TSO-C145 WAAS GPS Standard: Approaches, ADS-B, and Certification

TSO-C145 is the FAA’s minimum performance standard for airborne GPS sensors that use a satellite-based augmentation system such as the Wide Area Augmentation System. Equipment built to the TSO-C145 WAAS GPS standard delivers the accuracy and integrity needed for instrument flight, including approaches with decision altitudes as low as 200 feet above the runway. The technical requirements come from RTCA/DO-229D, and the current revision of the standard is TSO-C145d.¹

What the Standard Covers

A Technical Standard Order is an FAA document that sets the minimum performance a piece of aviation equipment must meet before it can be marked TSO-approved and installed on a civil aircraft. TSO-C145 applies specifically to GPS/SBAS sensors, classified as Class Beta equipment. These are the receiver and signal-processing components that take in GPS satellite signals along with SBAS corrections and output a refined position solution.²

RTCA/DO-229D, Change 1 is the underlying performance specification. It spells out the signal-processing algorithms, accuracy thresholds, integrity monitoring, and environmental test conditions the equipment must satisfy.² The standard has been revised several times (C145a through C145d). The FAA has not withdrawn authorization for earlier revisions, so manufacturers holding approvals under older versions can continue production.³

TSO-C145 Versus TSO-C146

This distinction trips up pilots and aircraft owners regularly. TSO-C145 covers only the GPS/SBAS sensor. TSO-C146 covers a more complete package: either an integrated sensor and navigation computer combination (Class Gamma) or an “ILS look-alike” configuration (Class Delta) that can drive course deviation indicators the same way a traditional ILS receiver does.²

Most panel-mount GPS navigators in general aviation cockpits hold a TSO-C146 authorization because they combine the sensor with a display and navigation computer in one unit. A standalone sensor that feeds position data to a separate flight management system or display would carry a TSO-C145 authorization. Both TSOs reference the same DO-229D performance standard, so the underlying accuracy and integrity requirements are identical. The difference is about system architecture, not signal quality.

What WAAS Adds to GPS

Standalone GPS is accurate enough for many purposes, but it lacks the integrity monitoring needed for instrument flight. You need to know not just where you are, but how confident the system is in that answer, and you need an immediate alert if the answer goes bad. That is what WAAS provides.

WAAS uses a network of precisely surveyed ground reference stations across North America that continuously monitor GPS satellite signals. The stations detect errors caused by ionospheric disturbances, satellite clock drift, and orbital inaccuracies, then calculate correction messages. The corrections are uplinked to geostationary satellites, which broadcast them on the same frequency as GPS. A TSO-C145 or TSO-C146 receiver applies these corrections automatically, tightening its position solution from roughly 10-15 meters of standalone GPS accuracy down to about 1-2 meters.

The more important improvement is integrity. Older non-WAAS GPS receivers relied on Receiver Autonomous Integrity Monitoring (RAIM), which cross-checks signals from multiple satellites to detect a faulty one. RAIM works, but it provides only horizontal error checking and cannot deliver the vertical integrity needed for approaches with vertical guidance. WAAS delivers both horizontal and vertical protection levels, continuously computed and compared against the alert limits for the current phase of flight. When a WAAS receiver is operating within the SBAS coverage area, it can also revert to Fault Detection and Exclusion (FDE) if WAAS service degrades, using whichever method provides the better protection level.²

One practical benefit pilots notice immediately: WAAS eliminates the requirement to perform preflight RAIM prediction checks. With a WAAS-equipped receiver operating in WAAS coverage, that step goes away. You should still check NOTAMs for WAAS outages.⁴

Approaches You Can Fly

WAAS-capable GPS equipment certified under TSO-C145 or TSO-C146 unlocks the full range of RNAV (GPS) approach procedures. The differences among them come down to where the lateral and vertical guidance originate and how precise they are.

LNAV

Lateral Navigation approaches provide only left-right course guidance. The pilot descends to a Minimum Descent Altitude using a step-down or dive-and-drive technique. Any IFR-approved GPS can fly LNAV, including older non-WAAS units, but with non-WAAS equipment a preflight RAIM check is required.⁵

LNAV/VNAV

These approaches add vertical guidance, giving the pilot a glidepath to follow rather than a series of altitude restrictions. The vertical path can come from either WAAS or a barometric altitude source. When baro-VNAV generates the glidepath, temperature limitations may apply because barometric altitude readings shift in non-standard temperatures. LNAV/VNAV minimums are published as a Decision Altitude rather than an MDA.⁵

LP

Localizer Performance approaches use WAAS-enhanced lateral guidance without vertical guidance. LP procedures are published where terrain or obstructions prevent an LPV procedure from being developed. Because WAAS narrows the obstacle clearance surface compared to LNAV, LP approaches can often achieve lower minimums than LNAV at the same airport, even without a glidepath.⁶

LPV

Localizer Performance with Vertical Guidance is the primary payoff of the standard. LPV uses satellite-derived lateral and vertical guidance, with angular course sensitivity that increases as the aircraft approaches the runway threshold, behaving much like a traditional ILS localizer and glideslope. LPV decision altitudes can go as low as 200 feet above the runway, matching ILS Category I minimums.⁷ LPV approaches can be published at airports with no ILS infrastructure at all, because the guidance comes entirely from satellites.

Misleading LP or LPV guidance is classified as a severe major to hazardous failure condition, which is why the integrity requirements for these approach types are so stringent.³

Serving as an ADS-B Out Position Source

Since January 1, 2020, aircraft operating in most controlled airspace in the United States, including Class A, B, and C, within 30 nautical miles of major airports, and generally at or above 10,000 feet MSL in Class E airspace, must be equipped with ADS-B Out.⁸

An ADS-B Out transmitter alone is not enough. The transmitter needs a high-integrity GPS position source that meets the performance requirements of 14 CFR 91.227. The FAA recommends a WAAS GPS compliant with TSO-C145 or TSO-C146 for this purpose because these units provide sufficient performance to satisfy those requirements.⁹ If you are upgrading avionics for ADS-B compliance, a TSO-C145 or TSO-C146 GPS source handles both the ADS-B position requirement and WAAS approach capability in one installation.

Installation and Certification

Installing GPS/WAAS equipment in an aircraft is a major alteration under 14 CFR Part 43. Changes to the basic design of navigation equipment approved under a Technical Standard Order that affect performance characteristics fall within the regulatory definition of a major alteration.¹⁰

The standard path to approval is a Supplemental Type Certificate (STC). An STC documents that a specific GPS unit has been evaluated and found compatible with a particular aircraft make and model, covering wiring, antenna placement, and integration with other avionics. For most popular aircraft and GPS combinations, an STC already exists. When no STC is available, the installation can be approved through a field approval process using FAA Form 337, which documents the major alteration and requires review by the local Flight Standards District Office.¹¹

Regardless of the approval method, the installation must include a revised Aircraft Flight Manual Supplement or Pilot’s Operating Handbook supplement. That document specifies the operational limitations of the installed equipment and lists exactly which approach types (LNAV, LNAV/VNAV, LP, LPV) are authorized. The aircraft logbooks must be updated to reflect the alteration.

Unlike older navigation installations, GPS/SBAS equipment carries no limitation requiring other positioning and navigation systems to remain onboard. The FAA still encourages operators to retain backup navigation capability against GPS outages or interference.² With GPS/SBAS equipment you can also flight plan an alternate airport using an RNAV approach to LNAV minimums rather than requiring a ground-based approach at the alternate.³

1
Federal Aviation Administration. AC 20-138D – Airworthiness Approval of Positioning and NavigationIses
2
Federal Aviation Administration. AC 20-138D – Airworthiness Approval of Positioning and Navigation Systems
3
Federal Aviation Administration. AC 20-138B – Advisory Circular
4
AOPA. RAIM Issue Brief
5
Federal Aviation Administration. RNAV (GPS) Approaches
6
Federal Aviation Administration. Wide Area Augmentation System
7
Federal Aviation Administration. Required Navigation Performance (RNP) Approaches (APCH)
8
eCFR. 14 CFR 91.225
9
Federal Aviation Administration. Installation
10
eCFR. Appendix A to Part 43, Title 14
11
Federal Aviation Administration. Field Approval Process