# Time to First Fix Robustness of Global Navigation Satellite Systems: Comparison Study
## 1 Introduction  
A global navigation satellite system (GNSS) provides a constellation of satellites that transmit location and timing signals to any part of the globe, on land, sea, or air, allowing the receiving equipment to determine its coordinates, altitude, velocity, and current time with great accuracy, 24 h a day and in all weather conditions [1]. The availability of this position, velocity, and time (PVT) solution enables the development of navigation, tracking, transport, synchronisation, emergency, geomatics, and agricultural applications, among others [2]. Nowadays, there are four GNSSs available worldwide, each developed and governed by a different country or union of countries. The first GNSS that became fully operational, in 1993, was the Global Positioning System (GPS), originally Navstar GPS, which is owned by the United States [3]. Afterwards, in 1995, Russia completed its GNSS, which is called GLONASS [4]. Later, in the 21 st century, both China and the European Union each deployed their own GNSSs, which are named BeiDou [5] and Galileo [6], respectively. From the user perspective, all four GNSSs are complementary, meaning that signals from any constellation can be used together to compute a PVT solution [7, 8]. For this purpose, all of them provide signals with the same type of information, but the technology for their generation and transmission varies from one to another [9]. This implies that, fora receiver, it may be easier to acquire and track signals to calculate its first position when using a GNSS in comparison to the others, mainly under adverse conditions. This aspect is especially important for the low-cost GNSS chipsets of mass-market devices, which usually undergo economic, size, or power constraints. These receivers may not be constantly powered and, for the sake of simplicity and legacy, tend to work with the standard frequency band and do not implement countermeasures against signal deterioration. This is the case of market segments such as smart agriculture, wildlife tracking, health and lifestyle, search and rescue, emergency services, smart cities, intelligent logistics, and environmental monitoring [10]. For this reason, the aim of this study is to assess the robustness of the time to first fix of the different GNSSs, analysing the performance of a low-cost receiver in providing a useful initial PVT against different levels of harshness. The time to first fix (TTFF) is the time required for a GNSS receiver to start up, acquire satellite signals, obtain navigation data, and calculate its current position [11]. The TTFF is a parameter of high interest for those applications where having a first position fix within a few seconds is more important than waiting for a more accurate solution [12]. In this sense, for mass-market devices, a low TTFF offers significant advantages for the aforementioned segments as follows:  
* Improved power efficiency: GNSS modules can switch off sooner after deriving a fix, reducing consumption and extending battery life. Relevant applications include wildlife tracking, intelligent logistics, and environmental monitoring [13; 14; 15]. * Enhanced user experience: Users receive positioning data with minimal delay, ensuring smoother operations. This is particularly beneficial for smart agriculture, healthcare and lifestyle, and smart cities [16; 17]. * Faster response times: Rapid location acquisition enables quicker reactions in search and rescue operations and emergency services [18]. In this context, the TTFF robustness, i.e., the capability of obtaining a low TTFF under adverse conditions, varies among GNSSs, depending on both the number of operational satellites and the strength of their signals [19; 20]. Therefore, if a GNSS offers a high quantity of healthy satellites and powerful GNSS signals, it is more likely that the receiver can rapidly detect and track at least four satellites to compute the PVT solution. Moreover, the TTFF also depends on the starting conditions of the receiver.