Minimum Operational Performance Standards Research Articles

Towards mmWave Altimetry for UAS: Exploring the Potential of 77 GHz Automotive Radars

Precise altitude data are indispensable for flight navigation, particularly during the autonomous landing of unmanned aerial systems (UASs). Conventional light and barometric sensors employed for altitude estimation are limited by poor visibility and temperature conditions, respectively, whilst global positioning system (GPS) receivers provide the altitude from the mean sea level (MSL) marred with a slow update rate. To cater to the landing safety requirements, UASs necessitate precise altitude information above ground level (AGL) impervious to environmental conditions. Radar altimeters, a mainstay in commercial aviation for at least half a century, realize these requirements through minimum operational performance standards (MOPSs). More recently, the proliferation of 5G technology and interference with the universally allocated band for radar altimeters from 4.2 to 4.4 GHz underscores the necessity to explore novel avenues. Notably, there is no dedicated MOPS tailored for radar altimeters of UASs. To gauge the performance of a radar altimeter offering for UASs, existing MOPSs are the de facto choice. Historically, frequency-modulated continuous wave (FMCW) radars have been extensively used in a broad spectrum of ranging applications including radar altimeters. Modern monolithic millimeter wave (mmWave) automotive radars, albeit designed for automotive applications, also employ FMCW for precise ranging with a cost-effective and compact footprint. Given the technology maturation with excellent size, weight, and power (SWaP) metrics, there is a growing trend in industry and academia to explore their efficacy beyond the realm of the automotive industry. To this end, their feasibility for UAS altimetry remains largely untapped. While the literature on theoretical discourse is prevalent, a specific focus on mmWave radar altimetry is lacking. Moreover, clutter estimation with hardware specifications of a pure look-down mmWave radar is unreported. This article argues the applicability of MOPSs for commercial aviation for adaptation to a UAS use case. The theme of the work is a tutorial based on a simplified mathematical and theoretical discussion on the understanding of performance metrics and inherent intricacies. A systems engineering approach for deriving waveform specifications from operational requirements of a UAS is offered. Lastly, proposed future research directions and insights are included.

Performance Simulation Theory of Low-Level Wind Shear Detections Using an Airborne Coherent Doppler Lidar Based on RTCA DO-220

The performance simulation theory of low-level wind shear (LLWS) detection using an airborne coherent Doppler lidar (CDL) is shown. The simulation theory that this performance is based on is analogous to the turbulence and wind shear detections with an airborne Doppler radar, as specified in the radio technical commission for aeronautics (RTCA) document (DO)-220. Prior theoretical studies of CDL regarding: 1) signal-to-noise ratio (SNR) equation; 2) the relation between SNR and wind sensing performance; and 3) atmospheric parameters are fully utilized. An example simulation result under a practical condition satisfies the requirements to specify minimum operational performance standards (MOPS) in DO-220. Further, the simulation result is well aligned with past experimental results on the detectable range of LLWS. The results of this study can be utilized to establish MOPS for LLWS detections using an airborne CDL in the future.

SBAS Protection Levels with Gauss-Markov K-Factors for Any Integrity Target

<h3>Abstract</h3> According to aviation minimum operational performance standards (MOPS), protection levels for satellite-based augmentation systems (SBASs) are to be computed as the product of the estimated standard deviation of errors and a scaling, or K-factor. MOPS recognized that K-factors were originally chosen to be consistent with certain assumptions that may not hold under all conditions. Considering the limited applicability of aviation-based K-factors, it will be important to identify a more rigorous method for deriving new SBAS applications for road, rail, or maritime use. Here we describe an innovative method applicable to any integrity risk (e.g., at 10⁻⁷ for aviation or 10⁻⁵ for maritime applications) and time interval <i>T</i> (e.g., 150 seconds to 1 hour for aviation or 3 hours for maritime use). This new method relies on rigorous probability justification. No restrictive assumptions are needed for the time correlation pattern of the errors. The method is easy-to-implement and applicable to any type of integrity risk or time interval <i>T</i>.

An Evaluation of UAS Pilot Workload and Acceptability Ratings with Four Simulated Radar Declaration Ranges

Currently, minimum operational performance standards (MOPS) are being developed for a broader rangeof unmanned aircraft system (UAS) platforms, including smaller UAS that will feature onboard sensors that are low in size, weight, and power, otherwise known as low SWaP. The low SWaP sensors used to detect non-cooperative traffic will have limited declaration ranges compared to those designed for medium-to-large UAS. A human-in-the-loop (HITL) study was conducted examining four possible radar declaration ranges (i.e., 1.5 NM, 2 NM, 2.5 NM, and 3 NM) for a potential low SWaP sensor with a detect and avoid (DAA) system encountering various non-cooperative encounters in Oakland Center airspace. Participants had lower workload, particularly workload associated with temporal demand and effort, in scenarios that featured larger detection ranges. Furthermore, participants reported better ability to remain DAA well clear within the larger declaration range conditions, specifically with the 2.5 NM and 3 NM conditions.

An ADS-B Message Authentication Method Based on Certificateless Short Signature

The automatic dependent surveillance—broadcast (ADS-B) system adopts an open communication mode, and the lack of designed-in security measures in the ADS-B system makes it vulnerable to various types of attacks (jamming, spoofing, etc.). In view of the low-bandwidth and less-data-bit features of the ADS-B, this paper studies the integrity and authenticity of information by signing messages and proposes an ADS-B message authentication method based on certificateless short signature. This method uses short signature and does not require certificate management and has efficient performance. Compared with the existing approach, the computation costs of the proposed method in the signature phase are reduced by 1/2, and the signature length is reduced by 3/4. Additionally, we used the extended NS2 simulation platform to simulate 1090ES data link in different scenarios of the network; the simulation results show that our solution is suitable for minimum operational performance standard of ADS-B.

Integration of the ground-based augmentation system in continuous descent operations

Predictability is a necessity in today's commercial flight operations if delays are to be minimized. Possible mitigations, such as the implementation of continuous descent arrivals (CDAs), may help generate the required procedural predictability to curtail these delays. However, on the basis of its individual avionics suite, each aircraft follows a different barometric vertical descent path; furthermore, the aircraft may be given additional level-off segments by air traffic control to compensate for possible deviations in the vertical profile. Therefore, aircraft flying the current descent profiles are too unpredictable and thus the procedures cannot be implemented in complex airspace structures or when traffic is moderate to high. This study introduces a mitigation to this problem by taking the global positioning system (GPS) one step further by integrating the GPS-based ground-based augmentation system (GBAS) into the new optimized continuous decent arrival procedure, subsequently, allowing for the introduction of a geometrical vertical guidance, namely, the GBASCDA, which will make the aircraft flying the descent profile predictable. With the increase in availability of GBAS aircraft equipage, specifically the GPS landing system (GLS), and the approaching amendment of the Appendix E of the Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment–RTCA DO-253D, this idea of the integration of the GBAS on the arrival/descent segment as a viable solution may be finally coming within reach.

Utility of Visual and Auditory Warning Alerting for Traffic Avoidance during UAS Operations

Minimum Operational Performance Standards (MOPS) are being developed to support the integration of Unmanned Aircraft Systems (UAS) in the National Airspace System (NAS). Input from subject matter experts and multiple research studies have informed display requirements for Detect-and-Avoid (DAA) systems aimed at supporting timely and appropriate pilot responses to collision hazards. DAA alerting is designed to inform pilots of potential threats to “DAA well clear”; the two highest alert levels – caution and warning – indicate how soon pilot action is required and whether there is adequate time to coordinate with the air traffic controller (ATC). Additional empirical support is needed to clarify the extent to which warning-level alerting impacts DAA task performance. The present study explores the differential effects of the auditory and visual cues provided by the DAA Warning alert, and performance implications compared to caution-only alerting are discussed.

EXTENSION OF RTKLIB FOR THE CALCULATION AND VALIDATION OF PROTECTION LEVELS

Abstract. System integrity (i.e. the capability of self-monitoring) and the reliability of the positions provided need to be ensured within all safety critical applications of the GPS technology. For the sake of such applications, GPS augmentations, for example Space Based Augmentation Systems (SBAS) are to be applied to achieve the required level of integrity. SBAS provides integrity in a multi-step procedure that is laid out in the Radio Technical Commission for Aeronautics (RTCA) Minimum Operational Performance Standards (MOPS) for airborne navigation equipment using GPS. Besides integrity, SBAS also improves accuracy of positioning via broadcasting corrections to reduce the most important systematic errors on standalone positioning. To quantify integrity, the protection level is defined, which is calculated from the standard deviation of the models broadcast in SBAS. Air Navigation Service Providers, airspace users and aviation authorities need to evaluate the performance of GPS systems and their augmentations. This is a necessary step to define the conditions under which GPS systems can be operationally used and which operations can be supported. For this evaluation two proprietary software are used widely in Europe: Pegasus from Eurocontrol (Butzmühlen et al., 2001) and magicGemini from GMV. Both tools provide several functionalities such as computation of position simulating MOPS-compliant receivers and determination of GNSS augmentation attributes like accuracy, integrity, continuity and availability. RTKLIB is an open source GNSS data processing and analysis tool (Takasu, 2009). The actual version (2.4.3) of RTKLIB has SBAS augmented positioning mode, but no protection level calculation is included. There is an open source project on GitHub3, a fork of RTKLIB 2.4.2 version with an option for WAAS MOPS compliant position calculation, including protection level calculation, too. This was developed by the Houghton Associates, Inc. and tested on Cygwin platform. Their development was finished in 2014. We have merged the WAAS MOPS position calculation into the newer RTKLIB release (2.4.3 beta) and made closer integration into the original RTKLIB utility program RNX2RTKP. Our enhanced RTKLIB version is also available on GitHub4 as a fork of the original RTKLIB project of Tomoji Takasu. This enhanced version was developed and tested on Ubuntu Linux 14.04 and 16.04. Raw static and kinematic data were post-processed by our enhanced RTKLIB version. Calculated SBAS positions and protection levels were compared to the results of Pegasus and magicGemini. Although the RTCA standard defines the exact formula to calculate protection levels, the numerical results of the tested software are slightly different. Accurate tests regarding the possible sources of this kind of discrepancies were carried on in order to validate our open source solution. The aim of our work is to provide an open source alternative to the available proprietary software. The open source solution might be a good basis for the evaluation of GPS and SBAS performance monitoring.

Downstream Effects of Separation Assurance on Encounters Between Unmanned and Manned Aircraft

This paper documents a crucial piece of the ongoing effort to develop minimum operational performance standards for unmanned aircraft detect-and-avoid systems: the estimation of the rate of encounters between unmanned aircraft and manned aircraft operating under visual flight rules. In fast-time simulations that included both simulated future unmanned aircraft and actual present-day manned aircraft operating under visual flight rules, unmanned aircraft encountered manned aircraft operating under visual flight rules once every 5.3 unmanned aircraft flight hours. Modeled air traffic controller mitigations for conflicts between unmanned aircraft and manned aircraft operating under instrument flight rules only reduced the rate of encounters between unmanned aircraft and manned aircraft operating under visual flight rules by 0.2%, with no statistically significant or practical effect on encounter geometry characteristics. Analysis of the simulations without modeled air traffic controller mitigations showed that the highest rates of encounter and loss of well clear occurred at altitudes below 5000 feet. In addition, a surveillance range of 14.3 n mile was needed to detect all encounters between unmanned aircraft and manned aircraft operating under visual flight rules. A surveillance range of 3.6 n mile was necessary to detect all losses of well clear. These results primarily inform the safety case and the surveillance requirements for unmanned aircraft detect-and-avoid systems.

Orbital representations for the next generation of satellite-based augmentation systems

The landscape of the Global Navigation Satellite System (GNSS) is changing. New constellations are coming online, and a diversity of new signals are coming to the user space. Multi-frequency adds a means for ionospheric correction as well as robustness to jamming. Multi-constellation gives rise to better geometry and robustness to satellite failures. Systems which require a high degree of safety such as aviation require Satellite-Based Augmentation Systems (SBAS) to be used in conjunction with GNSS. As such, SBAS standards must be modernized to reflect the evolving GNSS environment. SBAS will deliver additional service on a new frequency at L5, giving the ideal opportunity to modernize the SBAS Minimum Operational Performance Standards (MOPS). Geostationary (GEO) satellites currently comprise the space segment of SBAS. However, GEOs remain at the equator limiting their visibility at the Poles. As activity in the Arctic is increasing, SBAS service in this region is of utmost importance to ensure safety. As such, it is desired that the next-generation L5 MOPS allow for orbit classes other than GEO. Orbital diversity for the delivery of SBAS corrections will allow for better visibility of this service on all places on earth. Here, we discuss the design and qualification of the L5 MOPS orbit messages, namely the ephemeris and almanac. These will support a multitude of orbit classes including all of those used today by both GNSS and SBAS.

Single Frequency WAAS Augmentation Observations (L1 vs. L5) on a Ground Based GPS L1 C/A Solution

This paper presents observations on the WAAS L1 and L5 signals quality and their impact on the robustness of the navigation solution by quantifying the contributions of each broadcasted differential correction. This work is undertaken with the intent of defining performance benefits of L5 by dual frequency WAAS users and is to provide useful material for Minimum Operational Performance Standard (MOPS) development. In this perspective, a study of the WAAS signal characteristics is first carried out. The information gathered is then used to compare various GPS solutions in terms of frequency diversity, satellite diversity, pseudorange noise and different signal corrections and their impacts. These solutions are compared against a reference standalone GPS solution. All statistics are computed with respect to a post-processed Novatel Waypoint Real-Time Kinematics (RTK) GPS L1/L2 semi-codeless static solution, considered as the reference. A discussion on some simplifications with respect to specifications (i.e. MOPS) that could be considered by receiver manufacturers closes the paper. It is confirmed that the current WAAS navigation message definition is the same on both the L1 and L5 frequencies, the latter further being Manchester coded, thus avoiding data ambiguity. The +5 dB SNR on L5 has minor impacts in terms of reliability and continuous availability in the presented scenarios, but would become especially beneficial in hostile environments, despite a greater number of pulsed interferers. Another demonstration is that the WAAS message varies slightly from one WAAS satellite to another, even if corrections are generated centrally. Finally, it is observed that WAAS and GPS signals pseudorange noise are comparable on a “per frequency” basis.

Modelling Tropospheric Delays Using the Global Surface Meteorological Parameter Model GPT2

In case of autonomous positioning the tropospheric delays are taken into consideration by ‘blind’ models. These models do not require any observed meteorological input parameters.Currently the Radio Transmission Commission for Aeronautics defines the Minimum Operational Performance Standards of GNSS, which includes a state-of-the-art blind tropospheric model, too.Recently a new surface meteorological parameter model, the GPT2 has been released. Our study investigates the performance of this new model in the estimation of tropospheric delays using a global radiosonde data set for the period of 2010-2013.Our results showed that GPT2 helped to remove more than 90% of the bias in tropospheric delay estimates found in the RTCA model. Moreover the uncertainties of the estimates are decreased by approximately 10%. The validation showed that the performance of the tropospheric modelling using GPT2 is more stable geographically compared to the performance of the RTCA model.

Assessing the Capability of Distance Measuring Equipment (DME) to Support Future Air Traffic Capacity

Alternative position, navigation, and timing (APNT) systems are needed by aviation to maintain operations for prolonged periods during an absence of Global Positioning System (GPS) service. This is especially important in the future as the Federal Aviation Administration (FAA) Next Generation Air Transportation System (NextGen) utilizes GPS to efficiently handle the anticipated twofold increase in air traffic over the next 15 years. Thus, an APNT system must be able to support the full capacity of future airspace. One candidate APNT system is distance measuring equipment (DME). Since DME has capacity limits, it is important to understand these limits and how they may be increased. This paper models DME capacity and uses the model to examine potential means of improving capacity. Results indicate that increasing DME station reply rate reduces availability for avionics at the current minimum operational performance standards (MOPS). However, with reasonable MOPS changes, DME could support anticipated air traffic levels. Copyright © 2012 Institute of Navigation.

Availability Impact on GPS Aviation due to Strong Ionospheric Scintillation

Strong ionospheric scintillation due to electron density irregularities inside the ionosphere is commonly observed in the equatorial region during solar maxima. Strong amplitude scintillation causes deep and frequent Global Positioning System (GPS) signal fading. Since GPS receivers lose carrier tracking lock at deep signal fading and the lost channel cannot be used for the position solution until reacquired, ionospheric scintillation is a major concern for GPS aviation in the equatorial area. Frequent signal fading also causes frequent reset of the carrier smoothing filter in aviation receivers. This leads to higher noise levels on the pseudo-range measurements. Aviation availability during a severe scintillation period observed using data from the previous solar maximum is analyzed. The effects from satellite loss due to deep fading and shortened carrier smoothing time are considered. Availability results for both vertical and horizontal navigation during the severe scintillation are illustrated. Finally, a modification to the upper bound of the allowed reacquisition time for the current Wide Area Augmentation System (WAAS) Minimum Operational Performance Standards (MOPS) is recommended based on the availability analysis results and observed performance of a certified WAAS receiver.

On-line Detection of Tracking Loss in Aviation GPS Receivers Using Frequency-Lock Loops

For high-precision real-time kinematics (RTK) and safety-of-life Global Navigation Satellite Systems (GNSS), it is critical to track the carrier reliably without interruptions. We develop hardware-oriented robust on-line cycle slip and loss detection logic for a single-frequency stand-alone aviation receiver. The moving average statistics of time-differenced frequency-lock loop (FLL) are shown to provide a robust and fixed detection threshold for cycle loss detection. A Monte-Carlo simulation shows that the detection performance remains robust when aviation dynamics from minimum operational performance standard (MOPS) are added to a simulated strong scintillation.

In-Flight GPS-Signal-Reception Anomalies of Helicopters

I T IS difficult to find appropriate installation points for global positioning system (GPS) antennas on helicopters, because structures such as rotors and engines that are located above the fuselage may degrade GPS positioning accuracy. Even for fixedwing airplanes that have no such obstacles on the fuselage, an inflight GPS antenna performance study has revealed that large bias errors can be caused by distortions in the GPS antenna pattern due to other antennas nearby [1]. The effects of such problems are not so great when GPS usage is limited to en-route navigation. However, they will be more significant in approach and landing with vertical guidance (APV) or precision approaches using a GPS satellite/ ground-based augmentation system (SBAS/GBAS), such as the Federal Aviation Administration’s GPS wide/local area augmentation system (WAAS/LAAS). Although the minimum operational performance standards (MOPS) for WAAS/LAAS airborne equipment [2,3] specify an airborne accuracy model that includes an airframemultipath based on flight test data from large commercial jet transports [4], there is little quantitative flight test data for helicopter GPS antenna performance, and it is thought to be difficult for helicopters to meet the MOPS specifications. This paper presents in-flight GPS-signal-reception anomalies, such as GPS signal masking and multipath that degrade positioning accuracy, observed using the Japan Aerospace Exploration Agency’s (JAXA) research helicopter.

TCAS-II를 응용한 무인항공기의 충돌회피기동 연구

본 논문에서는 항공기충돌방지장치인 TCAS-II를 응용한 무인항공기의 충돌회피를 다룬다. TCAS-II의 최소운용성능표준 문서에 기초한 시뮬레이션 프로그램을 구현하고, TCAS-II가 제공하는 다양한 수직회피 명령을 자동조종장치의 입력으로 변환하여 최대고도변화율이 상이한 두 가지 무인항공기의 충돌회피 성능을 비교분석한다. 낮은 최대고도변화율을 가진 무인항공기에 TCAS-II를 적용할 때 발생 가능한 Near Mid-Air Collision을 방지할 수 있는 방안으로서, 최대고도변화율과 고도를 고려한 개선된 알고리듬을 제안하였다. 이 알고리듬은 ADS-B 개념에 의한 보다 넓은 감시범위가 제공된다는 전제하에 사용될 수 있다. In this paper, the collision avoidance of Unmanned Aerial Vehicles(UAVs) by applying the Traffic alert and Collision Avoidance System II(TCAS-II) is introduced. The performances of two UAVs whose maximum vertical rates are different each other are compared and analysed by not only converting many TCAS-II commands into an autopilot input but also implementing a computer program based on the Minimum Operational Performance Standards for TCAS-II. As the alternative to a possible Near Mid-Air Collision for UAVs whose maximum vertical rates are low, we have proposed a modified algorithm considering the maximum vertical rate and altitude. The modified algorithm is available on the assumption that a wider surveillance range is provided by a ADS-B system.

WAAS Receiver Carrier Tracking Loop and Data Demodulation Performance in the Presence of Wideband Interference

The Minimum Operational Performance Standards (MOPS) for GPS I Wide-Area Augmentation System (WAAS) airborne equipment [l] specifies an interference mask and minimum signal conditions that will be valid for all phases of flight. In addition, demodulated WAAS word error requirements are specified. In support of the RTCA Special Committee (SC)-159 WAAS MOPS activities, a detailed phase-locked loop carrier tracking simulation and analysis for the WAAS satellite signals was performed using the current link budget assumptions. The high-fidelity, nonlinear carrier tracking loop simulations include the Viterbi decoder, and provide word error, bit error, and cycle slip rates as a function of the carrier to noise-plus-interference ratio. Based on the simulation data, it appears that there is approximately a 1 dB link budget gap, which must be addressed by the appropriate industry/government committees.

Analytical Derivation of Maximum Tolerable In-Band Interference Levels for Aviation Applications of GNSS*

This paper presents an analytic derivation of the maximum levels of radio frequency interference (RFI) that can be tolerated for aviation applications of GNSS (global navigation satellite system). The derivation, which is included in the document of RTCA Special Committee-159 Working Group 6 on RFI to GNSS, includes consideration of many factors not addressed comprehensively in previous analyses. Such factors include quantization loss, intermediate frequency (IF) filtering losses, modulation imperfections in the navigation satellites (GPS, GLONASS, and Wide-Area Augmentation System [WAAS]), Doppler loop stress, processing gain losses due to realizable spreading codes, and oscillator phase noise. The goal of this paper is to demonstrate that the interference levels provided in RTCA's Minimum Operational Performance Standards (MOPS) are indeed close to the maximum levels that can be tolerated for aviation applications by moderate-cost modern digital receiver designs.

WAAS Messaging System: Data Rate, Capacity, and Forward Error Correction*

This paper focuses on the design of the Wide-Area Augmentation System (WAAS) messaging system. Specifically, it explains why the data rate is 250 bits/s and the messages are 250 bits long. It also explains why forward error correction is used. The paper estimates the signal-to-noise ratio required for reliable WAAS operation, and discusses message availability when the WAAS satellite is at low elevation angles. This paper does not give a bit-by-bit description of the WAAS data format, nor does it detail the spread-spectrum codes used by WAAS. For these topics, the reader is referred to the current version of the Minimum Operational Performance Standards (MOPS) for WAAS avionics. This paper is based on the work of RTCA Special Committee-159, which is charged with studying the integrity of GPS and writing performance standards for the airborne equipment. In particular, this paper owes much to the efforts of Working Group 2, which is developing the WAAS MOPS.