The GPS Balance Sheet

Certified GPS is a huge boon to safety, but the navigators quirks can make muddling through approaches an ordeal


Avionics manufacturers have a GPS navigator for every purpose and budget. At this springs Aircraft Electronics Association show in Atlanta, yet more models were announced, overhanging the market with unprecedented choices for owners.

GPS sales are clearly hot, but only a small percentage of the GA fleet is equipped with certified GPS. Our best estimate is not greater than 10 percent have IFR navigators.

Many owners are fence sitters still skeptical about the value of spending six grand or more for an IFR navigator that seems to perform only a bit better than a 10-year-old loran. The reasons are many: the cost of admission, lack of utility, beefs about operating complexity and worries about jamming and failures. Plus, with new product cycles withered to mere months, obsolescence-at-purchase is a legitimate worry for many buyers.

But are these fair assessments or just ill-informed hangar gossip? If you buy into IFR GPS now, what can you expect out of the deal? Is the complexity-of-operation issue real or just a straw man erected by the airport Luddites?

Most importantly, are there some not-so-obvious safety issues here?

Value is in the eye of the beholder – we cant tell you how to spend your money – but its possible to construct a safety balance sheet and we think it clearly tips in favor of GPS, although perhaps not decisively so for all owners.

VOR and ILS Rule
For long-term planning purposes, know this: VOR, DME and ILS – and to a lesser extent, NDB – still form the backbone of the National Airspace System. A decade ago, the FAA hoped GPS would shoulder ground-based navaids aside by now, but concerns about GPS accuracy and signal continuity and integrity have pushed that transition well into the future, if it ever happens.

Concerns about the fundamental shortcomings of GPS loom so large that the FAA was forced to commission independent research by the Johns Hopkins Applied Physics Lab last year to assess the risks. Early this year, APL concluded that GPS could become the sole means of approved navigation if the FAA enchanced its Wide Area Augmentation System with additional geostationary satellites (four are needed, says APL) and developed a local area augmentation program for Category II and III precision approaches.

Like many an FAA high-tech program before it, WAAS is behind schedule and, depending on whos crunching the numbers, over budget. Meanwhile, TSO C146, the technical spec for WAAS receivers, seems to shimmer mirage-like on the horizon, promised more than a year ago but still missing in action, except for a draft.

The FAA recently announced partnership agreements with Raytheon to develop LAAS systems for precision approaches, with an aggressive timetable thats supposed to deliver Cat I precision capability at selected airports by late next year.

While that timetable might be achievable for demonstration purposes, many industry insiders are skeptical that actual field deployment will happen in the short term. So are we. Therefore, ILS still rules for the foreseeable future.

The Department of Transpor-tations most recent Federal Radio-navigation Plan calls for the phaseout of VOR/DME to begin in 2005, with completion by 2010. Conventional ground-based ILS is supposed to follow a similar phaseout schedule. The fact that industry leaders such as Garmin and IIMorrow (now UPS Aviation Technologies) are introducing new and sometimes integrated VOR and ILS receivers shows that commercial interests arent convinced that GPS sole means is around the corner.

Balance sheet for owners: VOR and ILS are still legal must haves for IFR flight, meaning its far too soon to plan the sole means GPS panel. But GPS can effectively substitute for DME and NDB and probably should for some owners.

Off the Airways
Setting aside the legalities for a moment, owners who opt for certified GPS tell us that it delivers utility on two counts: Time saving off airways navigation and, increasingly, instrument approaches into airports that previously had none and/or lower minimums to many airports that once relied only on NDB cloud busters.

Truth be told, for off-route navigation, pilots and controllers draw little distinction between IFR-approved GPS, VFR (and portable) GPS and loran. Whether the equipment code is an officially blessed /G or not, controllers routinely assume that any airplane can navigate direct at any time and, if operationally beneficial, theyll issue the appropriate clearance.

Not to encourage scoflawing, but theres little safety risk in navigating off airways with a $500 portable GPS. Off airways clearances require radar contact anyway, so the risk of truly losing your way is remote. If the legalities trouble you, convert your non-approved GPS navigation into a radar vector.

Whether with approved gear or not, direct navigation with GPS is an order of magnitude more accurate than VOR, especially at significant distances from the station. Moreover, GPS databases contain all airports, navaids and intersections, improving enroute flexibility and providing options not available with VOR and DME.

Balance sheet: For enroute purposes only, GPS is a definite safety enhancer. In practical terms, it makes little difference whether its certified or not.

On the Approach
If GPS shines unchallenged in any arena, its on non-precision approaches. But that advantage means little if you dont actually fly the approaches. Certainly some percentage of GPS owners – how large is anyones guess – use these expensive boxes purely to navigate fix-to-fix and not to fly approaches.

These pilots find the receivers simply too complex and baffling to set up in the heat of IFR battle. Thats not necessarily a fault, but its a shame to ignore GPSs growing capability as an approach aid.

Just look at the numbers: As of spring 1999, there are 2,029 standalone GPS approaches plus 3,533 conventional non-precision approaches that can be flown as GPS overlays. (As more standalones come on line at the rate of about 50 a month, overlays to the same runway or airport are being withdrawn.)

The majority of GPS standalones are at airports that already have some type of approach, but a few – probably less than 10 percent – are at airports that previously had no approach at all.

This is a plus on two counts – you can operate IFR into those airports and they become available for emergency diversions – and a negative on one count: IMC ops into small, poorly lighted airports with short runways definitely raise the risk level slightly, especially at night.

For safety and accuracy in IMC, ILS remains the Alpha male but GPS approaches run a close second, besting VORs accuracy in most cases, the exception being on-airport VOR procedures. NDB isnt even in the same universe in terms of potential fix displacement errors.

Consider the possibilities from the cockpit point of view: On a typical VOR approach, with the station 5 miles from the airport, one dot of CDI deflection is about 1000 feet of lateral displacement; the error increases with distance from the station. On a GPS in approach mode, the same deflection is 360 feet, everywhere. There is no distance-from-station penalty.

This translates to a much greater likelihood of arriving at the missed approach point on the runway centerline, which improves the chances of seeing the runway rather than fishing for it in poor visibility.

In most cases, that kind of accuracy also translates to lower MDAs at the majority of airports where a standalone replaces or supplements an overlay approach. The reasons are two: Not being constrained by ground navaid position, GPS approaches can be readily aligned with the runway. And because GPS TERPs obstacle protection areas become narrower toward the runway, they tend to encompass fewer obstacles, thus the minimum descent altitude is likely to be lower.

Lower MDAs improve the chances of success on a non-precision approach. Further, with the missed approach point determined by hard coded waypoint, not timing, theres no ambiguity about when to abandon a no-hope attempt for the runway. If you operate frequently into airports with only NDB approaches, a GPS receiver will fundamentally swing the safety equation in your favor.

But there is a dark side. All this swell accuracy comes at price, and its not just money. The latest generation GPS navigators – particularly the Garmin 430 – are simplified compared to the first-generation TSO C129 receivers. But they are still high workload devices that will tax the proficient pilot and overwhelm a rusty one.

Balance sheet: Reliable, improved approach accuracy with a higher degree of safety is there for the taking, but only for the pilot willing to stay sharp on the operation of the box.

Substitutions, Emergencies
Apart from approaches, the thing GPS is best at – whether approved for IFR or not – is finding an airport accurately in a hurry. Even the most rudimentary portables have nearest airport features that instantly display bearing and distance to the closest airports, along with critical data for commencing an instrument or visual approach.

IFR navigators are capable of quickly loading approach waypoints for a desperation play for the runway in dire straits. Approved or not, any GPS will get you close to a runway in any weather, under emergency circumstances. No other navigation system can match this capability, although loran comes close.

In addition, the FAA recently approved GPS as a substitution for ADF and DME, including for ADF-required notes on ILS approaches. Again, as a stand-in for ADF, theres no comparison; GPS is simply a far more accurate means of finding the fix and although GPS is susceptible to interference, outages (rarely) and RAIM flags, its not weather tender in the way ADF and loran are.

GPS in lieu of DME is a toss-up. The two systems are comparable in accuracy but DME is robust to the point of being bulletproof. On some ILS-DME approaches, GPS is not yet a legal substitute for DME and for some pilots, that will be argument enough to keep DME in the panel.

Balance sheet: GPSs accuracy makes it unparalleled as an emergency navaid and substitute for ADF. Its less convincing in place of DME but certainly suffices for owners who have bought it.

The Bottom Line
Pity the poor avionics buyer who mulls over a GPS purchase for six months to a year, finally decides on a box only to discover that another manufacturer has rolled out two more models at the same price with more features.

Unfortunately, that blistering pace of product development may not slow in the short-term future. The good news is that any navigator bought today – or even since GPS was approved for IFR five years ago – will still function to its full potential for the foreseeable future, including non-precision approaches. Its just that each round of new products will bring more capability, including the potential for GPS precision approaches within two to three years.

How does this impact the safety balance sheet? It doesnt. The instant obsolescence issue is primarily a feature/value issue. Its a safety issue only insofar as the newer boxes may be more intuitive to operate and therefore the pilot is less likely to screw up. A GPS navigator bought today will offer the same benefits 10 years from now but will be rapidly outpaced by boxes with more gizmos.

Adding up the balance sheet: A panel-mount IFR GPS is a definite safety enhancer in terms of position awareness and navigation and approach options. Except for ILS, nothing else comes close in accuracy.

But an IFR navigators approach utility is next to useless unless youre willing to train and stay proficient in setting it up on the fly, with regular practice and use in IMC, if possible.

In fact, due to the quirks most receivers have, some owners tell us youre worse off trying to muddle through approach set-up with a strange box than you would be opting for a conventional procedure, even an NDB. Save your money and stick with your old loran or a portable GPS in that case.

Otherwise, theres little question that IFR-approved GPS really is delivering on its promise to open up every dark corner of the airspace system to IFR operations, albeit slowly. The era of certified GPS as a must-have hasnt quite dawned yet, but you can see it from here.

Also With This Article
Click here to view “GPS Approaches: Why They Go Awry.”
Click here to view “GPS and Lower MDAs.”


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