## 2 System Design  
To realize satellite formation flying autonomously, the proposed scheme is based on a Master/Slave architecture, as shown in Figure 1.  
The teleccommand instructions and the ranging signals are transmitted from the Master satellite (MS) to the Slave satellite (SS) via uplink, which is operated on the S-band with highly precise ranging and low-speed communication. On the other hand, the ranging signals, telemetry signals, and payloads are transmitted from the SS to MS on via downlink, which is operated on the C-band with high precision ranging and high-speed communication.  
Figure 1: Structure of distributed spacecraft networking.  
In the uplink, the modulation of the two branches is QPSK. In the I branch, the spread spectrum ranging system is designed similarly to a GNSS system, and the signal format in the Q branch is similar to that of the I branch. The only difference between the two links is the data rate. By contrast, in the downlink, the modulation of the two branches is UQPSK. Then, the signal format of the I branch is set identically to that of uplink. However, the Q branch is modulated by BPSK directly. QPSK can be regarded as a special case of UQPSK, in which the power in two branches is equal, then the ICRS can be regarded as being based on UQPSK modulation (for simplicity, QPSK is replaced by UQPSK in the remainder of this paper).  
We note that the ranging results are derived from I branches and the demodulated data is obtained from Q branches. In receiver terminals, a series of operations are performed, such as acquisition, tracking, and calculating in the I-branches. Then, to realize the ISC missions, the demodulation in Q branch is accomplished with the aid of the measured Doppler frequency shifts, recovered carrier and clock, which are acquired from the I branch. The signals in the I and Q branches are orthogonal and non-interfering, and the UQPSK signal transmitted in the two links can be modeled as  
\[s_{uplink}(t)=\sqrt{2P_{u}}d_{lu}(t)x_{u1}(t)cos2\pi ft+\sqrt{2P_{u}}d_{Qu}(t)x _{u2}(t)sin2\pi ft, \tag{1}\]  
\[s_{downlink}(t)=\sqrt{2P_{ld}}d_{ld}(t)x_{d}(t)cos2\pi ft+\sqrt{2P_{Qd}}d_{Qd }(t)sin2\pi ft, \tag{2}\]  
where \(P_{u}\), \(P_{ld}\) and \(P_{Qu}\) are the transmitting signal power in uplink and the transmitting signal power of the two branches in the downlink, respectively; \(d_{lu}(t)\), \(d_{Qu}(t)\), \(d_{ld}(t)\), and \(d_{Qd}(t)\) represent the communication signals of two branches in two links, \(x_{u1}(t)\), \(x_{u2}(t)\), and \(x_{d}(t)\) represent the spread spectrum code of the two branches in uplink and I branch in downlink, respectively; and \(f\) represents the carrier frequency. In addition, due to the applied UQPSK modulation, the total power is distributed to the I branch and Q branch, and we have  
\[P_{Id}=\frac{1}{1+10\frac{\alpha}{10}}P_{d}, \tag{3}\]  
\[P_{Qd}=\frac{10\frac{\alpha}{10}}{1+10\frac{\alpha}{10}}P_{d}, \tag{4}\]  
where \(\alpha\) represents PWDF and \(\alpha=10\lg\left(P_{Qd}/P_{Id}\right)\). \(\alpha\) is often chosen as 10 dB in the Tracking and Data Relay Satellite System (TDRSS), and the power of the Q and I branches account for 0.91 and 0.09 of the total power, respectively.  
The primary functions of the UQPSK-based system are summarized in Figure 2 and as follows:  
1. Generating the modulated ranging and communication signals in the I and Q branches;
2. Receiving, acquiring, tracking the modulated signals, and obtaining the carrier phases and PN code phases from the I branches;
3. Transferring and demodulating high-speed communication data in the Q branches with the aid of the phase information in the I branches.