Lunar navigation systems are being developed to support sustained surface operations, but their early deployment phase may involve limited satellite visibility and degraded accuracy in orbit and clock determination compared with terrestrial global navigation satellite systems (GNSS). Relative positioning with a lunar-surface reference station can mitigate common error sources through differenced measurements. In conventional GNSS, this mitigation often relies on the assumption that the line-of-sight (LOS) vectors from the reference and user receivers to the same satellite are nearly identical. Because lunar navigation satellite ranges can be considerably shorter than GNSS ranges, this approximation may be insufficient for kilometer-level surface baselines. This paper analytically quantifies three error mechanisms resulting from non-identical LOS geometries: the deterministic bias of the identical-LOS-vector approximation, the residual projection of broadcast ephemeris errors into single-differenced measurements, and the satellite-position-evaluation-time error caused by receiver clock initialization. For each mechanism, a simple closed-form bound is derived as a function of the baseline length and satellite range and is verified against time-series evaluations for representative lunar orbits. For a 10 km baseline in a low lunar orbit, the approximation alone can bias positioning by several tens of meters, whereas single differencing suppresses broadcast ephemeris errors to the meter level, and the clock-initialization effect can reach the meter level or larger. These results indicate that in lunar-surface relative positioning, the single-differenced range should be evaluated from the exact receiver-dependent geometry rather than from a common LOS vector. Additionally, the derived bounds provide a practical basis for budgeting broadcast ephemeris and receiver clock synchronization requirements.

A GNSS (Global Navigation Satellite System) receiver is an attractive material to learn signal processing for wireless systems. In digital design using an FPGA, high level synthesis (HLS) has become common as a mean to implement a complicated algorithm rapidly. This paper presents an HLS implementation of the serial search method of GNSS signal acquisition. The implemented code supports any combination of parallelization for the three dimensions: PRN (Pseudo-Random Number) codes, Doppler frequencies, and code offsets of PRN. Learners will understand how to optimize code for HLS and how parallelization policies affect the synthesized circuit and system design. According to our evaluation with a PYNQ-Z1 evaluation board, when the degree of parallelization was set to 528, the proposed circuit completed the signal acquisition process for 32 satellites in 3.095 seconds, which was 507.6 times faster than the circuit without being parallelized. The number of required LUTs was 29,872, which was only 21.1 times larger.

Aiming to establish a long-term presence on the Moon, interest in new and robust communication and navigation capabilities on the lunar surface is greater than ever before. In response, lunar navigation systems (LNS) have been conceptualized and put into action by space agencies. While we expect these services to be fully functional before the late 2020s, many missions are set to begin surface exploration before the initiation of these services. To meet the immediate needs of early missions, our group proposed a small-start technology demonstration mission called the Lunar Navigation CubeSat (LunaCube). The mission aims to demonstrate a LNS transmitter and store-and-forward radio in the vicinity of the Moon and to provide lunar positioning services to lunar surface assets. The project was initiated with funding support from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and completed preliminary design and detailed design in 2021 and 2022 respectively. As a result of the design study, the satellites consist of a 6U-size structure, a 50W-class deployed solar panel, a X-band transponder, three-axis attitude control using a star tracker, sun sensors, gyro, reaction wheels, and reaction control system thruster, as well as orbit control using a water-based resistojet. The total wet mass is less than 14 kg.

Precise point positioning (PPP) is a high-precision global navigation satellite system (GNSS) positioning technology. Under PPP, the major research problems include the convergence time to obtain sufficient positioning accuracy from the start of positioning and the accuracy after convergence. However, most of these studies used archived correction data, and the manner of obtaining the correction data from satellite broadcasts in the actual environments was not discussed sufficiently because only a few low-cost GNSS receivers that can receive new signals are available. In this study, we evaluated the real-world performance of PPP in urban areas using Pocket SDR, which is a GNSS software define radio (SDR) receiver that consists of a low-cost (a few hundred dollars) RF front-end and an attached open-source GNSS-SDR receiver that supports GPS, GLONASS, Galileo, QZSS, BeiDou, NavIC, and SBAS.

We propose an accurate analytical non-gravitational force model of QZS-1, 2, and 4 for a precise orbit determination of these satellites. To construct an accurate disturbance model, we used a high-fidelity satellite geometry model and the thermal information provided by the satellite developer. They are the most detailed design information to be used to construct the analytical solar radiation pressure and thermal radiation pressure models ever for QZSS satellites. We applied the pre-computed geometry tensor method for solar radiation pressure modeling and constructed a simple box-wing-hat thermal radiation pressure model. In particular, this thermal radiation pressure is the first model constructed with realistic temperature information. Based on the analytical model, we also proposed a hybrid model combined with the empirical approach. The accurate force models were implemented on a precise orbit determination tool called MADOCA, and orbit determination experiments were performed for QZS-1, 2, and 4. The results show that the proposed analytical model has better accuracy in orbit determination than the currently published orbit products obtained by empirical disturbance models.

Multi-epoch double-differenced pseudorange observation (MDPO) is a dual-satellite lunar navigation algorithm specially designed for a precursor mission, using a minimum number of lunar orbiting small satellites to realize a GNSS-like radio navigation system for the Moon. In this study, we evaluated the performance of the MDPO algorithm by using real pseudorange measurements obtained from a pair of GNSS ground stations, one of which represented a lander, and the other a rover on the Moon. It was natural that the resulting positioning accuracy varied largely by satellite geometry, but the estimated error distributions of the double-differenced pseudorange observations were consistent and agreed with the predicted value. The results showed that the MDPO algorithm worked properly with the real GNSS observables and was capable of providing the expected navigation performance for future lunar exploration missions.

In this study, dual-satellite lunar global navigation systems that consist of a constellation of two navigation satellites providing geo-spatial positioning on the lunar surface were compared. In our previous work, we proposed a new dual-satellite relative-positioning navigation method called multi-epoch double-differenced pseudorange observation (MDPO). While the mathematical model of the MDPO and its behavior under specific conditions were studied, we did not compare its performance with other dual-satellite relative-positioning navigation systems. In this paper, we performed a comparative analysis between the MDPO and other two dual-satellite navigation methods. Based on the difference in their mathematical models, as well as numerical simulation results, we developed useful insights on the system design of dual-satellite lunar global navigation systems.

The authors regret that this paper [1] contains typographical errors in the sentence between Equation (16) and Equation (17), as well as in Equations (18), (20), (21), (34), (36) and (37), with respect to the point that they use the wrong notations t1 − tN, while the correct notations are tk − tk+N−1 [...]

Together with direct Global Navigation Satellite System (GNSS) signals, the signals reflected at the water surface can be received by an unmanned aerial vehicle (UAV). From the range difference between two GNSS signal paths, the height of the UAV above the water level can be geometrically estimated using the weighted least squares method, called GNSS reflectometry (GNSS-R) altimetry. Experimental low-cost GNSS-R altimetry flights with a UAV were conducted at the coast of Lake Biwa, Japan. Although the height estimated by the GNSS-R altimeter included large short-term noises up to 8 m amplitude, it agreed well with the UAV altitude measured by the post-processed kinematic positioning. By selecting better weight functions in the least square method and using sufficient temporal averaging, the GNSS-R altimetry achieved accuracy in the order of 0.01 m if a sufficient number of GNSS satellites with high elevation angles were available. The dependency of the results on the weight functions is also discussed.

This work describes in-orbit results of the Earth-observation missions of the microsatellites Hodoyoshi-3 and Hodoyoshi-4. These satellites were successfully launched from Russia by the Dnepr launch vehicle on June 19, 2014. Hodoyoshi-3 has observation devices that consist of a medium-resolution camera and a low-resolution one. Hodoyoshi-4 is equipped with a high-resolution multispectral camera that has a push-broom imaging sensor with a ground sampling distance of 6.3 m. The initial observations by Hodoyoshi-3 and -4 were successfully achieved. All cameras are regularly providing Earth-observation images. Due to limitations with the satellite attitude-determination system, geometric correction of the acquired images is carried out by registration to other satellite images. Hodoyoshi-3 and -4 can provide various Earth-observation results that consist of low-, medium-, and high-resolution images. These images are useful for disaster monitoring, vegetation monitoring, agriculture, and forest management.

This paper describes a triple-band global positioning system (GPS) receiver that simultaneously covers the L1, L2, and L5 frequency bands. The proposed receiver uses an image-rejection technique that can separate signals from the three frequency bands to three corresponding ports. It uses a single RF path containing a low-noise amplifier (LNA), and active and passive mixers with a pair of local oscillator signals. A triple-band GPS RF front-end chip was fabricated using 130 nm CMOS technology. The noise figure of this chip is less than 7 dB and its S11 coefficient is less than −10 dB in the 1.15-1.6 GHz frequency range. The power consumption of the LNA and mixers is 7.2 mW when using a 1.2 V supply voltage. The image-rejection ratio (IMRR) between L1 and the other (L2 and L5) band signals is 40 dB, while that between the L2 and L5 signals is 37–38 dB. To improve the IMRR between the L2 and L5 signals, we investigated the utilization of a digital compensation technique. This technique was confirmed to have improved the IMRR by about 12 dB.

We propose a Doppler pose estimation method for an indoor messaging system (IMES) ('pose' refers to both position and orientation). With this method, both the position and orientation of a receiver are estimated simultaneously by using Doppler shifts produced by moving a receiver antenna with two or more IMES transmitters. The proposed method is evaluated through the identical experiments conducted in two different locations. In these experiments, the position and orientation of the receiver is estimated using two transmitters, and the achievable accuracy is evaluated by changing the separation distance between the transmitter antennas. The experimental results demonstrate that a positioning accuracy higher than a few decimetres and orientation estimation accuracy of higher than a few degrees are achievable when the measurement condition is relatively good (i.e. when the proper separation distance is set between two transmitter antennas and cycle slips do not occur). We also conducted an analysis for the convergence of initial values (which are used for the iterative position and orientation calculation in the nonlinear least-squares method). The results show that the initial values basically converge to appropriate position and orientation values as long as an inverse matrix in the position and orientation estimation process can be calculated. Moreover, we analysed the effect of the number of transmitters on position and orientation estimation precision. The results show that, as the number of transmitters increases, the precision of the position and orientation estimation also increases, and the precision is particularly high in the area surrounded by the transmitters.

In order to achieve high precision GNSS (Global Navigation Satellite Systems) positioning, accurate modeling of the disturbance forces acting on GNSS satellites, including Japanese QZS-1 (Quasi-Zenith Satellite-1), is essential to minimize the contribution of the satellite orbit and clock errors. This study focuses on the development of analytical non-conservative force models for QZS-1 precise orbit determination, especially solar radiation pressure (SRP) and thermal re-radiation pressure (TRP) forces. In order to evaluate the performance of the proposed analytical model, the acceleration profile obtained by the traditional empirical SRP model was used as a reference. Results showed that TRP was major error sources and should be modeled separately from SRP. A ray tracing study also showed that the shadow effect of the large L-band antenna cover of the QZS-1 was not negligible and should be treated properly in the SRP calculation.