The ultimate access path: without it you miss out.
Can you answer the following: a) What type of instrument approach leads in shear numbers? b) What is the fastest growing type of instrument approach? c) What instrument-approach type offers minima as low as 200 feet ceilings- and visibility of a half-mile without a ground system to support it? d) What instrument- approach type will dominate approach options more and more in the coming years?
The right answer to all of the above questions is: The Lateral Precision with Vertical Guidance approach or LPV approach. LPV is a type of approach programmed and enabled for use by aircraft flying with a GPS enhanced by the Wide Area Augmentation System (WAAS). If you haven’t heard of this- and you do not have a WAAS GPS- you’re missing out.
ADS-B Out fulfillment is far from the only benefit that accrues from flying with a WAAS GPS. A WAAS GPS can be part of that fulfillment for the future while delivering here-and-now benefits too.
THE WAAS ADVANTAGE
So far- with the majority of ADS-B adapters embracing WAAS GPS as the onboard data source- it makes sense to examine the benefits. Created to enhance the accuracy of the standard GPS signals (without degrading Selective Availability turned on) WAAS employs a pair of geostationary satellites working with a network of about two-dozen ground stations.
With positions precisely surveyed and known to the system- the ground stations broadcast a correction signal to WAAS receivers based on distortions noted by the ground stations between what they know their location to be and where the basic GPS signals say they are.
The result is position accuracy to better than four meters – less than 15 feet laterally- less than 15 feet vertically and with velocity calculated to within a fraction of a knot. WAAS makes your GPS a primary navigational instrument – not a supplemental source as were approved non-WAAS GPS navigators.
This means you can file alternate destinations using the WAAS GPS as navigation and approach source. Activated nationally in 2003- WAAS GPS received an FAA performance standard in 2008 and today supports special WAAS LPV approaches exceeding 2-600 in number and growing at a rate of about 500 per year.
At many airports LPV provides guidance equivalent to the gold-standard Instrument Landing System of Localizer and Glideslope – and it’s in access to these and other improved instrument approaches that ADS-B adapters may find their greatest benefits.
LPV approaches closely mimic the gold-standard ILS approach- with one huge difference: no ground-based ILS hardware. Instead- the GPS receiver itself generates its own localizer and glideslope paths based on the approach created and programmed into the navigator.
LPV approaches can provide minima down to 200 feet and a half mile if the airport equips itself with appropriate lighting and marking; 300-foot ceilings and a half-mile require less ground infrastructure. And for many of the other new WAAS GPS-based approaches- little to nothing at all is required aside from runway lights- end market lights or such.
These approaches improve on- or replace non-precision approaches based on everything from NDBs to VORs and DMEs. In many cases they improve on existing GPS non-precision approaches- if they can’t deliver a full LPV approach. Additionally- WAAS GPS holds potential for some aircraft to qualify for operations using Required Navigation Performance (RNP) standards.
RNP opens up arrivals and departures of complexity and dimensions unsupportable by most other navigation sensors – providing lower minima in many a terrain- or traffic-constrained situation.
RUNWAY ENDS: THE KEY PARAMETER
For decades the gold standard in instrument approaches has been the Instrument Landing System. A technology based on two different VHF radio signals- the ILS signals are detected by a VHF navigation receiver. First- the localizer signal guides the pilot to align the aircraft with the runway centerline; second- the glideslope signal provides an invisible glide path set up to take the aircraft a few hundred feet beyond the runway threshold. ILS signals (when functioning optimally) are so precise that typical approach minima of 200 feet above the ground with 3/8th of a mile horizontal visibility is nominal.
Some ILS approaches are designed for aircraft equipped to land in 100/3/8th- while still others can land in zero-zero conditions – if- that is- the aircraft and the ground systems are appropriately equipped and the crew is trained for these approaches. So with such a stellar system available- why don’t all airports boast an ILS system to all their runway ends? The answers to this question form much of the basis for the growing appeal of the LPV approach.
First up- ILS installations are somewhat costly – about $1.5 million per runway end is today’s nominal price. A further half million will need to be spent for the installation per runway end. Many a small-community airport simply lacks the funds to invest in a single ILS- let alone the $4 million you’d need to cover both ends of one runway.
And federal Airport Improvement Program money is contingent on the community airport demonstrating the need (namely traffic volume). If the airport doesn’t attract traffic levels above a threshold- the FAA won’t approve an application for a federally funded ILS install.
It’s in the installation itself that further issues arise. First- since there’s a limited number of frequencies available for the two complimentary transmitters of an ILS- proximity to other ILS systems plays a role – as does frequency pollution from other navigation aids.
The system must be installed properly- with clear areas and no terrain issues that can interfere with the ILS signals – or worse- cause them to mislead the ILS receiver. So site suitability and preparation becomes a key issue. Many a runway end suffers with a combination of terrain challenges- frequency pollution and a lack of money available to upgrade it. Fortunately- none of these issues directly impact the creation of a LPV approach.
THE ESSENCE OF THE LPV APPROACH
For a WAAS-based LPV approach- terrain preparation or blockage isn’t much of an issue – as long as a clear glidepath can be flown. There’s also no ground stations involved. Only lights and runway markings must match up to the level of the LPV approach. Otherwise- the system is almost entirely space-based.
‘Almost’ forms an important part of the previous sentence- because WAAS is not an entirely space-based system. WAAS uses ground-based stations that know- to within inches- their exact location on the planet. Those ground stations compare the accuracy of GPS signals received from the satellite constellation and- through a pair of geostationary satellites- send corrected signals to cockpit GPS navigation receivers.
The accuracy levels attainable actually exceed those of the gold-standard ILS. So with no ground-equipment to purchase- no site-preparation work required- and no frequency issues in play- an LPV approach can be surveyed- created- proven- implemented and published as a legal approach for about 1/30th of the cost of an uninstalled ILS. Compared to an installed ILS the cost of an LPV approach amounts to about 2.5 percent. In terms of actual figures- according to FAA and MITRE Corp. that’s $50-000. The goal of the FAA is to have an LPV approach available for every runway end at every public airport in the national system.
Even private airports may find the costs acceptable. The only sticking point for some is that to get the approach minima down to the same level as an ILS requires runway lights- markings- and terrain clearance on the glideslope. Even without- however- approaches to 300 feet and less are abundant.
WHAT DOES IT LOOK LIKE?
Flown on a system such as Honeywell’s KFD 840 primary flight display – or with any other appropriate display indicator – the activated LPV approach works through the WAAS GPS to drive a localizer-type indicator and guide the pilot to the runway centerline.
In flight it essentially looks exactly like a Localizer indication – except for the icon designating “LPV” as the approach type. A second glideslope-like bar guides the pilot to fly on the glide path that will take the airplane to about the same point on the runway as the old ILS (often a little closer to the runway threshold- but not so close as to create a risk of touching down short of the runway end).
Essentially- the LOC marker moves left or right as the plane drifts right or left of the centerline while the GS marker shows whether the aircraft is above or below the optimal glidepath to the runway. All in all- the LPV approach looks pretty much exactly the same as the ILS; only the designation changes.
Remember: the Lateral Precision with Vertical Guidance approach or LPV approach uses only WAAS-corrected GPS signals and the aircraft’s on-board WAAS GPS navigator working with an appropriate navigation indicator. No ground systems are needed.
While the LPV approach may be the ‘new kid on the block’- LPV approaches already outnumber ILS approaches – and WAAS GPS offers still more to enhance any form of flying. For example- the WAAS GPS and other equipment can form the foundation to win approval for Required Navigation Performance operations (RNP).
RNP approaches and- in particular arrival and departure procedures- require less precision than an LPV approach- but enough to make the qualification demanding to obtain. Essentially RNP arrivals and departures use the accuracy benefits of WAAS working with FMS or autopilot and other guidance to keep the aircraft on a precisely defined path to- or from an airport – precise to within a few hundred feet.
RNP arrivals and approaches are being created to help streamline traffic in busy airspace- as well as to provide safe paths through airspace complicated by obstructions and/or terrain – but without the need for the less-accurate VHF navigation beacons we call VORs; they aren’t accurate enough for the job. WAAS- however- delivers accuracy levels of about 20 feet.
The advent of WAAS has also enabled the creation of other approach types that- like LPV and RNP- are ultimately designed to replace thousands of non-precision approaches- NDB approaches and the costly- complicated series of arrivals and departures tied to the ground network of navigation beacons.
VNAV- RNAV- LPV and RNP combined offer the benefits of access impossible to match with the old system of navigation beacons- cockpit receivers and panel indicators. (More than a few pilots of my acquaintance have been in contact to ask about LPV approaches after finding that they couldn’t get into an airport because the runway lacked an ILS approach- or because conditions were below available non-precision and NDB approaches – and they learned their aircraft lacked the WAAS GPS they could have used to fly the available WAAS-based approach.)
We’ll close out this feature in the same way we started it… with a selection of questions: Do you know what navigation system you can select to gain several thousand new runway ends to access in instrument conditions? Are you aware of what navigation system can move you toward RNP capability- or what navigation system can improve on your direct-to capabilities beyond your current IFR GPS? What navigation system can serve as the position-data source for an ADS-B Out system?
Again- there’s one answer to the questions: A WAAS GPS.
WAAS APPROACH TYPES
(In increasing levels of accuracy)
• LNAV: The most basic non-precision approach that resembles the kind of pre-WAAS GPS approaches you may have been flying all along.
• LNAV + V: This approach has an extra feature in that the glideslope needle comes alive. This gives you a stabilized approach- but does not give you obstacle clearance.
• LNAV/VNAV—or L/VNAV: This approach gives you lateral and vertical guidance down the final approach course. This is not a precision approach like an ILS- but you are assured of clearance above terrain and obstacles.
• LPV: This is the most accurate WAAS approach- giving you lateral and vertical guidance to decision altitudes as low as 200 feet agl. On an LPV approach- accuracy is 16 meters laterally- and four meters vertically. Currently- there are nearly 2-700 LPV approaches.
CREDIT: Dave Higdon from AVBUYER