## 2 System Concept and Dual One-Way Measurements  
As stated in the Introduction, there have been many missions that exploited Lagrange orbits because they offer a stable, convenient, and cost-effective location for missions that require a long-term presence in space or a fixed view of a particular region of the celestial sphere. In these particular orbits, it is possible to exploit relative tracking measurements to estimate relative and absolute orbit states for all participating spacecraft simultaneously if at least two spacecraft are deployed [7]. Usually, these measurements are conducted exploiting different communication bands such as X-band or K-band, or exploiting inter-satellite optical links. In general, if the additional spacecraft are not used for other services, it can be assumed that only the main spacecraft have an onboard system able to process the measurements, allowing small, low complexity and cheap floating anchors, used for inter-satellite measurements. Figure 1 illustrates an example in which two spacecraft are deployed in a Lagrange orbit. Different inter-satellite measurements can be performed by exploiting an ISL. The most used technique is the conventional two-way ranging that is also used by ground stations: it consists of sending an up-link signal coming from the TX-satellite (which could be a sequence of discrete tones or a pseudo-noise sequence); this signal is then received and re-transmitted back by the RX-satellite and received by the TX satellite, and this exchange allows the computation of the two-way propagation time [12]. Another technique is the telemetry-based ranging that provides a round-trip time derived from the telemetry stream [13; 14]. This method does not require a return ranging signal, but it provides, as part of the telemetry stream, the delay between the acquired ranging signal and the start of the next telemetry frame. Both techniques provide a round-trip delay time solution at the signal source. Finally, an alternative technique is based on time transfer, as in the CCSDS Proximity-1 Space Link Protocol [15]: users can exchange epochs between satellites and derive the round-trip time using dedicated algorithms [16]. All of these techniques can be implemented, in principle, in any communication band, including the optical one. Finally, another effective way for two-way ranging can be the use of a satellite laser ranging technique exploiting retro-reflectors on the spacecraft [17]. In this work, the use of a different technique is proposed: the so-called dual one-way. It has the advantage of producing not only an estimation of the satellite range but also an estimation of the satellite clock biases. Referring to Figure 2, each spacecraft is equipped with a transmitter and a receiver to exchange ranging signals and with a reference clock to measure the Time of Transmission and the Time of Arrival of these signals. The working principle is mutated from the one used in GNSS: pseudo-random codes are transmitted from one spacecraft and received in the other exploiting a Delay Lock Loop (DLL) to measure the signal Time of Arrival (TOA) and a Phase Lock Loop (PLL) to measure the signal Doppler shift (Frequency Of Arrival-FOA). These measurements are used to estimate the range, the range rate, and the clock bias at regular time intervals (\(\Delta T\)); the measurements are then accumulated for a given time span (\(T\)) and used to solve the OD&T problem in the main spacecraft. The orbiting spacecraft exchange data and measurements exploiting the ISL and, if they have a precise OD&T, can be used to transmit to the celestial body Navigation and Telecommunication signals. In more detail, the range between two satellites is defined as the linear distance between the spacecraft and can be measured by exploiting the Time Of Arrival (TOA), if the TOA measurements taken on the anchors are transmitted back to the main spacecraft.