Components of GPS systems. The INS detects the navigation information position, velocity, and attitude of moving bodies using an accelerometer and gyroscope. The precision and cost of an INS are exponentially proportional and its error increases with time. However, it is more stable than other systems because it can obtain information about three axes, X, Y, and Z, and has no communication problems.
It has the advantage of not being affected by the external environment, and providing highly accurate and continuous navigation data for short voyage durations. The GPS is a system that determines the position and velocity of moving bodies by measuring the pseudo range to moving bodies from at least 4 of the 24 satellites orbiting the Earth.
The GPS is relatively low-priced, does not accumulate errors with time, and the error range is fixed. However, its disadvantage is that the performance drops if there is severe jamming, or the number of visible satellites is less than four. Furthermore, visibility and communication may become restricted [ 9 ]. If you design a navigation system using only one method, high costs are required to obtain the desired performance.
However, high-performing sensors can be obtained at low cost if you consider complementary performance. Thus, the navigation system of ground equipment and the orbiter of a space probe are designed to complement each other. Furthermore, this can make the overall navigation system more robust than if using only one method [ 10 ]. One characteristic of these integration systems is that the performance can be enhanced by improving the tracking performance of the code and carrier tracking loops of the GPS receiver.
Data measured from the GPS receiver pseudo range, pseudo range rate and the INS data position, velocity, attitude include many errors. The loosely coupled system can be smaller and faster than the tightly coupled system. However, noises also become amplified. The performance of loosely coupled and tightly coupled integration is the same if the GPS availability is good throughout the test run.
When GPS availability is poor, as in an urban canyon, tightly coupled integration performs better than loosely coupled integration. The positional accuracy is best when both code and Doppler measurements are used, and is worst when only Doppler measurements are used.
The velocity accuracy is also best when both code and Doppler measurements are used, and worst when code only measurement is used. The analysis of space orbital motion generally means the integration of the nonlinear orbital motion equation. For this analysis, special perturbation [ 12 ], which is a numerical method, or general perturbation, which is an analytical method, is used.
Special perturbation has a small error due to numerical integration during orbit propagation, but requires a high-performance onboard computer. General perturbation has a small computational load during orbit propagation, but the numerical integration error increases substantially with time. To improve this shortcoming, low-orbit satellites generate a residual, which is the difference between the reference orbit and the true orbit.
The residuals, which exhibit periodic characteristics, are approximated using the coefficients of trigonometric and Fourier functions, before being transmitted to the satellites along with the reference orbital elements. The satellite computer can improve the precision of orbit propagation and greatly reduce the computational load by generating a reconstruction orbit [ 13 , 14 ] using coefficients and reference orbital elements received from the ground station.
However, the approximation method using residuals cannot be applied to deep space probes for the Moon and Mars because they do not rotate around the Earth repeatedly, like satellites. As a solution, an auxiliary navigation system using B-spline data compression has been proposed [ 3 ]. The data compression rate must be increased because deep space communication is expensive and limited in communication time. For Earth and lunar orbits where takeoff and landing occur, intensive control and communication are essential due to the danger and unpredictability.
However, in the transition segment, which is stable and accounts for the majority of navigation, communication time can be saved, and stable navigation data can be received by sending compressed orbit data calculated on the ground to the probes. Satellite orbit compression using the Fourier technique, and development of an auxiliary navigation system using this technique, have mainly been studied using a low-orbit satellite model [ 1 , 2 , 12 , 15 ].
The overall operation concept of an onboard orbit propagator is shown in Figure 7. First, an actual orbit is created through accurate modeling and numerical integration of orbital motions, which is available on the ground. Then, a reference orbit is created that is sufficiently close and has a known solution.
After defining the residual, which is the difference between the reference orbit and the actual orbit, a few approximate functions are obtained by reflecting the characteristics of orbital motion. The corresponding coefficients are sent to the satellite.
The satellite determines the position and velocity of the satellite by substituting the pre-embedded numbers in the onboard computer Orbit Reconstruction. Diagram of onboard orbit propagator operation. The orbit precision predicted through such orbit reconstruction is closely related to the selection of the reference orbit and residual reproduction function. However, the selection criteria for the reference orbit and residual reproduction function must be based on the calculation power of the satellite onboard computer, the data transmission protocol between the ground station and the satellite, and the required precision of the orbit propagation result.
Essentially, orbit data is created by numerical integration using the initial time, position, and velocity data on the ECI coordinate system, as well as precise orbit modeling.
The data determined in this way is converted to orbit elements, and the reference orbit is established. The general reference orbit can be expressed in first or second order polynomials, as shown below, and the coefficients are interpolated using the least squares method. Here, the coefficients are determined using the least squares curve fit or a similar technique based on the precise orbit prediction data of actual orbits created by numerical integration.
However, in the case of a near-circular orbit, the argument of perigee cannot be defined or the curve fitting may be inaccurate.
The residual means the difference between the actual orbit data and the designed reference orbit Figure 8. The Fourier series coefficient must be determined, and the least squares regression is used for this purpose. Definition of various orbits. The coefficients determined above are uploaded from the ground station to the satellite through communication. Using the reference orbit coefficient and residual coefficient received from the ground station, the onboard computer in the satellite reconstructs the orbit.
The velocity data is indirectly calculated from the position data, or can be directly created using the same method. The compressed orbit elements are converted to velocity and position data through the DCM, etc. In the case of lunar exploration satellites, they activate trans-lunar injection TLI in the parking orbit of the Earth to enter the lunar transfer orbit. Once a lunar exploration satellite enters the lunar transfer orbit, the satellite is tracked with ground antennae around the world, and orbit determination is performed by processing the obtained tracking data.
Communication with a lunar probe corresponds to deep space communication, and a representative example is the Deep Space Network DSN. Four antennae are in currently in operation at three locations Goldstone, Madrid, and Canberra [ 16 , 17 , 18 ]. The antennae and communication range of DSN are shown in Figure 9.
The orbit data must be compressed as much as possible because communication is very limited in both time and range. Therefore, an algorithm for efficient orbit data compression should be developed. Deep space network facility. After orbit determination, the coordinates and velocity data of the calculated orbit are compressed as control points through the proposed procedure based on a B-spline.
Compared to the conventional method, this method can stably compress even bulky data at a higher compression rate. This data is sent to the probe in the form of compressed parameters. The probe reconstructs the orbit based on the received parameters.
Methods by which the lunar exploration satellite leaves the earth orbit and enters the lunar orbit include direct transfer, phasing loop transfer, and weak stability boundary WSB. The B-spline approximation method creates a three-dimensional curve with position and velocity data over time 4-D , excluding time, and then time is reconstructed by linear interpolation using the B-spline. The initial degree and control points are given, and the parameters are estimated based on the given point data.
Control points are obtained from the estimated parameters and corresponding points, and the curve is reconstructed. After performing linear interpolation for the reconstructed data in line with the time, it is compared with the original data, and the control points and degrees are increased until they enter the error range.
Then, the drive command is given with the position and velocity data of the x, y, and z axes for time t. Earth is surrounded by navigation satellites. Credit: NOAA. GPS can be used to keep an eye on dangerous natural hazards, too! Tsunamis GPS can help provide early warning of tsunamis. Credit: mnlamberson. GPS is used to monitor volcanoes. Credit: Earth Uncut Productions Ltd. The aftermath of Earthquakes can be rapidly monitored using GPS. Orbits 'R' Us! Telling a pine from a maple To determine the attitude of a satellite.
This can be accomplished by comparing the measurements obtained from different antennas. To collect GPS measurements that will allow a precise reconstitution of the orbit of the satellite. To collect GPS measurements that can be used to reconstitute the characteristics of the medium travelled through by the signal: ionosphere and troposphere.
Future uses: The relative navigation of two spacecraft currently being validated. The tracking of the launch and early-orbit phases of rockets. The tracking of re-entering spacecraft, even to the point of autonomous landing. EGNOS will complement the GPS system in order to provide European users with increased availability, integrity and accuracy for real-time applications such as aircraft navigation. It is the main positioning instrument envisioned for the Automated Transfer Vehicle AT , both for absolute navigation and for navigation relative to the International Space Station.
This includes the flight dynamics activities needed to achieve and maintain their desired orbit and attitude. Scientists were proposing to install a permanent network of precise ground-based GPS receivers that would allow the monitoring of the movement of the Earth's surface in order to better understand plate tectonics and local deformations that are the cause of earthquakes.
The data from these receivers could be processed in order to obtain precise orbits for the GPS satellites that would be used by geodesists in regional deformation studies. Additionally within the IGS Terms of Reference was a provision of support for other applications, including scientific satellite orbit determination. The assets which ESOC could contribute to the IGS were its network of ground stations in which receivers could be installed and its expertise, supported by in-house developed software, in precise orbit and geodetic parameter estimation.
The first receiver was installed in Maspalomas Spain in June Fig. Precise estimation software was extended to include GPS measurement types for both ground-based and spacecraft-borne receivers.
We have been providing data and increasingly precise GPS products for the last five years. Currently we provide: raw measurement data from our six ground stations, precise orbits of the GPS spacecraft, Earth orientation parameters polar motion, length of day , station coordinate solutions for those stations included in our analysis, GPS satellite clock information.
Figure 2. It includes a network of ground GPS receivers, the necessary communication interfaces to allow the remote operation and data downloading from ESOC, and the processing and analysis software needed to format the data and to obtain the precise products Fig. The system is highly automated, and includes an easy to operate interface for the retrieval and the processing of the data Fig.
Figure 4. This panel is used to monitor the daily retrieval tasks. Other recent developments are: The calculation of global and local ionospheric models that can be used to correct one-frequency ranging and altimeter data.
The implementation of a sequential filter to estimate spacecraft trajectories using the precise products obtained by the IGS analysis activities. This support involves the following activities: Support of critical real-time GPS applications GPS has been proposed as the absolute and relative positioning system for spacecraft going to the manned International Space Station. For this application it is clear that the ground segment cannot be in-the-loop for the calculation of real-time trajectories of the spacecraft involved because of the unavoidable delays that this will create.
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