The Global Positioning System (GPS) and Galileo will transmit signals on similar frequencies, that is, the L1---E1 and L5---E5a frequencies. This will be beneficial for mixed GPS and Galileo applications in which the integer carrier phase ambiguities need to be resolved, in order to estimate the positioning unknowns with centimeter accuracy or better. In this contribution, we derive the mixed GPS + Galileo model that is based on "inter-system" double differencing, that is, differencing the Galileo phase and code observations relative to those corresponding to the reference or pivot satellite of GPS. As a consequence of this, additional between-receiver inter-system bias (ISB) parameters need to be solved as well for both phase and code data. We investigate the size and variability of these between-receiver ISBs, estimated from L1 and L5 observations of GPS, as well as E1 and E5a observations of the two experimental Galileo In-Orbit Validation Element (GIOVE) satellites. The data were collected using high-grade multi-GNSS receivers of different manufacturers for several zero- and short-baseline setups in Australia and the USA. From this analysis, it follows that differential ISBs are only significant for receivers of different types and manufacturers; for baselines formed by identical receiver types, no differential ISBs have shown up; thus, implying that the GPS and GIOVE data are then fully interoperable. Fortunately, in case of different receiver types, our analysis also indicates that the phase and code ISBs may be calibrated, since their estimates, based on several datasets separated in time, are shown to be very stable. When the single-frequency (E1) GIOVE phase and code data of different receiver types are a priori corrected for the differential ISBs, the short-baseline instantaneous ambiguity success rate increases significantly and becomes comparable to the success rate of mixed GPS + GIOVE ambiguity resolution based on identical receiver types.

https://www.tpbin.com/Uploads/Subjects/bbd79792-dcb8-4346-85b9-16e329b92746.pdf

Characterization of between-receiver GPS-Galileo inter-system biases and their effect on mixed ambiguity resolution

A global navigation satellite sensor system (GNSS) and gyroscope control system for vehicle steering control comprising a GNSS receiver and antennas at a fixed spacing to determine a vehicle position, velocity and at least one of a heading angle, a pitch angle and a roll angle based on carrier phase position differences. The roll angle facilitates correction of the lateral motion induced position errors resultant from motion of the antennae as the vehicle moves based on an offset to ground and the roll angle. The system also includes a control system configured to receive the vehicle position, heading, and at least one of roll and pitch, and configured to generate a steering command to a vehicle steering system. The system includes gyroscopes for determining system attitude change with respect to multiple axes for integrating with GNSS-derived positioning information to determine vehicle position, velocity, rate-of-turn, attitude and other operating characteristics. A vehicle control method includes the steps of computing a position and a heading for the vehicle using GNSS positioning and a rate gyro for determining vehicle attitude, which is used for generating a steering command. Alternative aspects include multiple-antenna GNSS guidance methods for high-dynamic roll compensation, real-time kinematic (RTK) using single-frequency (L1) receivers, fixed and moving baselines between antennas, multi-position GNSS tail guidance (“breadcrumb following”) for crosstrack error correction and guiding multiple vehicles and pieces of equipment relative to each other.

Multiple-antenna GNSS control system and method

A global navigation satellite sensor system (GNSS) and gyroscope control system for vehicle steering control comprising a GNSS receiver and antennas at a fixed spacing to determine a vehicle position, velocity and at least one of a heading angle, a pitch angle and a roll angle based on carrier phase position differences. The system also includes a control system configured to receive the vehicle position, heading, and at least one of roll and pitch, and configured to generate a steering command to a vehicle steering system. The system includes gyroscopes for determining system attitude change with respect to multiple axes for integrating with GNSS-derived positioning information to determine vehicle position, velocity, rate-of-turn, attitude and other operating characteristics. Relative orientations and attitudes between motive and working components can be determined using optical sensors and cameras. The system can also be used to guide multiple vehicles in relation to each other.

GNSS and optical guidance and machine control

A GNSS integrated multi-sensor guidance system for a vehicle assembly includes a suite of sensor units, including a global navigation satellite system (GNSS) sensor unit comprising a receiver and an antenna. An inertial measurement unit (IMU) outputs vehicle dynamic information for combining with the output of the GNSS unit. A controller with a processor receives the outputs of the sensor suite and computes steering solutions, which are utilized by vehicle actuators, including an automatic steering control unit connected to the vehicle steering for guiding the vehicle. The processor is programmed to define multiple behavior-based automatons comprising self-operating entities in the guidance system, which perform respective behaviors using data output from one or more sensor units for achieving the behaviors. A GNSS integrated multi-sensor vehicle guidance method is also disclosed.

Gnss integrated multi-sensor control system and method

Apart from the gradiometer as the core instrument, the first ESA Earth Explorer Core Mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will carry a 12-channel GPS receiver dedicated for precise orbit determination (POD) of the satellite. The EGG-C (European GOCE Gravity-Consortium), led by the Technical University in Munich, is building the GOCE HPF (High-level Processing Facility) dedicated to the Level 1b to Level 2 data processing. One of the tasks of this facility is the computation of the Precise Science Orbit (PSO) for GOCE. The PSO includes a reduced-dynamic and a kinematic orbit solution. The baseline for the PSO is a zero-difference procedure using GPS satellite orbits, clocks, and Earth Rotation Parameters (ERPs) from CODE (Center for Orbit Determination in Europe), one of the IGS (International GNSS Service) Analysis Centers. The scheme for reduced-dynamic and kinematic orbit determination is based on experiences gained from CHAMP and GRACE POD and is realized in one processing flow. Particular emphasis is put on maximum consistency in the analysis of day boundary overlapping orbital arcs, as well as on the higher data sampling rate with respect to CHAMP and GRACE and on differences originating from different GPS antenna configurations. We focus on the description of the procedure used for the two different orbit determinations and on the validation of the procedure using real data from the two GRACE satellites as well as simulated GOCE data.

Precise Orbit Determination for the GOCE Satellite Using GPS

The navigation system described here utilizes GPS and Galileo satellite signals combined with Inertial Navigation Systems (INS), where a Coupled Antenna (CAN) provides both GNSS and Inertial Measurement Unit (IMU) data to a Highly Integrated GNSS-Inertial (Hi-Gi) receiver. Such receiver makes use of a high fidelity relation between GNSS unprocessed Correlator Output (COUT) I and Q data and the user trajectory, and inertial sensor data, which in turn are combined within a Kalman Filter (KF). The KF determines the navigation solution that is also used to provide feedback to the receiver demodulation signal processing stage, thus eliminating the need of dedicated structures such as Delayed Locked Loops (DLL) and Phase Locked Loops (PLL), allowing a significant improvement in navigation performance. The improvement allows this system to provide high quality measurements and operate in circumstances where usual techniques are not usable; for example during satellite signal interruption due to obstruction, or in very high dynamics or even in attenuated signal environments, due to, for example, the canopy of trees. The KF also makes use of particular Galileo signal characteristics, lock detectors and Coupled Antenna that allow the system to operate in such environments.

Highly integrated gps, galileo and inertial navigation system

Design of a High Sensitivity GNSS receiver for Lunar missions

INTRODUCTION
Reception of weak GNSS signals in challenging environments using techniques such as Assisted GNSS has become a reality and today such techniques are considered important enablers of GNSS receivers in mobile devices [1]. Furthermore, the use of GPS and Galileo signals for indoor navigation is also receiving increased attention in recent studies which push the limits of minimum signal to noise ratios [2].
Space users have also seen remarkable achievements, where GPS signal reception has been confirmed and reported in GEO orbits [3] and seen as enabler for increased spacecraft autonomy in GEO orbits [4]. The study of weak GNSS signal reception techniques for lunar missions appears as a logical sequential step which is further supported by the growing international efforts for lunar exploration, which typically involve radiometric range and Doppler measurements from the earth to perform orbit determination.

CONTEXT AND MOTIVATION
The first missions flying to the Moon after the Apollo era were the NASA's Clementine (launched in 1994) and Lunar Prospector (launched in 1998). Both missions searched for lunar polar ice deposits and explored unknown areas thus producing more detailed surface maps.
The interest in Lunar exploration missions then started to spread out in the international community and SMART-1, the first European mission to the Moon, was launched in 2003. Since then, the European Space Agency has fostered investigation on Lunar Exploration Missions. Lunar Lander is an example of such mission [5], which has reached CDR in 2012 for which DEIMOS has been actively involved as co-prime for the G&C subsystem during this descent phase and prime for the HDA subsystem. ESA’s interest in moon exploration continues with the recent agreement for the provision of a service module for NASA Orion uncrewed spacecraft Exploration Mission-1 (EM-1) in 2017.
Additionally, DEIMOS has lead the GNSSGEO feasibility study for ESA, for the use of GNSS in high orbits (GEO, GTO, HEO) for autonomous orbit determination. In this study, a GNSS receiver simulator and receiver design has been developed including detailed design of signal processing algorithms. Orbit Determination (OD) in GEO and large portions of HEO orbits down to 18 dB-Hz was as shown as an alternative to traditional radiometric based OD, with potential cost savings and simplifications in the mission.

SIGNIFICANCE OF WORK
In the frame of an ESA study, the Lunar GNSS project investigates the use of weak GNSS signals from existing GPS and future Galileo towards future lunar exploration missions for real-time position, navigation and timing information. More specifically, GNSS signals could be used if receivers are complemented with advanced processing signal and filtering techniques, allowing acquisition and tracking down to very weak signal to noise ratios.
The paper presents the main challenges of a GNSS receiver in different phases of a mission to the moon including Moon Transfer Orbit and Low Lunar Orbit, including expected signal strengths, DOP and number of visible satellites. Factors such as GNSS antenna radiation patterns (transmitter and receiver), frequency, constellations, spacecraft attitude, integration times and TT&C link characteristics are considered. Requirements for the Lunar Lander mission based on conventional radiometric measurements from the earth as well as onboard sensors and then presented.
An overview of selected high sensitivity techniques which take benefit of Galileo signals and modernized GPS to cope with the challenges is then presented followed by the description of an orbital filter complementing GNSS measurements in closely-coupled fashion. 
The paper also describes a dedicated test platform which allows demonstrating the main functional and performance capabilities for weak signal navigation. Such platform includes both the capability to use realistic IF data and to execute high fidelity simulations of the signal processing and navigation functions of an AGGA4 based chipset (ESA’s future generation GNSS receiver). A first batch of simulation results for a selected trajectory of the Lunar Lander mission is presented, highlighting the achievable navigation performance throughout the trajectory.
In conclusion, this paper contributes to the understanding of how GPS and Galileo could be used for orbit determination in lunar missions, which may have an impact on the use of ground segment assets (ground station use times, operations manpower and equipment, etc.).

REFERENCES
[1] Mulassano P., Dovis F. “Assisted Global Navigation Satellite Systems: An Enabling Technology for High Demanding Location-Based Services”. In: Location-Based Services Handbook / Ahson S. A., Ilyas M. CRC Press, Boca Raton, FL (USA), pp. 279-298. ISBN 9781420071962, 2011
[2] Vecchione et al., “DINGPOS, A GNSS-based Multi-sensor Demonstrator for Indoor Navigation”, IEEE/ION PLANS 2010, Myrtle Beach, USA, April 2010
[3] Moreau M. C. et al, “Results from the GPS Flight Experiment on the High Earth Orbit AMSAT OSCAR-40 Spacecraft”, ION GNSS, Sept. 2002
[4] Lorga J.F.M et al., ”Autonomous orbit determination for future GEO and HEO missions”. In: 5th ESA Workshop on Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing (NAVITEC), 2010, Noordwijk (NL), 8-10 Dec. 2010.
[5] Richard Fisackerley, Alain Pradier: “The ESA Lunar Lander Mission”, AIAA, 2011

Weak GNSS Signal Navigation to the Moon

Factored Correlator Model: A Solution for Fast, Flexible, and Realistic GNSS Receiver Simulations

Increasing the GNSS receiver sensitivity and robustness in very harsh environments has become a central topic for the GNSS community. This paper addresses the design of a very High Sensitivity (HS) acquisition scheme for weak GNSS signal detection down to 5/10 dB-Hz in the particular space environment of the Lunar Low Orbit. A theoretical analysis for the determination of optimal extended coherent integration time and non-coherent accumulations allowing the achievement of pre-defined acquisition metrics will be presented and then validated by means of simulations. Complete data navigation bits and secondary code synchronization achieved thanks to the use of external aiding sources or self-assistance techniques is considered along the analysis which will provide results for weak GNSS signal acquisition in the GPS L1/L5 and Galileo E1/E5 frequency bands. Presence of a Doppler aiding source is considered and a detailed study on the required Doppler aiding accuracy is also presented.

Joao S. Silva

Papers

Highly integrated gps, galileo and inertial navigation system

Design of a High Sensitivity GNSS receiver for Lunar missions

Weak GNSS Signal Navigation to the Moon

Factored Correlator Model: A Solution for Fast, Flexible, and Realistic GNSS Receiver Simulations

Design of a very High Sensitivity acquisition system for a space GNSS receiver