Table 2. Summary of WAAS Performance En route through Nonprecision Approach LPV Availability 99.999% 99.9% Horizontal Navigation System Accuracy (95%) 33 m
16 m Vertical Navigation System Accuracy (95%) Unspecified 20 m SIS Integrity Probability of misleading information in an hour = 10- 7 Probability of misleading information during an approach = 10-7 Service volume U.S. Airspace Contiguous U.S., parts of Alaska
(4) WAAS General Operational Limitations. The requirements were developed for GPS/WAAS to be used within the SBAS coverage without the need for other radio navigation equipm ent appropriate to the r oute to be flown. Outside of the SBAS coverage, GPS/WAAS equipment reverts to GPS-only operation.
(5) WAAS Equipm ent Classes. RTCA /DO-229C, sectio n 1.4, defines the different classes of GPS/ WAAS equi pment to support different equipm ent configurations.
c. Ground-Based Augm entation System (GBAS) . To im prove the accuracy, integrity and availability of GPS signals, ICAO has defined Standards and Recommended Practices (SARPs) for ground-based augm entation system s (GBAS). The FAA developed the Local Area Augm entation Syst em (LAAS) to provide GBAS service at designated airports throughout the U.S. Th e LAAS is intended to support a level of navigation service that require s g reater p erformance than that prov ided by WAAS.
LAAS is being developed to ensure interopera bility. Sim ilar to th e WAAS concept that incorporates the use of comm unication satellites to broadc ast a correction m essage, the LAAS will broadcast its cor rection m essage via very high f requency (VHF) radio broadcast f rom a groun d-based tr ansmitter. Th e loc alized coverage ar ea will sup port multiple operations to an airport. The LAAS will complement WAAS at locations where WAAS is unable to m eet existing navigation and landing requirements. As the LAAS i s further developed, it will provide a level of service sufficient to support CAT II/III precision approach and landing operations.
8. APPROVAL PROCESS . This paragraph describes the general approval process applicable to GNSS e quipment intended for IFR navigation (paragraph 9 provides guidance addressing installati on of GNSS equipm ent for VF R use). Unique approval issues associated with G PS for Oceanic/R emote Navigation and GLONASS are defined in appendices 1 and 2, respectively.
a. TSO Authorization (TSOA).
(1) Several TS O’s have been written for GNSS-related equipm ent and components (reference paragraph 6a of this AC ). While the TSO process operates on the basis of self-certificatio n by the ap plicant, it is beneficial to involve the ACO (both Par 7 Page 9
engineers and flight test pilots) in the evaluation of the equipment as early as possible for two primary reasons:
(i) Obtaining a TSOA does not ensure that the equipment will satisfy all of the applicable requirements when it is installed; and
(ii) The design of GNSS equipment has become increasingly more complex and workload intensive with no standardized interface with the pilot. The human factors evaluation of the equipment is subjective.
(2) It is recomm ended that the m anufacturer elect to obtain an STC for their equipment concurrent with obtaining the TS OA. This has proven to be an effe ctive means of involving the ACO and identifying issues associated with installing the article.
(3) The installation guidelines should refere nce standard practices used in the installation and should address the unique GNS S issues in paragraphs 11 through 20 of this AC. The installatio n instructions should identify each in stallation configuration that is supported, including the GNSS antenna(s), each autopilot make/model, other GNSS or RNAV equipm ent that is insta lled o n the ai rcraft, and any other articles to which the equipment will in terface (e.g., ex ternal swit ches, barometric altim eter, TAW S). In addition, the installation instructions shoul d address the critical areas unique to each installation, such as antenna placem ent, unit placem ent (for controls/display ), and electromagnetic compatibility.
(4) The operations m anual must address th e operation of the equipm ent. For installations accomplished in accordance with the installation instructions, the operations manual should also cover the operation of related com ponents (e.g., rem ote source selector) for every installation configuration. If there any lim itations associated with the operation of the equipm ent, a sample Airp lane/Rotorcraft Flight Manual Supplem ent (A/RFMS) should also be provided (e .g., the TSO-C129(AR) lim itation for other navigation equipment appropriate to the route to be flown).
b. First-Time Airworthiness Approval . Each TSO’d article should obtain a firsttime airworthiness approval through TC or STC.
(1) The initial certification of GNSS e quipment involves extensive engineering and flight test evaluations as described in paragraphs 11 through 23 of this AC.
Applicants are encouraged to obtain AC 21-40, an Application Guide for Obtaining a Supplemental Type Certificate.
(2) It is recomm ended that the ACO reque st a S pecial Certification Review (SCR) Team when evaluating GNSS stand-alon e navigation equipment for the first tim e, typically as part of an STC. The SCR team should be coordinated through AIR-130. The team assists in applyin g a comm on stand ard, which will result in a more consistent evaluation.
c. Installation of GNSS Equipm ent. For installation of TSO’d articles after the first-time STC, the extent of FAA involve ment and review for a given installation depends on the characteristics of the installation. The insta ller should comply with the guidelines described in paragraph 15 th rough 20, and should ensure continued compliance with the relevant certification regulations.
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(1) Evaluation of installation. The installa tion should be evaluated to classify the installation and determine the type of approval vehicle. The following guidelines are provided:
(i) Major change to type design: Pe r 14 CFR Part 21.93, the applicant should determine if the installation has an appr eciable effect on the st ructural strength or operational characteristics affecting the airwor thiness of the product. An exa mple is installation in m ultiple-pilot aircraft where an other, dissimilar area navigation system is also installed (see paragr aph 18f), which should be thoroughly evaluated under the TC/STC process unless interm ixing of the sa me GNSS equipment was part of the initia l design approval or specifically addresse d under an approved m odel list (e.g., the Brand-A, model X instructions m ay adequately address intermixing with Brand-A model Y equipment).
(ii) Major alteration: Per 14 CFR Part 43 Appendix A, installation of GNSS equipment m ay be a m ajor alteration requir ing FAA-approved data pertinent to that installation. For example, insta llation of the GNSS antenna m ay im pact structural strength, and the GNSS equipm ent m ay be co nnected to other equipm ent that has not been previously determined to be compatible or for which the installation instructions are not adequate to ensure com patibility under all conditions. If the installation is determined to be a m ajor alteration, the appl icant can obtain an STC or a field approval using FAA Form 337 (Major Repair and Alteration).
(iii) Minor alteration: Under the origin al AC 20-138, installation of GPS equipment required the use of approved data (u nder an STC or m ajor alteration) because GPS was a new and unique technology. However, since GPS technology is now common and considerable experience has been obtained in the installation of GPS, approved data for every installation is no longer appropriate. Instead, installations that do not qualify as major alterations above should be accom plished as minor alterations. These installations should be based on acceptable data including the following:
(A) Data previously approved as app licable to the aircraft (e.g., STC, Approved Model List).
(B) Data previously approved as applicable to a dif ferent make/model aircraft (e.g., an initial STC obtained by the equipm ent m anufacturer), provided the installation is installed in accordance with the manufacturer’s instructions, any equipment interfaces are adequately address ed in th e installation instructions for the GNSS equipment and the equipm ent with which it is interfacing, and the equipm ent is installed in accord ance with the guidelin es in this AC. For exam ple, installation of GNSS navigation equipm ent that only interfaces w ith an antenna, power, ground, an external HSI/CDI with a single source selector switch, a nd a left/right (deviation-based) autopilot would typically be considered a minor alteration.
(2) For these installations, any A/RFM(S) associated with the original approved data should be used as the basis f or an A/ RFM(S) for this inst allation. Lim itations imposed on the GNSS equipm ent during the initial approval should be i mposed for follow-on equipm ent approval unless an FAA re-evaluation of the relevan t is sues determines that they do not apply. An A/RFM(S) is not necessary if it was not part of the original installation, if the operating m anual addresses the use of the equipm ent and Par 9 Page 11
associated installed component s (e.g., rem ote source selecti on), and there are no unique limitations associated with the particular installation.
(3) The installation perform ance tests (paragraphs 22 and 23) m ust be successfully completed. The installation perf ormance tests are usually conducted in two parts, the ground checks and the flight checks. T he aircraft can be approved for return to service following the require d ground checks, however the GNSS equipm ent cannot be used for flight under Instrum ent Flight Ru les (IFR) until the flight checks have been accomplished and would have to be placarded as such.
(4) Upgrading previously-approved installations.
(i) WAAS Upgrade. GPS TSO-C129(AR ) equipment may be upgraded to TSO-C145a or TSO-C146a compliance. Manufacturers who provide an upgrade path are encouraged to use an approved m odel list to stream line the approval of subsequent installations. The guidance in paragraph 8c(1 ) also applies to the upgrade of previously installed equipment.
(ii) Additional GNSS equipm ent. Som e applicants ar e expe cted to re tain existing TS O-C129(AR) equipm ent and add TSO-C146a equipm ent. Com patibility between the equipm ent should be established in accordan ce with paragraph 18f. The original TSO-C129(AR) installation should not be re-evaluated, except f or modifications made as part of the TSO-C146a installation. The re-evaluation should be to the affected functions or perform ance. For example, if the equipment shares a GNSS antenna there will be additiona l signal loss f or the original equipment and com pliance to the o riginal TSO-C129(AR) installation instructions should be verified.
(iii) Re-use of GNSS antenna. W AAS or LAAS equipm ent may be added to an airc raft (as an addition or as m odification to existin g GPS equipm ent) with a previously installed GNSS antenna. This m ay be accomplished by including the antenna under the WAAS or LAAS TSO authorizati on, by establishing com patibility under the WAAS or LAAS TSO authorization, or de monstrating com patibility a t tim e of installation (see p aragraph 17a). I f a W AAS or LAAS TSOA holder dem onstrates compatibility with a previously-approved antenna (approved under TSO-C129(AR)), the installation instructions should state the part number of the compatible antenna. Note that compatibility to support LPV or GLS operations cannot be addressed without new design data on the antenna (see paragraph 11e).
(iv) Marking. When equipm ent is u pgraded to a new TSO (e.g., from TSO-C129(AR) to TSO-C146a), the m arking must be modified to reflect the new configuration and approval. This marking ch ange can be accom plished by a certificated person in accordance with the T SO authoriz ation holder’s instru ctions (e.g., service letters or service bulletins). A certificated person in this context is defined as one who is authorized under 14 CFR section 43.3, Persons authorized to perform m aintenance, preventative maintenance, rebuilding, and alterations.
9. GNSS EQ UIPMENT LIMITED TO VFR USE. GNSS equipm ent m ay be installed on a no-hazard basi s as a supplem ent to VFR navigation. S uch installations need only to verify that the GNSS installation does not introduce a hazard to the aircraft
(e.g., properly secured for crashworthiness, not com bustible, etc). GNSS installations limited to VFR USE ONLY should be evaluated under the criteria described in paragraph Page 12 Par 8
8c(1) to classify the type of aircraft/appliance m odification. The only exception is that, for installations where the GNSS equipm ent does not in terface with other equ ipment (except a dedicated remote indicator if app licable), the installation can be accom plished without any reference to previously approve d data (i.e., the equipm ent does not need a TSO or prior STC). Installa tion guidelines for such an in stallation are prov ided in appendix 5 of this AC. Loss of or m isleading VFR navigation information is considered a m inor failure condition; therefore, Soft ware Development Assurance Level D is acceptable. A readable placard stating "GPS limited to VFR use only" must be installed in clear view of the pilot, unless th e equipment automatically displays this m essage on start-up and pilot ac tion is required to clea r the message. An A/RFM(S) is not required since the placard or display contains the equipment limitation.
10. TSO AUTHORIZ ATIONS. This paragraph provides guidance on granting TSO Authorizations f or GNSS-rela ted equipm ent. Only issues that are u nique to G NSS equipment are discussed.
a. Applicability of TSO-C129(AR).
(1) TSO-C129(AR) define an accep table standard for GPS equipm ent intended to be used as a supplement to other navigation systems.
(2) There is no plan to cancel TSO-C129a , and m anufacturers may continue to obtain new TSO authorizations un der TSO-C 129a. In addition, th ere is no plan to withdraw TSO-C129(AR) authorizations, so manufacturers of TSO-C129(AR) equipment can continu e to m anufacture that equipm ent, and the equipm ent is still eligib le f or installation in accordance with the guidance in this AC.
b. Multiple TSO Authorizations. Multiple TSO authorizations m ay be granted for equipment that accomplishes multiple functions. However, the num ber of TSO markings on GNSS e quipment should be m inimized to avoid confusion during installation and approval. T he following com binations of TS O authorizations should not be granted, as the TSOs either contradict each other or are superfluous to a TSO with broader scope:
(1) TSO-C129(AR) must not be combined with TSO-C145a or TSO-C146a;
(2) TSO-C145a m ust not be com bined with TSO-C146a Class 1, 2 or 3 equipment. The com bination of TSO-C145a with TSO-C146a Class 4 equipm ent is permitted; and
(3) TSO-C146a must not be combined with TSO-C115(AR).
(4) Any class of TSO-C129(AR) may be combined with TSO-C161, and should be marked accordingly.
c. TSO-C145a and TSO-C146a Deviations.
(1) Under TSO-C129(AR), m any deviat ions have been granted to manufacturers due to the relative immaturity of the TS O, which significantly mod ified the industry standard (RTCA/DO-208). A pplicants for TSO-C145a and TSO-C146a should not expect the sam e latitude when requesting deviatio ns. These TSOs are based on the industry standard (RTCA/DO-229C) wh ich has been internationally harmonized and received a good deal of scrutiny since it was first published in January 1996.
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(2) When requesting a deviation to these TS Os, the applicant must substantiate how their implem entation is equivalent. A primary objective of RTCA/DO-229C is to standardize the interface a nd provide com patibility across equipm ent from different manufacturers. Therefore, the statem ent of e quivalency for deviations to the pilot interface requirem ents must also include how the im plementation is operation ally compatible with other GPS/WAAS equipment.
(3) Deviations that are granted to these TSOs will be made available and will be incorporated into the ne xt revision of the TSO to ensure th at all m anufacturers have the same opportunity to deviate.
d. Technical Data. Paragraph 9c of Order 8150.1A, Technical Standard Order Procedures states that the ACO m ay elect to do a cursory review of the TSO subm ittal package. Due to the complexity of GNSS equipment, the ACO should:
(1) Evaluate manufacturer supplied bench test data on the equipm ent and verify that the tests com ply with the relevant procedures a nd requirem ents (TSO-C129, and sections 2.4 and 2.5 of RTCA/DO-229C and RTCA/DO-253A as applicable) and that the equipment has passed applicable tests.
(2) Verify co mpliance with the appropr iate environm ental qualification standards and tests in RTCA/DO-160(AR).
(3) Verify compliance to th e design assurance levels consistent with the f ailure condition hazard classification. If the criteria in AC 23-1309-1C for different Classes of Part 23 airc raft is app lied, the ins tallation in structions m ust inc lude a summary of the approval and a lim itation. For exam ple, TSO-C146a Class 3 equipm ent (GPS/WAAS precision approach stand-alone equipm ent) with software devel oped to RTCA/DO-178B Level C would state the following:
“This equipment complies with p aragraph 3a of TSO-C146a for Class I, II, and III aircraft as defined in AC 23-1309-1C. This equipm ent is not eligible for installation in Part 25 or Part 23 Class IV airc raft unless the precision approach m ode is disabl ed through the following equipm ent configuration setting: ….”
(4) Verify compliance with the appropriate software qualification standards and tests in RTCA/DO-178B.
(5) Verify the approval of a data pro cess capable of updating the navigation database for GNSS stand-alone naviga tion equipm ent (TSO-C129(AR) Class A, TSO-C146a). RTCA/DO-200A defines an acceptable m eans of e nsuring that th e aeronautical data process does not corrupt data, from the publication of data by a State to its application in the equipment. The data quality requirements must be defined, including any lim itations or inco mpatibilities between the navigation functi on and the database
(e.g., incompatible ARINC 424 leg types, leg length limitations).
(6) The ACO should review the installati on manual. The m anufacturer should include all known installation limitations, and speci al attention should be given to reviewing those limitations in light o f the var iety of different instal lation configurations that are supported.
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(7) For TSO-C146a, the pilot’s guide should be reviewed by an FAA flight test pilot f amiliar with the equipm ent to ensure it is com plete, con cise, and eas y to understand.
(8) For TSO-C146a, the quick reference guide should be review ed by an FAA flight test pilot familiar with the eq uipment. Recommended f ormat and content f or this guide can be found in appendix 3 of this AC.
(9) For TSO-C146a, the training program should be reviewed by an FAA flight test pilot familiar with the equipment. This training program may be a series of manuals, a videotape, interactive CD-ROM, etc. Due to the inherent complexity in area navigation operations, it is im portant that these training materials are useful and help pilots learn to operate the equipment.
e. Major/Minor Design Changes. The following changes are among those generally considered by the FAA to be major ch anges (per 14 CFR Part 21.611). After consultation with the ACO on a specif ic change, the ACO and manufacturer m ay determine that particular change is a minor design change.
(1) Changes to the navigation or integrity algorithms; and
(2) Changes to navigational display form ats and navigation/function operating procedures.
11. EQUIPMENT P ERFORMANCE – GPS ANTE NNA. This paragraph provides guidance related to the basic functions a nd perfor mance of the GNSS antenna. The antenna does not have to be installed in an ai rcraft to com ply with the m eans defined in this paragraph. Typically, this guidance will be applicable to the m anufacturer who is seeking a TSO Authorization (TSOA). If th e manufacturer does not obtain a TSOA, this paragraph would apply as part of the initial airworthiness approval. These issues do not need to be revisited for antennas that hold a previously granted TSO authorization under TSO-C129(AR) or TSO-C144, or minor m odifications to such equipment. However, the applicant may elect to address these issues to demonstrate that the antenna is compatible with a WAAS or LAAS installation.
a. TSO-C144, Airborne Global Positio ning System Antenna, defines an acceptab le means of com pliance for a GNSS antenna. Applicants shou ld com ply with RTCA/DO-228, Change 1.
b. Antennas authorized under TSO-C129(AR). A num ber of antennas were authorized u nder TSO-C129(AR), before TS O-C144 becam e available. This paragraph addresses the performance of these antennas.
(1) Antennas authorized as part of GPS receiver TSOA. Antennas authorized in conjunction with a GPS receiv er under TSO- C129(AR) m ay continue to be in stalled along with the associated receiv er. These antennas m ust not be installed with a different receiver unless the antenna part number is listed in the receiver installation instructions as being compatible.
(2) Antennas authorized independently fr om a GPS receiver. TSO-C129(AR) does not provide an adequate standard to en sure interoperability between GPS antennas, Par 10 Page 15
and m ust not be applied to anten nas wit hout reference to a specific GPS receiver.
However, some antennas have been approve d under an authorization to TSO-C129(AR) without reference to a specific GPS receiver. These antennas should not be installed with any new receivers (i.e., one for which a pr evious TC/STC does not exist) unless the antenna part number is listed in the receiver installation instructions as being compatible.
Antenna manufacturers should obtain TSO-C144 for their antennas as quickly as practical to ensure compatibility with receiver equipment and to facilitate installation.
c. Lightning Qualification. The RTCA/DO-160D category for lightning induced transient su sceptibility that bes t represents the trans ient induced in the antenna cable during the Direct Effects of Lightning test (Section 23 of RTCA/DO-160D) should be identified for comparison to the receiver susceptibility at time of installation.
d. Signal Power/Loss. The following issues should be addressed in the installation instructions.
(1) Passive antennas authorized under T SO-C144. Passive antennas are those without a pream plifier (reference RTCA /DO-228, section 2.2.1). The m aximum (and minimum, if applic able) signal loss between the antenna output por t and receiver input port must be defined, to yield the m aximum and minimum receiver s ensitivities defined in section 2.1.1.10 of RTCA/DO-229C. This information should be described as a function of the type and lengths of interc onnecting cable and type and num ber of connectors or specified as the limit of cable loss in dB.
(2) Active antennas authorized under T SO-C144. An active antenna is defined as one that includ es a low noise power am plifier as p art of the antenna ass embly (reference RTCA/DO-228, section 2.2.2). The inst allation instructions must identify the maximum a nd m inimum output power (m ay be specified as pre-am plifier gain) and amplifier noise figure. These guidelines can be compared to the minimum and maximum power inputs defined in the GNSS receiver in stallation instructions. Guidelines on the loss associated with reco mmended cables and co nnectors should be provided so that the installer can determ ine the m aximum and m inimum signal power at the receiver port.
This range will be compared to the range specified by the GNSS receiver manufacturer at time of installation. In addition, the installation ins tructions shou ld identify any conditions under which out-of-band em issions (e.g., SATCOM) c an create in-band intermodulation products that may interfere with receiver operation.
(3) Antennas authorized w ith a specif ic receiver (TSO-C145a, TSO-C146a, TSO-C161). The installation instructions m ust identify the m aximum (and m inimum, if applicable) signal loss between the antenna ou tput port and receiver input port to yield the m aximum and m inimum receiver sensitiv ities def ined f or the rece iver. This information should be described as a function of the type and lengths of interconnecting cable and type and number of connectors or specified as the limit of cable loss in dB.
e. Differential Group Delay. The differential group dela y is defined within section
2.1.4.5 of RTCA/DO-229C and section 2.3.6.4. 1of RTCA/DO-253A. The differential group delay of the antenna m ust be docum ented in the installation instructions for those antennas intended to support LPV and GLS approach operations. Antennas have filters that could affect the d elay. The dif ferential group delay of the antenna is defined in section 2.2.3 of RTCA/DO-228.
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12. EQUIPMENT PERF ORMANCE – GNSS SENSOR. This paragraph provides guidance related to the basic functions a nd perfor mance of the GPS sensor, including those sensors that integrate W AAS or LAAS. This paragraph also applies to sensors embedded within stand-alone navigation eq uipment. A ppendix 2 provides unique guidance material for GPS sensors that inte grate GLONASS. The sensor does not have to be installed in an aircraft to com ply with the m eans defined in this paragraph.
Typically, this guidanc e will be ap plicable to the m anufacturer who is seeking a TSO Authorization (TSOA) for TSO-C129a, TSO-C145a, TSO-C146a, TSO-C161, or TSO-C162. If the m anufacturer does not obtain a TSOA, this paragraph would apply as part of the initial airwor thiness approval. These issues do no t need to be rev isited f or equipment that holds a previously grante d TSO authorization under TS O-C129(AR), or minor modifications to such equipment.
a. Acceptable Means.
(1) GPS. TSO-C129(AR) defines an accep table means of com pliance for GPS equipment that is inten ded to supp lement other navigation equipm ent on the aircraft.
With one exception, GPS equipment that complies with this standard has a limitation that the aircraft must be equipped with other m eans of navigation appropriate to the route to be flown. The exception is the use of this equipment for oceanic and remote operation, if several additional features are provided as described in appendix 1 of this AC.
(2) GPS/WAAS. TSO-C145a and TSO-C146a, which reference RTCA/DO-229C, define an accep table m eans of com pliance for this equipm ent whe n operating in GPS or GPS/ WAAS modes. RTCA/DO-229C is a complete set of standards for GPS a nd GPS/ WAAS. While applic ants m ay elect to use portions of RTCA/DO-229C to supplement the earlier standards for TSO-C129(AR) equipment, it is not appropriate to obtain TSO-C129(AR) au thorization in conjunction with TSO-C145a or TSO-C146a equipm ent. GPS/ WAAS equipment that com plies with the standa rds in DO-229C does not have a lim itation that the ai rcraft must be equipped with other m eans appropriate to the route to be flown.
(3) GPS/WAAS/LAAS sensor. TSO -C145a and TSO-C146a define an acceptable m eans of com pliance f or th is eq uipment when operating in th e GPS or GPS/WAAS m odes. When operating in th e G PS/LAAS mode, the equipm ent should comply with the requirements of TSO-C161 and Section 2 of RTCA/DO-253A.
(4) GPS/LAAS. TSO-C129(AR) and TSO-C145a define two acceptable m eans of compliance for this equipment when operating in GPS mode (i.e., outside the coverage of a selected LAAS ground facility). W hen operating in the GPS/L AAS m ode and conducting a GLS approach, the equipm ent mu st com ply with the requirem ents of Section 2 of RTCA/DO-253A. When operating in GPS/LAAS mode and not conducting a GLS approach, the equipm ent m ust comp ly with the sensor requirem ents of RTCA/DO-253A (for position, velocity and time) and the navigation requirem ents of TSO-C129(AR). For Class B1 and B2 equipm ent, the requirem ents in TSO-C129(AR) paragraph (a)(4)(vii) do not apply when a LA AS-correction position is being used. For Class C1 and C2 equipment, the requirements in TSO-C129(AR) paragraph (a)(5)(vii) do not apply when a LAAS-corrected position is being used. GPS/LAAS equipm ent carries Par 12 Page 17
the basic GPS lim itation that th e airc raft must be equipped with other m eans of navigation appropriate to the route to be flown.
b. Compatible GNSS Antennas . Compatible antennas should be identified. This may include a generic reference to TSO-C144- compliant antennas, re fer to a specific GNSS antenna (part number) that is included as part of the receiver design approval (e.g., part of receiver TSOA), or a part of another receiver design approval (also by part number). When using an antenna that is pa rt of a separate design ap proval, the receiver manufacturer should use the antenna specifi cation provided by the antenna m anufacturer and should coordinate the com patibility finding with the antenna m anufacturer.
Otherwise, m inor design changes to the antenna could be m ade by the antenna manufacturer that would adversely impact the operation of the GNSS equipment.
c. Non-Standard Mask Angle. All equipment has a mask angle, the elevation angle above the horizon at which the GNSS receiver c onsiders satellites for use in the so lution.
The accuracy and reliability of GPS signals is affected by a num ber of factors, inclu ding shadowing of the signal (by a portion of the aircraf t or loc al terrain), the GPS antenna gain and atmospheric effects (ionosphere and troposphere).
(1) A 5 degree m ask angle is standa rd (7.5 degree m ask angle for TSO-C129(AR)). Sensors may track a satellite below 5 degrees, but must not incorporate it in the navigation position soluti on unless the accuracy is d emonstrated for satellites at the design m ask angle. The sensor m ay use a weighted position solution, w hich deweights satellites at low elevation angles based on the accuracy demonstrated for those elevation angles.
(i) For equipment that ass umes that GPS selective availability is activ e (assumed pseudorange accuracy of ≥33m r ms), the ionospheric, tropospheric, and airframe multipath errors can be considered ne gligible down to 2 degre es. The use of a mask angle below 2 degrees requires validation of the error models for these effects.
(ii) For GPS equipm ent that takes adva ntage of the discontinuance of selective availability (assu med pseudorange accuracy of <33m r ms), the aviation community has validated error m odels down to a 5 degree m ask angle (see RTCA/DO-229C, Appe ndices A and J). The use of a m ask angle below 5 degrees requires validation of the error models for these effects.
(iii) The accuracy must account for th e errors caused by receiv er noise and interference. This m ay be accomplished by calibrating the pseudorange accuracy w ith a measured signal-to-noise ratio, or by using a dem onstrated tracking accuracy at low elevation an gles. To dem onstrate tracking accuracy, the actual an tenna characteristics below 5 degrees m ust be used when dem onstrating com pliance to the perform ance characteristics defined in TSO-C129(AR) , RTCA/DO-229C or RT CA/DO-253A. This means that the received GPS signal power must be adjusted down to reflect the minimum antenna gain relative to -4.5dBic for a sta ndard antenna, while th e interference power cannot be adjusted dow n as the location of the interference em itter is unknown. Any associated limitations on the installation (e.g., standard ground plane) should be identified in the installation instructions.
(2) Prediction programs and ava ilability assessments intended to support flight planning functions must use the equipment mask angle except as follows:
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(i) In order f or the prediction program to take advantage of the discontinuance of selective ava ilability f or sa tellites b elow 5 degr ees, the p rediction program must use the dem onstrated track ing accuracy as described in parag raph 12c(1)(iii) ( including a ccounting f or the spec ific antenna characteri stics and effects of interference). TSO-C144 gain characterist ics may be used if the antenna is TSO-C144 compliant. Alternately, antenna ch aracteristics below 5 degrees m ay be identified, a specification for low-elevation angle performance defined, and the antenna m ust obtain a design and production approval accord ing to that specification. This m ay be accomplished by including the antenna as part of the GNSS manufacturer’s TSOA.
(ii) For en route, term inal and nonprecision approach operations, the predicted mask angle cannot be below 2 degrees.
(iii) For precision approach and LPV, th e predicted m ask angle cannot be below 5 degrees.
d. Interference Testing. The GPS and WAAS si gnal acquisition and track performance, W AAS message lo ss rate, in tegrity and ac curacy requ irements will be verified during m inimum signal conditions an d defined interference conditions. These tests must be conducted in acco rdance with th e test conditio ns and procedures sp ecified in TSO-C129(AR) or RTCA/DO-229C, as appropriate.
e. Prediction Programs. Unlike conventional ground- based navigation aids which have a direct correlatio n between a facili ty o utage (VOR f ails) an d the loss of an operational capability (VOR approach is unava ilable), GPS satellite failures have a nonintuitive impact on operational capability. The operator is responsible for considering the effects of satellite outag es during f light planning. Predictio n programs and availab ility assessments are essential to enabling the operator to fulfill this responsibility.
(1) The FAA plans to pro vide NOTAM service for all app roach operations using TSO-C145a or TSO-C146a equipm ent, and for GLS operations using LAAS equipment.
(2) For oceanic/remote operation b ased on GPS, a prediction p rogram must be provided to determ ine if GPS satellite outages have an operational im pact on oceanic/remote operations.
(3) For basic R NAV operations in Europe , a prediction program should be provided (see AC 90-96 for additional info rmation). Alternatively, basic R NAV operations can be supported by the ge neric Eurocontrol prediction tool (http://augur.ecacnav.com/).
(4) For non-precision approach operations using TSO-C129(AR) equipm ent, manufacturers m ust provide a prediction availability based upon RAIM. For nonprecision approach operations using GPS/ WAAS equipment, this capa bility should be provided to support GPS operations outside of the WAAS coverage area.
(5) Prediction Program Requirements. Th e following guidelines apply to any prediction program that is intend ed to assess the operational impact of satellite outa ges for flight planning purposes (it does not appl y to the prediction described in paragraph
(a)(3)(xv)4a of TSO-C129(AR)).
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(i) This prediction program can be prov ided on any processing platfor m (in the GPS equipment or not).
(ii) The prediction program software s hould be developed in accordance with at least RTCA/DO-178B/EUROCAE 12B, level D guidelines.
(iii) The program should use either a RAIM algorithm identical to that used in the airborne equipment, or an algorithm based on assumptions for RAIM prediction that give a more conservative result.
(iv) The pr ogram s hould c alculate RAIM availabil ity based on the sa me satellite mask angle used in the equipm ent, but not less than 2 degrees f or en route and terminal operations or 5 degrees for precision approach or LPV operations.
(v) The progra m shoul d have the capab ility to manually desi gnate GP S satellites which are expected to be out-of-service for the intended flight.
(vi) For en route predictions, the program should allow the user to select the intended route (including declared alternates) and the time and duration of the flight. The maximum outage of RAIM (and FDE, if applicable) should be provided, and any predicted o utages of navigation capability must be identif ied. Note that f or oceanic/remote operations, the m aximum duration of an FDE outage cannot exceed 34 minutes for RNP-10 operation (see Order 8400.12a) and 51 minutes for MNPS operation (see AC 90-94). For B-RNAV operations, th e maximum acceptable duration of a RAIM outage is 5 minutes (see AC 90-96).
(vii) For approach predictions, the program should allow the user to select the destination and exp ected time of arrival, and provide a RAIM availability p rediction over an interval of at least +15 minutes computed in interval s of 5 m inutes or less about the ETA.
f. Interface Requirements
(1) Interface to m ulti-sensor naviga tion system s. Equipment requirements applicable to the integration of a GNSS sensor with a multi-sensor navigation system are in TSO-C129(AR) (Class B and C equipment) and TSO-C115b.
(2) Interface to non-navigation systems. GNSS sensors may provide output to a variety of applications, including terrai n awareness and warning sy stems (TAWS), automatic dependent surveillance (ADS), and m oving map displays for situational awareness. The interface should b e specifie d, and com patible equipm ent iden tified to streamline installation approvals.
13. EQUIPMENT PERFORMANCE – STAND-ALONE NAVIGATION EQUIPMENT. In addition to the guidance in this paragraph, stand-alone navigation equipment incorporates an antenna and a sens or and should comply with the guidance in paragraphs 11 and 12 of this AC. T hese issues do not need to be revisited for equipment that holds a previously granted TSO aut horization under TSO-C129(AR), or m inor modifications to such equipment.
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