A simulation-based analysis of the 746th Test Squadron’s Locata non-GPS based positioning system for navigation testing at White Sands Missile Range (WSMR).
SEAN ABRAHAMSON, KALYN JONES, 746TH TEST SQUADRON JOSEPH MURPHY, JESSE SCHLOSSER, ANSYS GOVERNMENT INITIATIVES
The 746th Test Squadron (746 TS) utilizes a non-GPS based positioning system (NGBPS) developed by Locata Corporation to provide high-accuracy navigation solutions in GPS-denied environments. The Locata system broadcasts signals that are processed by an onboard, airborne receiver during flight testing. Post-processing techniques are applied to improve the precision of the navigation solution. This unique infrastructure of ground pseudolites at White Sands Missile Range (WSMR) provides a useful resource for testing and evaluation in GPS-denied scenarios by providing a localized, off-band signal for reference. However, the availability of this high-accuracy solution is limited by the placement of transmitters on range and the location of the antenna on the aircraft as it traverses the field, emphasizing the need for careful mission planning.
To address these challenges, the 746 TS has used the Ansys Government Initiatives (AGI) System Took Kit (STK), a software tool for modeling and analyzing systems, to simulate the performance of the Locata system. By incorporating flight profiles with attitude information, STK enables the estimation of the system’s performance. This simulation-based approach has led to significant improvements in mission planning and post-mission analysis. The 746 TS can ensure flight paths are optimized for the Locata system, leveraging the capabilities of STK to model the effects of body masking and aircraft dynamics to develop profiles that maximize the accuracy and reliability of the Locata navigation solution.
This article explores the application of STK in simulating the Locata system’s performance, highlighting the benefits of this approach in mission planning and post-mission analysis. The results of this simulation-based approach are presented, demonstrating the potential for improved navigation system testing and evaluation at WSMR.
Full Spectrum PNT Testing and Evaluation
Positioning, navigation and timing (PNT) systems are critical components of modern military operations. With increasing challenges to GPS availability through Navigation Warfare (NavWar) threats, the ability to test and evaluate navigation systems in realistic NavWar environments has become essential.
The 746 TS, based at Holloman Air Force Base in New Mexico, specializes in the testing and evaluation of PNT systems to support national defense operations across all environments.
The squadron’s mission is centered on delivering agile, efficient, and reliable PNT and NavWar test and evaluation services. This encompasses precision laboratory testing, hardware integration, ground and flight testing, and open-air NavWar assessments, providing a comprehensive, end-to-end PNT test and evaluation capability.
The significance of NavWar testing has been demonstrated by the large scale and persistent GNSS jamming in Eastern Europe over the past several years. As adversaries continue to develop and deploy GPS jamming capabilities, the need for resilient navigation systems and effective testing methodologies becomes increasingly critical. NavWar testing ensures military systems can be properly evaluated in realistic denied environments before deployment.
Reference System Challenges
The 746 TS uses an Ultra High Accuracy Reference System (UHARS) to provide time, space, position information (TSPI) reference data to compare to the systems under test. This reference system must maintain accuracy levels approximately 10 times better than the systems being tested (targeting an accuracy of <10 cm per axis 1-sigma), even in GPS-denied environments.
The UHARS incorporates multiple navigation subsystems including:
• All in view GNSS civilian receiver with differential post processing (multiple constellations across multiple frequency bands)
• Military GPS signal receiver
• High performance navigation grade inertial navigation system (INS)
• Advanced anti-jam antenna electronics
• NGBPS (using Locata technology)
Despite this diversity of systems, maintaining high accuracy in a fully jammed environment remains challenging and largely relies on the performance of the NGBPS.
Locata NGBPS
The Locata system deployed at WSMR consists of a network of ground-based transmitters (referred to as LocLites) that broadcast GPS-like signals in the 2.4 GHz Industrial, Scientific and Medical (ISM) band. These signals are received by an airborne receiver as well as other LocLites to maintain synchronous time across the system (Figure 1). The receiver mounted in the test aircraft processes the signals to determine the aircraft’s position and velocity.
Key features of the Locata system include:
• Code and carrier phase measurements for high-precision positioning
• Meteorological data integration to correct tropospheric errors
• Time synchronization between all transmitters (TimeLoc™ technology)
The system operates independently of GPS, making it ideal for providing reference positioning in GPS-denied environments. The signals are processed in real-time during flight testing but require additional post-processing to enable the full, high-accuracy navigation solution from the system.
Locata Operational Limitations
While the Locata system is capable of providing an accurate solution for positioning in GPS-denied environments, it has several operational considerations that must be addressed through careful mission planning:
• Geometric Constraints: High-accuracy position solutions are only available where the Precision Dilution of Precision (PDOP) remains below 3.0. This requires the platform to have line-of-sight to enough LocLite transmitters to provide well distributed geometry. The geometry and corresponding PDOP tend to improve when the aircraft’s receiver antenna is surrounded by LocLite transmitters (Figure 2). If PDOP exceeds 3.0 due to loss of signals during banking maneuvers, the system must re-converge on a solution.
• Carrier Phase Ambiguity Resolution: While the Locata receiver generates a real-time code solution, it is usually not sufficiently accurate to evaluate the system under test. High-accuracy post-processed solutions resolve carrier phase ambiguities, but that process requires sufficient time and appropriate flight patterns within the Locata network to converge on a solution. If too many LocLite signals are lost during maneuvers, the system must restart the ambiguity resolution process, requiring additional motion and time.
These considerations emphasize the need for careful mission planning to ensure flight paths maintain optimal geometric relationships with the LocLite network. The aircraft must generally operate within the bounds of the LocLite network to achieve favorable PDOP values. Additionally, the location of the antenna on the aircraft and the aircraft’s attitude during flight significantly impact signal reception, as banking maneuvers can cause the aircraft body to mask signals from certain LocLites, degrading the positioning solution. An example of a flight that demonstrates well how these mission planning parameters affect the availability of a high-accuracy Locata solution is shown in Figure 3.
STK Modeling Approach
AGI has developed the STK set of software tools, which provide a comprehensive environment that enables high fidelity test environment modeling of the test environment, including the test platform and Locata system. STK’s physics-based modeling capabilities allow for accurate representation of:
• Aircraft dynamics and attitude during flight
• Signal propagation from pseudolites to the airborne receiver
• Aircraft body masking effects on signal reception
• Antenna placement and radiation patterns
• Computation of geometric parameters such as PDOP
These capabilities enabled the 746 TS and STK to partner together to create a scenario within STK that simulated a test flight for one of the primary 746 TS flight test platforms (C-12J) under various flight conditions with the Locata system present to investigate the estimated effects of the flight profile on the Locata system performance.
Model Development
The STK model developed for the Locata system includes:
1. Pseudolite Network: Ground-based transmitters placed at their actual surveyed locations on WSMR.
2. Aircraft Model: Detailed representation of the test aircraft, including:
• Outer mold line geometry.
• Flight dynamics at different speeds and altitudes.
• Antenna placement.
• Signal masking due to aircraft body during turns determined by C-12J flight characteristics.
3. Flight Profiles: Planned flight paths with detailed attitude information, including banking angles during turns.
For the initial setup, the pseudolite network transmitters were modeled as single points at the appropriate coordinates of the transmit antennas. However, each LocLite site has two transmit antennas located near each other and initial versions of the scenario only account for one of these. It turned out that this impacted the PDOP estimates and a second antenna location was later added to each site. The current version of the model does not account for the antenna gain pattern of the transmit or receive antennas, though this could easily be added in the future using built in tools within STK.
For the aircraft model, a 3D outer mold line of the C-12J was integrated. Using measurements from the actual test platform, the Locata receive antenna was placed on the model at its installed location relative to the nose of the model. Based on the antenna location, the STK software can estimate a line-of-sight mask that indicates the area above which signals would no longer be able to reach the antenna. This mask is used to estimate when the receiver would no longer be able to track a LocLite signal based on the orientation of the aircraft during the flight profile. It was discovered that even small changes in the antenna location relative to the aircraft body frame played a significant role in the estimates of what signals would be available. For example, moving the location vertically down by 1 inch had a significant impact on the estimated body mask during banks (Figure 4).
The goal of flight profile optimization is to maintain PDOP values below 3.0 throughout the flight to ensure continuous high-accuracy solutions. This requires optimizing several parameters:
• Flight altitude
• Airspeed
• Bank angle
• Flight path geometry
Initial flight profiles were planned at three altitudes: 24,500 feet, 17,000 feet and 10,000 feet MSL and included both a racetrack and figure-eight type flight path at each altitude. These profiles were modeled in STK to evaluate PDOP performance when using STK’s Aviator tool to estimate the appropriate attitude of the flight test platform based on the C-12J flight characteristics. The initial estimated PDOP is shown in Figure 5.
Based on the initial results from the STK model, it became clear the planned air speed and subsequent banking required to maintain turns at lower altitudes would likely lead to a loss in the ability to track certain Locata sites. This led to an inflation of the PDOP above the required threshold. Investigating different altitudes, it was estimated that flying at an altitude at or above 11,000 feet MSL would lead to more consistent performance and reduce the risk of exceeding the desired PDOP threshold (Figure 6). Based on the feedback from the modeling efforts, the actual mission flew at 11,500 feet MSL altitdue.
In addition to modeling altitude effects on estimated Locata performance, STK also enabled exploration on how changes in air speed might affect performance. By increasing the air speed, the required bank angle to maintain the turn would increase and therefore there was a greater risk of losing track of sites that enable the PDOP geometry to stay below the desired threshold. As an example, at 12,000 feet and an airspeed of 202 knots, STK estimated there were significant sections of the flight path that would surpass the desired PDOP threshold (Figure 7).
Flight Test Results
Test Execution
In April, a dedicated flight test was conducted to demonstrate performance of the Locata system. Given the results from the modeling effort, the racetrack and figure-eight flight profiles were flown at altitudes of 11,500 feet, 17,000 feet and 24,000 feet MSL (Figure 8).
PDOP Performance
In post-flight analysis, the actual as-flown trajectory of the flight was input into STK to determine the estimated PDOP values. Within STK, the predicted PDOP values were time-aligned directly with the actual PDOP values reported by the Locata receiver during the flight and compared. The results showed good correlation between predicted and actual PDOP values as shown in Figure 9, with all values remaining below the critical threshold of 3.0 throughout the flight.
The turns revealed the major contributor to the changes in PDOP during the mission was the attitude of the aircraft and that STK modeled this well. During a 15.6-degree bank-angle turn, the model predicted a loss of line-of-sight to two Locata sites in the north because signals fell above the estimated body mask. This behavior was reflected in the actual Locata receiver data from that turn (Figures 10-13).
The successful maintenance of PDOP below 3.0 throughout the flight confirmed the effectiveness of the STK-based mission planning approach. This result is particularly significant as it demonstrates the proper tools for mission planning enabled a continuous high-accuracy positioning using a non-GPS based positioning system. This tool will greatly improve the ability to provide a high-accuracy position solution even in a GPS denied environment for future test events.
Signals Tracked Performance
Other performance parameters of the Locata system were compared to similar values from STK to see if the model could further predict performance. One specific parameter of interest was the actual number of tracked and used LocLite signals as reported by the Locata receiver. This was compared to the STK estimate of which sites would have line-of-sight as shown in Figure 14. While the STK model uses a rigid masking approach, the real-world Locata system likely experiences a bit of diffraction and may track LocLite signals outside of when the STK model predicts that it will maintain track. While overall, the signal tracking pattern matched relatively well between STK prediction and reality, there were significant periods of the flight profile where fewer LocLites were tracked and used than what should have had line-of-sight based on the STK model. This does indicate there are potentially some unmodeled effects such as the time it takes for the receiver to track a signal when it regains line-of-sight or how it determines whether the signal is used in the solution.
Future Work
Following validation of the STK modeling approach with the C-12 platform, several areas for development have been identified to enhance mission planning and post-mission analysis capabilities. The development of more sophisticated aircraft models is a key area of focus, with plans to incorporate detailed flight characteristics and antenna placement to improve model fidelity. Additionally, the inclusion of antenna gain patterns will enable the simulation of receive power levels, providing a more comprehensive understanding of the system’s tracking performance.
The STK-based approach can be applied to other platforms, including another common 746 TS flight test aircraft, the T-38C. This will involve assessing the performance of different antenna configurations and analyzing the impact of high-dynamic flight profiles on the Locata system. The analysis will focus on optimizing antenna placement and evaluating system performance during high-dynamic maneuvers.
The STK tool is also capable of integrating NavWar assets such as jammers to enable more comprehensive mission planning and analysis for navigation systems being tested in GPS contested environments. This is a crucial next step as jamming assets can be optimized to provide the ideal jamming environment while also making sure flight profiles can maintain the desired Locata performance.
Conclusion
This work demonstrates the effective use of STK for modeling, mission planning, and analysis of the Locata pseudolite system at WSMR. The STK-based approach has enabled the 746 TS to optimize flight profiles, predict system performance during flight-testing maneuvers, investigate model accuracy against flight-test data, and identify areas for enhancement. The high correlation between STK predictions and actual flight-test results provides evidence this approach will be effective for continued mission planning and analysis.
By leveraging STK’s capabilities to model aircraft body masking effects and simulate signal propagation, the 746 TS may develop a robust methodology for evaluating navigation system performance in NavWar environments. This approach provides a foundation for optimizing pseudolite-based positioning systems and enhancing test capabilities for increasingly complex NavWar scenarios.
Disclaimer
References to non-federal entities do not constitute or imply Department of Defense or Air Force endorsement of any company or organization.
Authors
Sean Abrahamson is the Lead Engineer for TSPI Reference Systems at the 746 TS. He holds a Bachelor of Science in Engineering Physics from New Mexico State University, specializing in Aerospace Engineering, and a Master of Science in Electrical Engineering with a focus on Guidance, Navigation and Control from the Air Force Institute of Technology. His expertise spans the test and evaluation of PNT technologies, including strategic-grade inertial sensors, integrated INS/GPS navigation systems, and antenna electronics systems.
Kalyn Jones serves as the Chief Scientist for the 746 TS. Her experience includes leading the 746 TS Operations Flight, developing instrumentation packages for the test and evaluation of guidance and navigation systems, and managing Radar Cross Section testing for the United States Air Force. She earned her Bachelor of Science in Electrical Engineering from the New Mexico Institute of Mining and Technology and her Master of Science in Electrical Engineering from Johns Hopkins University.
Joe Murphy has been a 29-year member of the technical and leadership staff at Ansys Government Initiatives (AGI), maker of the Systems Tool Kit (STK) family of physics based mission modeling and engineering software products. He has led a business area with AGI over the past 12 years in development of a Test and Evaluation Tool Kit (STK/TETK) product line designed to help test practitioners achieve greater efficiency and effectiveness in test operations in event planning, execution and post-flight verification analysis. Part of a larger effort in support of digital engineering transformation initiatives, Murphy has helped various industry partners and customers integrate these tools and methods with specific efficiency, enabling strategies into T&E disciplines and test teams within various programs of record.
Jesse Schlosser is an Application Engineer for Ansys Government Initiatives (AGI) supporting the US Air Force and US Navy accounts. Schlosser supports the entire Digital Mission Engineering (DME) portfolio with a focused expertise on the air and test and evaluation capabilities. She graduated from Widener University in 2023 with a BS and MS in Mechanical Engineering.
