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When it comes to GPS approaches, not all devices are created equal. This is especially true for older, non-WAAS GPS receivers. While flying a GPS approach seems simple, it can actually be one of the more complicated approaches you’ll fly. Understanding GPS approach minimums is critical to a successful approach.
Early GPS Approaches
In a way, GPS approaches used to be simpler. The first GPS approaches just overlays of existing approaches with the same minimums. For example, in the mid-90s, when I started flying, an approach could be named “NDB or GPS RWY 6” or “VOR or GPS RWY 24.” As the FAA began to phase out NDBs, they would rename these overlay approaches “GPS RWY 6,” but the approaches still had the same fixes and minimums.
Even when dedicated, stand-alone GPS approaches began to appear, they only listed two minimums, one of them the circling minimum. So if you were flying a GPS approach back in then, you just had to decide if you were straight-in or circling and pick the appropriate line on your approach plate. Determining your Minimum Descent Altitude was no more complicated than for a VOR or NDB approach.
Like VOR and NDB approaches, the Height Above Touchdown (HAT) for these new GPS approaches varied with the location as each airport environment has different obstacles. VOR based approaches have a 250′ Required Obstacle Clearance (ROC), while NDB based approaches have at least a 300′ clearance. Standalone GPS approaches also have a 250′ ROC, but with their better lateral navigation, they would often have lower minimums than ground-based navaid approaches. Getting down to 250′ above the ground, however, was unlikely to happen. The airport would have to have no nearby obstructions at all.
Then in 2000, the FAA began to certify GPS approaches with vertical guidance. These approaches used the barometric altitude information from the aircraft’s altimeter to calculate a vertical path for the approach. This allowed properly equipped aircraft to fly GPS approaches with a glideslope similar to an ILS. Instead of an MDA, LNAV/VNAV approaches have a Decision Altitude like an ILS. Note that in spite of having a glideslope and DA, these approaches are still considered non-precision.
You can still fly an LNAV/VNAV approach without the glideslope (making it an LNAV approach), just as you can fly a localizer approach without the ILS glideslope. That means that there were now three minima lines to choose from: LNAV/VNAV, LNAV, and Circling. For most instrument pilots, this wasn’t a problem because it mirrored the ILS approaches that we were already used to flying.
With VNAV, GPS approach minimums are still dictated by the same 250′ obstacle clearance as other non-precision approaches. However, sometimes obstacles farther away from the airport can cause an odd situation where the LNAV minimum is lower than the LNAV/VNAV minimum. For example, on the RNAV (GPS) RWY 13 approach into Harrisburg, Pennsylvania (MDT), the LNAV/VNAV decision altitude is higher than the MDA for the LNAV and Circling minimums (more on this below).
The Introduction of WAAS
In 2003, the FAA approved the use of the Wide Area Augmentation System (WAAS) for use in general aviation. WAAS is a form of Satellite-Based Augmentation System (SBAS) that uses ground stations to improve the accuracy of the GPS location data. These stations use satellites to transmit GPS correction data to compatible GPS receivers. WAAS improves the GPS location information from approximately 20 meters to better than 2 meters. This paved the way for much improved instrument approaches.
Approaches using WAAS are called LPV or Localizer Performance with Vertical guidance. (Note that Garmin uses Lateral Precision with Vertical guidance.) As the name implies, these approaches act like localizers with glideslopes: in other words, an ILS-like approach.
A localizer path is like a wedge extending on either side of the runway centerline. The left and right sides of this wedge are indicated on your OBS by a full-scale deflection of the CDI. As you get closer to the runway, the width of the wedge gets smaller. At the threshold, the path is 700′ wide. For an LPV approach, the GPS receiver calculates this “wedge” rather than receive it from a radio navaid. The glideslope works in the same way, only vertically instead of horizontally.
This is an improvement over the LNAV GPS approaches. On the older approaches, the approach path from the final approach fix to the runway is a constant 0.3NM (1,800′) wide. With LPV, the course is also 0.3NM wide at the final approach fix, but it narrows as you get closer to the runway, just like a localizer. At the threshold, the LPV path is the same width as a localizer, 700′. The glide path on an LPV approach also narrows like the glide path on an ILS.
This is great news for instrument pilots because, unlike the older LNAV/VNAV approaches, LPV approaches become more accurate as you approach the runway. The bad news is that now there are up to four minima lines on a GPS approach plate, each with a different minimum altitude. Worse, some of those altitudes are Decision Altitudes while others are Minimum Descent Altitudes.
Putting It All Together
With all of these different options on a GPS approach plate, how do you know which one to choose? Well, the first thing you have to do is determine the capabilities of your equipment. If you have a non-WAAS GPS without baro-VNAV (typical for an older panel-mount GPS in a small GA airplane), then you only have one option: the LNAV minimum. You would fly it like a localizer-only approach, descending to MDA (leveling off at any stepdowns along the way), level off, then fly to the Missed Approach Point. In the example below, that would be 1,140′ MSL.
On the other hand, if you have a baro-VNAV, you can use the LNAV/VNAV minimums and follow the glideslope on your OBS to the DA (1,128′ in our example). As I mentioned above, the LNAV MDA is sometimes lower than the DA of the LNAV/VNAV. While a complete discussion of how approaches are built is beyond the scope of this article, the short answer is because of where obstacles are on the approach. If you see this situation and are flying the LNAV, you should be aware that there may be one or more obstacles below the MDA that you’ll have to avoid on your own.
A slightly longer, but still simplified, explanation has to do with how the Obstacle Clearance Surface (OCS) is defined. For an LNAV approach, this is an imaginary horizontal line drawn at the height of the highest obstacle between the Final Approach Fix and the runway. Generally, add 250 feet to that and you get your MDA. But for an LNAV/VNAV approach, the OCS is a sloped line beginning approximately one mile from the threshold. The OCS slope is shallower than the baro-VNAV glide path because of how temperature affects barometric altimeters. The higher you are above the ground, the bigger the error. If an obstacle penetrates the OCS, the DA must be placed before the obstacle so the pilot can visually avoid it while descending.
Temperature adds another problem to LNAV/VNAV approaches. The approach designers can only account for so much variance from temperature-induced altimeter errors. The result is that most LNAV/VNAV approaches are limited by temperature. The chart notes will state something like “For uncompensated Baro-VNAV systems, LNAV/VNAV NA below -17°C or above 54°F.” If the outside air temperature is outside the specified range, you’ll have to use the LNAV minimums and ignore the glideslope.
An interesting side note to this is that WAAS-capable GPS receivers can also fly LNAV/VNAV approaches with no need for baro-VNAV. So if the approach doesn’t have an LPV minimum, you can fly the LNAV/VNAV approach. Bonus points because WAAS-GPS receivers are not limited by the temperature restrictions of baro-VNAV systems. They use WAAS GPS data to generate the glideslope rather than altimeter data.
Finally, if you have a fully WAAS-capable GPS receiver, you can fly the approach to LPV minimums. In our example, that would be 944′. Fly it like you would an ILS to the DA. Generally, LPV approaches can have lower minimums than an LNAV/VNAV approach. The greater accuracy of the LPV approach results in a smaller OCS. So obstacles inside the LNAV approach path may be outside of the LPV path. Likewise, the narrower glide path on an LPV approach can put the OCS above obstacles that may penetrate the OCS on an LNAV/VNAV approach.
In 2006, the FAA has authorized LPV approaches down to 200′ above the touchdown zone if obstacle clearance and airport infrastructure requirements permit it. For now, though, most LPV approaches have a higher DA than an ILS on the same runway. Most of them still get you below 500′ though, which is good enough for most weekend pilots.
Another Wrinkle
Not all GPS approaches have vertical guidance available. This can happen when there is insufficient obstacle clearance for an LPV or LNAV/VNAV procedure. In these cases, you will find just LP, LNAV, and Circling minimums. If the LP minimums are the same as the LNAV minimums, the chart may only show LNAV.
Note that the LP (Localizer Performance) minimum is an MDA like the basic LNAV minimum. You would fly both the same, just choose the minimum that matches your equipment and remember that the LNAV has a fixed width path, while the LP has an angular path.
One thing to be careful of with these procedures. Some GPS receivers will provide an advisory glideslope for LP and LNAV approaches. You’ll see this as LP +V or LNAV +V. In these cases, your GPS receiver is generating a glideslope for you to follow. Just remember that it doesn’t guarantee obstacle clearance. You still have to abide by any MDA or stepdown altitude limits on the procedure.
One last item to be aware of: WAAS-enabled GPS receivers verify the GPS signal prior to the Final Approach Fix. If the unit determines that the signal is insufficient for the LPV approach, it will downgrade to LNAV. In this case, you must adhere to the LNAV minimums. No vertical guidance will be provided. Be sure to check your GPS before you start your descent at the Final Approach Fix.
So What’s My Minimum?
Understanding GPS approach minimums seems complicated until you realize what the requirements are for each one. Here’s a quick checklist to determine which one to use:
- Does the approach have an LPV line and do you have a WAAS-capable GPS receiver and is it indicating LPV?
- Follow the glideslope to the LPV DA
- Does the approach have an LNAV/VNAV line and do you have a baro-VNAV enabled GPS receiver and is the outside air temperature within the operating range for the approach?
- Follow the glideslope to the LNAV/VNAV DA
- Does the approach have an LP line and do you have a WAAS-capable GPS receiver and is it indicating LP or LP +V?
- Descend to the LP MDA and fly to the Missed Approach Point
- Do none of the above apply?
- Descend to the LNAV MDA and fly to the Missed Approach Point
Flying GPS approaches isn’t hard, once you understand which minimums to use. Don’t get yourself in trouble, though, by picking the wrong one. Just because you can fly an LPV approach doesn’t automatically give you the lowest minimums. Make sure your GPS receiver is in LPV mode and not LNAV mode. Likewise, don’t forget to check the outside air temperature if your plane is baro-VNAV equipped. And finally, make sure the minimums you are using match the approach you are flying.
What About RNP Approaches?
Technically, all GPS approaches are RNP (Required Navigation Performance) approaches. What most people mean when they refer to RNP approaches though, are those titled RNAV (RNP) instead of RNAV (GPS). RNAV (RNP) approaches are more correctly called RNP Authorization Required Approaches. The AIM discusses both in section 1-2-2.
In short, RNP AR approaches have similar minimums to LPV approaches, but require precise approach paths that may include curves and other turns to avoid obstacles or conflicting airspace. In addition to the required equipment and performance capabilities, special training is needed to receive authorization to fly these approaches. So unless you work for an airline or a corporate flight department, chances are you’ll never fly one. At least for now.
The Future of GPS Approaches
With the introduction of the Ground Based Augmentation System (GBAS; formerly known as Local Area Augmentation System or LAAS), the FAA is designing precision GPS approaches. Using exact known locations, ground stations installed at an airport can provide extremely accurate GPS correction information to nearby aircraft. This improves GPS accuracy to better than one meter, leading the way to ILS-level minima, including Category II and III approaches in the near future.
GBAS Landing System (GLS) approaches are already available in newer airliners flown by major carriers. Although GBAS is only installed and approved at two public use airports in the US at this time—Newark (EWR) and Houston (IAH)—many other airports are in the process of adding it. Laguardia, JFK, SEATAC, and San Francisco International are all looking to install systems this year. Europe, with dozens of installations already, is making a big push in 2020 to install GBAS systems all across the continent.
Unfortunately, the FAA has decided not to fund GBAS installations (for now). That means that in the US only larger airports that can afford to purchase the equipment will have GLS approaches in the foreseeable future. I expect that situation to change, however, as the FAA decommissions more older navaids and smaller airports begin to push for federal funds to help upgrade their approaches.
While it may be a while before your flight school’s Cessna 172 can fly a GLS, as availability increases and equipment costs decrease, expect to see GLS capable equipment available for General Aviation in the future. Cat-III approaches for the masses? Probably not. But having approach minimums of 200′ at more small airports will be a huge benefit to the frequent instrument flier.
You can find more information on the history of GPS approaches here: GPS Approach Minima – How Low Can You Go?