# LEO-Augmented GNSS Based on Communication Navigation Integrated Signal
## 1 Introduction  
A Low Earth Orbit (LEO) constellation has been adopted in satellite communication systems. Considering the high CN0 of the signal received on the ground and high moving velocity of the satellite, GNSS can be augmented by LEO in many aspects. When GNSS is augmented by the LEO satellite system, receivers can realize positioning using the Doppler of signal. Geometric dilution of precision (GDOP) performance can also be improved for positioning with pseudorange. It can also improve the performance of Precise Point Positioning (PPP). Based on these advantages, LEO-augmented GNSS is getting more attention.  
There are many LEO satellite communication systems that have been constructed or are under construction, including IRIDIUM [1], GlobalStar [2], Hongyan [3, 4], Luojia-1A [5], and so on. GlobalStar is a LEO satellite communication system based on signal transponder. Hongyan and Luojia-1A are still under construction. IRIDIUM is still one of the most important LEO satellite communication systems, and it has developed the iGPS function.  
[PERSON] et al. [6] and [PERSON] et al. [7] analyzed the performance of LEO-augmented GPS. These analyses are based on the constellation of IRIDIUM, and they show that LEO-augmented GPS shows great improvement in float carrier phase positioning and carrier cycle ambiguity resolution comparing to GPS only. [PERSON] et al. [8] shows generalized analysis of the performance of LEO-augmented GNSS in resolution of integer cycle ambiguities. These papers show that the performance of GNSS can be greatly augmented by LEO satellite system. However, the disadvantage of these analyses is that they are based on simulated measurements. They assume that the pseudorange and carrier phase measurements can be got and they did not consider the signal modulation that LEO satellite system adopts.  
Hongyan and Luojia-1A have considered the integration of communication and navigation. However, in the literature, only the performance of navigation signal is provided. The research on integration of communication signal and navigation signal is still not enough. IRIDIUM developed the function of satellite time and location (STL) [9] on the basis of the communication function. The IRIDIUM satellites launch STL burst signals so that receivers could get the pseudorange by measuring the time of arrival (TOA) of the signal. The beginning of a STL burst is manipulated to form a continuous wave (CW) marker, and the remaining in the burst is organized into pseudorandom sequences. But the performance of STL signal is limited under the current design.  
There has been some work focusing on the integration of navigation signal and communication signal. [PERSON] et al. [10] proposes the combination of OFDM and PSK or BOC for the signal modulation of future GNSS. However, this only considers the transmission of navigation messages, and it cannot be used in satellite communication systems. [PERSON] et al. [11] developed an integrated navigation and communication system based OFDM modulation. However, this is only for special occasion.  
To augment GNSS, the LEO system should supply navigation signal together with communication signal. With the limited bandwidth, the communication signal and navigation signal has to share the bandwidth. Therefore, the integration of the communication signal and navigation signal means the allocation of frequency resource, time resource and power. In this paper the integrated communication signal and navigation signal is considered on the basis of the modulation of IRIDUM. The signal modulation satisfies the satellite communication requirements. The navigation signal is added to the existing communication signal and it is considered with the least influence on the communication system. The following is the quantitative analysis.