# A Low-Ambiguity Signal Waveform for Pseudolite Positioning Systems Based on Chirp
## 1 Introduction  
Global Navigation Satellite System (GNSS) positioning accuracy degrades significantly in indoor and urban canyon environments due to severe signal attenuations and multipath effects, which has been motivating researchers to explore alternative technologies for location-based service (LBS) in recent years [1; 2; 3; 4]. GNSS-like positioning technologies, for example, pseudolite positioning systems (PLs) and GNSS repeater systems, have been extensively investigated for indoor and urban applications [5; 6; 7]. A pseudolite system employs signals the same as or similar to GPS signals for ranging [8; 9]. As an essential design consideration of a positioning system, the signal modulation method directly affects the inherent performance of the system [10]. GNSS-like technologies are widely investigated for positioning in GNSS signal blocked situations, e.g., indoor, urban canyons, and open-pit mines, etc. It is well known that high accuracy and flexibility are the basic requirements for such applications. The portable transmitters, e.g., PLs powered by batteries, and receivers have become more and more significant, which made us to explore low-power consumption solutions. BPSK signals with a larger bandwidth can provide better tracking precision,but require correspondingly greater transmitter power to conserve integrated energy per code epoch [11]. High order BOC signals may provide high tracking performance without shortening the chip width so that will not increase the power consumption. The large side-peaks of its ACF (auto-correlation function), however, may lead to false locking. Therefore, it is of practical significance to design a modulation method to meet the requirements. Chirp signal sweeps over a frequency band in a certain time duration, with which one can spread the spectrum without shrinking the chip width, so that it will not increase the power consumption compared with BPSK and quite suitable for a GNSS-like system which is low data-rate. Recent advances in surface acoustic wave (SAW) devices have shown that chirp generators and matched filters can be implemented by using completely passive, low-cost SAW chirp delay lines [12]. Nanotron Technologies (Berlin, Germany) has brought forth some mature chirp integrated circuits (IC) at a price of about 40 Euro per chip, that work in the industrial, scientific and medical (ISM) radio band. In addition, SEMTECH (Camarillo, CA, USA) and SAGE Millimeter (Torrance, CA, USA) have also developed chirp products that work in various radio bands. Chirp signals have been widely used in radar systems [13; 14; 15]. Owing to their properties of high compression gain, good resistance to narrowband interferences and multipath effects [12; 14], chirp signals have also been used extensively in other fields, such as measurement [16], ultra-wide band communication [17] and optical fiber communication [12]. Furthermore, the chirp signal is also introduced for positioning and navigation. Typically, Nanotron has presented an positioning system based on CSS (chirp spread spectrum, a globally patented design of the company), and successfully used in open-pit mining, and its positioning products, nanoLOCs, are widely used in indoor positioning [18]. What's more, [19] employs chirp in an RFID indoor positioning system, some experiments also have been done by researchers and obtained some useful results [18; 20; 21]. Another signal named absolute phase modulation (APM), which is based on non-linear chirp, has also been introduced to positioning systems [22]. Most chirp-based positioning systems are two-way ranging systems. They have several shortcomings, such as limited system capacity and the need for more base-stations, compared with one-way ranging systems. Based on the linear chirp technique, we present a modulation method named chirped pseudo-noise (ChPN). Its principle is similar to BOC modulation by replacing the BOC sub-carrier with one period of chirp.