## 1 Introduction  
Over the past few decades, there have been a number of studies on Global Navigation Satellite Systems (GNSS). Many people around the world nowadays utilize GNSS receivers on mobile devices for various kinds of applications. With the spread of location-based services and applications, the need for position information has grown even in places where GNSS signals are not available, such as indoors, in mountainous or urban canyons, and in other GNSS shadow areas. Many studies have been done to increase the availability of GNSS in these challenging environments [1; 2; 3]. A pseudolite is a GNSS-like signal generator and transmitter that can be used as an additional signal source to augment GNSS. Using pseudolites has many advantages over other methods [4; 5; 6]. Employing multiple pseudolites, it is even possible to develop an independent positioning system where the GNSS signals do not reach. However, there are some drawbacks to using pseudolites in positioning systems, including the following [7]:  
(1) Near/far problem  
(2) Phase noise  
(3) Receiver modifications  
This study focuses on the receiver modification problem. The receiver modification problem refers to the need to modify GNSS receiver hardware or firmware to acquire accurate data using a pseudolite-based positioning system. In a pseudolite-based system, only GNSS receivers developed for special uses can calculate their own positions, because the pseudolite positions cannot be delivered in ephemeris format. Therefore, legacy GNSS receivers such as those in smartphones or mobile devices cannot be used with pseudolite signals.  
To overcome this problem, a study was done that suggested a modified navigation message to deliver a pseudolite's position information [8]. The modification is simple yet tricky. Pseudolites 'cheat' the GPS receivers by filling in some ephemeris parameters with particular numbers to make the resulting coordinates independent from time and fixed at required values. However, the scale factor and the number of data bits used in GPS ephemeris data format is limited [9]. Therefore, it is impossible to make the pseudolite position perfectly independent from time, and the accuracy and precision of the pseudolite position remains limited.  
This paper proposes a new positioning system for pseudolite signals that does not require any modification of legacy GPS receivers. The proposed system uses pseudolites which generate each channel signal of the GPS simulator and legacy GPS receiver without any modification. The user position is estimated using the output of the legacy GPS receiver and the information about pseudolites and simulated GPS satellites. Section 2 shows the limits of expressing pseudolite positions in GPS ephemeris format. Section 3 proposes a new positioning algorithm using pseudolite signals. The system concept and detailed algorithms are described. Simulation results will be presented in Section 4. The test was performed using simulated measurements and live GPS L1 data with additional manual delays. The results show that an accurate position can be obtained using the proposed algorithm without any receiver modification. For convenience, the GPS L1 signal with C/A code was assumed during the study.