navigation systems with an analog FGS (for example, autopilots) may present potential hazards and is not recommended. However, applicants that desire to integrate these systems in legacy aircraft will need to perform a thorough engineering evaluation to ensure compatibility. The AFM(S)/RFM(S) must reflect any limitations resulting from the installation.
c. Operational compatibility should be verified between the positioning and navigation equipment and the FGS. Protection should exist in the positioning and navigation equipment or the FGS to keep the aircraft within the approved operational envelope (e.g., roll, pitch, speed, and altitude). See the relevant guidelines for the particular aircraft (e.g., the latest revision of AC 23-17, AC 25.1329-1, AC 27.1329 contained in AC 27-1, and AC 29.1329 contained in AC 29-2).
d. A continuous and unambiguous indication must be provided of the FGS modes actually in operation, as well as those that are armed or enabled. This includes a clear indication whenever the mode has been armed either manually by the flight crew, or automatically by the system.
e. The positioning and navigation equipment must be compatible with the FGS modes of operation. For example, the arm, engage, and disengage sequence of the positioning and navigation equipment should be consistent in the annunciation and engage/disengage timing of the FGS. This is particularly important during GPS/SBAS-augmented GNSS approaches that include different levels of service.
f. If the positioning and navigation equipment is connected to a reduced bank selector, the equipment bank angle, like the FGS, must not be limited by this switch selection during approach. Consistency in the operation must be maintained.
g. Some aircraft operate in areas of the operational flight envelope where buffet onset can occur when normal maneuvering load factors are present (e.g., at higher altitudes). This is a concern since the positioning and navigation equipment can automatically sequence waypoints and initiate turn maneuvers that may or may not have the pilot’s attention. If buffet protection is not automatically provided in the positioning and navigation equipment or FGS
(e.g., bank angle limiting, or cruise altitude limiting based upon buffet margin), an AFMS/RFMS limitation should be added to the performance section, prohibiting cruise operation at weight/altitude combinations that result in buffet margins less than 1.4g.
h. For those aircraft that have incorporated bank angle limiting, consideration should be given to whether the positioning and navigation equipment is defining a path that cannot be flown or does not conform to the published procedure.
i. If the aircraft includes a synthetic vision system (SVS) or enhanced vision system (EVS) displaying flight guidance, the positioning and navigation output to the SVS or EVS function should meet the appropriate system performance criteria in this AC. The latest revision of AC 20-167, Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment provides additional guidance for these systems.
14-7. Interface to Magnetic/True Switch.
If a magnetic/true heading reference switch is installed in the aircraft, the positioning and navigation equipment should be driven by the same switch to maintain consistency in the displays and operation for both manual and automatic heading reference changes. Magnetic heading is the primary heading reference.
Note: Magnetic heading is the primary heading reference in the U.S. However, aviation authorities in other countries do use true heading as the primary heading reference in areas of extreme latitude.
14-8. Interface to Air Data and Inertial Reference Systems.
a. If there is an air data source select switch, when the pilot de-selects an air data source, the de-selected source must no longer be used by the positioning and navigation equipment. This becomes important for GNSS barometric-aided FDE since erroneous altitude inputs to the FDE algorithm could lead to improper satellite FDE.
b. If there is an inertial source select switch, when the pilot de-selects an inertial source, the de-selected source must no longer be used by the positioning and navigation equipment.
c. If the positioning and navigation equipment requires barometric corrected (or pressure) altitude data for certain operations (identified in the equipment manufacturer’s installation manual/instructions), the installation should provide an automatic altitude input from the air data system to the positioning and navigation equipment such that additional pilot actions are not required.
d. If barometric corrected altitude is required by the positioning and navigation equipment and an automatic input of the barometric corrected altitude is not available, an alert for the pilot during approach must be provided indicating the need to enter the barometric pressure setting in the equipment. If the equipment requires multiple entries to enter the correction, the workload should be evaluated during an approach.
14-9. Antenna/Sensor Compatibility.
a. Using the guidelines in the equipment manufacturer’s installation manual/instructions, verify the antenna installation is compatible with the positioning and navigation equipment. This may be accomplished by one of the following:
(1) The antenna (part number) is identified in the positioning/navigation equipment manufacturer’s installation manual/instructions as compatible.
(2) The positioning/navigation equipment manufacturer’s installation manual/instructions state that the equipment is compatible with an appropriate antenna that has a TSOA/LODA (refer to Table 2 for GNSS antennas). Compare the RTCA/DO-160 (appropriate revision) category for lightning-induced transients specified in the antenna 111
installation instructions with the category defined in the receiver installation instructions. Also, for active GNSS antennas, a signal power budget must be defined that takes the output power at the low-noise amplifier (pre-amplifier) of the antenna (as defined in the antenna installation instructions) and matches that to the receiver dynamic range and sensitivity (as defined in the GNSS equipment installation instructions).
b. Use the guidelines in the equipment manufacturer’s installation manual/instructions when determining what cables and connectors to use to connect the antenna to the receiver, including cable lengths that result in acceptable loss of signal power.
c. Ensure antenna and positioning/navigation equipment compatibility for aircraft equipped with SATCOM. Some equipment may have been qualified to less stringent interference levels intended for aircraft installations without SATCOM.
14-10. Interference - Electromagnetic Compatibility (EMC).
a. The equipment must not be the source of objectionable conducted or radiated interference, or be adversely affected by conducted or radiated interference from other equipment or systems installed in the aircraft.
b. Proper sensor equipment grounding is essential to ensuring EMC.
c. When possible, do not install positioning/navigation sensor(s) near a VHF radio.
d. The following paragraphs identify potential sources of interference and means of mitigating that interference:
(1) VHF communications harmonic, spurious, and local oscillator harmonics can cause interference. VHF interference can be mitigated by:
(a) Installing filters at the output of the VHF transmitter to prevent antenna-to-antenna interference. Such filters should have an insertion loss of 2 dB or less, or the installed VHF transceiver performance will have to be re-evaluated.
(b) Installing the positioning/navigation equipment as far away as feasible from any VHF transmitter equipment (to prevent case-to-case interference).
(c) Replacing the VHF equipment.
(2) An emergency locator transmitter (ELT) can re-radiate DME or VHF signals that can interfere with GNSS. Notch filters on the ELT antenna cable or replacing the ELT can eliminate this problem.
(3) DME has been known to cause interference to GNSS. Replacing the DME transceiver can eliminate the problem for some equipment.
(4) Direction finding equipment may cause interference. Moving the direction finding antenna to the belly of the aircraft has been found to eliminate this problem.
14-11. Interface Requirements.
a. Ensure sensor compatibility for aircraft systems using the position output (e.g., FMS, automatic dependent surveillance system, etc.). Systems should use the same bus standard (e.g., ARINC 429 with ARINC 743 data, RS-232 as applicable).
b. Operation of the positioning/navigation equipment must not adversely affect the performance of, or interfere with, other installed aircraft equipment.
c. Some applications will require the positioning/navigation sensors to provide a position output at a 5 Hz rate. However, sensors with a 1 Hz rate augmented by inertial aiding can be considered for use in applications requiring 5 Hz output.
d. Positioning/navigation equipment (particularly installations that use GNSS sensors) may provide output to a variety of applications, including TAWS/HTAWS, automatic dependent surveillance, and moving map displays. Output of navigation position, velocity and time must support the intended function for which it is used under any foreseeable operating condition. It is important to note for ground based navigation aids that the airworthiness approval alone does not guarantee performance since it is dependent upon the operational environment. This is significant when interfacing the positioning and navigation equipment to systems dependent upon the navigation accuracy and integrity statements.
e. The guidance material in this AC for sensors applies to all of these interfaces, and should be used in conjunction with the guidance material for the application. For specific information relating to:
(1) TAWS equipment, see TSO-C151(latest revision) and the latest revisions of AC 23-18, and AC 25-23.
(2) HTAWS equipment, see TSO-C194(latest revision) and Miscellaneous Guidance 18 in the latest revisions of AC 27-1 and AC 29-2.
(3) Electronic map displays, see TSO-C165(latest revision) and the latest revisions of AC 20-159, AC 23.1311-1, and AC 25-11.
(4) ADS-B equipment, see TSO-C166b, RTCA/DO-260B for 1090 Extended Squitter; and, TSO-C154c and RTCA/DO-282B for Universal Access Transceiver. AC 20-165 provides guidance information on ADS-B.
Chapter 15. Installation Considerations - GNSS.
15-1. Introduction.
a. GNSS equipment is particularly susceptible to out-of-band SATCOM emissions and in-band inter-modulation between multiple channel SATCOM installations. GNSS equipment must not be installed in aircraft with multiple SATCOM channels unless the simultaneous use of interfering frequencies can either be prevented or demonstrated not to interfere with the operation of the GNSS equipment. Note that some equipment is not compatible with SATCOM installations at all; this should be noted in the equipment manufacturer’s installation manual/instructions.
b. In addition, improperly used or installed GNSS re-radiators can present misleading information to GNSS equipment. Equipment manufacturers may provide mitigation against the use of erroneous data for GNSS position and navigation solutions. Possible measures include:
implementing or enabling cross-checks of GNSS sensor data against independent position sources and/or other detection monitors using GNSS signal metrics or data. It is incumbent upon airworthiness approval applicants to determine the method chosen by the equipment manufacturer and its adequacy for the aircraft integration.
15-2. Navigation Displays.
a. Navigation Display(s) for Retrofit Installations.
This guidance reflects the AC 20-138A clarification policy memorandum signed on July 30, 2007 and replaces that memorandum.
(1) Table 8 in paragraph 12-2.a classifies misleading information during GNSS CAT I and LP/LPV approaches as a hazardous failure condition; and paragraph 12-2.b states applicants must conduct a safety assessment of the installation to verify that design errors and failure modes meet the probability requirements for that failure class. The guidance in paragraph 15-2.a only applies when installing retrofit GPS/SBAS LP/LPV or GPS/GBAS CAT I GNSS landing system capability in aircraft that have a previously certified horizontal/vertical course deviation indicator(s) or an electronic display (or displays) approved for ILS approaches.
The intent is to apply this guidance when introducing GPS/SBAS LP/LPV or GPS/GBAS GLS capability into aircraft equipped with ILS course deviation indicator(s) or display(s) that were approved (TSOA/LODA, STC/PMA or type approval (TC/amended TC)) on or before July 30, 2007.
(2) Applicants may use the existing display(s) or course deviation indicator(s) for GPS/SBAS LP/LPV or GPS/GBAS GLS course deviations and annunciation data when adding GPS/SBAS LP/LPV or GPS/GBAS GLS capability into existing approved aircraft type designs, either by retrofit (STC, Form 337) or forward fit in the factory (amended TC), that were previously approved with ILS. It is not necessary to demonstrate compliance with the end-to-end design assurance levels of a ‘hazardous’ failure condition for the mechanical or electronic course deviation displays. Using the same path and device that was previously used to display ILS data for displaying GPS/SBAS LP/LPV or GPS/GBAS GLS data yields an equivalent level of safety to the original ILS certification basis. This equivalency provides 114
relief from paragraphs 12-2.a, 12-2.b, and data submittal requirements related to a ‘hazardous’ failure condition during LP/LPV or GLS approaches for the existing display(s) or course deviation indicator(s).
(3) This guidance may be applied to aircraft equipped with a single or dual horizontal/vertical course deviation indicator, or electronic display(s) with level C software.
However, this guidance cannot be applied to the equipment that computes and outputs the GPS/SBAS LP/LPV or GPS/GBAS GLS deviation data, as GPS/SBAS and GPS/GBAS do not have the same failure modes as ILS in generating the deviations. The installation is considered ‘not significant’ with respect to 14 CFR 21.101 when installing GPS/SBAS LP/LPV or GPS/GBAS GLS capability per this guidance.
b. Displays used for waypoint sequencing, start of a turn, turn anticipation, active waypoint, distance to active waypoint, desired track and actual track (track angle error), and automatic mode switching should be located within the pilot’s primary field of view, or on a readily accessible display page.
(1) Displays used for loss of integrity monitoring, TO/FROM indication, and approach mode annunciation should be located within the pilot’s primary field of view.
(2) Traditionally, 14 CFR part 23 airplanes with “classic” analog instrumentation in the “basic T” arrangement have included the center radio stack within the allowable field of view to satisfy this guidance. There is no intent for this AC to change that long-standing guidance.
Note: Primary field of view is being incorporated as a standardized term across all documents. The primary field of view definition should be broad enough to include the center radio stack on 14 CFR part 23 airplanes with “classic”, basic ‘T’ instrumentation.
c. For some “stand-alone” equipment, the display can be remote from the navigation computer and GNSS sensor. These displays must be compatible with expected outputs from the navigation computer to provide the proper deviation indications to the pilot. Both GNSS and display manufacturers are encouraged to provide a list of compatible GNSS/display equipment, as appropriate, in their installation manuals as this has been shown to greatly aid installation approvals. The installation instructions approved as part of the STC or the installation manual approved by the TSOA for GNSS equipment and displays should provide appropriate equipment compatibility lists and/or compatibility standards. Additional combinations can be established later through a simple amendment to the STC installation instructions or TSOA installation manual.
Note: In addition to or instead of compatibility lists, equipment manufacturers may decide to provide detailed compatibility standards information on their equipment’s interface characteristics.
15-3. Failure Indications.
a. It is important to install the equipment such that all failure conditions result in appropriate displays.
b. For installations not incorporating an external navigation display (CDI, HSI, etc.), complete blanking of the CDU display is acceptable.
c. Presentation of a failure/status annunciation (flag or integrity annunciation) does not require removal of navigation information from the navigation display.
d. A unique annunciation of the loss of integrity monitoring should be provided.
Consideration may be given to the aircraft’s overall cockpit alerting philosophy to reduce the number of independent indicators.
Note: Paragraph 15-3.d does not apply to GPS/SBAS Class Delta-4 LPV operations because this equipment only functions during the final approach segment (or its extension) where loss of integrity monitoring is defined as a loss of navigation. A normal loss of navigation indication is sufficient.
15-4. Documenting Effects of GNSS Outage.
a. GNSS outages are a normal operating condition in areas with routine interference testing, and can occur anywhere in the NAS due to unintentional interference. The aircraft-level effect from losing GNSS positioning, velocity, and timing is a complex problem that depends on the GNSS equipment design, the degree of integration with other systems, and where the installations take place. GNSS outputs are being integrated into a variety of functions beyond traditional navigation data. For example, GNSS PVT is being used for TAWS/HTAWS, synthetic vision systems, ADS-B, and micro-electro-mechanical system (MEMS) accelerometers/sensors in air data attitude heading reference system (ADAHRS) inputs to electronic primary flight displays. There are a wide range of integrations from legacy aircraft with self-contained GNSS navigation units and a simple autopilot interface to new production aircraft with digital cockpits using integrated modular avionics providing the advanced functions mentioned above and more (including potential GNSS time applications).
b. TC and STC applicants must document, in the AFM(S)/RFM(S) normal section, the aircraft-level effects if GNSS PVT is not available or the outputs are disrupted. The AFM(S)/RFM(S) must describe the effects on any affected system from losing GNSS outputs;
the indications that should be expected if GNSS outputs are disrupted; and the flight crew or pilot procedures. TC and STC applicants should not use GNSS for both primary and back-up critical systems that would result in a single point of failure due to a GNSS outage.
Additionally, installers should be alert for equipment upgrade combinations that may result in GNSS as a single point of failure.
c. TSOA applicants must include, in their required operating guide, a description of the effects of GNSS outage on the functions in the article. The installation instructions must 116
include a description of the GNSS outage effects on the outputs from the article so the installer can evaluate the effects at the aircraft level.
15-5. Installed Performance Capability.
a. It is essential that procedures or optional functions an aircraft is not capable of performing are either removed from the GNSS equipment database, or otherwise inhibited, even if the avionics do support the function (RNP AR procedures or procedures with RF legs for example). The AFM(S)/RFM(S) must contain an appropriate entry for any limitations (see paragraph 12-7 and 12-8).
b. Helicopter-specific RNAV(GPS) approach procedures to LNAV, LNAV/VNAV, LP, or LPV minima can have unique characteristics. For the most part, helicopter-specific RNAV(GPS) procedures that contain unique features are published as Special Instrument Approach Procedures. Examples of unique criteria are: steep descent gradients, speed restrictions, and turns at the FAF greater than allowed in public approach procedures. The GNSS equipment standards generally support the TERPs criteria for helicopter-specific RNAV(GPS) procedures, but that does not ensure installed performance. It is incumbent upon the airworthiness approval applicant to demonstrate the rotorcraft GNSS integration is capable of meeting the desired performance for the intended function, or provide appropriate RFM(S) limitations. Particular attention should be directed toward the autopilot integration and whether or not the autopilot, coupled to all flight axes, is able to support the intended function.
c. Data Quality Requirements (DQRs) for installed databases are normally addressed at the equipment design level. The DQRs applicable to the intended function must be confirmed at the aircraft level during airworthiness approval. A Type 2 LOA may be used to ensure the database requirements, including tool qualification, is appropriate for the intended function of the installed equipment. A Type 2 LOA also provides evidence the data quality characteristics are described and valid for the compatible equipment listed on the LOA (see paragraph 19-8).
15-6. G PS Equipment.
a. GPS equipment only uses a RAIM algorithm to provide integrity and has reduced availability of an integrity-assured position solution at the minimum, guaranteed satellite constellation configuration compared to equipment using GPS/SBAS or GPS/GBAS augmentation.
b. When installing GPS equipment that uses RAIM for integrity during instrument meteorological conditions (IMC), there must be a performance limitation in the installation manual (or equivalent installation documentation) for inclusion as a limitation in the AFM(S)/RFM(S). The limitation is to have other navigation equipment available appropriate to the operation.
c. There must also be a flight planning performance limitation in the installation instructions (or equivalent installation documentation) for inclusion as a limitation in the AFMS/RFMS. This limitation must indicate the equipment has a performance limitation necessitating an alternate airport flight planning operational mitigation (see appendix 5 for an 117
example). The Aeronautical Information Manual provides some operational flexibility regarding the flight planning operational mitigation for equipment that has FDE capability.
Refer to paragraphs 1-1-17b.5 and 1-2-3d in the Aeronautical Information Manual for specific operational guidance.
d. The limitation language can provide further reference to the Aeronautical Information Manual for additional information.
Note 1: See appendix 1 for methods to remove the limitation for other navigation equipment in oceanic and remote areas.
Note 2: The performance limitations in paragraphs 15-6.b and 15-6.c do not have to use the specific language above or the example in appendix 5. The intent is to indicate the equipment has a performance limitation and adequately capture that limitation in the AFM(S)/RFM(S).
15-7. GPS/SBAS Equipment.
This paragraph provides guidance related to the installation of GPS/SBAS equipment supporting LP/LPV approaches. This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).
15-7.1 Limitations.
a. When installing equipment that uses GPS/SBAS for integrity during instrument meteorological conditions, there must be a performance limitation in the installation instructions (or equivalent installation documentation) for inclusion as a limitation in the AFMS/RFMS. This limitation must indicate the equipment has a performance limitation necessitating an alternate airport flight planning operational mitigation (see appendix 5 for an example). The Aeronautical Information Manual provides some operational flexibility regarding the flight planning operational mitigation for operators that also have baro-VNAV capability. Refer to paragraphs 1-1-18c.9 and 5-4-5m.7(h) in the Aeronautical Information Manual for specific operational guidance.
Note: The performance limitation in 15-7.1a does not have to use the specific language above or the example in appendix 5. The intent is to indicate the equipment has a performance limitation and adequately capture that limitation in the AFMS/RFMS.
b. The limitation above can provide further reference to the Aeronautical Information Manual for additional information.
15-7.2 LP Approach.
a. It is not acceptable to install TSO-C145a/C146a equipment that hasn’t been certified with LP capability in such a way as to make the equipment perform LP approaches. To do so is forcing the equipment to perform an unintended, non-certified function that is a violation of the original certification basis.
b. TSO-C145a/C146a equipment manufacturers may provide LP capability through software changes. The guidance above in paragraph 15-7.2a is not intended for equipment updated by the manufacturer.
c. GPS/SBAS installations of Class 3 or Class 4 equipment that complies with TSO revision ‘b’ or later with a deviation to not include LP capability must have an appropriate limitation included in the AFM(S)/RFM(S).
d. There are no unique LP flight testing requirements for airworthiness applicants that have performed LPV flight testing. LP uses the same signal and horizontal performance capability that GPS/SBAS provides during LPV. Airworthiness applicants with LPV data may use similarity or analysis as the basis for approving LP capability on the aircraft. The only installation considerations are that LP is clearly and unambiguously annunciated, and the FGS is able to accept horizontal-only approach guidance inputs (see paragraph 21-2.2.f ).
Note: An LP approach is somewhat analogous to a localizer-only approach in that localizer-only uses the horizontal ILS component to provide horizontal guidance.
However, LP and Localizer are not operationally similar because LP is not published on the same approach chart as LPV.
15-7.3 Update Rate.
For sensors and stand-alone equipment that support an LP/LPV approach capability (Class 3 equipment of TSO-C145(AR), Class 3 and 4 equipment of TSO-C146(AR)), the position used for the approach should be computed and output at a minimum 5 Hz rate per the requirements in RTCA/DO-229D, Change 1. If the equipment provides a 1 Hz output, the installation must use inertial aiding or some other equivalent method to obtain deviations at a minimum 5 Hz rate.
15-7.4 Performance Monitoring.
Sufficient information must be provided in the flight deck to allow the pilots to monitor the approach operation progress and safety. This in-flight performance monitoring must include:
a. Unambiguous indication of the intended path for the approach (e.g., LP/LPV approach type, approach identifier, active waypoint); and 119
b. Unambiguous indication of the position of the aircraft with respect to the intended path (e.g., cross track and vertical deviation information).
15-7.5 System Status.
The flight crew must have a means to evaluate the aircraft equipment’s capability to accomplish the approach operation. The flight crew must be advised of failed aircraft systems or equipment affecting the decision to continue to the destination or divert to an alternate.
Note: System status considerations also apply to IFR departure operations such as standard instrument departures. The flight crew must be advised of failed aircraft systems or equipment affecting the decision to depart.
15-7.6 Annunciations.
a. For installations containing more than one approach navigation source, the navigation source (for example, ILS, GPS/SBAS, baro-VNAV, etc.) selected for the approach must be positively indicated in the primary field of view. Consideration should be given to the overall aircraft-level annunciation philosophy.
Note: Aircraft requiring two pilots must have this
annunciation at each pilot station.
b. The approach type (LP/LPV, LNAV/VNAV, LNAV) must be clearly annunciated to the pilot prior to and throughout the entire approach in the primary field of view.
(1) It is acceptable for a primary flight display to annunciate localizer/glideslope using an abbreviation such as LOC/GS when the aircraft integration includes GPS/SBAS LPV, GBAS GLS Category I, and ILS. There must be an unambiguous indication the navigation source is provided by GPS/SBAS, GPS/GBAS, or ILS, as appropriate, in the primary field of view.
(a) LNAV/VNAV and LNAV approach types must still be annunciated, when applicable, if the applicant uses LOC/GS for LPV, or GLS Category I, or ILS.
(2) It is acceptable for the primary flight display to annunciate azimuth/elevation using an abbreviation such as AZ/EL when the aircraft integration includes GPS/SBAS LPV, GPS/GBAS GLS Category I, and MLS. There must be an unambiguous indication the navigation source is provided by GPS/SBAS, GPS/GBAS, or MLS, as appropriate, in the primary field of view.
(a) An unambiguous navigation source annunciation is still necessary if there is no MLS capability on the aircraft and AZ/EL is being used for a combination of only LPV and GLS Category I.
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(b) LNAV/VNAV and LNAV approach types must still be annunciated, when applicable, if the applicant uses AZ/EL for MLS, LPV, or GLS Category I.
c. Changes in the approach type annunciations should be very clear and distinctive to quickly catch the flight crew’s attention. This is so the flight crew knows and understands that there has been a change in service level and will make adjustments as needed to fly the appropriate approach minimums.
d. Aircraft installations containing both GPS/SBAS-provided vertical guidance and baro-VNAV vertical guidance capability should be carefully evaluated to ensure the annunciations clearly indicate the approach service provided and the sensor driving the vertical display (see paragraph 18-5).
15-7.7 Accessibility.
a. Single equipment installations must be accessible and usable by the pilot. For aircraft requiring two crew members, the equipment should be accessible and usable by either pilot located at a pilot or copilot crew station.
b. Dual (or more) equipment installations must have a convenient and expedient way to update data, information and programs, and a means to ensure consistency between the equipment (e.g., a “cross-load” capability can ensure identical data).
15-7.8 GPS/SBAS Glidepath Low Deviation Alerting Function.
a. To help prevent controlled flight into terrain, manufacturers and operators of aircraft that install a GPS/SBAS LPV and LNAV/VNAV approach capability are highly encouraged to install a glidepath deviation alerting function.
Note: There is no differentiation between LPV approaches using a 35m VAL or 50m VAL in the FAS datablock. The guidance above is intended for all LPV approaches.
b. If implemented, GPS/SBAS vertical performance monitoring and alerting based on deviation guidance outputs (i.e., FTE) can be used to provide a glidepath deviation alerting function. The alert must be prominent and easily recognized by pilots and may be either aural or visual or both. Manufacturers may propose other alternatives.
15-8. GPS/GBAS VDB Receiver.
This paragraph provides guidance related to the installation of a VDB receiver, including those embedded within a multi-mode receiver (MMR). This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).