration error, the error s R due to noise is typically
required to be smaller than 1 m.
For telemetry, the key parameter is the ratio of the
available signal energy per bit Eb to the noise spectral
density N0 , where both Eb and N0 are referenced to
the same point in the receiving chain. The ratio Eb =N0
that is required for successful detection of telemetry
depends on the error-correcting code in use on the
link. Eb =N0 ! 3.0 dB is a reasonable constraint for the
typical error-correcting code used with deep-space
telemetry [10].
For downlinks to the DSN, both telemetry and
ranging are phase modulated onto the carrier in such a
way that a residual carrier is also present. Carrier synchronization, an essential part of the detection process
in the receiver, is done with a PLL tracking the residual carrier. The SNR rC in the carrier loop should be
better than 10 dB [10].
48

fRC

Nonregenerative
ranging þ
Telemetry

The required PT =N0 also depends on link parameters.
The values used for the link parameters are listed in
Table 2. fRC is the frequency of the range clock. BC is the
noise-equivalent bandwidth of the receiver's carrier loop.
fR is the phase deviation of the downlink carrier, within
the spacecraft transponder, due to signal plus noise in the
transponder's ranging channel. rTR is the SNR in the
transponder's ranging channel. These link parameters
have all been assigned typical values for the sake of the
analysis presented here. In any event, regenerative and
nonregenerative ranging are assigned the same set of values for the link parameters fRC , BC , and fR ; thus, the relative performance of these two ranging schemes may be
judged by these three values.
The telemetry modulation index is another important
link parameter. In this study, it is treated as a variable,
over which an optimization is done. For this reason, it
does not receive a value in Table 2.
The SNR rTR has a large effect on the relative performance of the two ranging schemes. For nonregenerative
ranging, the bandwidth of the transponder's ranging channel is usually about 1.5 MHz, so as to pass a range clock
with frequency fRC ¼ 1 MHz. The uplink noise is (like
the downlink noise) white, so the noise power in the ranging channel is the noise spectral density of the uplink noise
times this large ranging-channel bandwidth. At planetary
distances, rTR is, for nonregenerative ranging, very small,
and under these conditions, most of the power in the ranging channel is noise, rather than signal. Therefore, for nonregenerative ranging, substantial downlink power is
wasted in noise modulation sidebands. The assignment
rTR ¼ À25 dB is typical for nonregenerative ranging at
planetary distances.
For regenerative ranging, the code-tracking loop in the
transponder will have a narrow bandwidth (1 Hz is a practical value), so the SNR in the transponder's ranging channel is large, even at planetary distances, and there is a
negligible amount of noise modulated onto the downlink
carrier.
The PT =N0 that is required to support both ranging
and telemetry equals the minimum PT =N0 that is consistent with all the performance constraints of Table 1. With

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