16
Global Positioning System (GPS)
Precise point-to-point navigation is possible using satellite navigation systems that can compute aircraft position and altitude accurately by comparing signals from a global network of navigation satellites. The first global positioning systems (GPS) were designed for the U.S. Department of Defense, but in the early 1990s, GPS was made available for civilian use. Later, full system accuracy was also made available.
Basically, three elements make up GPS:
1. a space element, consisting of a constellation of satellites orbiting the earth every 12 hours in six orbital planes, at an altitude of 11,000 NM (21,300 km);
2. a satellite control ground network responsible for orbital accuracy and control; and
3. a navigation receiver in the aircraft (many are small enough to be hand held) capable of receiving and identifying several satellites at a time.
Each satellite transmits its own computer code packet on frequency 1,575.42 MHz (for civilian use) 1,000 times a second. The satellite constellation typically guarantees that at least four satellites are in view and usable for positioning at any one time from any position on earth. GPS pinpoints an aircraft’s horizontal position in lat.-long. coordinates which is similar to other long-range navigation systems, for instance the VLF/Omega. In the case of most aviation units, it then turns the information into a graphical moving map display which shows the aircraft’s position in relation to surrounding airspace on an LCD screen. Most GPS receivers can also display a CDI presentation, along with track, present position, actual time (to an accuracy of a few nanoseconds), groundspeed, time and distance to the next waypoint, and the current altitude of the aircraft.
GPS units have been approved for both en route and approach navigation, but not all units are approved for anything other than situational awareness. IFR units must have their databases updated on a regular basis to remain IFR certified; however, some GPS receivers may be used for IFR en route without a current database, provided the fixes are verified with another form of navigation (VOR).
Nonprecision GPS approaches are available at most U.S. airports today. Precision GPS approaches are also now available which uses a ground station to augment the satellite signals. This wide area augmentation system (WAAS) will allow GPS to be used as the primary NAVAID from takeoff through to approach.
Figure 16-1 Signals from satellites are received to establish an aircraft’s position.
Some manufacturers have produced multi-function displays (MFDs) which combine data from conventional flight instruments and on-board fuel/air data sensors for light aircraft. Typical GPS panels are shown in figure 16-2.
Figure 16-2 GPS and NAV management receivers.
As stated earlier, GPS has three functional elements:
- a space segment;
- a control segment; and
- a user segment (the airborne receivers).
Figure 16-3 The GPS consists of three basic segments.
Space Segment
The space segment consists of a constellation of approximately 24 satellites orbiting the earth at an altitude of 11,000 NM (21,300 km) in six strategically defined orbital planes. Each orbital plane consists of four operational satellites and a spare satellite slot. The objective of the GPS satellite configuration is to provide a window of at least five satellites in view from any point on earth.
Figure 16-4 The relative orbital positions of GPS satellites.
The satellites orbit at an inclination angle of 55°, taking approximately 12 hours to complete an orbit, and the orbital position of each satellite is known precisely at all times.
Note. As a point of interest, the GPS space segment consists of so-called Block II and IIA satellites and upgraded versions known as Block IIR satellites. The service they provide is identical as far as a user is concerned. They will be the basis of the system for at least the next decade.
Pseudo-Random Code
Each satellite transmits its position and precise time of transmission, and a separate signal is used by the receiver to establish range from the satellite. This is achieved by the satellite RF carrier transmissions being modulated with a 50 bit/second navigation message and a unique encoded signal known as a pseudo-random code. It repeats itself every millisecond and is used by the GPS receivers to recognize and track individual satellites for ranging purposes.
There are two types of pseudo-random code:
- a coarse acquisition (C/A) code (sometimes referred to as the standard positioning service (SPS)) available for general civilian use, which provides accuracy in the order of 100 meters in position and 140 feet in altitude with a 95% probability given a quality receiver; and
- a precision (P) code (also known as the precise positioning service (PPS)), which permits extremely precise position resolution (formerly available for authorized military users only but now available to all users).
As will be discussed, a minimum of three satellites is required to determine a two-dimensional fix if altitude is known. For a three-dimensional fix, four satellites are required. The navigation message contains information on satellite ephemeris, GPS time reference, clock corrections, almanac data, and information on system maintenance status.
Control Segment
The controlling authority is the United States Department of Defense. By letter of agreement between the United States Government and ICAO, civilian access via the C/A code only is permitted on a no-cost basis for the foreseeable future. The deliberate degrading of the accuracy of the system for civilian users, i.e., the standard positioning service (SPS) accessed via the C/A code, is known as selective availability (SA).
Note. In early 2000, the U.S. Department of Defense turned SA off.
The control segment includes monitoring stations at various locations around the world, ground antennas and up-links, and a master station. The stations track all satellites in view, passing information to a master control station, which controls the satellites’ clock and orbit states and the currency of the navigation messages.
Satellites are frequently updated with new data for the compilation of the navigation messages transmitted to system users. Assuming the current level of space vehicle technology, the planned life span of a GPS satellite is around seven to eight years.
User Segment (the Receiver)
As previously mentioned, the receiver identifies each satellite being received by its unique pseudo-random code, i.e., the C/A code for civilian operations. It then starts to receive and process navigation information. Ephemeris data takes about 6 seconds to transmit, but almanac data takes about 13 seconds. For this reason, almanac data is stored in the receiver’s memory. During operation, almanac data in the receiver is changed on a continuous basis. On start-up, the receiver recalls the data that was last in memory on the preceding shutdown. From this information and the stored almanac data, the receiver determines which satellites should be in view and then searches for their respective C/A codes. It then establishes ranges to the satellites, and by knowing their position, computes aircraft position, velocity, and time. This process is known as pseudoranging.
Range determination is a simple matter of measuring the period between the time of transmission and the time of reception of each satellite’s C/A code and multiplying that time interval by the speed of light in free space. The GPS receiver, in fact, does this by emitting its own code at the same time as the satellite’s and uses it and the time the signal from the satellite is received to establish the time interval. Timing is critical. This is the reason why the time reference is provided by synchronized, high-precision atomic clocks in the satellites.
Fixing Position
A three-dimensional position in space (position and altitude) is accomplished by the receiver determining where it must be located to satisfy the ranges to four or more appropriately positioned satellites. A two-dimensional fix requires only three satellites in view if altitude is known. The synchronization of the receiver’s time reference with that of the satellite is important in this process.
Timing errors are detected and eliminated by the receiver’s computer. Figure 16-5 shows a two-dimensional position established, assuming the respective clocks are synchronized perfectly.
Figure 16-5 Two-dimensional fix established with perfect timing.
However, if the receiver’s clock is, say, one second fast, as is the case in figure 16-6, the period between transmission and reception with respect to each of the three satellites interrogated will be sensed initially as taking one second longer. This will be represented as a gross error in all three ranges and thus, rather than producing a precise fix, will create a very large area anywhere in which the receiving aircraft could be positioned. The receiver’s computer senses this and immediately begins a trimming process until it arrives at an answer which allows all ranges to arrive at the one and only position possible. This process automatically eliminates the effect of receiver clock error for subsequent tracking and position fixing.
Figure 16-6 Effect of receiver clock error of one second on a two-dimensional fix.
Receiver Design
The capability of making range calculations to three, four or more satellites has an impact on the design, cost, and accuracy of GPS receivers, namely, whether they are single-channel receivers operating sequentially or the more expensive and accurate receivers providing multiple channels operating simultaneously. GPS receivers approved as a supplemental- or primary-means navigation aid have multiple channels and come under the provisions of an FAA Technical Service Order (TSO C129). IFR/primary navigation certification specifications for GPS equipment include a requirement for multiple receiver channels and a navigation integrity monitoring system, known as receiver autonomous integrity monitoring (RAIM).
Receiver Autonomous Integrity Monitoring (RAIM)
RAIM is a special receiver function which analyzes the signal integrity and relative positions of all satellites which are in view, so as to select only the best four or more, isolating and discarding any anomalous satellites. At least five satellites must be in view to have RAIM find an anomalous situation and six to actually isolate the unacceptable satellite.
There are two types of RAIM messages: one indicates that there are not enough satellites available to provide RAIM; another indicates that RAIM has detected a potential error which exceeds the limit for the current phase of flight.
When operating, it ensures that the minimum acceptable level of navigation accuracy is provided for the particular phase of flight. In the process, it ensures that a potential error, known as the position dilution of precision (PDOP) or geometric dilution of precision (GDOP), is minimized. The PDOP depends on the position of the satellites relative to the fix. The value of the PDOP determines the extent of range and position errors.
When the satellites are close together, the tetrahedron formed covers a large area and results in a high PDOP value (see figure 16-7).
Figure 16-7 Poor satellite geometry resulting in high PDOP.
However, when the selected satellites are far apart, the area covered by the tetrahedron is much more compact, resulting in a lower PDOP value and therefore greater accuracy. A PDOP value of less than six is acceptable for en route operations. A value of less than three will be required for nonprecision approaches.
Figure 16-8 Good satellite geometry resulting in low PDOP.
If RAIM is not available, another type of navigation and approach system must be used, another destination selected, or the trip delayed until RAIM is predicted to be available on arrival.
Barometric Aiding
Barometric aiding is the process whereby the digital data of the pressure altimeter is used by the GPS receiver as, in effect, the range readout of a (simulated) additional satellite. It is only applicable when there are less than five satellites in view and RAIM alone cannot be effective. Barometric aiding provides additional redundancy and RAIM capability and therefore increases the navigation coverage of GPS.
Masking Function
The masking function in the GPS receiver software ensures that any satellites in view which lie below a fixed angle of elevation relative to the receiver are ignored. This is due to the range errors that will be generated because of the greater distances that their signals will have to travel through the ionosphere and troposphere to reach the receiver. The fixed angle stored in the receiver is known as the mask angle. In some receivers, it is selected automatically by the receiver, depending on the strength of the transmitted signals at low angles of elevation, receiver sensitivity and acceptable low-elevation errors. When fixed, it is typically set at around 7.5° (figure 16-9).
Figure 16-9 Mask angle.
Receiver Displays
Displays for the pilot vary from one GPS unit to another. Flight planning data is usually entered via an appropriate keypad on a control display unit (CDU) or control panel. The usual navigation information (position, track, groundspeed, EET, and, with a TAS input, TAS and wind) is displayed. The unit must also be capable of showing satellite status, satellites in view and being tracked, the value of PDOP, RAIM status, and signal quality.
Operating Modes
GPS receivers normally provide three modes of operation:
- navigation with RAIM;
- navigation (two or three dimensional) without RAIM; and
- loss of navigation (annunciated as DR in some receivers).
Differential GPS
The accuracy standards available for 95% of the time have already been mentioned. However, for the GPS to be of any value as a primary navigation source for precision approach/departure operations, a much higher order of accuracy is required. Furthermore, the higher accuracy standard should be available 99.99% of the time. We know that GPS is capable of providing unprecedented levels of accuracy with P-code access, or the PPS. This standard of accuracy is now available to civilian users, assuming direct interrogation of GPS.
One ingenious way of improving the accuracy available for civilian users is with an enhancement known as differential GPS (DGPS). The GPS receiver is installed at a ground station located in the terminal area. The station compares the GPS computed position with the actual (surveyed) position of the station and determines the difference, if any, which would be common to other airborne GPS receivers operating in the area. The station transmits the appropriate error-correction signal by data links to the aircraft, with the result that an accuracy in the order of 10 meters is achievable. Figure 16-10 shows the simplicity of the concept.
Figure 16-10 Differential GPS.
This enhanced standard of accuracy is acceptable for nonprecision instrument procedures but not for precision approaches. However, a lot of research and development work is being undertaken, particularly by the FAA, to improve the accuracy even further. In fact, the FAA have confidently predicted that Category II and III precision approach navigation capability using GPS will be possible in the future.
Figure 16-11 Concept of wide area differential GPS.
As well as developing differential GPS for precision operations, a much wider network of ground receivers is being developed for en route operations, with geostationary navigation receiver and communication satellites and relays. The enhanced network is known as a wide area augmentation system (WAAS).
Note. It is important to point out that GPS (GNSS) is still a developing technology as far as civil air operations are concerned. At the time of publication, GPS equipment meeting system integrity standards and operated in accordance with specified limitations and procedures is approved as a primary-means navigation aid for IFR en route operations and specified IFR arrival procedures.
Operations Without RAIM
If RAIM is lost, the accuracy of the system is unacceptable for both navigation and ATC separation purposes. Loss of satellite reception and RAIM warnings may occur due to aircraft dynamics (changes in pitch or bank angle). Antenna location on the aircraft, satellite position relative to the horizon, and aircraft attitude may affect reception of one or more satellites. RAIM availability should always be checked. RAIM information can be obtained for a period of 3 hours (ETA hour and 1 hour before to 1 hour after the ETA hour) or a 24-hour duration at a particular airport. FAA briefers will provide RAIM information for a period of 1 hour before to 1 hour after the ETA, unless a specific time frame is requested by the pilot. If flying a published GPS departure, a RAIM prediction should also be requested for the departure airport. RAIM can also be predicted using the GPS receivers in the aircraft.
If RAIM is not available, another type of navigation and approach system must be used, another destination selected, or the trip delayed until RAIM is predicted to be available on arrival. On longer flights, pilots should recheck the RAIM prediction for the destination during the flight.
If a RAIM failure/status annunciation occurs prior to the final approach waypoint (FAWP), the approach should not be completed. The receiver performs a RAIM prediction by 2 NM prior to the FAWP to ensure RAIM is available at the FAWP as a condition for entering the approach mode. The pilot should ensure the receiver has sequenced from “Armed” to “Approach” prior to the FAWP (normally occurs 2 NM prior). Failure to sequence may be an indication of the detection of a satellite anomaly, failure to arm the receiver (if required), or other problems which preclude completing the approach.
If the receiver does not sequence into the approach mode or a RAIM failure/status annunciation occurs prior to the FAWP, the pilot should not descend to minimum descent altitude (MDA). Rather, the pilot should proceed to the missed approach waypoint (MAWP) via the FAWP, perform a missed approach, and contact ATC as soon as practical. Refer to the receiver operating manual for specific indications and instructions associated with loss of RAIM prior to the FAF.
If a RAIM failure occurs after the FAWP, the receiver is allowed to continue operating without an annunciation for up to 5 minutes to allow completion of the approach. If the RAIM flag/status annunciation appears after the FAWP, the missed approach should be executed immediately.
GPS Approaches
Pilots who have used GPS for en route navigation know that the system is accurate and easy to use. Many pilots started using GPS in their early VFR training and then integrated its use into IFR flying. The promise of GPS goes well beyond en route navigation: together with additional ground-based equipment, it has the potential of eventually providing every airport with an ILS-quality instrument approach.
Traditionally there have been two types of instrument approaches: precision and nonprecision. The difference between the two is an electronic glide path. Precision approaches have an indicator on the flight deck that tells the pilot if they are above, below, or on the proper glide path to the runway. The most common of these precision approaches is the Instrument Landing System (ILS). GPS has added a third type: a “semi-precision” approach. These approaches are called “approach with vertical guidance” (APV) because they have a glideslope indicator on the flight deck, but they are not as accurate as a full ILS — at least not yet.
GPS Overlay Nonprecision Approach
When GPS was first used for instrument approaches, the procedure was simply placed over an existing nonprecision approach. The original GPS approach procedures provided authorization to fly nonprecision approaches based on conventional, ground-based NAVAIDs. Many of these approaches have been converted to stand-alone approaches, especially as NDBs have gone off the air and the few that remain are identified by the name of the procedure and “or GPS.” These GPS nonprecision approaches are predicated upon the design criteria of the ground-based NAVAID used as the basis of the approach. As such, they do not adhere to the RNAV design criteria for stand-alone GPS approaches; therefore they are not considered part of the RNAV (GPS) approach classification for determining design criteria (see figure 16-12).
Figure 16-12 Traditional GPS overlay approach.
GPS Stand-Alone/RNAV (GPS) Approach
RNAV (GPS) approaches are named so that airborne navigation databases can use either GPS or RNAV as the title of the approach. This is required for non-GPS approach systems such as VOR/DME based RNAV systems. In the past, these approaches were often called stand-alone GPS. They are considered nonprecision approaches, offering only Lateral Navigation (LNAV) and circling minimums and have a minimum descent altitude (MDA). The RNAV (GPS) Runway 18 approach for Alexandria, Louisiana incorporates only LNAV and circling minimums (see figure 16-13). A GPS approach that has LNAV minimums is not necessarily more accurate than traditional NDB or VOR nonprecision approaches. To get the greater benefit of GPS for instrument approaches, greater accuracy is needed. A ground-based antenna will hone in the satellite signals and narrow down the accuracy; this is called the Wide Area Augmentation System (WAAS). WAAS makes it possible for GPS to deliver not only lateral guidance but also vertical guidance — like a glide slope.
Figure 16-13 Alexandria International (KAEX), Alexandria, Louisiana, RNAV (GPS) Rwy 18.
Wide Area Augmentation System
The introduction of WAAS, which became operational on July 10, 2003, gives even lower minimums for RNAV (GPS) approaches by providing electronic vertical guidance and increased accuracy. As its name implies, WAAS augments the basic GPS satellite constellation with additional ground stations and enhanced position integrity information transmitted from geostationary satellites. This augmentation improves both the accuracy and integrity of basic GPS and may support electronic vertical guidance approach minimums as low as 200 feet HAT and ½ SM visibility. WAAS covers 95 percent of the country 95 percent of the time.
Vertical Navigation
One of the advantages of some GPS and multi-sensor FMS RNAV avionics is the advisory VNAV capability. Traditionally, the only way to get vertical path information during an approach was to use a ground-based precision NAVAID like the ILS glide slope. Modern RNAV avionics can display an electronic vertical path that provides a constant-rate descent to the minimum altitude of the approach.
Since these systems are advisory and not primary guidance, the pilot must continuously ensure the aircraft remains at or above any published altitude constraint, including step-down fix altitudes, using the primary barometric altimeter. The pilots, airplane, and operator must be approved to use advisory VNAV inside the FAF on an instrument approach.
VNAV information appears on selected conventional nonprecision, GPS, and RNAV approaches. It normally consists of two fixes (the FAF and the landing runway threshold), FAF crossing altitude, a vertical descent angle (VDA), and may provide a visual descent point (VDP). (See figure 16-14.) The published VDA is for information only, advisory in nature, and provides no additional obstacle protection below the MDA. Operators can be approved to add a height loss value to the MDA and use this derived decision altitude (DDA) to ensure staying above the MDA. Operators authorized to use a VNAV DA in lieu of the MDA must start a missed approach immediately upon reaching the VNAV DA, if the required visual references to continue the approach have not been established.
A constant-rate descent has many safety advantages over nonprecision approaches that require multiple level-offs at step down fixes or manually calculating rates of descent. A stabilized approach can be maintained from the FAF to the landing when a constant-rate descent is used. Additionally, the use of an electronic vertical path produced by onboard avionics can serve to reduce controlled flight into terrain (CFIT) and minimize the effects of visual illusions on approach and landing.
Figure 16-14 VNAV information.
In order to achieve the lowest minimums, the following requirements of an entire electronic vertical guidance system, including satellite availability must be satisfied: clear obstruction surfaces; Advisory Circular 150/5300-13, “Airport Design”; and electronic vertical guidance runway and airport requirements. The minimums are shown as DAs, since electronically computed glide path guidance is provided to the pilot. The electronically computed guidance eliminates errors that can be introduced when using barometric altimetry.
A greater degree of accuracy is achieved when WAAS is used for both lateral and vertical navigation — not just vertical, as with the LNAV/VNAV approach. Combining WAAS lateral and vertical guidance produces an approach called the Localizer Performance with Vertical guidance, or LPV approach. This approach has minimums similar to an ILS but is limited to areas where terrain is not a challenge. Approach minimums as low as 200 feet HAT and ½ SM visibility is possible, even though LPV is still considered a semi-precision and not a precision approach. However, as with precision approaches, LPVs have a DA like an ILS approach.
Full precision approach status will become available when the Local Area Augmentation System (LAAS) becomes operational. LAAS further increases the accuracy of GPS and improves signal integrity warnings because it is a ground station designed for use at a specific airport or runway.
Precision approach capability requires obstruction clearance and approach lighting systems to meet Part 77 standards for ILS approaches. LAAS-assisted approaches are called GNSS (global navigation satellite system) Landing System, or GLS. Some approach charts have GLS included in the minimums section, but with the designation “NA” (not authorized). For now, in approach charts there is only a place holder for GLS, but when GLS approaches are in use they might be transferred to an altogether different chart.
RNAV (GPS) approach charts presently can have up to four lines of approach minimums: LPV, LNAV/VNAV, LNAV, and Circling. Figure 16-15 shows how these minimums might be presented on an approach chart, with the exception of GLS.
GPS has opened the door to many new approach possibilities, which in turn increase the chances that a pilot will be successful in flying an approach to a landing at their destination airport. However, there are more types of approaches to become familiar with.
Nonprecision approaches have no electronic glide slope and their lowest altitude is called the minimum descent altitude (MDA). These approaches include the NDB, VOR, VOR/DME, Radar, SDF, LDA, and LNAV.
Semi-precision approaches, or APVs, use WAAS to add vertical guidance. This means that the pilot has an electronic glide slope, yet the accuracy is not sufficient to be considered full precision. APVs include LNAV/VNAV and LPVs and their lowest altitude, as with an ILS, are called decision altitudes (DAs).
Precision approaches have an electronic glide slope that meets the highest accuracy standards. Today the most common of these is the ILS, but when the LAAS are implemented, the GLS will also become available.
Figure 16-15 RNAV (GPS) electronic vertical guidance approach minima.
Multiple Approaches to the Same Runway
GPS has made it necessary to change the way instrument approaches are named. Traditionally, one NAVAID would provide one instrument approach to a single runway or to a circle-to-land. Now there can be more than one GPS instrument approach to the same runway. To eliminate confusion, instrument approaches with the same guidance are annotated with an alphabetical suffix beginning at the end of the alphabet and working backwards for subsequent procedures. Figure 16-16 depicts two GPS instrument approaches to the same runway which are designated as the “Z” and “Y” approach.
Figure 16-16 FAA Z and Y approach charts, Salem, Oregon.
Required Navigation Performance
As air traffic congestion increases, it will be necessary to increase the accuracy of navigation so that more airplanes can fit into the same airspace. To achieve this goal, a set of Required Navigation Performance (RNP) criteria is being established. When an airplane flies through particular airspace it must navigate to within a pre-set tolerance. It does not matter which navigation equipment the pilot/airplane uses to meet the tolerance, RNP only requires that it be met. The RNAV system that will be most often used to achieve the RNP tolerance will be GPS.
The operational advantages of RNP include accuracy and integrity monitoring, which provide more precision and lower minimums than conventional RNAV. RNP DAs can be as low as 250 feet, with visibilities as low as ¾ SM. Besides lower minimums, benefits of RNP include improved obstacle clearance limits and reduced pilot workload. When RNP-capable aircraft fly an accurate, repeatable path, ATC can be confident that these aircraft will be at a specific position, thus maximizing safety and increasing capacity.
To attain the benefits of RNP approach procedures, a key component is curved flight tracks. Constant radius turns around a fix are called “radius-to-fix legs,” or RF legs. These turns, which are encoded into the navigation database, allow the aircraft to avoid critical areas of terrain or conflicting airspace while preserving positional accuracy by maintaining precise, positive course guidance along the curved track. The introduction of RF legs into the design of terminal RNAV procedures increases airspace and allows procedures to be developed to and from runways that are otherwise limited to traditional linear flight paths — or in some cases, not served by an IFR procedure at all. Navigation systems with RF capability are a prerequisite to flying a procedure that includes an RF leg. Refer to the “Notes” box in the pilot briefing portion of the approach chart in figure 16-17.
In the United States, all RNP procedures are in the category of Special Aircraft and Aircrew Authorization Required (SAAAR). Operators who want to take advantage of RNP approach procedures must meet the special RNP requirements outlined in FAA Advisory Circular 90-101, “Approval Guidance for RNP Procedures with SAAAR.” Currently, most new transport category airplanes receive an airworthiness approval for RNP operations. However, differences can exist in the level of precision that each system is qualified to meet. Each individual operator is responsible for obtaining the necessary approval and authorization to use these instrument flight procedures with navigation databases.
Figure 16-17 RNAV (RNP) approach procedure with curved flight tracks.
Review 16
Global Positioning System (GPS)
1. The space element of GPS consists of how many satellites orbiting the earth?
2. How often do these satellites complete an orbit?
3. At what altitude are these satellites located?
4. At least how many satellites must be observed for a GPS three-dimensional fix?
5. Compared to a sole means navigation system, which two performance requirements are not necessarily satisfied in a primary navigation system?
6. For civilian GPS operations, what is the pseudo-random code used? What is the service provided known as?
7. How is the range from a satellite determined?
8. What feature of the TSO-approved GPS system provides additional redundancy and RAIM capability?
9. What are the three operating modes normally provided by a GPS receiver?
10. How are ionospheric effects offset by the GPS receiver?
11. How are tropospheric effects minimized by the GPS receiver?
12. What should all data entered into the GPS, either manually or automatically, be checked against?
GPS Approaches
13. What is LNAV and VNAV?
14. What is WAAS and LAAS?
15. What is RNP?
16. What is GLS?
Refer to the RNAV (GPS) RWY 18 approach at Murfreesboro, Tennessee for questions 17 through 20. See figure 16-18.
17. What is the LNAV minimum descent altitude and visibility requirement for a Category A aircraft?
18. If a pilot flies the RNAV (GPS) RWY 18 approach, but the wind favors a landing on runway 36, what procedure should the pilot use to make a safe landing on runway 36?
19. Why are both the GLS and LNAV/VNAV approach minimums listed as “NA” on the RNAV (GPS) RWY 18 approach?
20. What is indicated when the letters Z or Y appear in the title of an instrument approach? RNAV (GPS) Z RWY 12
Figure 16-18 RNAV (GPS) RWY 18 approach at Murfreesboro, Tennessee.