WAAS and precision-GPS equipment, procedures and regulations are covered in TSO C146a. The technical and operational benefits of a WAAS interface are many, and the new WAAS GPS engines, processors and software are reliable and highly accurate. WAAS not only enhances GPS accuracy but also corrects the errors in GPS signals caused by ionospheric effects.

WAAS provides less than 1.5-meter horizontal accuracy and 3-meter vertical accuracy. Class 3 WAAS sensor capability means no other navigational gear is required. If this sounds like putting all the eggs in a single basket, it is. These owners are relying completely on quality installations.

Operational WAAS also doesnt require user to check RAIM-receiver autonomous integrity monitoring-which was an early

GPS system for alerting a pilot if the GPS signal quality had gotten too poor for navigational use. All WAAS GPSs use Fault Detection and Exclusion (FDE), which detects satellite failure and ignores the failed satellites. This ultimately affects the signal integrity and performance of the unit, and may mean the box will only offer you an LNAV approach with no vertical guidance rather than the LPV you were hoping for. Thats because the boxes are smart enough to “downgrade” a WAAS GPS approach back to a basic non-precision GPS approach 60 seconds prior to reaching the FAF. This possibility is why the FAA is so particular of the placement of mode annunciation that alerts the pilot to GPS messages.

If a WAAS outage is predicted and NOTAMd, then you should use the FDE program in the Garmin desktop simulators, the on-board RAIM prediction of the GPS itself, or using tools such as www.raimprediciton.net to see if youll have adequate GPS signal at your destination at your planned arrival time. Weve found that downgrades do happen, but not often and WAAS outages are a truly rare event.

On the flip side, WAAS GPS receivers can be sensitive to stray RF noise. Strobe noise, com radio interference, pulse noise and cell phones are a few enemies. We recall a certain Cessna Skyhawk with a WAAS unit that would lose satellite reception when flying in wet clouds. Seems the location of the antenna-directly aft of the windscreen-was being affected by precipitation static. Moving the antenna back another foot solved the problem. Garmin engineers admit that WAAS for small airplanes can cause some challenges given the close quarters of installed antennas and other systems.

What were once called GPS approaches are being renamed

RNAV approaches, with the non-precision GPS approach minimums called LNAV (lateral guidance). The addition of WAAS means the unit can display a glidepath the way youd expect with an ILS approach. These come in two official flavors: the LNAV/VNAV and the LPV. Without getting into the details, both function the same way from a pilots perspective-you fly them to a decision altitude (DA) just like an ILS-but the LPV will let you fly to lower minimums if your GPS offers it. A properly interfaced WAAS panel-mount navigator can fly all of these procedures.

Garmin has also added “advisory guidance” to many LNAV approaches that they annunciate LNAV+V. This causes some confusion as its really just a path you can follow to a reasonable visual descent point, but the approach is still technically a non-precision approach with an MDA, not a DA. The FAA has an entire section dedicated to WAAS procedures in AIM section 1-1-20. Its a must read for existing owners and

for those contemplating WAAS GPS upgrades.

If youve flown RNAV approaches youll know that the signal quality and resultant needle action is far more stable than a traditional ILS approach. Theres no scalloping that can cause grief with a coupled autopilot, glideslope oscillation as you get close to the pavement. Given our choice between flying an ILS or an LPV to the same runway, the choice is a no-brainer.

Its no secret that Garmin owns the GPS market with two of the most popular panel units ever made: the GNS 400W- and 500W-series navigators. These are WAAS-equipped right out of the box. The G1000 integrated suite has LRU GPS systems that are essentially remote GNS units. Early G1000s were not WAAS capable and upgrading them will empty the wallet. More on that later.

The first panel navigator with WAAS was the CNX80/GNS480 series, but Garmin pulled the plug on them after they bought UPS-AT. Remaining units on the used market are capable but orphaned machines. Garmin-AT still supports these systems with open arms, but we would have to think long and hard about dropping our coin on one at this stage.

Honeywells KLN94 standalone color GPS doesnt have a WAAS engine and it probably never will. You can still purchase a Garmin GPS155XL and com-equipped GNC300XL. They cant be upgraded to WAAS either, and adding the accessories and parts to satisfy the old C129 (A1) standard will drive the price up to a questionable level.

There are plenty of used GNS430 and 530 units on the used market that can be upgraded to WAAS. Garmin does the upgrade for $2995, plus $800 if the trip to the factory reveals any discrepancies that need to be fixed. If an upgrade is your game plan, we say dont pay more than four grand for the used GPS to start. Remember, youll still need to buy an appropriate navigational indicator. In some applications, youll need other pricey accessories that can easily snowball an otherwise routine although complex installation.

Since WAAS GPS outputs lateral, vertical and nav flag data youll need a navigational indicator. There are several indicator options here. Garmins GI106A (made by Mid Continent Instruments) is the stand-alone indicator for displaying the data, as well as analogue VOR/LOC/glide-slope information which is switched internally by the navigator.

Bendix/King has the KI209A that can work with Garmin navigators and most HSI systems will work, too. Of course, you can go all out and display the WAAS data on an Aspen PFD, Garmin G500/600 PFD, Sandel EHSI or other EFIS display. If you have an older Bendix/King EFIS 40, the software can be upgraded to properly play the GPS glidepath data. The trip to Honeywell will cost a few thousand.

Nearly any autopilot that couples to a traditional ILS signal can fly a GPS glidepath. Diligence is critical when wiring this interface, however, because changes to autopilot wiring modification of the autopilot computer may be needed. Some inexperienced shops have turned out upgrades where autopilots wont fly the GPS glidepath.

Some autopilots arent fully capable. S-TECs popular System 30 and 50-series systems cant fly any glideslope, ILS or GPS. They will accept roll steering commands with the ST-901 GPSS steering system, and in our view this is a worthy option as it

greatly improves autopilot tracking ability, and adds the ability to anticipate turns and fly procedure turns and holds. For non-S-TEC autopilots there are generic roll steering systems made by Icarus and DAC International. External roll steering add-ons can add several thousand dollars to an installation and wont be included in a bid for a basic installation.

Aspens EFD1000Pro or Garmin G500/600-series PFDs have integral GPS roll steering activated with a push of a button. In this case, the concern is more how well the PFD itself plays with your autopilot.

Pilots are growing accustomed to this hands-crossed approach automation. Its important to discuss the autopilot interface with the installing shop so you understand what capabilities you will or will not have, after all the wires are crimped.

While your non-WAAS Garmin is taking its $2995 factory vacation, the avionics shop will replace the legacy GA56 GPS antenna with a new WAAS antenna that sports the same mounting holes but has a wider footprint. Accessing this antenna can be easy or it can be bloody hell, depending on the design of the headliner and overhead. Single-piece headliners like those found in many Beechcraft models could be the worst since theyre susceptible to sagging after removal and reinstallation.

Any old, single-shielded coaxial cabling must be replaced with twin-shielded, low-loss cable. Owners underestimate this task

and are often shocked by the labor price.

A remote mode annunciation might be required if the GPS is too far from the pilots field of view. The shop will need to access the units main connector to wire lamps which could mean sizeable teardown and associated expense.

The paperwork and final testing chase shouldnt be underestimated as its billable time. The existing GNS Flight Manual Supplement is replaced with an updated version, and final testing includes an evaluation flight. If the aircraft is on the AML for the installed system and there were no deviations to the criteria stated, there should be no need for an FAA field approval.

While the lucky might escape a complete WAAS upgrade for under $4000, expect a non-glass cockpit project to be at least $5000 including the cost of the box upgrade. No matter what, be sure the shop offers an accurate proposal after they evaluate the existing installation.

The step up to WAAS in a glass cockpit has been a controversial and heated topic for owners because the costs are shocking. Garmin and Hawker Beechcraft offer an upgrade path for non-WAAS G1000 G58 Baron and G36 Bonanza aircraft. Each GIA63W LRU for the suite is $11,495 and Garmin offers a $1750 credit for the legacy GIA63 thats removed. Theres also the GDU1045 MFD thats exchanged for $11,595 minus $1000 credit for the old MFD. Thats just north of $30,000 in parts before a technician even picks up a screwdriver. Cessnas Service Bulletin SB07-34-07 lists the GPS/WAAS modification kit for upgrade at $18,295 for single-engine G1000 models. Manpower is listed at approximately 11.5 hours.

Any Avidyne/Garmin-equipped Cirrus aircraft not equipped with WAAS will need the standard GNS factory upgrade, but some will need extra antenna work performed by Cirrus service centers. Depending on the software level of the Avidyne Entegra displays, factory software upgrades will be required. Were talking thousands here, too. While its hard to imagine such capable glass-cockpit aircraft without WAAS the upgrade pricing is staggering to say the least.

Summarizing these findings is easy: If you want WAAS, there arent that many options and all cost some real coin. For new installations, Garmins 400W and 500W units set the standard. A new installation of a 430W starts at around $12K while a 530W demands around $15K. Upgrading an existing Garmin shaves some thousands. But these units still wont show airways or high-resolution weather. For airplanes that havent seen a radio upgrade in years the 430W is a good value, in our view.

You could buy Bendix/Kings KLN94 but were getting vibes that this is a dead-end product without any WAAS potential. The same could be said for Garmins GPS155XL and 300XL. The long-promised Honeywell KSN770 NMS system will have WAAS and could threaten the Garmin navigator monopoly, but it could be another year before we see it. A lot can happen in a year. Well keep you posted.