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GNSS Constellation Specific Monthly Analysis Summary: December 2024

Feb 2025 | No Comment

The analysis performed in this report is solely his work and own opinion. State Program: U.S.A (G); EU (E); China (C) “Only MEO- SECM satellites”; Russia (R); Japan (J); India (I)

Narayan Dhital

Actively involved to support international collaboration in GNSSrelated activities. He has regularly supported and contributed to different workshops of the International Committee on GNSS (ICG), and the United Nations Office for Outer Space Affairs (UNOOSA). As a professional employee, the author is working as GNSS expert at the Galileo Control Center, DLR GfR mbH, Germany

Introduction

The article is a continuation of monthly performance analysis of the GNSS constellation. Please refer to previous issues for past analysis. From the application side, the new topic that is addressed is the usages of GNSS PVT solutions for the Terminal Area Energy Management of spaceplanes, re-usable space vehicles and unmanned air vehicles.

Analyzed Parameters for December, 2024

(Dhital et. al, 2024) provides a brief overview of the necessity and applicability of monitoring the satellite clock and orbit parameters
a.. Satellite Broadcast Accuracy, measured in terms of Signal-InSpace Range Error (SISRE)
b. SISREOrbit ( only orbit impact on the range error), SISRE (both orbit and clock impact), and SISREPPP (as seen by the users of carrier phase signals, where the ambiguities absorb the unmodelled biases related to satellite clock and orbit estimations. Satellite specific clock bias is removed) (Hauschlid et.al, 2020)
c. Clock Discontinuity: The jump in the satellite clock offset between two consecutive batches of data uploads from the ground mission segment. It is indicative of the quality of the satellite atomic clock and associated clock model.
d. URA: User Range Accuracy as an indicator of the confidence on the accuracy of satellite ephemeris. It is mostly used in the integrity computation of RAIM.
e. GNSS-UTC offset: It shows stability of the timekeeping of each constellation w.r.t the UTC.
f. Terminal Area Energy Management: TAEM is a critical phase in the flight of re-entry vehicles, where precise navigation and control are essential to ensure a safe and controlled descent. Defining TAEM based on GPS involves leveraging the highprecision positioning data provided by the GPS to manage the vehicle’s energy and trajectory as it transitions from hypersonic to subsonic speeds.

Note:- for India’s IRNSS there are no precise satellite clocks and orbits as they broadcast only 1 frequency which does not allow the dual frequency combination required in precise clock and orbit estimation; as such, only URA and Clock Discontinuity is analyzed..

Note: The author is an experienced air navigation engineer with flight deck experience in assessing the navigation performance of the aircraft. The flight deck pictures provided below are taken from public sources and not directly from author’s work due to ethic and contractual reasons.

In the previous issues, the application of GNSS in time synchronization and orbit estimation for LEO satellite missions was explored. It is fascinating that the GNSS has played a crucial role also in the space re-entry vehicles during their missions, including the last few space shuttle missions. GNSS PVT solutions are integral to the navigation and control of re-entry vehicles, launch and liftoff vehicles, unmanned aerial vehicles (UAVs), and overall terminal area energy management (TAEM) as shown in Figure F1 and Figure 2. These systems leverage the integration of GNSS with Inertial Measurement Units (IMUs), Air Data Systems, Distance Measuring Equipment (DME), and VHF Omnidirectional Range (VOR) within the Flight Management System (FMS) as shown in Figure F2 to provide a comprehensive and accurate navigation solution. The flight computer or the FMS utilizes data from these sensors to continuously update the vehicle’s position, velocity, and attitude, ensuring precise navigation and control throughout the mission. GNSS provides high-precision positioning, which is crucial for the accurate execution of flight paths and maneuvers. However, in the event of a GNSS outage or failure, the accuracy of the navigation is degraded, and it may not always meet the required performance standards, such as Required Navigation Performance (RNP), based on the combination of IMUs, DMEs and VORs. Figure F3 shows the variation in achieved accuracy as shown by the flight computer for different sensors. The Schuler tuning is applied in the IMU/IRS of aircraft to correct for the Earth’s curvature and reduce drift but the sensor vibration, temperature effect and initialization errors still accumulates the drift over time to degrade the positioning. This degradation in the absence of GNSS poses a significant challenge for the envisaged integration of space traffic and air traffic management, considering the frequent launch, lift-off and re-entry of space vehicles envisaged in the coming decades. As shown in Figure F3, the accuracy of the IMU (also of DME, VOR to a larger extent) degrades significantly requiring larger separation minima in the airspace. Still, the integration of IMU, Air Data, DME, and VOR ensures redundancy and maintains navigation integrity, allowing the FMS to continue providing reliable navigation solutions. In nominal situation, the usage of the GPS (Figure F3 and F4) allows higher accuracy enabling efficient airspace management and precision operations for terminal approach and final landing. [1] provides a detail analysis on the use of GPS for the re-entry approach and landing of Dream Chaser Orbital Vehicle (like the Space Shuttle).

Re-entry vehicle dynamics are typically described by a set of nonlinear differential equations (as shown in the equations below) that account for the vehicle’s motion in three dimensions over a rotating Earth. These equations include:

Equations of Motion: These describe the translational and rotational dynamics of the vehicle. The translational motion is governed by Newton’s second law, considering gravitational, aerodynamic, and thrust forces. The rotational motion is described by Euler’s equations, accounting for the vehicle’s moments of inertia and external torques.

Energy Management: The TAEM phase involves managing the vehicle’s energy to ensure a safe and controlled descent. This includes optimizing the trajectory to minimize dynamic pressure and thermal loads. The control inputs, such as angle of attack and bank angle, are adjusted to manage the vehicle’s total energy and ensure it follows the desired flight path.

Path Constraints: The trajectory planning must satisfy various constraints, such as maximum dynamic pressure, thermal limits, and structural integrity. These constraints are incorporated into the trajectory optimization algorithms to ensure the vehicle remains within safe operational limits.

The bank angle and angle of attack play crucial roles in the TAEM phase.The bank angle is used to control the lateral trajectory and manage the vehicle’s cross-range capability. By adjusting the bank angle, the vehicle can perform controlled turns, which helps in aligning with the desired landing corridor. The angle of attack, on the other hand, is used to control the lift and drag forces acting on the vehicle. By varying the angle of attack, the vehicle can manage its descent rate and maintain the desired flight path. These control inputs are essential for ensuring a smooth transition from hypersonic to subsonic speeds, maintaining control and stability throughout the descent.

The two important concepts in the TAEM are:

Heading Alignment Cone (HAC): As the vehicle approaches the landing site, it uses the heading alignment cone, a virtual truncated cone, to align with the runway centerline. The vehicle maneuvers into the HAC to ensure it is properly oriented for the final approach & landing.

S-Maneuver: The S-maneuver is used to dissipate excess energy and adjust the vehicle’s trajectory. By performing a series of S-shaped turns, the vehicle can manage its speed and alignment, ensuring it follows the optimal path to the landing site.

Equations: Kinematic model for the re-entry vehicles [2].

Figure F1 shows the different phases of re-entry, approach and landing of space vehicle. Based on the Space Shuttle missions, it also gives representative use of GPS and other sensors at different altitude and speed.

[4] and [5] provide an overview on the use of GPS along with other sensors that enabled the Space Shuttle re-entry, approach, and landing. It is also highlighted how the GPS augmented system can even enable precision approach and auto landing capability. Regarding the terminal approach, instead of TACAN used in earlier Shuttle missions, DME-DME or standalone GPS LNAV/VNAV Area Navigation up to 0.1 RNP (the accuracy can be met as shown in Figure F3 and F4) are more beneficial for contemporary missions. Further, GNSS derived ADSB provides continuous surveillance information in addition to the primary surveillance radar. GPS for Space Shuttle was used with stand-alone mode. However, with current state-of-the-art technology, augmented GPS (i.e. SBAS) provides better accuracy and most importantly higher confidence in the solution. Once the vehicle is brought down to the TAEM exit point, the final approach and landing procedures can be supported with combinations of SBAS (Approach Procedure with Vertical Guidance (APV I/APV II)), GBAS, ILS, DME-DME and Vision-Based Landing System. Overall, GNSS plays a crucial role.

In summary, the operation of re-entry procedures begins from the de-orbit burn and as such, the most important requirement is to identify a suitable de-orbit burn zone, during either orbit ascending or descending. One of the main design parameters driving the de-orbit burn is the cross-rang capability.

After the de-orbit burn, the high energy (high kinetic and potential energies) of the re-entry vehicle needs to be properly balanced before preparing the final approach and landing. In Figure F1 above, the experience from the Space Shuttle mission is used to distinguish different phases of the re-entry, TAEM, Heading Alignment, energy dissipation S-maneuvers, and final approach & landing. The analogy to these phases (from TAEM entry point onwards) in the routine aircraft operation is the stabilized approach of the aircraft from the en-route phase through the Initial Fix (IF) and Intermediate Approach Fix (IAF) to the Final Approach Fix (FAF). GNSS based approaches and landing like LPV that translate the above concept can be used as shown in Figure F5. It is the certified and published flight procedures for space shuttle approach and landing. The GPS standalone positioning is capable to support the LNAV/VNAV approach while SBAS, the augmented GPS positioning, is required to support the LPV approach.

Monthly Performance Remarks:
1. Satellite Clock and Orbit Accuracy:
▪ The performance of all constellations looks similar to the last month. There is a slight improvement in Beidou 3 orbit quality.
▪ For GPS, as usual there are couple of unusable satellites due to maneuver. For Galileo, E16 and E23 are not considered as there are still in test phase.
▪ For IRNSS, URA value distribution for all satellites shows low spread than in previous months.
2. UTC Prediction (GNSS-UTC):
▪ All constellations show stable UTC prediction with minor variations. GLONASS showed a strong deviation.

References

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BIMP (2024 a) https://e-learning.bipm.org/ pluginfile.php/6722/mod_label/intro/User_ manual_cggtts_analyser.pdf?time=1709905608656

BIMP (2024 b) https://e-learning.bipm.org/mod/ folder/view.php?id=1156&forceview=1

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Note: References in this list might also include references provided to previous issues.