10 considered a major change and requires an application for a TSO-C115b TSOA/LODA.
(1) TSO-C115b only addresses TSO-C129 Class B and C sensors because it was published prior to TSO-C129a, TSO-C145(AR), TSO-C161(AR), and TSO-C196. It is acceptable to integrate any GNSS sensor with TSO-C115b equipment.
(2) Applicants with equipment that has an existing TSO-C115b approval may add an incomplete system TSO-C129a Class A or TSO-C146c Class Gamma TSOA/LODA for the navigation computer functions. The navigation computer requirements in TSO-C129a or TSO-C146c will take precedence if there are any conflicts with the requirements in TSO-C115b (deviations to TSO-C115b need to be approved). See paragraphs 3-1 and 3-2 respectively for additional information on TSO-C129a and TSO-C146c incomplete system TSOA/LODAs.
b. Multi-sensor systems must also comply with the appropriate TSO requirements for the individual sensor; for example, TSO-C66c, Distance Measuring Equipment(DME) Operating Within the Radio Frequency Range of 960 to 1215 Megahertz, for DME equipment.
Refer to AC 90-100 (latest revision), U.S. Terminal and Enroute Area Navigation (RNAV) Operations, for operational approval of terminal and enroute area navigation systems for use in the United States national airspace system.
Note: The multi-sensor navigation system must comply with TSO-C115b (or a later revision) when integrating GNSS sensors. It is not acceptable to use equipment certified to earlier revisions (see paragraph 3-5.b(1)).
3-5. Multiple TSO Authorizations.
a. Multip le TSO authorizations may be granted for equipment that accomplishes multiple functions. However, the number of TSO markings on positioning and navigation equipment should be minimized to avoid confusion during installation and approval.
b. The following combinations of TSO authorizations must not be granted, as the TSOs either contradict each other or are superfluous to a TSO with broader scope:
(1) TSO-C115/C115a with TSO-C129(AR), TSO-C145(AR)/C146(AR), TSO-C196, or TSO-C161a/C162a).
(2) TSO-C129(AR) Class A with TSO-C146(AR) Class Gamma.
(3) TSO-C129(AR) Class B or Class C with TSO-C145(AR), TSO-C146(AR) or TSO-C196.
(4) TSO-C196 with TSO-C145(AR) or TSO-C146(AR).
Note: The intent is that combinations not specifically prohibited by paragraphs 3-5.b(1) through 3-5.b(4) are acceptable.
Chapter 4. Equipmen t Performance - Advisory Vertical Guidance.
4-1. Introduction.
Position ing and navigation equipment may provide vertical path deviation guidance indications on a non-essential, not-required basis as an aid to help pilots meet barometric altitude restrictions. Equipment that has this capability typically uses GNSS or baro-VNAV, but may use any method to generate the vertical path information. Advisory vertical guidance does not provide approved vertical guidance deviation indications for operational credit. Only vertical guidance deviation indications for LNAV/VNAV or LPV approach procedures are approved for operational credit.
Note 1: Advisory vertical guidance is an optional capability implemented at the equipment manufacturer’s discretion for enroute, terminal, and/or approach operations; not a requirement for positioning and navigation equipment.
Note 2: Paragraph 4-1 does not apply to instrument landing system (ILS).
Note 3: We recommend that equipment manufacturers provide a means for pilots to inhibit advisory vertical guidance to support nonprecision approach training requirements.
4-2. Implementations.
a. It is accep table to provide advisory vertical guidance as a descent aid during oceanic/remote, enroute, and terminal operations.
b. GPS equipment without GPS-provided approved vertical capability (TSO-C129(AR), TSO-C196, TSO-C145/C146(AR) Operational Class 1) and no baro-VNAV integration in the aircraft may provide advisory vertical guidance deviation indications on any approach using the LNAV line of minima.
c. Some aircraft integrations may use GPS equipment without GPS-provided approved vertical capability (TSO-C129(AR), TSO-C196, TSO-C145/C146(AR) Operational Class 1) to provide lateral path deviation indications and an approved baro-VNAV installation to provide vertical path deviation indications for an approach to the LNAV/VNAV line of minima.
(1) This integration constitutes approved vertical guidance to conduct a charted instrument approach procedure to the LNAV/VNAV line of minima and should not be confused with advisory vertical guidance applications.
(2) There may be occasions where it is operationally advantageous to use the LNAV line of minima rather than the LNAV/VNAV minima during an instrument approach procedure. It is acceptable for approved baro-VNAV installations to provide advisory vertical 11
guidance when using the LNAV line of m inima. However, during these operations, the flight crew must use the primary barometric altimeter as the primary reference for compliance with all altitude restrictions associated with the instrument approach procedure; including compliance with all associated step-down fixes.
Note 1: Baro-VNAV integrations may use non-GPS RNAV position sources to generate lateral path deviations for approaches that do not require GPS.
Note 2: Baro-VNAV is subject to temperature and pressure altitude (i.e., correct altimeter setting) performance limitations that could potentially cause advisory vertical path guidance to fall below step-down fixes on LNAV approaches.
d. During RNAV (GPS) instrument approach operations, TSO-C145/C146(AR) Operational Class 2 equipment may provide advisory vertical guidance when the procedure defines only the LNAV line of minima (i.e., procedures without a charted LNAV/VNAV and/or LPV line of minima). GPS/SBAS Operational Class 2 equipment is not capable of performing LP approaches, but may provide advisory vertical guidance for the LNAV line of minima on instrument approach procedures that define both an LNAV and LP line of minima. GPS/SBAS Operational Class 2 equipment may not provide advisory vertical guidance on instrument approach procedures with an LNAV line of minima published with LNAV/VNAV and/or LPV lines of minima. The reason is due to potential confusion over advisory versus approved vertical guidance and which line of minima applies. This human factors issue is even more critical when GPS/SBAS equipment “fails-down” to LNAV minima during the final approach segment if vertical guidance is lost during an LNAV/VNAV approach.
Note: There is no intent to preclude using baro-VNAV advisory vertical guidance for an LNAV approach provided it is selected prior to the final approach fix. See paragraph 17-5 when integrating baro-VNAV with GPS/SBAS vertical guidance.
e. During RNAV (GPS) instrument approach operations, TSO-C145/C146(AR) Operational Class 3 equipment may provide advisory vertical guidance when the procedure defines only the LNAV and/or LP line of minima (i.e., procedures without a charted LNAV/VNAV and/or LPV line of minima). GPS/SBAS Operational Class 3 equipment may not provide advisory vertical guidance on instrument approach procedures with an LNAV line of minima published with LNAV/VNAV and/or LPV lines of minima. The reason is due to potential confusion over advisory versus approved vertical guidance and which line of minima applies. This human factors issue is even more critical for GPS/SBAS equipment that has a “fail-down” mode to LNAV minima during the final approach segment.
Note 1: LP approach procedures will never be published with other lines of minima that contain approved vertical guidance (i.e., LNAV/VNAV or LPV). LNAV and LP lines of minima can be published on the same approach 12
chart; and, it is acceptable to provide advisory vertical guidance during approach operations using these lines of mi nima.
Note 2: There is no intent to prevent using baro-VNAV advisory vertical guidance for an LNAV approach provided it is selected prior to the final approach fix. See paragraph 17-5 when integrating baro-VNAV with GPS/SBAS vertical guidance.
f. TSO-C146(AR) Operational Class 4 (Functional Class Delta) equipment provides an “ILS look-alike” capability for LPV and LP only. GPS/SBAS Operational Class 4 equipment indicates LP by flagging the vertical guidance indications. It is acceptable for Class Delta equipment to provide advisory vertical guidance during approach procedures to LP minimums. However, it is not acceptable for Operational Class 4 equipment to provide advisory vertical guidance when the equipment “fails-down” from LPV to LNAV minima during the final approach segment due to potential confusion over advisory versus approved vertical guidance and which line of minima applies.
Note: Per RTCA/DO-229D it is not appropriate for GPS/SBAS Operational Class 4 equipment to “fail-down” from LP to LNAV.
g. Advisory vertical guidance may be provided during very high frequency omni-directional range (VOR) and non-directional beacon (NDB) instrument approach operations (including overlays with “or GPS” in the title) and also for older GPS instrument approach operations titled “GPS RWY XX”.
4-3. Limitatations.
a. There are no TSO requirem ents or MOPS standards regarding advisory vertical guidance minimum performance or how advisory vertical guidance is generated (see notes).
Positioning and navigation equipment manufacturers can generally use their discretion when choosing to implement this capability. However, the equipment manufacturers cannot claim any operational or certification credit for advisory vertical guidance. When implementing advisory vertical guidance, the equipment manufacturer must provide an equipment limitation in the installation/instruction manual for inclusion in the airplane flight manual supplement (AFMS)/rotorcraft flight manual supplement (RFMS) and may also include the information in the operations manual. The limitation should state the following or be equivalent:
“Advisory vertical guidance deviation information is only an aid to help pilots comply with altitude restrictions. When using advisory vertical guidance, the pilot must use the primary barometric altimeter to ensure compliance with all altitude restrictions, particularly during instrument approach operations.”
Note 1: RTCA/DO-229D, referenced by the GPS/SBAS TSOs, does provide some cursory requirements related to advisory vertical guidance.
Note 2: The pilot using the primary barometric altimeter to ensure compliance with altitude restrictions is generally considered the mitigation for misleading or erroneous advisory vertical guidance information (i.e., 14 CFR XX.1309 compliance). However, each installation should be individually evaluated.
b. The AFMS/RFMS and/or installation manual limitation does not preclude positioning and navigation equipment from outputting advisory vertical guidance deviations to an autopilot provided applicable autopilot requirements are satisfied. However, demonstrating autopilot compatibility or compliance is beyond the scope of this AC (see the latest revision of AC23-17, Systems and Equipment Guide for Certification of Part 23 Airplanes, AC 25.1329-1, Approval of Flight Guidance Systems, AC27-1, Certification of Normal Category Rotorcraft, or AC29-2, Certification of Transport Category Rotorcraft, for autopilot guidance information).
c. Positioning and navigation equipment manufacturers should consider providing a method to differentiate advisory vertical guidance indications from those used for approved vertical guidance to reduce potential crew confusion. If practical, the vertical deviation indications for each mode of vertical guidance should be unique and distinct.
Chapter 5. Equipmen t Performance - GNSS.
5-1. Introduction.
a. GNSS The TSOs and MOPS for various types of GNSS equipm ent have been published over a span of several years. Despite efforts to harmonize all the GNSS documents, there are differences among the documents because of this publishing time gap; particularly for characterizing the broadband external interference noise environment. The broadband external interference noise is defined in RTCA/DO-235B, Assessment of Radio Frequency Interference Relevant to the GNSS L1 Frequency Band, dated March 1, 2008. Table 1 below summarizes the effective noise density for acquisition, tracking, and re-acquisition that GNSS equipment manufacturers should use when certifying new equipment. Equipment not meeting the effective noise density in table 1 (e.g., TSO-C129) may experience performance degradation as more GNSS satellites are launched. Equipment intended for use beyond 2020 in applications such as ADS-B should be qualified using the effective noise density shown in table 1.
Receiver Function Effective Noise Density (dBm/Hz) Initial Acquisition (GPS Only)* -172.2 GPS Tracking and Reacquisition -171.9 SBAS Tracking and Reacquisition -172.8 Table 1. Effective Noise Density for All GNSS Sources
b. It is acceptable to run GPS/SBAS testing separately using a total GNSS noise (IGNSS) of -172.8 dBm/Hz for accuracy verification and/or collection of the GPS/SBAS message loss rate data.
c. TSO-C145b/c, TSO-C146b/c and TSO-C196 equipment use a slightly earlier broadband external interference noise standard that is very close to the table 1 effective noise density. This equipment should not experience any difficulty with the increasing noise environment. However, manufacturers should use the values listed in table 1 when conducting the test scenarios described in RTCA/DO-229D and RTCA/DO-316 respectively.
Note: The respective MOPS and TSOs will be updated with the latest noise standards during the next revision cycle.
d. TSO-C145a/C146a equipment complies with RTCA/DO-229C that uses the broadband external interference noise characterized by a previous RTCA/DO-235 revision.
TSO-C145a/C146a equipment may experience performance degradation when other GNSS services are launched because future interference management and prevention will be based on the RTCA/DO-235B-defined broadband external interference noise environment.
e. TSO-C129(AR) equipment has no broadband external interference noise requirements comparable to RTCA/DO-235B. This equipment is more likely to experience 15
perform ance degradation when other GNSS services are launched because future interference management and prevention will be based on the RTCA/DO-235B-defined environment.
5-2. GPS (TSO-C129(AR) and TSO-C196 receivers).
a. TSO-C129(AR) and TSO-C196 GPS equipm ent may be used on RNAV ‘T’ and ‘Q’ routes and for RNAV approaches to LNAV minimums within the contiguous United States.
AC 90-100 (latest revision), contains specific information on performance requirements for RNAV routes.
b. Equipment requirements for ‘T’ and ‘Q’ routes in Alaska differ due to infrastructure and other unique characteristics of Alaskan airspace. Special Federal Aviation Regulation (SFAR) 97 does not permit using TSO-C129(AR) and TSO-C196 equipment on ‘T’ or ‘Q’ routes. However, there is an ongoing program to either revise or delete the SFAR 97 requirements. See enroute charts, Notices to Airman Publication (NTAP), and Notice to Airmen (NOTAMs) for latest equipment requirements in Alaska.
5-2.1 Antennas.
a. It is recommended that receiver m anufacturers seeking antennas for equipment certified to TSO-C129(AR) and TSO-C196 use antennas compliant with the applicable antenna TSOs described below (see paragraph 5-5.4 for a complete discussion on antenna re-use/replacement). However, combinations where both the antenna and receiver were certified together under TSO-C129/C129a will need a new certification if antennas meeting the original requirements are no longer available. Using a different antenna specification constitutes a major design change that requires a new TSOA/LODA.
(1) Either passive or active antennas are acceptable for TSO-C129(AR) and TSO-C196 GPS equipment. The current TSOs for passive and active antennas are listed below;
but antennas previously certified to TSO-C144 are also acceptable (see Table 2).
(2) TSO-C144a, Passive Airborne Global Navigation Satellite System (GNSS) Antenna, that references RTCA/DO-228, Change 1, paragraph 2.2.1, defines an acceptable passive GPS antenna. A passive antenna is defined as one that does not include a low noise power amplifier as part of the antenna assembly.
(3) TSO-C190, Active Airborne Global Navigation Satellite System (GNSS) Antenna, that references RTCA/DO-301, Active Airborne Global Navigation Satellite System (GNSS) Antenna, dated December 13, 2006, defines an acceptable active GPS antenna. An active antenna is defined as one that includes a low noise power amplifier as part of the antenna assembly.
b. To aid compatibility findings, antenna manufacturers should identify the antenna minimum DC supply current, and nominal amplifier gain/tolerance for active antennas in the installation instructions. Receiver manufacturers need this information to define installation instructions that integrate the antenna and receiver. Additionally, the antenna manufacturer should identify the antenna minimum radiation pattern gain at 5 degrees and the antenna minimum gain/equivalent input noise temperature (G/T) ratio in the installation instructions so 16
that receiver manufacturers can take advantage of performance better than the minimum MOPS requirement. Documenting the improved performance facilitates the antenna/receiver installation and integration.
5-2.2 Sensors.
a. TSO-C129(AR) and TSO-C196 are position ing and navigation systems with equipment limitations for the aircraft to have other navigation equipment available appropriate to the operation and for alternate airport flight planning (see paragraph 14-5).
Note: TSO-C129(AR) Class A equipment is a sensor/navigation computer combination.
b. TSO-C129(AR) systems will not have an equipment limitation for the aircraft to have other navigation equipment in oceanic and remote areas provided additional requirements are met (see appendix 1).
c. TSO-C196 systems will not have an equipment limitation for the aircraft to have other navigation equipment in oceanic and remote areas provided the operator obtains a fault detection and exclusion (FDE) prediction program (see paragraph 5-2.3).
d. The intent for TSO-C196 equipment is to take advantage of signal processing improvements found in RTCA/DO-229D without the GPS/SBAS requirements. TSO-C196 equipment is intended as a direct replacement for Class B & C sensors certified to TSO-C129(AR). Additionally, TSO-C145(AR) equipment can also replace TSO-C129(AR) Class B & C equipment.
Note 1: Applicants will have to establish sensor/antenna compatibility when replacing TSO-C129(AR) sensors with either TSO-C196 or TSO-C145(AR) sensors.
Note 2: GPS/SBAS TSO-C146(AR) Class Gamma equipment is considered the direct replacement for a Class A sensor/navigation computer combination certified to TSO-C129(AR).
5-2.3 Prediction Program.
a. GPS satellite failures have a non-in tuitive impact on operational capability unlike conventional ground-based navigation aids that have a direct correlation between a facility outage (VOR fails) and the loss of an operational capability (VOR approach is unavailable).
The operator is responsible for considering the effects of satellite outages during flight planning. Flight Standards defines the operational requirements to review NOTAMs, publish aeronautical information, or perform pre-departure receiver autonomous integrity monitoring (RAIM) or FDE availability checks. Prediction programs and availability assessments are essential to enabling the operator to fulfill this responsibility. The FAA provides these services through NOTAMs for TSO-C145(AR)/C146(AR) equipment for domestic navigation operations. The FAA plans to provide these services for all GNSS TSO equipment to support 17
ADS-B out. Paragraph 5-2.3 applies to operations that require a prediction program .
b. The following guidelines apply to any prediction program applicable to a particular model TSO-C129(AR) or TSO-C196 receiver that is intended to assess the operational impact of satellite outages for flight planning purposes. Some operators are required to have an approved prediction program. A prediction program with the characteristics described below is acceptable.
Note: The guidelines also apply to prediction programs for TSO-C145/C146(AR) if the equipment is intended for oceanic/remote operations without limitations (see appendix 1 and paragraph 5-3.2.a.
c. TSO-C129(AR) equipment may have either a fault detection only algorithm or an FDE algorithm. TSO-C196 equipment is required by RTCA/DO-316 to have an FDE algorithm. The prediction program must include the appropriate capability for the intended receiver.
d. Operational guidance on predictive RAIM requirements may be found in the Aeronautical Information Manual (or other FAA guidance). The best performance model most likely comes from the original GPS equipment manufacturer.
Note: The FAA/Volpe prediction website did not include FDE prediction capability at the time this AC was published. However, the FAA/Volpe website is scheduled to include FDE prediction capability soon.
e. The prediction program can be provided on any processing platform (in the GPS equipment or not).
f. The prediction program software should be developed in accordance with at least RTCA/DO-178B level D guidelines or equivalent.
g. The program should use either an algorithm identical to that used in the airborne equipment, or an algorithm based on assumptions for RAIM or FDE prediction that give a more conservative result.
h. The program should calculate RAIM or FDE availability based on the same satellite mask angle used in the equipment, but not less than 2 degrees for enroute/terminal operations or 5 degrees for approach.
i. The program should have the capability to manually or automatically designate GPS satellites which are expected to be out-of-service for the intended flight.
j. The program should allow the user to select the intended route (including declared alternates) and the time and duration of the flight. The program should identify the maximum RAIM or FDE outage time, and any predicted navigation capability outages.
(1) For approach operations, the m aximum RAIM or FDE outage duration cannot exceed 5 minutes.
(2) For RNAV 1 and 2 operations, the maximum RAIM or FDE outage duration cannot exceed 5 minutes when TSO-C129(AR) or TSO-C196 receivers are used as the only means of RNAV 1 and 2 compliance.
(3) For oceanic/remote operations, the maximum FDE outage duration cannot exceed 25 minutes for RNP-4 operation, 34 minutes for RNP-10 operation, and 51 minutes for minimum navigation performance standards (MNPS) operation
Note: Prediction programs must use FDE for oceanic/remote operations. See appendix 1.
k. For approach predictions, the program should allow the user to select the destination and expected time of arrival, and provide a RAIM or FDE availability prediction over an interval of at least +15 minutes computed in intervals of 5 minutes or less about the ETA.
l. Prediction programs supporting ADS-B out should meet the requirements in 14 CFR § 91.227. The program should identify the maximum RAIM or FDE outage time, and any predicted surveillance capability outages.
5-3. GPS/SBAS (TSO-C145/C146(AR) receivers).
5-3.1 Antenna.
a. This parag raph provides guidance on acceptable GPS/SBAS antennas. See table 2 for a list of acceptable standard antenna TSOs matched with GPS/SBAS receiver TSO revision and operational class.
b. TSO-C190, that references RTCA/DO-301, defines an acceptable GPS/SBAS antenna.
Note: RTCA/DO-229D, paragraph 2.1.1.10 allows receiver manufacturers to designate manufacturer-specified antennas that take advantage of unique antenna performance rather than using the “standard” antenna defined by TSO-C190.
c. A number of “passive” and “active” antennas were authorized under TSO-C144 for TSO-C145a/C146a compliant GPS/SBAS receivers. These antennas remain acceptable for those receiver installations according to their original TSOA/LODA. Receiver manufacturers are encouraged to use a TSO-C190 antenna whenever a replacement becomes necessary for a TSO-C144 antenna.
d. Major modifications to TSO-C145a/146a receivers that require a new TSOA/LODA upgrade to TSO revision ‘c’ (for operational class 2, 3, and 4) must use either a TSO-C190 or a receiver manufacturer-specified antenna that meets current requirements in RTCA/DO-229D 19
(such as, s ensitivity and dynamic range, geostationary orbit satellite (GEO) bias, broadband interference, and group delay).
5-3.2 Sensor and Sensor/Navigation Computer Configuration.
a. GPS/SBAS equipm ent has two different TSOs depending on whether the equipment is designed as a position sensor only; or, a sensor/navigation computer combination.
TSO-C145c defines an acceptable standard for GPS/SBAS sensors (Class Beta) while TSO-C146c defines a complete sensor/navigation computer configuration (Class Gamma) and an “ILS look-alike” (Class Delta). Both TSOs reference RTCA/DO-229D for operations within and outside of the GPS/SBAS service provider coverage area. When outside of a GPS/SBAS service provider’s coverage area the receivers can revert to using FDE for integrity. The receiver will use GPS/SBAS integrity or FDE; whichever provides the best protection level.
GPS/SBAS equipment will not have any limitations in oceanic and remote areas provided the operator obtains an FDE prediction program (see paragraph 5-2.3).
Note: GPS/SBAS equipment may continue using GEO data and range measurements when outside the GPS/SBAS service provider’s coverage area per the requirements in RTCA/DO-229D (RTCA/DO-229C for TSO revision ‘a’ equipment).
b. GPS/SBAS equipment that complies with the standards implemented by TSO-C145(AR)/C146(AR) does not have an equipment limitation for the aircraft to be equipped with other navigation systems appropriate to the operation. With GPS/SBAS equipment it is also acceptable to flight plan an alternate using an RNAV approach to LNAV minimums at the alternate airport rather than a ground-based approach aid (see paragraph
14-6.1). Additionally, GPS/SBAS equipment may be used on RNAV ‘T’ and ‘Q’ routes within the contiguous United States and Alaska. AC 90-100 (latest revision), contains specific information on performance requirements for RNAV routes. These capabilities should be clearly indicated in receiver manufacturer’s operating instructions.
c. Although GPS/SBAS does not have an equipment limitation for other navigation systems onboard the aircraft, the receiver manufacturer’s operating instructions should encourage operators to retain back-up navigation systems to guard against outages or interference events.
d. GPS/SBAS equipment that complies with the ‘b’ or ‘c’ TSO revision has the capability to do LP approaches. TSO-C145a/C146a manufacturers may provide LP capability in their equipment. LP approaches use the horizontal accuracy and integrity values of LPV but do not provide vertical guidance. The intent is to provide LP approaches only at locations where some issue prevents implementing LPV vertical guidance; so, LP should be the exception rather than the rule. Therefore, LP is not operationally similar to a localizer-only approach on an ILS because LP and LPV are mutually exclusive. That is, LP and LPV lines of minima will never be on the same approach chart.
5-3.3 Navigation Database - Data Conversion for GPS/SBAS L NAV/VNAV Approaches.
a. For LNAV/VNAV approaches collocated with LPV approaches, GPS/SBAS equipment must use the final approach segment (FAS) data block to define the final approach segment as described in RTCA/DO-229D. But, a FAS data block is not provided for stand-alone LNAV/VNAV approaches. The vertical path for such approaches is defined by the threshold location, threshold crossing height, and glidepath angle. However, the altitude of the threshold is published as the height above/below mean sea level (MSL), rather than as the height above/below the WGS-84 ellipsoid. Therefore, the published MSL height (interpreted as a height above/below the WGS-84 geoid) must be converted to height above/below the WGS-84 ellipsoid prior to being used. Actual decision altitude (DA) must be based on the barometric altimeter.
b. The data conversion is not considered alteration of data per RTCA/DO-200A, Standards for Processing Aeronautical Information, dated September 28, 1998, paragraph
2.4.2, since it changes the reference system but not the actual vertical location.
c. Receiver manufacturers may consider performing the conversion within the GPS/SBAS equipment or using a database process that provides threshold crossing height directly as height above/below the WGS-84 ellipsoid for GPS/SBAS approaches. However, GPS/SBAS integrations with baro-VNAV will need to address the issue of WGS-84 ellipsoid versus MSL height. This can be done through conversion or storing threshold data as both WGS-84 ellipsoid and MSL height to use for GPS/SBAS or baro-VNAV as appropriate.
5-4. GPS/GBAS (TSO-C161a/C162a receivers).
The GPS/GBAS equipm ent supports Category I precision approach and, optionally, a GPS/GBAS differentially corrected positioning service (DCPS). When the GPS/GBAS avionics supports both levels of service, whether or not the DCPS is available is determined by the GPS/GBAS ground station. All GPS/GBAS ground stations provide precision approach service; but, not all ground stations are expected to provide DCPS.
5-4.1 Antennas.
a. TSO-C190 that references RTCA/DO-301, defines an acceptable GPS/GBAS antenna.
b. TSO-C162a GPS/GBAS equipm ent also requires a VHF antenna to receive the GPS/GBAS satellite, path, and ground facility information broadcast. Installed antennas approved for ILS localizer/VOR can be used to receive the VDB signals. Unlike ILS ground antenna arrays, a GBAS VHF data broadcast transmitter may be located anywhere on the airport or in its vicinity. This means the VHF broadcast must be received from any direction.
The ILS localizer/VOR antennas may, or may not, be included under TSO-C162a. See paragraphs 12-1.d, 14-7, and 20-2.b for installation considerations.
Note: RTCA/DO-253C gives GPS/GBAS equipment manufacturers an option for an antenna/receiver 21