# Time to First Fix Robustness of Global Navigation Satellite Systems: Comparison Study
## 2 Materials and Methods  
In order to carry out the proposed study and achieve a fair comparison between the four GNSSs, the very same geometry of satellites and signal conditions need to be applied many times to different configurations of the GNSS receiver. For this purpose, we design and prepare the test setup depicted in Figure 1, which comprises the following elements:  
* GNSS antenna: A full-spectrum choked-ring antenna, Leica AR20 with a dome, is employed to capture the signals of all GNSSs. The antenna is installed on the roof of a building at the campus of the Spanish National Institute for Aerospace Technology (INTA) located in Torrejon de Ardoz. * GNSS recorder and playback system: Live GNSS signals are recorded and later reproduced to carry out all the test runs with a Spirent (Spirent Communications plc, Crawley, UK) GSS6450 multi-frequency record and playback system [26]. The radio-frequency spectrum that is acquired includes the main band of each GNSS, which are always available in any low-cost receiver:
* BeiDou B1: 1561.098 MHz;
* GPS L1 and Galileo E1: 1575.42 MHz;
* GLONASS G1: 1602.018 MHz. The configuration applied in the Spirent GSS6450 for recording is as follows:
* Central frequency: 1583.604 MHz;
* Bandwidth: 50 MHz;
* Sample resolution: 8 bits. * RF attenuator: This playback system also includes an output attenuation function that allows us to simulate hypothetical degradations of the power of the signals due to external factors. * GNSS receiver: It must allow the automation of all the test runs by the remote configuration and management of the following:
* Cold-start execution;
* Configuration of the GNSS (BeiDou, Galileo, GLONASS, or GPS) to be used;
* Access to National Marine Electronics Association (NMEA) 0183 messages [27]. For this aim, a representative low-cost receiver, specifically a u-blox (u-blox AG, Thalwil, Switzerland) ZED-F9P with firmware version 1.32 [28], is used [29; 30; 31]. This receiver tracks and acquires signals of operational satellites of all generations simultaneously. For the test, its navigation mode is configured with the stationary dynamic platform model and the automatic position fixing mode. Moreover, in order to detect as many satellites as possible, no minimum values for the elevation angle and the signal-to-noise ratio (SNR) are set in its navigation input filters. On the other hand, its navigation output filters, which control the quality of the PVT, are kept with their default values: 25 for maximum dilution of precision (DOP) and 100 m for minimum positioning accuracy. In order to achieve representative results, the test plan covers four complete days. Specifically, a recording of 10 min is acquired every hour starting at 00:00 UTC of 4 November  
Figure 1: Diagram of the test setup. ber 2023 until 00:00 UTC of 8 November 2023. In total, 97 10-min recordings, containing the actual spectrum from 1558.604 MHz to 1608.604 MHz, are acquired. In this way, although only one location is used, many different constellation configurations of the four GNSSs are covered. The test duration and number of recordings are limited by the data storage capacity available, as the acquisition of each 10-min signal produces a file of more than 63 GB (over 6 TB in total). Another relevant factor of the test is the hypothetical attenuation of the signal power in challenging scenarios. For this purpose, not only is the original radio-frequency spectrum used, but also several attenuated versions of it. In particular, 11 different attenuation values, from 0 dB to 20 dB in steps of 2 dB, are applied. In this way, the results show the behaviour of the GNSSs against different levels of adverse conditions. In addition, as the behaviour of the receiver tracking satellites is not deterministic, 10 iterations are carried out with each recording and attenuation level in order to assure the reliability of the results. In total, since we use 4 GNSSs, 97 recordings, 11 attenuation levels, and 10 iterations, 42,608 test runs, that can last up to 10 min each, are carried out.