INNOVATION
Aircraft Landings: The GPS Approach George Dewar Nav Canada
The Global Positioning System, You might not notice it from the passenger ple, during the night and when landing and considered by many to be the greatest seat, but the next time you fly, your pilot taking off in low cloud ceilings. When flying advance in aviation since the invention could be using GPS to navigate the plane. IFR, pilots rely on navigation instruments in of the jet engine, will revolutionize Many regulatory agencies, including Trans- conjunction with instructions from air traffic the operation of aircraft all around port Canada and the U.S. Federal Aviation control agencies issuing clearances for routes the world. Not only will it direct Administration (FAA), have sanctioned the and altitudes. In congested areas, air traffic pilots to the vicinity of an airport, use of unaugmented (stand-alone) GPS as a controllers essentially “steer” aircraft using it will also be able to guide a plane supplemental navigation system for all radar vectoring. along a runway approach route and phases of flight, including nonprecision To ensure an orderly flow of traffic, most even permit automatic landings. approaches and landings. Unaugmented GPS phases of flight must follow a track, or flight To enable more efficient operations, does not have the accuracy or integrity path, that coincides with ground-based navi- air navigation service providers are required to use it as a primary navigation sys- gation facilities. This requirement has always designing new approach procedures tem for precision approaches; however, in been a major constraint on IFR operations. for aircraft using GPS. In this month’s remote areas and over oceans, GPS can be The ability to navigate with a high degree of column, George Dewar examines used as a primary system. accuracy, independent of ground-based facil- these new GPS approaches and how Within the next few years, the Wide Area ities, is considered the most beneficial aspect they differ from approaches using and Local Area Augmentation Systems of GPS with regard to aviation. conventional navigation aids. (WAAS and LAAS) will come into use, per- Precision and Nonprecision Approaches. Aeronau- Dewar obtained his “wings” mitting GPS use as a primary system for all tical navigation involves four phases of in 1962 and since then has had a phases of flight and, in the case of LAAS, flight: en route, terminal, approach, and number of aviation-related jobs, enabling aircraft landings with extremely departure. En route navigation encompasses including air traffic controller, limited visibility. In the meantime, the use of point-to-point navigation, usually between commercial pilot, and aeronautical unaugmented supplementary GPS in the cities. Departure navigation is the portion of information officer. He is currently, cockpit is growing exponentially. flight from take-off to the en route phase. Nav Canada’s Airspace Standards Flying with GPS, especially in the last And terminal navigation refers to the transi- Team Leader for the Atlantic Region, stages of flight (the approach and landing) is tion between the en route phase and the headquartered in Moncton, New different from conventional flying, and spe- approach, or landing phase. For approaches, Brunswick. He holds an Airline cial procedures have been developed for GPS there are two basic procedures: precision and Transport Licence for fixed-wing and use. In this article, we’ll examine those pro- nonprecision. a commercial license for rotary-wing cedures and how they are designed. But first, A navigation system permitting a preci- aircraft, and he regularly flies a let’s learn the rules of the “road.” sion approach provides both lateral (horizon- Cessna Citation business jet. This tal) and vertical guidance to a decision article represents Dewar’s personal APPROACH BASICS altitude/height (DA/H). If the required visual views, which are not necessarily those Just as we have rules to govern how we drive references, such as approach lights or the of Nav Canada. our cars and thereby maintain order on the runway environment, are not in view at the “Innovation” is a regular roads, pilots have rules to maintain order in DA/H, a pilot must execute a missed column featuring discussions about the air. There are actually two sets of rules — approach — that is, a specified, controlled recent advances in GPS technology visual flight rules (VFR) and instrument routing away from the runway. and its applications as well as the flight rules (IFR) — and they apply to air- A nonprecision approach (NPA) provides fundamentals of GPS positioning. craft both small and large, depending on the lateral guidance only and uses a minimum The column is coordinated by situation. descent altitude (MDA). MDA is defined as Richard Langley of the Department VFR operations require the pilot to fly an altitude below which an aircraft must not of Geodesy and Geomatics Engineering clear of clouds and conduct landings and descend until visual reference has been estab- at the University of New Brunswick, takeoffs in good visual conditions. This is lished (see Figure 1). Typically, MDAs range who appreciates receiving your basically a “see and be seen” situation. GPS from 300 to 500 feet, but the actual value comments as well as topic suggestions allows VFR pilots to maintain an accurate depends on the presence of obstacles such as for future columns. To contact him, track if conventional navigation aids don’t hills, buildings, or towers in the airport’s see the “Columnists” section on exist or map reading is difficult. vicinity. page 4 of this issue. IFR operations can be conducted in condi- The International Civil Aviation Organi- tions of zero or restricted visibility, for exam- zation is in the process of introducing a new
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approach term — a nonprecision instrument approach procedure with vertical guidance (IPV). IPV bridges the gap between the Precision approach — instrument landing systems (ILS) precision approach lateral and vertical guidance and the nonprecision approach lat- Final approach eral-only guidance, allowing procedure Sumspot Airport, fix elevation 100 feet designers to isolate obstacles that constrain above sea level the landing limits on a typical NPA. It recog- Glidepath ILS localizer + glidepath nizes the capability of modern aircraft to + + Tower does not penetrate OCS establish onboard systemÐderived vertical Obstacle clearance + surface (OCS) + 310 ft. Decision height: 300 feet guidance using a navigation database and + + barometric altimetry. IPVs can be based on + GPS, and some are already being evaluated by commercial operators in VFR weather Nonprecision approach — VHF omnidirectional range (VOR) conditions. Final approach fix CONVENTIONAL PROCEDURES Large urban centers typically have an airport 250 feet required obstacle clearance VOR equipped with expensive precision approach + + Minimum descent altitude = facilities, such as an instrument landing + 310 + 250 = 560 feet + 310 feet system, which allows operations in weather +
+ conditions of 200-foot ceilings and 0.5- + mile visibility or less. Some of these airports have facilities installed that allow appropri- Figure 1. Profile views of a nonprecision VHF omnidirectional range (VOR) proce- ately-equipped aircraft and trained flight dure and a precision instrument landing system (ILS) procedure illustrate how lower crews to land under conditions of very landing limits are achieved with a glide slope. low visibility without any weather ceiling restrictions. An NPA based on GPS would not signifi- lated to aid identification. An automatic distance to the station from the signals’ cantly enhance operations at these large direction finder on the plane establishes round-trip travel time. This technique uses urban airports. Instead GPS’s potential is the bearing of the transmitter with respect to frequencies in the 960 Ð1215-MHz range. In currently greatest at second- or third-level the aircraft heading. some cases, DMEs have been installed and locations where the traffic counts do not If an NDB is placed on the approach path collocated with NDBs in an effort to achieve warrant expenditures on the more conven- to a runway, it is possible — though rarely lower MDAs. tional navigation aids. Nonetheless, to under- achieved — to obtain an MDA as low as 300 stand the benefits of GPS NPAs, it is feet above the runway. Typically, the pres- GPS APPROACHES important to review how conventional navi- ence of obstacles along the approach Conventional navigation aids have a high gation tools are employed in nonprecision path necessitates higher MDAs. If it is not degree of reliability, but this is offset by costs approaches. possible to locate the NDB on the runway of installation and ongoing maintenance. In VORs and NDBs. One of the most common approach path, the situation becomes even the early part of this decade, therefore, vari- ways of providing NPA capability is with a worse because of the requirement to assess a ous agencies that regulate aviation began to VHF omnidirectional range (VOR) or a larger area for obstacles. An NDB placed at formulate specific plans to take advantage of nondirectional beacon (NDB) system. VORs the airport requires obstacle assessment in an the less expensive and more reliable naviga- are primarily used as en route navigation aids area that may extend to as much as 15Ð20 tion potential of the growing GPS constella- for operations that demand relatively precise nautical miles. In some extreme cases, tion. But the existing design standards and guidance. In some cases, though, VORs are MDAs may be above what is required to fly criteria for instrument approach procedures located in such proximity to an airport that under the visual flight rules! (IAPs) based on existing navigation aids they can also provide approach guidance. A VOR has an inherent advantage over an could not be used to define the obstacle VOR stations transmit modulated signals NDB because of smaller obstacle assessment assessment areas required to accommodate in the 108Ð117.975-MHz band. A receiver areas resulting from more precise guidance. GPS (see Figure 2). on board an aircraft measures the relative In addition, most VORs have associated “T” Configuration. GPS NPAs can be config- phase of the signal modulations to determine distance measuring equipment (DME) that ured in many ways, with the most common the azimuth of the aircraft relative to the can provide an aircraft with more precise configuration referred to as the “T” and com- transmitter. positioning along flight paths, enabling posed of at least five segments delineated by NDBs are also employed as en route and obstacles behind the aircraft to be excluded geographic waypoints (see Figure 3). A way- approach navigation aids, and because of from assessment. DMEs work on a two-way point is nothing more than a ground point comparatively low costs, their use is wide- ranging principle. An interrogator in the air- with assigned coordinates. Essentially, a GPS spread. These nondirectional transmitters craft sends out a pulsed signal that a DME NPA is a series of waypoints joined by tracks, operate in the low- and medium-frequency ground station picks up. The station then with each waypoint contained within an bands (190Ð435 kHz and 510Ð535 kHz replies with a similar signal. The DME obstacle clearance area (see Figure 4). Each in North America). The signals are modu- instrument on board the aircraft computes the obstacle clearance area has primary and sec- www.gpsworld.com GPS WORLD June 1999 69 INNOVATION
ondary zones, and the extremities of succes- specified point along the final approach, he as a base for plotting data — usually sive areas are joined to create a procedure’s or she must fly the missed approach segment obtained from databases maintained by aero- various segments. The typical configuration to the missed approach holding waypoint. nautical information agencies — about consists of two initial segments, one interme- While situations requiring a missed approach human-made structures. In addition to basic diate, one final approach, and one missed are relatively uncommon, they do occur drafting tools, a computer program capable approach. (especially in bad weather conditions), so the of calculating geographic positions is In the initial segments, an aircraft transi- missed approach is an integral part of any essential. tions from the en route phase to the terminal procedure. The starting point for designing most pro- phase. The intermediate segment is designed cedures is the first usable portion, or thresh- to allow an aircraft to reduce speed and con- DESIGN PROCESS old, of the runway. Threshold positions and figure for approach and landing. The speed There are numerous methods used to design runway profiles are obtained from airport reduction can be significant and the interme- GPS NPAs, with differences mostly in the operators and plotted on the procedure chart. diate segment must be long enough to allow degree of automation adopted during the In addition, designers also need to take into the gradual reduction of air speed. At the design process. The most basic method account such factors as terrain, traffic pat- final segment, the aircraft stabilizes at the relies on drafting the procedure on a map. terns, noise-sensitive areas, restricted zones approach speed and descends toward the run- Typically, a designer will use a 1:50,000- (for example, military areas), and aircraft way. In the event a pilot does not obtain the scale topographical chart, which provides performance. With these basics in place, the visual references required for landing by a terrain information as well as functions designer, whose primary mission is to
Figure 2. NDB (left) and GPS (right) procedures for runway 05 at Middle Georgia Regional Airport, Macon, Georgia. Slightly lower landing limits are obtained with the GPS procedure.
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achieve the lowest MDA consistent with Final Segment. The final segment (see Figure The designer begins by locating natural safety, can begin to develop the GPS 5) starts at the final approach waypoint obstacles and plotting the positions of approach procedure. He or she starts with the (FAWP) and ends at the missed approach human-made obstructions in the general area final segment to make optimizing MDA as waypoint (MAWP). MAWP is usually of the runway approach. He or she can then easy as possible. located at the landing runway threshold. use transparent templates, scaled to various segment lengths, to determine how to mini- mize the effect of those obstacles. DELTA Ideally, the final segment should be aligned with the extended runway centerline (RCL), but, if this alignment results in too Flyby waypoint high an MDA, the designer has the option of realigning the approach track as much as 15 "T" Fly-over waypoint degrees from RCL. Realignment can result in some obstacles falling outside the segment or within the secondary areas, lessening the FOXTR required obstacle clearance (ROC). Another option is to use a stepdown way- CHARL BRAVO ALPHA point to divide the segment, thus isolating an
DELTA: Initial approach waypoint (IAWP) Segment From To obstacle. In some extreme cases, the final ECHOO: Initial approach waypoint (IAWP) Initial DELTA CHARL segment can be made as long as 10 nautical CHARL: Intermediate approach waypoint (IWP) Initial ECHOO CHARL miles to effectively narrow the areas and BRAVO: Final approach waypoint (FAWP) Intermediate CHARL BRAVO reduce ROC. However, this long final ALPHA: Missed approach waypoint (MAWP) Final BRAVO ALPHA approach invokes other procedural penalties, FOXTR: Missed approach holding waypoint (MAHWP) Missed ALPHA FOXTR such as the excessive time required to fly the procedure. ECHOO Once the final segment is configured, one Figure 3. A typical GPS “T” configuration. Waypoints are usually identified using must calculate MDA. In the primary area, five-letter names. a 250-foot ROC is added to the height of the controlling (highest) obstacle within the
Approach waypoints (WP)
NM = nautical miles 25 Missed approach .5NM waypoint
07 MAWP .3NM
.5NM .5NM
FAWP .3NM Maximum length 10 nautical miles (NM) SDWP ROC ROC 250 feet 250Ð0 feet 1NM 1NM Optimum descent: Terminal waypoints 300 feet per NM Maximum: 400 feet per NM 1.5NM Secondary Primary Secondary 1NM IWP MAHWP Final approach waypoint MATWP Secondary Primary ROC 1NM 2NM ROC 250 feet 250 feet 0 feet 1.5NM
1NM IAWP Figure 5. The primary and secondary areas plus operational parameters of a final approach segment. The required 2NM 4NM obstacle clearance (ROC) within the primary area is 250 feet. This reduces Figure 4. Fix displacement tolerance areas (shaded) and obstacle clearance areas linearly to zero feet at the periphery of associated with the various waypoints used in a GPS procedure the secondary area. www.gpsworld.com GPS WORLD June 1999 71 INNOVATION
final approach segment. This value is then rounded up to the next highest 20-foot Direct Route increment. ROC in a secondary area is 250 MAHWP MAHWP feet at the inner boundary and zero feet at the Width 6 nautical Width miles (NM) at 2 NM Width outer boundary — resulting in a 125-foot 15 NM at 4 NM at ROC if an obstacle lies halfway between the 15 NM 15 NM No secondary Secondary area two boundaries. area reduction reduction A designer must also account for the max- 1000 1000 imum allowable rate of descent in developing feet feet the final segment to enable an aircraft to + + MDA MDA smoothly transition to level or ascending 620 340 flight. This maximum rate is 400 feet per feet feet nautical mile, which corresponds to a glide slope of about 4 degrees with respect to a horizontal plane. Ideally, an aircraft should descend at 300 feet per nautical mile (an approximately 3-degree glide slope), but in an extreme case, a procedure could require a 1,600-foot decent in a 4-mile final segment, thus yielding the maximum rate. Missed Approach Segment. Coincident with con- figuring the final segment, the designer must ensure that the target MDA will allow for obstacle clearance in the missed approach segment. The missed approach segment starts at MAWP and ends at the missed approach holding waypoint (MAHWP), MAWP MAWP where an aircraft either transitions to an Secondary area reduction Secondary area reduction for obstacle assessment for obstacle assessment en route phase leading to another airport or not permitted permitted enters a holding pattern while awaiting clear- ance to commence another approach. Figure 6. Two types of missed approach segments (from the missed approach Approximately half the GPS IAP standards waypoint [MAWP] to the missed approach holding waypoint [MAHWP]) showing the manual issued by FAA is devoted to describ- lower minimum descent altitude (MDA) possible using secondary area reduction ing the various missed approach options available to the designer. The most desirable MDA both with and without secondary area designer can start calculating waypoint posi- option is one that is easiest to fly — usually a reduction. tions. If the final approach and missed straight segment that essentially continues the When the designer has located the obsta- approach segments are aligned with the final segment. If obstacles prevent a straight cles in the primary area, he or she determines extended RCL, the position of FAWP and segment, the designer can elect to design a whether there is an obstacle identification MAHWP can be calculated employing both turning segment or even a combination of surface rising at 1:40, penetrating the missed runway threshold positions. The designer straight and turning segments. In a missed approach segment. This surface starts at the calculates the forward bearing of the runway approach procedure using a turn, a missed farthest point of the MAWP obstacle clear- and then uses the runway reciprocal bearing approach turning waypoint (MATWP) is ance area at an elevation 250 feet below along with the final segment’s required established. Although the available options MDA. If an obstacle crosses this surface, the length (usually 5 nautical miles) to determine are many, all are based on the requirement designer would be forced to raise MDA to the FAWP position. If the intermediate seg- that an aircraft maintain a minimum climb ensure a 250-foot ROC above that obstacle. ment is aligned with the final segment, the rate of 200 feet per nautical mile, which pro- If there is positive track guidance, an obstacle position of the intermediate waypoint (IWP) vides increasing absolute altitude above a in the secondary area is assessed using the can also be calculated using the reciprocal of 1:40 obstacle clearance slope throughout the 1:40 surface to a point on the primary/ the runway bearing and the total length of the entire missed approach segment. secondary boundary line and then a 1:12 final and intermediate segments. The The areas assessed for obstacles in a basic slope. (It is assumed that the probability of an MAHWP position is calculated using the straight missed approach start at MAWP and aircraft deviating into the secondary area is missed approach segment length and the run- splay to a width of 6 nautical miles on either reduced through positive track guidance, thus way’s forward bearing. side of the required flight path along a justifying the less restrictive 1:12 slope.) When an operational necessity requires a 15Ðnautical mile track distance. If the aircraft Obstacles falling under the 1:12 slope have procedure configuration other than the basic maintains a prescribed track while flying less impact and thus may allow the designer “T,” the designer may have to deal with one the segment (positive track guidance), to lower the MDA. of four possible scenarios when placing the designer can use secondary areas to mini- Waypoint Position Calculation. With the obstacle MAWP: 1) MAWP is at the threshold, but mize the effect of obstacles much the same assessment of the final and missed approach the final segment track is not aligned with the as in the final segment. Figure 6 illustrates segments complete, and assuming that runway; 2) MAWP is on the extended RCL the effects of a 1,000-foot obstacle on MAWP is coincident with the threshold, the but displaced to a position prior to the thresh-
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old; 3) MAWP is on the extended centerline points that join various segments in a GPS an operational evaluation before it can be with the final segment track but not aligned nonprecision approach. As the names imply, published. Different service providers have with the runway; or 4) MAWP is displaced an aircraft will either go directly over a way- various types of organizations set up to from the centerline and, in accordance with point before transitioning to the next segment design and publish procedures. FAA criteria, the final segment track extends or transition to a succeeding segment by turn- An operational pilot may design a proce- to the runway threshold. In all cases, MAWP ing prior to a waypoint. dure as well as complete the flight check. Or a is used as the reference for calculating Turning prior to a waypoint is referred to technician may design the procedure for a FAWP and MAHWP positions, but the run- as automatic turn anticipation, which is flight inspection group to operationally evalu- way threshold becomes the reference for cal- defined by FAA, in its GPS criteria, as “the ate. In either case, the flight inspection will culating the position of MAWP. Calculating capability of GPS airborne equipment to assess the procedure’s “flyability” and con- the MAWP position, in these situations, has determine the point along a course, prior to a firm the presence of obstacles, waypoint posi- to account for distance from the threshold turn waypoint, where a turn should be initi- tions, and GPS signal availability. The and, if applicable, the final segment track’s ated to provide a smooth path to intercept the procedure for a given airport will only be displacement from the reciprocal of the run- succeeding course, and to enunciate the released for publication after a flight inspec- way bearing. information to the pilot.” The two waypoints tion pilot has certified that it is safe and Intermediate Segment. With the final segment designated as flyover in a procedure are flyable. in place and the FAWP calculated, the MAWP and MAHWP, whereas the rest of System Errors. The development of standards designer must decide the length and align- the waypoints are flyby. Automatic turn and criteria for any navigation system takes ment of the intermediate segment, which anticipation for flyby waypoints requires the into account total system error (TSE). TSE is starts at the earliest point of the IWP obstacle designer to expand the obstacle clearance the result of two error components: naviga- assessment area and ends at the plotted areas if there are turns of more than 15 tion system error (NSE) and flight technical FAWP position. This transition between the degrees at IWP, FAWP, or MATWP. error (FTE). Regulatory agencies standardize terminal and approach phases basically and publish NSEs, which are based on the allows a reduction in airspeed, constraining FLIGHT INSPECTION signal characteristics of a particular naviga- the designer to a maximum allowable descent With the paper phase of the design process tion source. FTE compares the actual aircraft rate of 300 feet per nautical mile. complete, the procedure now has to undergo position to the required position. The intermediate segment must be 5Ð15 nautical miles long and aligned within 30 degrees of the final segment. ROC in the pri- mary area is 500 feet with secondary reduc- tions allowed. If the segment is realigned to gain an operational advantage, such as avoid- ing obstacles, FAWP is used as a reference, with the bearing change and distance deter- mining the IWP position. Initial Segment. The initial segment starts at the earliest point of the initial approach way- point obstacle assessment area and ends at the plotted IWP position. The segment has no standard length but should not exceed 50 nautical miles. In addition, alignment relative to the intermediate segment should be 120 degrees or less. In the primary area, 1,000 feet of ROC is applied, and in the secondary area, ROC is 500 feet at the inner boundary decreasing to zero feet at the outer boundary. To allow approaches from either direction, the “T” configuration will have two initial segments placed at 90 degrees to the interme- diate segment. If the designer wishes to devi- ate from the “T,” he or she can develop procedures without initial legs or with more than two initial segments to accommodate unique traffic flows or extreme terrain.
FLYBY, FLYOVER WAYPOINTS With all segments in place, the designer must address an aspect of GPS procedures that is not a factor in the design of procedures based on conventional navigation aids. Flyby and flyover denote the two basic types of way- www.gpsworld.com Circle 48 GPS WORLD June 1999 73 INNOVATION
To obtain accuracy measurements, analysts the data to establish a profile or “distribution” as a standard deviation (sigma or SD) about usually employ a “truth system,” such as a of errors from which they calculate TSE. the mean. Two SDs correspond to a 95- laser tracker, to gather data from a large num- With TSE and NSE known, FTE can be percent chance that the true value is within ber of flights (more than 100). They then use determined. All errors are usually expressed the stated range. The 95-percent value is usually regarded as acceptable for defining primary area dimensions when assessing FURTHER READING obstacles in the various IAP segments. The For an introduction to GPS use in aviation, see necessary total protection from obstacles, Ⅲ Aviator’s Guide to GPS, by Bill Clarke, 3rd edition, McGraw-Hill Book Company, taking into account a large number of air- Ð7. This 10 ן New York, 1999. craft, has been targeted at 1 For an overview of U.S. air navigation policy, see value corresponds to about 5.3 SDs or Ⅲ 1996 Federal Radionavigation Plan, published by the U.S. Departments of Trans- 99.99999 percent. The total area considered portation and Defense, DOT-VNTSC-RSPA-97-2/DOD-4650.5, Washington, D.C., 1997. A for obstacle protection is then said to capture PDF file of the plan is available online from the U.S. Coast Guard Web site: approximately 5.3 SDs of the population.
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