Standalone GPS L1 C/A Receiver for Lunar Missions

Abstract

Global Navigation Satellite Systems (GNSSs) were originally introduced to provide positioning and timing services for terrestrial Earth users. However, space users increasingly rely on GNSS for spacecraft navigation and other science applications at several different altitudes from the Earth surface, in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and feasibility studies have proved that GNSS signals can even be tracked at Moon altitude. Despite this, space remains a challenging operational environment, particularly on the way from the Earth to the Moon, characterized by weaker signals with wider gain variability, larger dynamic ranges resulting in higher Doppler and Doppler rates and critically low satellite signal availability. Following our previous studies, this paper describes the proof of concept “WeakHEO” receiver; a GPS L1 C/A receiver we developed in our laboratory specifically for lunar missions. The paper also assesses the performance of the receiver in two representative portions of an Earth Moon Transfer Orbit (MTO). The receiver was connected to our GNSS Spirent simulator in order to collect real-time hardware-in-the-loop observations, and then processed by the navigation module. This demonstrates the feasibility, using current technology, of effectively exploiting GNSS signals for navigation in a MTO.

Orbital Filter Aiding of a High Sensitivity GPS Receiver for Lunar Missions

“…These studies assumed that all signals in the considered GPS antenna sidelobes can be used for navigation as demonstrated with in-orbit measurements in [10]. The results shown in Figure 3, which shows the same results obtained in Figure 5 of [7], have revealed that, by assuming a 0-dBi receiver antenna gain, the fourth strongest GPS L1 C/A signal (at least four ranges are required to compute the 3D position) at receiver antenna never goes below a power level of À168.5 dBm, neglecting a few lower peaks. Therefore, a very high sensitivity receiver architecture is required.…”

“…Note that the term "altitude" here always refers to the distance of the receiver from Earth's surface. As illustrated in Figures 5 and 6 (and equivalently in figures 6 and 7 of [7]), the analysis of the dynamics has shown that the carrier frequency can be affected by Doppler shifts and Doppler rates of up to 20 kHz and up to 3 Hz/s, respectively, at the Moon altitude (when the signals are weaker) and of up to 60 kHz and up to 65 Hz/s at the very beginning of the MTO (when the signals are stronger). Note that the spikes at the beginning of the orbit (of up to 60 kHz and up to 65 Hz/s) are due to the very high velocity of the receiver at the perigee of the MTO.…”

“…Although indirect ways of reaching the Moon exist, in this study as in [7], we have considered a simple direct MTO as reference kinematics of the GPS receiver. This trajectory corresponds to the light blue portion of Earth to the Moon trajectory illustrated in Figure 2.…”

“…Our previous studies [6,7,9] have explored the GPS L1 C/A signal characteristics in detail during the considered MTO. These studies assumed that all signals in the considered GPS antenna sidelobes can be used for navigation as demonstrated with in-orbit measurements in [10].…”

“…For the first milestone, we have investigated the characteristics of the GNSS signals and analyzed the feasibility of using them for a Moon mission; this has been described in [6]. The achievement of this milestone provided all the requirements to design and then develop a GNSS signal processing engine as a proof of concept for lunar missions, which is the second achieved milestone, described in [7]. However, the navigation performance achievable by using only unfiltered GNSS receiver observations is very coarse, and only a few signals can be tracked and decoded.…”