## 1 Introduction
In addition, as increasing numbers of available GNSS satellites will transmit L-band navigation signals in the future, this technique offers the advantage of enabling Earth surface observation with unprecedentedly high temporal resolution when compared with other traditional satellite missions. We can now forecast that the future state-of-the-art GNSS-R Earth observation system will consist of several LEO observatories and approximately 120 GNSS satellites from multiple constellations. Unlike active space-based remote sensing techniques, the observation areas of the GNSS-R system are centered on specular reflection points on the Earth's surface between the GNSS transmitters and the LEO receivers rather than on the nadir points. Many researchers worldwide have studied GNSS-R through various simulations and analyses. In 1998, several criteria for altimetry using reflected GPS signals were addressed, including the required signal strength, the required delay characteristics and an algorithm for computation of the specular reflection position between the GPS and the LEO receiver [26]. In 2003, the TOPEX/Poseidon and Challenging Miniisatellite Payload (CHAMP) satellites were used as GPS-R observatories and the distributions of ocean reflections from GPS satellites were simulated [27]. [PERSON] et al. [28] proposed that a resonance orbit could be used to enhance the ground track density of GNSS-R LEO satellites. In 2006, [PERSON] and [PERSON] [29] presented the performances of two proposed GNSS-R altimetry space missions. The results showed that the required antenna gain in the case of 700-km orbit height was higher than that in the case of 500-km orbit height [29]. In 2017, [PERSON] et al., analyzed the altimetric root mean square error of ESA (European Space Agency) proposed GEROS-ISS (GNSS Reflectometry, Radio Occultationand Scatterometry-International Space Station) mission for the minimum (330 km) and maximum (460 km) ISS orbital heights [30]. In recent years, some researchers have begun to consider the spatial and temporal resolutions of GNSS-R anew because of the NASA (National Aeronautics and Space Administration) mission called the Cyclone Global Navigation Satellite System (CYGNSS). At the end of 2016, NASA launched the CYGNSS mission, which used eight micro-satellites; each individual observatory can track up to four parallel reflected GPS L1 signals [24]. The medium and mean revisit times for CYGNSS missions were computed using an empirical distribution. The results showed that the median time is 2.8 h and the mean revisit time is 7.2 h when calculated across the entire 13-day cycle. [PERSON] et al. [31] showed that the mean revisit time over the equatorial regions with an approximate latitude range of \(\pm\)38\({}^{\circ}\) is approximately 5 h, but they may have adopted a different strategy from that of [PERSON] et al. [24]. In addition, [PERSON] et al. [31] performed a coverage and revisit time simulation for a possible GNSS-R constellation consisting of 24 polar orbiting satellites, with each of these satellites tracking up to 10 parallel reflections from both GPS and Galileo satellites. The average revisit time for this system was less than 2 h globally. However, researchers have seldom studied the potential performance of GNSS-R when four GNSS systems are fully operational and detailed simulation methods and results have not been published to date. The precursor of CYGNSS is TDS-1 mission launched in July 2014, which carried a GNSS-R payload called the Space GNSS Receiver Remote Sensing Instrument (SGR-ReSI). An updated revision of the SGR-ReSI was selected as payload on each satellite of CYGNSS mission. This instrument on TDS-1 could receive GPS L1 and L2C signals and gathered many useful and important spaceborne GNSS-R data for scientific research [32]. Much of the data from TechDemoSat-1 GNSS-R experiment has been made available at the MERRByS website and analyzed to perform geophysical parameter retrievals [14; 15; 18].