Instrumentation & Measurement Magazine 25-1 - 88

A Method for Measuring the
Group Delay of an On-Orbit
Satellite Transponder
Wei Wang, Xuhai Yang, Weichao Li, Fen Cao, Pei Wei, Liang Chen, and Zhigang Li
A
s one of the important parameters of the satellite
transponder, the group delay of the transponder
not only determines the magnitude of the signal
transmission delay that is generated by the system or network
but is also closely related to the signal transmission distortion,
which has a substantial impact on the signal transmission
quality. When the satellite is in orbit, it is difficult to measure
this index due to the particularity of its position. To overcome
this problem, this paper proposes a method for measuring the
group delay of an on-orbit satellite transponder. By using two
sets of ground station antenna systems that are installed in
parallel to conduct the observation of the transponders to be
measured simultaneously in the mode of " receiving its own
station-disseminated signal, " the measurement results of the
group delay of the transponders can be obtained after the calculation.
Based on the orbit determination by the transfer
tracking system of the National Time Service Center, a group
time delay measurement experiment of the C5B transponder
of the APSTAR-7 satellite in orbit was conducted. The results
of an analysis of the data demonstrate that this method can effectively
measure the group delay of a satellite transponder
and that the root mean square of the two measurement results
is less than 0.2 ns, which provides a useful reference for further
optimization and improvement of the orbit determination accuracy
of the system and provides a performance evaluation
service for satellite companies.
Introduction to Satellite Transponders
Satellite transponder is a connecting device that provides a
link between a satellite transmitting antenna and a receiving
antenna. It is the center of satellite communication. Its function
is to enable the satellite to receive, process and retransmit signals
[1]. As one of the main components of the satellite payload,
the basic requirement of the transponder is the transmission of
the radio signal effectively and reliably for all stations with
minimal additional noise and distortion and with sufficient
working frequency bandwidth and output power [2]. According
to the frequency conversion method and the transmission
signal form, satellite transponders can be divided into three
types: single-frequency-conversion, dual-frequency-conversion,
and on-board processing transponders.
A single-frequency-conversion transponder, which amplifies
the input signal directly, converts the frequency to the
downlink frequency, and transmits it to the ground through
the antenna after power amplification. This type of transponder,
which is also known as a transparent transponder, has
a simple structure and reliable performance and has been
widely used in scenarios with limited payload and power
supply.
A dual-frequency-conversion transponder, which converts
the received signal to an intermediate frequency (IF) after limiting
the amplitude, converts it to the downlink frequency,
and finally transmits it from the antenna to a ground station
through a power amplifier; this type of transponder has the advantages
of high forwarding gain, stable circuit operation, and
narrow IF band, but it is not suitable for multicarrier operation.
An on-board processing transponder, which performs
signal processing in addition to signal transmission, has the
advantage of flexibility and the disadvantage of a complex
structure. In two-way satellite time and frequency transfer
(TWSTFT), the first transparent transponder is typically used.
The satellite transponder is the space part of the TWSTFT
equipment configuration, and research on its performance is
more difficult and complex than that on ground station equipment
[3]-[5]. The group delay characteristic is an important
index of a satellite transponder. In a satellite transponder test,
the group delay measurement is an important test item for
measuring the phase linearity of a continuous signal that is
passing through the transmission system. It not only determines
the magnitude of the signal transmission delay that is
generated by the system but is also closely related to the signal
transmission distortion. If the group delay characteristic of
This work was supported in part by the CAS " Light of West China " Program under Grant XAB2018YDYL01,
and in part by the National Natural Science Foundation of China under Grant 12073034.
88
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
February 2022
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