# The Availability of Space Service for Inter-Satellite Links in Navigation Constellations
## 1 Introduction  
The Global Positioning System (GPS) was the first of several global navigation satellite systems (GNSS). Others include the GLONASS, GALILEO, and BeiDou systems. With the progress of science and technology, human activities are expanding from Earth into space, and even to deep space, so various types of spacecraft have emerged. Navigation through ground stations presents some drawbacks because they have insufficient coverage and poor geometric dilution of precision (GDOP) for high orbit users. However, space-based navigation systems such as GPS do not have these problems. GPS is used for precise orbit determination [1; 2], while the feasibility of GPS when applied in geosynchronous orbit (GEO) is exploited [3], and the results of tests conducted on the GPS receiver called PiVoT (for positioning, velocity and timing) that is designed to operate in high Earth orbits (HEO) is presented [4]. The GPS service volume is divided into the terrestrial service volume (TSV) and the space service volume (SSV). The former begins at the surface of the Earth and extends to 3000 km altitude, while the latter consists of the medium Earth orbit (MEO) SSV extending from 3000 km to 8000 km in altitude, and the HEO/GEO SSV extending from 8000 km to 36,000 km in altitude [5]. Since the GNSS currently in use were developed for terrestrial-based users, their availability is excellent in the TSV. However, it is poor for users in MEO or HEO orbits, especially those orbits that are higher than the navigation satellites. Normal receivers need to be improved in order to adjust for the weak signals and this increases the difficulty of development. Inter-satellite links (ISLs) are used to shorten the end-to-end delay in low Earth orbit (LEO) satellite communication system [6]. The flower constellations theory is applied to the design and optimization of constellations maximizing the global coverage and the network connectivity viaISLs [7], the use of ISLs in polar and near polar circular orbit constellation is examined [8], and the ISLs routing is discussed [9; 10]. However, ISLs are used to provide ranging and allow communication between the navigation satellites. ISLs can also be used to improve the accuracy of navigation system for positioning, velocity and timing, achieve integrity monitoring and autonomous navigation, and increase flexibility and expansibility. ISLs working on ultra-high-frequency (UHF) band were first introduced to enhance service performance in GPS. Other GNSS, such as GALILEO and BeiDou, also plan to establish ISLs [11; 12]. The next generation GPS will update its ISLs to Ka or V band (approximately 23 or 60 GHz) [13]. High gain directional antennas will be needed in order to overcome the propagation loss caused both by the longer propagation path and by the higher carrier frequency, scanning antennas will be required to address the relative motion of the navigation satellites, and time division multiplexing will be used to increase the flexibility of the ISLs. Therefore, we can schedule some or all of the ISLs to serve users outside the navigation constellation at specific times, which is referred to as the inter-satellite link extension application (ISLEA). Current research is focused inside the navigation constellation. A new approach for determining the precise orbit and ephemeris of the GNSS using an ISL was developed [14], a method that changes non-coherent integration to coherent integration was proposed to increase the sensitivity of ISL signal acquisition [15], an approach for allocating power in inter-satellite ranging of the navigation constellation, which can degrade the total power consumption to 55.3%, was presented [16], etc. However, there are few studies on the ISLEA. The SSV of the GPS main lobe is given by the received power, availability, and pseudorange accuracy [5]. Similarly, both the main lobe and the side lobe of the BeiDou SSV are characterized in terms of the minimum received power, satellite visibility, pseudorange pacuracy, and geometric dilution of precision (GDOP) [17].