NextGen
NextGen
We are in the midst of the most significant advancement in aviation technology in more than sixty years. NextGen (Next Generation Air Transportation System), the FAA s airspace modernization program, is designed to overhaul the National Airspace System. This complex, multi-faceted air traffic system is a comprehensive transformation. The advancements that NextGen presents to the aviation community are on a scale surpassing even that which was provided by the introduction of Radar. Avilon TM vor ils gps waas nextgen 1950s 1960s 1990s 2005 2015
What s it for? NextGen will create capabilities that make business and commercial air transportation safer and even more reliable by increasing airspace capacity, reducing delays and reducing aviation s environmental impact through lowering fuel burn and aircraft noise on the ground. It will do this with two main ingredients one on the ATC side and the other inside the aircraft. NextGen has been described as the antidote to the gridlock in our congested air traffic system. The operational advantages will be far-reaching to those aircraft that are NextGen equipped. In fact, many of the following benefits are already starting to manifest themselves: Safety Capacity Efficiency Access Arrivals Departures Source: Federal Aviation Administration (FAA) Improving Traffic Flow in Complex Airspace A key NextGen goal is to safely improve efficiencies in metropolitan areas that have multiple airports and complex air traffic flows. The Dallas, Texas metroplex is a good example of the problem. The number of aircraft flying in North Texas airspace makes it one of the busiest in the country due to the close proximity of Dallas Fort Worth International (DFW) and Dallas Love, in addition to Arlington and other GA airports. If runway capacity alone was a solution, DFW s seven runways would have solved it. But adequate airfield capacity only exacerbates the related issues of routing and separation.
As part of the NextGen program, airspace designers were tasked with redrawing arrivals, departures, and traversing air traffic using defined path concepts. To do this, they examined the historical track data showing where the controllers were already directing the traffic manually. Then, they designed and published procedures to improve upon those paths. With 80 new RNAV procedures designed, they flipped the switch permitting equipped aircraft to precisely track what was before a scatter plot. The results were dramatic. Controllers no longer had the need to direct aircraft leg by leg, reducing both controller and pilot workload. Arrivals and departures are now flown more efficiently, in less time, less miles and with less fuel burn. A yearly fuel savings of 2.6 million gallons has been achieved. NextGen s intent is to provide improved airspace efficiency to match the airfield capacity, at lower cost. Its features and benefits are being expanded nationally. Although Performance Based Navigation (PBN) started as an air-carrier experiment, it will rapidly expand to all IFR aircraft. The significant side effect is that in the future, unequipped aircraft will be penalized, directly or indirectly, for the inability to participate in the new airspace. Providing Approaches to Terrain Challenged Airports With NextGen s defined 3D paths and PBN in the aircraft, it is now practical to provide approach procedures to airports where it has been previously impossible. This can make a big difference in access to rural communities. In the first famous U.S. RNP implementation, Alaska Airlines reduced weather diversions in Juneau, Alaska by an average of 2 flights per day, resulting in a substantial benefit to the airline, the community, and the public. This was accomplished by the use of a curved approach and curved missed-approach path and RNP containment which significantly lowered landing minimums. This same benefit is now beginning to proliferate to smaller airfields, first in the form of straight-in LPV and to be further enhanced by curved-approach RNP with vertical guidance. This will permit simultaneous approaches and departures to occur in IMC to runways which, in the past, had conflicting straight-in airspace overlap requiring traffic sequencing. This will simplify both approaches and departures and eliminate ATC delays in such situations.
RNAV STARs 100 309 209% RNAV SIDs 175 416 137% Q/T Routes 73 190 160% RNAV/RNP Approaches 2,568 104% 5,032 2009 2014 2009 2014 RNAV SIDs/STARs Q/T-routes Source: Federal Aviation Administration (FAA)
Data Communications (Data Comm) Another aspect of NextGen is the further deployment of Data Comm, allowing controllers to send digital messages to pilots in the cockpit. Flight crews can also send digital messages, such as requests and reports, to controllers on the ground. International Access The FAA is working with ICAO to develop unified standards worldwide. An important operator benefit is that all ICAO countries are working on the same basic plan to enhance airspace management. So, an aircraft system that is NextGen capable will be able to make use of PBN not just in the US, but worldwide. What s Happening Now? ADS-B ADS-B is the first step in the NextGen system. It provides high accuracy aircraft position information to air traffic control without the requirement for radar coverage. ADS-B allows more aircraft to operate safely in crowded enroute and terminal airspace. PBN Routes and Procedures As PBN becomes the primary means of navigation across the national airspace system, many of the procedures and standards we use are changing to make use of the enhanced capability. Progress has already been made as illustrated here. What s Next? After ADS-B Out, expect PBN to proliferate, and expect preferential ATC treatment for NextGen equipped aircraft. Aircraft equipped only for plain-vanilla GPS and ILS will pay a price in the ability to fit into the PBN flow.
Sandel Avilon and NextGen NextGen Architected Avilon TM Avilon is a new design, NextGen architected from the inside out. Sandel has provided for the combination of PBN, ADS-B, and Data Comm, along with unique pilot interface and safety enhancements. Sandel Avilon addresses all of the airborne elements of NextGen for retrofit applications. PBN Data Comm ADS-B Path-Centric Control Concept Avilon is built upon the fundamental concept of navigating a continuous 4-D path in a PBN-centric National Airspace System. Pilot management of the path is via Sandel s exclusive Path Guidance Panel, which has carefully designed mode control selections and integrated lateral and vertical preview displays. Lateral Navigation (LNAV) is the primary lateral control mode with tactical Track and Heading mode selections available. Vertical Navigation (VNAV) is the primary vertical mode with tactical Flight Path Angle and Speed modes also available. Time of Arrival Control (TOAC) is accomplished by speed command to achieve the time objective. Performance Based Navigation Avilon is capable of a complete set of PBN functions for current and future operations. Enroute, terminal, and approach navigation is supported: Q, T, legacy airways, point-to-point great circle, RNAV 1 SIDs and STARs, ILS, RNAV(GPS), and RNAV (RNP) 0.3 and lower. Climb operations can be conducted as a performance climb, based on specific aircraft performance, via a specified indicated airspeed that is pilot selected or defined in the flight plan, or to a pilot selected flight path angle.
Before Delays Congestion After Reliability Smooth Traffic Flow Better Use of Airspace
Descents in the enroute and terminal areas can be flown using barometric VNAV that is compliant with flight plan constraints. Geometric descent guidance is provided for all arrival and approach operations. Path Guidance Panel (PGP) This patent-pending panel features two integrated LCD screens designated for Lateral and Vertical path preview information for the specific purpose of always keeping the pilot in-the-loop. The guiding principle used in the design and operation of these displays is that they must unambiguously show to the pilot what the path guidance is currently doing and what it is about to do next. Path Guidance Panel For example, the Lateral PGP display may show the current lateral track with a graphical depiction showing clearly whether that track will or will not capture a subsequent leg in the flight plan. The Vertical PGP display may show the current aircraft flight path angle and a graphical depiction of where that flight path angle intercepts and captures the selected altitude. The pilot selectable guidance modes are easy to understand and are graphically integrated with the flight plan path depiction. Vertical Flight Display (VFD) Avilon features the Sandel designed vertical flight display that greatly increases the information available to a pilot for awareness and control of the vertical flight path. This display integrates a number of data elements in graphical form, including terrain, vertical path constraints, target altitude, approach minimums, actual aircraft flight path angle, desired flight path angle (as defined in the flight plan or as selected by the pilot), minimum climb gradient for departure, and a power-control cue. Vertical Flight Display
The Vertical Flight Display has sensitivity tailored for vertical path capture and tracking. This display will permit manual tracking of the RNAV - RNP vertical path. Vertical Situation Display (VSD) A Vertical Situation Display (VSD) compliments the VFD by providing a long range view of the vertical path at the same selected range as the HSD. Its purpose is to do for vertical what the HSD does for lateral that is, depict the vertical situation clearly and unambiguously. Horizontal Situation Display (HSD) The Horizontal Situation Display, in additional to conventional features, shows a path-based representation including RF turns at flight plan intersections, and RF turns which are part of PBN procedures. Vertical Situation Display Its turn noodle is specifically adapted to permit manual tracking of the RNP lateral path. Horizontal Situation Display ADS-B through Mode S Avilon has been designed with ADS-B via Mode S extended squitter that is fully compliant with the FAA ADS-B rule. The use of Mode S Transponder ensures that all Avilon aircraft will have unrestricted access to all classes of airspace in the US National Airspace System and worldwide. 978-In Avilon includes a 978Mhz data receiver, providing for the reception of the Traffic Information Service broadcast (TIS-B), and Flight Information Service broadcast (FIS-B). The combination of Mode-S transponder and 978-In provides best of both world coverage of ADS-B features. Data Comm Ready Avilon has been designed with internet connectivity in mind by the inclusion of a built-in Data Comm LRU with cockpit WiFi capability. When a reasonably priced airborne internet connection is available, Avilon is Internet Ready.
Avilon and Safety Studies of the first generation of Technically Advanced Aircraft show little to no improvement in accident statistics. Sandel believes that some key areas needed to be thoughtfully addressed in the design of Avilon. First, the vertical flight situation needs to be clearly communicated for improved awareness and control. This has been provided in the VFD. safety IS OUR #1 priority Second, management of the flight path should be simple with a clear understanding of the current and future path, both lateral and vertical. This has been accomplished with the design of the Path Guidance Panel. Third, provisions need to be made for upset avoidance and recovery, which is provided for with the EMER button. And finally, the integrated design must provide a high level of reliability and redundancy to minimize failure conditions that result in degraded modes of operation. All of these concepts have been addressed in the Avilon design. Upset Recovery Avilon features a capable guidance system with sophisticated autopilot and flight director control laws. These control laws incorporate a variety of protections aimed at reducing the opportunities for upset conditions. Avilon provides autopilot or flight director-assisted recovery in the event of an upset of the aircraft beyond certain defined parameters. Redundancy and Reliability The architecture of the Avilon system has been designed with a high level of system redundancy to ensure that full capability is available, even in the event of significant sub-system failures. The Avilon sub-system components are integrated with a triple redundant and dissimilar data bus structure. All flight data is available independently from each bus. A fourth dedicated standby bus supports the operation of the Path Guidance Panel displays as standby Primary Flight Display and standby Navigation Display.
A typical Avilon installation has 3 to 4 large format displays, each with three physical bus connections that provide each display with a triple redundant data exchange capability. Multiple Avilon system functions are distributed across each display unit, with each capable of providing flight management, autoflight, all display formats, and full user interface. The software design methods employed in Avilon provide a functional assurance level that is greater, in many cases, than current certification requirements. This ensures that Avilon operators enjoy a high level of data integrity.
PBN
THE Case FOR PBN PBN can eliminate many limitations on the present generation of ground-based NAVAIDs used in instrument operations: Ground stations have to be installed and maintained at great expense. The location of the ground stations is critical and siting is problematic. Because the system is angular, the airspace protections provided around the course centerline increased with distance from the ground transmitter, wasting airspace. Only straight-line routes and approaches are supported, making the system incapable of simultaneous approaches in multi-airport locations with conflicting extended centerlines. Conventional Routes RNAV Routes RNP Routes Ground-based Navaids Flexible Waypoints Curved Paths Linear Containment Limited Flexibility Increased Efficiency Highly Optimized A Practical Definition PBN is the umbrella term that includes RNAV and RNP. RNP was developed to further refine the concept of RNAV with the objective of increasing access, efficiency, and capacity objectives without specifying a particular navigation technology required in the aircraft. Instead, a performance requirement is specified for each particular route. The aircraft systems are tasked with using any navigation positioning technology onboard and available to meet the navigation performance requirement. This is a paradigm shift.
The aircraft can use GPS, GPS-WAAS, GPS with Inertial GPS with Baro, Multi-mode with DME-DME, etc. The design rests squarely on the side of the aircraft navigation system designer. When regulators talk about PBN, whether they are referring to either RNAV or RNP, they are generally saying that the desired operation is contingent on the navigation performance capability of the aircraft and pilot. In other words, the operation is performance based. This is an accurate simplification. FAA AND PBN The FAA s PBN strategy was introduced in its 2003 Roadmap for Performance Based Navigation, the main thrust of which was to promote the use of PBN procedures in the NAS. The FAA s early initiatives also included the introduction of Q-Routes in the enroute domain and RNAV departure and arrival procedures for the terminal domain. These procedures were typically designed as overlays of historical vector patterns and existing conventional (ground-based) procedures to accelerate the availability of published PBN procedures. The July 2006 update to the Roadmap for Performance Based Navigation served as a call to action for both FAA and industry by continuing to focus on the wide propagation of PBN routes, procedures, and approaches throughout the NAS. Additionally, the strategy included bringing Required Navigation Performance (RNP) procedures, which grew out of the need for airport access to the terrain-challenged airports of Alaska, into operational use in the lower 48 states. The FAA s PBN vision at that time, simply stated, was RNAV everywhere and RNP where needed. The FAA is currently working on updating the PBN plan and is working closely with the industry, including Sandel Avionics, to develop a new strategy for Performance Based Navigation in the NAS. The stated goal is to use the PBN implementation experience gained over the last 15 years to focus on areas for improvement. A number of those areas are of significant importance to corporate aviation and regional air transport. They include: Reducing the time for flight procedures to be developed. This will be significant in that many airports that currently are on a waiting list for performance-based navigation procedures could expect to see that waiting period significantly reduced or eliminated. Reducing the dependence upon a network of legacy navigational infrastructure that does not support performance-based navigation and is costly to maintain.
CURRENT PBN INstrUMENT FLIGHT PROCEDURE INVENTORY Even as the national PBN strategy is being finalized to accelerate the production of PBN procedures and expand to all NAS operations, it is worthwhile to note how much work has been done already as evidenced by an impressive number of PBN flight procedures. These PBN Instrument Flight Procedures are deployed across the National Airspace System and are providing benefits to users in enroute, terminal, and approach operations. As of October, 2015, the total inventory of all Instrument Flight Procedures (IFPs) in the NAS was 34,387. This includes both Instrument Approach Procedures (IAPs) and Airways. Over half of that total, or 15,622 were PBN IFPs, including PBN IAPs and Airways. The effective result of this large inventory of PBN procedures is that many airports in the US are served by an instrument approach and in most cases with vertical guidance to a Decision Altitude (DA). Both of these outcomes are important to insure reliable air transportation at the highest levels of safety. To be more specific, there are approximately 2700 airports with at least lateral instrument guidance from a PBN IAP, and approximately 1700 airports that have LPV and/or LNAV/VNAV vertical guidance to a DA.
Technical Reference
Technical Reference RNAV Position Estimation A very important function of the RNAV system is to estimate the current position of the aircraft using one or many sources. The accuracy and integrity of the position estimate must be known. Both values must be equal or better than the requirement for the intended operation. Once a navigation operation has commenced, a sufficient level of accuracy and integrity must be continuously available to complete the operation. Most modern systems can continuously manage numerous position sensors as a sub-system to get the best position result. In the truest sense of the word synergy, the positioning performance often obtained by an RNAV system is better than what is possible from any individual positioning sensor technology. Let s take a look at the different positioning technologies commonly managed by RNAV systems and discuss the accuracy, integrity, availability, and continuity of each. Global Navigation Satellite System (GNSS) There are currently four global navigation satellite constellations. By 2020, there will be at least 124 global navigation satellites in orbit comprising four separate, compatible and complimentary constellations. Each constellation will provide equivalent or better positioning performance as we have with GPS today. It is reasonable to expect that aviation users will enjoy an increase in performance in satellite navigation over GPS today, particularly with respect to availability, integrity, and continuity. For this reason, it is worth mentioning a bit about these three other constellations. The Russian GLONASS constellation has been in various stages of deployment since the early 1990s, and has been fully deployed and globally available for a few years. GLONASS is comprised of a nominal set of 24 satellites and in ideal conditions offers a level of performance nearly equivalent to GPS.
The European Galileo constellation is in deployment phase with 7 satellites in orbit of a planned 30 by the year 2020. Galileo is a civilian system that will provide 1-meter accuracy to all users with a Safety-of-Life Integrity signal provided to users who require high integrity. The Chinese BeiDou constellation is also in the deployment phase with 18 satellites in orbit of a planned 35 by the year 2020. It is intended to provide 10-meter accuracy for all users on the globe. The Unites States Department of Defense (DOD) Global Positioning System (GPS) has been deployed and in use for over 20 years and was the first truly successful GNSS. This system has 35 satellites in orbit providing better than 5-meter accuracy to users across the globe. Since GPS is the only GNSS currently in use in the NAS, most of the discussion on GNSS will focus on this system. The introduction of GPS for air navigation in the early 1990s has been a big enabler for RNAV in the NAS, especially for remote and oceanic operations. GPS Accuracy The accuracies offered by GPS have continually improved over the last twenty years. The official commitment by the United States DOD is for a nominal set of 24 satellites. For a variety of reasons, the GPS constellation has had many more than the planned nominal set; for example, today s constellation has 35. With this abundance of satellites, excellent positioning performance is obtained by user receivers across the globe. The Standard Positioning Service (SPS) of GPS today is monitored and reported in a quarterly report to the FAA. The latest report, published July 31, 2015, indicates that the nominal GPS accuracy observed at sites across the NAS varies from 2-meter to 7-meter accuracy in the horizontal and from 4-meter to 7-meter accuracy in the vertical. For aircraft navigation operations on departure, enroute, arrival and approach (down to 200 feet AGL), these position errors of GPS are inconsequential in practice. It s amazing to think how far we have come in the last twenty years! Integrity Integrity, or the assurance of truth, in the position calculations of a GPS receiver is an essential element of aircraft operations that depend on the
signal for navigation. The big concern for GPS users is an erroneous position computation due to some type of undetected satellite failure. This is not likely, but the consequences are potentially significant. It takes a minimum of four satellites in view for a GPS receiver to compute a position. This includes three satellites to establish a fix, and the fourth satellite to resolve the clock error (this clever concept in the original US DOD design was key to keeping GPS receiver costs low atomic clocks are expensive!) The basic concept of RAIM is simple. With additional satellites in view, multiple combinations of four can be used to compute positions and the results compared as a type of independent monitor. Any errors beyond those attributed to differences in geometry could be the result of a satellite failure. Of course there are a number of other error sources, such as ionospheric propagation effects, random noise, etc. The math folks manage all this with models and computations so that for a given set of satellites, the RAIMequipped GPS receiver outputs an integrity parameter called Horizontal Integrity Limit (HIL). This number is a radius that is centered on the computed GPS position that the receiver guarantees to the user (e.g. RNAV system) contains the true position, at least to the degree of certainty necessary for most aviation applications. Due to the abundance of satellites in the current GPS constellation, the ability of an aircraft s GPS receiver to independently monitor the satellite constellation with the RAIM function is really good. For example, the most demanding use of autonomous GPS in aviation today is RNP AR 0.1 operations. Referring again to the July 31, 2015 SPS report, many locations across the NAS report that the HIL necessary to support RNP 0.1 is available 100% of the time. The worst case reported was 99.61% availability! It is interesting to note that RAIM provides integrity for horizontal position computations only. Integrity levels sufficient for vertical positioning in approach operations have only been possible with augmentation of the basic signals through various external monitoring schemes. Industry experts are increasingly optimistic that aircraft GNSS receivers using dual frequency, multi-constellation satellites will be able to provide, in the next decade, lateral and vertical integrity levels equivalent to that provided today by Space Based Augmentation Systems (SBAS). This is an extension of the original RAIM concept and is generically referred to as an Aircraft Based Augmentation System (ABAS).
Availability For reasons already mentioned, the availability of the necessary GPS accuracy and integrity to conduct RNAV operations is almost 100 percent. There are exceptions related to the intentional denial of the service due to periodic testing by the US military over specified geographic areas. These periods and areas affected are documented in NOTAMs and can be accessed as part of a pilot s pre-flight planning. As discussed previously, the availability of integrity is the more limiting concern. For the most demanding operations, such as RNP AR approaches at RNP levels below 0.3, additional measures are required in order to ensure that the GPS integrity (HIL) will be available at the start of the operation. Special programs have been developed to help pilots and dispatchers forecast the HIL for a particular location and time. Time is important because the geometry of the constellation is continually changing since each satellite is orbiting the earth twice a day. In certain locations with high terrain, even the effects of terrain obscuring satellites as aircraft descend to an airport are modeled for accurate forecasting. This forecast of HIL is part of the pre-flight activities for these operations. Continuity The ability of the GPS receiver to continually provide sufficient accuracy and integrity for an RNAV system to complete a navigation operation is, like the availability, nearly 100%. There are certain RNAV operations in areas of high terrain where the complete failure of a GPS receiver could be hazardous. Consequently, a second receiver may be required. Areas of high terrain could also mask portions of the sky that are well above the horizon. Consequently, the number of visible satellites could be reduced so that HIL is increased above the value necessary for the operation. In these instances, a multi-sensor RNAV system design would increase the effective availability. SBAS/Waas The Standard Positioning Service provided to users by the US DOD does not provide sufficient vertical integrity for instrument approach operations. Two different schemes have been developed to augment the autonomous GPS signal in space to provide higher integrity for more demanding instrument approach operations close to the ground.
The FAA named the US deployment of Satellite Based Augmentation System, aka the Wide Area Augmentation System (WAAS). The concept is quite simple observe the actual position calculation errors at known sites on the ground and re-broadcast the corrective information to user receivers over a wide area (from geo-synchronous satellites). Additionally, any GPS satellite failures can be detected and users notified much faster than with autonomous systems. WAAS became available for aviation users in the US in 2003, and since then, almost all GPS receivers have been designed to receive the WAAS (SBAS) enhancements to the GPS signal-in space. SBAS has been deployed in Europe (EGNOS), Japan (MSAS), India (GAGAN), and is planned in Russia (SDCM). Many areas in the Northern Hemisphere are covered by the signals from these SBAS systems, but to date, no systems have been deployed to support aviation operations south of the equator. Given the future expectations from dual frequency, multi-constellation ABAS, SBAS may remain a northern hemisphere regional system. There are numerous advantages offered to users in the SBAS scheme. Let s quickly review of the various elements of WAAS (SBAS) available to support operations in the NAS: Accuracy There is a small improvement, but quite frankly, not a very meaningful (for aviation users) improvement in horizontal position accuracy with WAAS. The horizontal accuracy of WAAS reported in the July 2015 WAAS Performance Analysis Report varies from 0.7 m to 1.4 m across the NAS. For scientists (and people spreading fertilizer in Iowa!) this may be exciting, but the difference between the GPS SPS horizontal accuracy of 2m to 7m and the WAAS horizontal accuracy of 0.7m to 1.4m with respect to the runway width is minor. So why did the FAA spend an estimated $2.2 billion to deploy WAAS and then continue to spend $110 million annually to maintain the system? The answer is that the FAA s primary objective in the US with WAAS was to support a pseudo-ils navigation operation called LPV. The lateral and vertical accuracy requirements to fly an ILS are 16m horizontal and 4m vertical. This horizontal accuracy requirement is easily met with the GPS SPS, but not quite adequate in the vertical. WAAS vertical accuracy across the NAS, as reported in July 2015, varies from 0.9m to 2.8m, while GPS SPS vertical accuracy varies from 4 m to 7m across the NAS in the same time period.
Integrity The integrity improvement provided by WAAS is much greater than that available from RAIM in GPS SPS. The integrity necessary to fly ILS-type approaches with GPS is currently only possible with augmentation like WAAS. The integrity terminology has changed a bit in the WAAS scheme. Receivers compute and transmit horizontal and vertical integrity information to users as Horizontal Protection Level (HPL), and Vertical Protection Level (VPL). The HPL for WAAS is basically the same concept as HIL computed by RAIM in the unaugmented GPS operation. For ILS-type approaches (LPV), WAAS has to provide not only sufficient lateral and vertical integrity but also notify users in a timely manner (within 6.2 seconds) if either HPL or VPL exceed the alarm limits for the approach. In the July 2015 WAAS Performance Analysis Report it was reported that at no time did the maximum lateral or vertical error observed exceed the HPL or VPL computed by receivers at monitor sites across the NAS. Availability In the July 2015 WAAS Performance Analysis Report, sufficient lateral and vertical integrity was available to support LPV approaches down to 200 feet minimums 100 percent of the time at many stations across the NAS. Availability decreases at the edges of the coverage area with the lowest availability reported at about 95% in Cold Bay, Alaska. The availability increases for less demanding WAAS navigation operations such as LPV with minimums greater than 200 feet and LP, which is equivalent to a localizer approach with no vertical guidance. For example, the LP approach supported by WAAS in Cold Bay, Alaska is available 99.98% of the time. Like GPS SPS, there are periods when WAAS is not available due to the intentional denial of the service during periodic testing by the US military over specified geographic areas. These periods and areas affected are documented in NOTAMs and can be accessed as part of a pilot s pre-flight planning. Continuity The ability of WAAS to continually support the necessary accuracy and integrity to complete a navigation operation is nearly 100 percent across the NAS. Like the GPS SPS, WAAS can be affected by terrain between the user receiver and the satellites. An added challenge is that the WAAS broadcast of information is from geo-synchronous satellites over the equator. At high latitudes, these geo satellites are lower on the horizon and subject to terrain masking in these regions. As mentioned before, multi-sensor RNAV architectures will become important to mitigate these effects at high latitude.
Ground-Based Augmentation System (GBAS) This is an augmentation of GPS that, like SBAS, is quite simple in concept. The actual position calculation errors are monitored by receivers at known locations, and the information necessary to correct the position errors is re-broadcast as a VHF Data Broadcast to aircraft in the local area (out to 60 NM or so). Because the user receiver and the monitoring site are in such close proximity, the accuracy and integrity (including the time to alarm) enhancement to GPS is further improved over WAAS (SBAS) and is sufficient to support landing (autoland) operations, which are the most demanding from an accuracy and integrity standpoint. GBAS, also called Local Area Augmentation Service (LAAS) by the FAA, is in the early stage of deployment across the NAS with a few sites operational in 2015. These installations are being deployed with the view that landing operations will be supported at these locations for aircraft with the necessary GPS Landing System (GLS) airborne equipment. INERTIAL NAVIGatioN SYSTEM The Inertial Navigation System (INS) was originally introduced into aviation to provide navigation in oceanic and remote areas and is currently not authorized as a primary means of navigation within the NAS. The real value of an INS for PBN operations is as an independent sensor input to a multi-sensor RNAV system. The navigation characteristics of an INS are complimentary to other sensors such as GPS. An inertial system is essentially a dead-reckoning system that is initialized or aligned at a defined point at the departure airport. Accelerations and resultant velocities in each of three orthogonal axes are used to calculate the change in position over time from the starting point. Key to the success of these systems is the ability to detect small linear and angular accelerations. Accuracy The short term accuracy of a ring-laser based inertial navigation system is excellent. Due to the dead-reckoning operation of the system, position errors accumulate over time that degrade the accuracy. The current ring-laser generation of these inertial systems in commercial aviation is certified to maintain less than 2 NM of position error per hour of operation, but the actual operational performance is much better. It is not uncommon to see less than 1 NM of error in these systems after several hours of flight.
Integrity An INS does not output any position integrity information, but in dual or triple installations, the attitude and acceleration information can be monitored to detect any degradation. Availability The positioning performance of the INS is entirely self-contained and thus the availability of the INS position information is related directly to the reliability of the unit. Current production long-range aircraft are equipped with multiple INS units to improve dispatch availability for oceanic and remote operations. DME Until GPS was introduced in the early 1990 s, the best RNAV position performance was obtained by DME updating. The RNAV system computes a position by tuning DME stations in the vicinity and using the intersection of the arcs defined by the observed distance from each station. A database of DME stations that includes at a minimum the latitude, longitude, elevation, and frequency of each station is required. The RNAV system must also have control of a navigation radio to tune and receive the distance information. Accuracy The geometry of the ground stations is an important factor in the computed accuracy of the position. The best accuracy results from stations that are orthogonal to each other. In areas of a limited population of DME ground stations, the accuracy will likely decrease as the RNAV system is not able to tune optimum combinations of DME stations. In areas of abundant DME stations, RNAV systems that have access to specially designed scanning radios can rapidly tune multiple DME station pairs that improve the position accuracy. With high quality receivers and optimum station geometry, the best accuracy performance of a DME-updated RNAV system is approximately 0.2 NM. The minimum position accuracy requirements required for an RNAV system with DME updating is 2.8 NM for enroute, 1.7 NM for terminal, and 0.3 for approach. It is interesting to note that while DME updating has been overshadowed by the advent of GPS, the FAA is planning a big future for the technology. As the FAA looks ahead to a PBN future that is increasingly dependent on space-based navigation, malicious denial of service is a growing concern. The signal from a DME station is significantly stronger than GPS, which makes it an excellent option to ensure a resilient backup navigation
infrastructure. The FAA is working on plans to rationalize the distribution of DME ground stations to ensure sufficient RNAV position performance for future PBN enroute and terminal operations in selected airspace to support basic service to designated airports. Integrity This important parameter of navigation is provided by periodic flight inspection of the ground station to ensure it is within tolerance. The RNAV navigation database with DME station location and frequency must be processed and managed with defined data quality standards. Availability In order to ensure availability, sufficient DME stations must be distributed across the area of intended operation. Because the signal from DME is line-of-sight, terrain can be a factor for certain operations. The geometry of DME stations relative to the planned flight path is especially important when the population of DME is limited. In these circumstances, availability may be affected if a particular DME is unavailable. DME position updates are not available for oceanic operations for obvious reasons. Continuity The primary factor in continuity of DME position computations for RNAV systems is the reliability of the ground station hardware and power supplies. In limited circumstances, such as descending on an arrival or approach in high terrain, continuity of this function could be affected by terrain masking. One final note: DME/DME position computations are sometimes referred to as Rho/Rho positioning. This comes come the spherical coordinate system where the first term in a spherical coordinate is the Greek symbol Rho, for distance from the origin of the spherical coordinate reference. VOR/LOC Both VOR and LOCalizer are used in combination with DME in limited cases by RNAV systems to estimate position. These are sometimes referred to as Rho Theta updates. Rho, being the Greek symbol used for distance in a spherical coordinate system and Theta being the Greek symbol for azimuth.
Accuracy Because of the radial spread of the VOR signal, position updates are limited to a maximum DME to ensure that the accuracy meets the RNAV position requirements of 2.8 NM for enroute, 1.7 NM for terminal, and 0.3 for approach. Because the maximum distance can be as little as 25 NM, the utility of VOR/DME is quite limited. Much better accuracy is available from LOC/DME because of the reduced radial spread of a localizer, but there are typically numerous conditions that must be satisfied before the updates can be computed, which limits the opportunity. The integrity, availability, and continuity of VOR/LOC are the same as DME above. BARO ALTIMETRY Barometric altitude is the primary vertical position sensor for RNAV systems. It is important to note that the basis for vertical separation (aircraft-to-aircraft and aircraft-to terrain) in the entire NAS is based on barometric altimetry. The only exception to this is the final approach segment of a precision approach. In this case, the vertical position may be provided by augmented satellite navigation (SBAS or GBAS), or an ILS. True Altitude Given Atmospheric Pressure (pressure altitude) Indicated Altitude 2000 ft 1520 ft 3000 ft 2000 ft 1000 ft High OAT Standard OAT Low OAT
Accuracy A modern digital air data computer provides an accurate measure of pressure altitude without the mechanical errors of prior generations of altimeters. However, accuracy is ultimately dependent on the value of the local airport pressure (QNH). Another important factor in accuracy is temperature variations from standard. At temperatures higher or lower than standard, the aircraft altitude computation will have an error that increases with altitude above the source of the QNH. At temperatures higher than standard, the computed aircraft altitude will be higher than indicated and at temperatures lower than standard, the computed aircraft altitude will be lower than indicated. This is a long-understood fact with barometric altimetry. However, RNAV systems add a new complication with the use of barometric altimetry to provide vertical navigation in arrivals and approach operations. Because the actual VNAV descent angle can change in cold or hot temperatures, the approach phase of flight is of particular concern. A nominal 3 degree approach angle could vary 2.5 to 3.8 degrees in cold and hot conditions, which introduce particular concerns for each condition. RNAV temperature compensation capability is common, which helps mitigate this issue. VNAV temp lower than standard Nominal 3 Glidepath VNAV temp higher than standard Integrity There is no built-in integrity in the typical altimeter system. An improperly set altimeter due to crew error or an erroneous or stale airport report of QNH will result in vertical position errors. Operational procedures such as cross-check of altitude while on a glideslope at known fixes provide some mitigation.
It is interesting to note that with the advent of Terrain Awareness and Warning Systems (TAWS), techniques have been developed and certified to mitigate barometric altimetry error by managing multiple vertical position sources. Availability Barometric altimetry is a globally available system for vertical positioning at altitudes above the specified transition altitude. For altitudes below the applicable transition level, the availability of barometric altimetry for terminal and approach operations is typically dependent on a local QNH setting. Continuity The continuity of barometric altitude positioning is a function of the reliability of the digital air data system. HEADING/TAS Heading and true airspeed are important inputs for an RNAV system to dead-reckon a position in the absence of other position sources. Accuracy Dead-reckoning accuracy degrades relatively quickly over time as errors accumulate. The better the quality of heading and TAS, the slower the increase in position error. Integrity This position computation has no quantitative integrity and would be suitable only as a position source of last resort for an RNAV system. This could allow an RNAV system to provide some reasonable guidance for an aircraft to climb to a safe altitude during a missed approach or departure with a total loss of normal position sources. Availability and continuity would be a function of the reliability of the heading and air data source of the airplane. Path DEFINITION Besides the position computation, the RNAV system has to define a desired lateral and vertical path for the planned navigation operation.
Lateral The RNAV system defines a lateral path in a variety of ways. A simple example of this is the Direct-To. In a case where the pilot wants to go to a location or waypoint that is directly ahead, the RNAV system predicts the lateral path as a great circle route to get there. It is a bit more complicated if the desired waypoint is somewhere other than directly ahead. The RNAV system will need to compute a turn in the shortest direction to the waypoint. The turn is computed as a predicted ground track that is a function of the commanded bank angle and ground speed. The ground speed is computed using the TAS and actual or predicted winds through the turn. Preplanned routes can be flown by the FMS through routes or procedures stored in the navigation database. Over the last several decades, industry standard groups have defined data standards so that very detailed lateral paths can be defined using combinations of waypoints and leg types. This process has significant complexity because of the objective to have RNAV systems define and fly lateral paths that precisely overlay these procedures. Legacy navigation procedures such as, Climb to an altitude on YYY heading and then go direct to XYZ, or Fly direct to XYZ VOR and track YYY radial to XX distance are examples that are possible using the defined navigation data standards. Industry standards for RNAV system navigation data defines nearly 30 different leg types that can be used. RNP systems use a much smaller set of leg types to accomplish the same path objectives. Once a flight plan is entered, a modern RNAV system design will use all available information to define a continuous lateral path from the departure runway to the landing runway. Every change in the desired lateral track along the flight plan will have a precise radius of turn predicted based on the track change angle, ground speed, and commanded bank angle. For the highest level of precision, forecast winds are needed along the route, along with performance predictions for the aircraft to provide the altitude and true airspeed at the beginning and end of each turn segment. Vertical The RNAV system should also define a vertical path for the planned flight. A performance model for the aircraft and engines in the RNAV system are highly desirable. (We will stay with the RNAV moniker to describe the system even though the combination of RNAV with aircraft and engine databases are much more accurately described as Flight Management Systems (FMS)).
With accurate performance models for the aircraft and engine combination, along with atmospheric (wind and temperature) predictions, the RNAV system can define the vertical path along the defined lateral path in consideration of crossing restrictions ( at or above, at, etc.) and optimum cruise altitude. For descents, the RNAV system should define the top of descent to comply with all speed and altitude constraints on the arrival and approach. RNP allows certain vertical paths to be defined continuously between waypoints. This added capability can be advantageous for ATC and the airplane where descents need to transit high traffic density airspace. A PBN-equipped airplane has the information and performance capability necessary to execute such procedures. With a loaded flight plan, a mature RNAV system design will use all available information to define a continuous vertical path from the departure runway to the landing runway. Temporal It will become increasingly important for RNAV systems to predict accurate time estimates for every point along the 3-D flight plan. These time predictions will be used for conflict-detection much further ahead than the current airspace management practices. Controlled Time of Arrival will become an important tool as the future strategic management of airspace will be based on 4-dimensional trajectories. Path DISPlay Another important function of the RNAV system is to share the flight plan path predictions with onboard sub-systems for display and use by the flight crew and, in the not too distant future, users outside of the aircraft to support the transition to 4-D trajectory based airspace operations. There are three essential pieces of information that must be communicated through the path display medium. The first and most basic is the need to define the centerline of the desired path. Secondly, it is important to display the deviation from the centerline. And finally, the maximum deviation limits for that particular segment of flight must be communicated. Lateral The first and most common means to display RNAV lateral path information is the Course Deviation Indicator (CDI), which of course has been carried over from the previous era of conventional navigation. The course centerline,
deviation left and right, and maximum deviation limit scales are all clearly communicated by the CDI. Flying a lateral path with a CDI has been described as looking through a soda straw. You have all of your focus on the present condition with no means to anticipate what s ahead. As PBN applications expand and become more complex, the Nav Display (ND) or MAP display offers many advantages. With a well-designed ND, the pilot has the ability to see the same essential information provided by the CDI, including course centerline, deviation, and lateral deviation limits. The desired path display must provide the complete details of the lateral path as it has been predicted through the complete flight plan. With this level of fidelity, the pilot can see an upcoming turn, which minimizes gross deviation errors or blunders while manually controlling the aircraft. Also, hazards to the aircraft, such as weather, terrain, and other traffic can be overlaid on the lateral path information to increase situational awareness. Vertical The vertical path defined by the RNAV system is typically displayed on a Vertical Deviation Indicator (VDI) on the Primary Flight Display (PFD) in a manner similar to ILS glideslope. Some synthetic vision displays provide vertical path display information as goal posts, wire frame symbols, or other type of perspective symbology in an attempt to communicate the desired vertical path for the aircraft. None of these methods are ideal. It can be concluded that this is an area that will be improved in future cockpit displays. Temporal The display of the required/control time of arrival (RTA/CTA) at a future point in the flight plan is at a nascent stage of development. Aircraft currently equipped with this feature display all of the necessary information in digital form: the RTA/CTA, time error, and earliest/latest time at the fix. Expect to see a lot of development in this area in future system designs so that the flight crew has a clear, unambiguous display of the necessary temporal information to effectively manage the trajectory to the objective.