Aquote from the EAA Magazine
The important thing to know is that LPV guidance in your instrument panel behaves just like an ILS signal. Other RNAV or GPS approaches present linear guidance, meaning one dot of deviation equals the same distance off the centerline no matter how far you are from the runway or waypoint. The LPV, like an ILS, shows angular deviation, so any tracking error shown on your instruments narrows as you near the runway. Like an ILS, when you get to the runway threshold, a dot of deviation off centerline equals only a few feet of actual distance from the center.
If it were up to me, I’d use linear deviation guidance for all approaches because I think that method is easier to fly. The “sensitivity” of the linear display stays the same all the way to the runway, while the LPV, like the ILS with angular deviation, “cones in” the closer you get to the decision point. But the FAA people who created the LPV specs wanted LPV to behave exactly like an ILS, so we pilots don’t need to learn anything new. Old dogs, new tricks – that sort of thing.
In spite of having angular deviation, I have found the LPV easier to fly than an ILS because the GPS-derived LPV guidance is rock steady. You don’t realize how much a typical ILS signal is wandering around until you fly with LPV on one display and raw data ILS on the other. Because it is an analog signal, and its radio beams are susceptible to all sorts of reflection and distortion, the ILS guidance can never be as steady as the LPV, which comes from a new GPS position and velocity calculation being made several times each second.
All of this has taken on new significance to me given the several potential GPS interference sources that are in the news. If I lose GPS guidance en route, it is important because that is my primary source, but there is a lot of time and several navigation alternatives to fall back on. If nothing else, the controller’s radar can provide guidance en route to get you where you are going. But if GPS signals are interfered with while you are flying a few hundred feet above the ground on an instrument approach in the clouds, snow, or murk, that is a different and much more critical matter.
A potential wide-scale GPS interference source is a new 4G broadband Internet network called LightSquared that has been given preliminary approval by the FCC. LightSquared’s proposed thousands of transmitters would operate on frequencies closely adjacent to the GPS frequencies. The GPS industry is very concerned that the comparatively powerful transmitters of the network will cause widespread interference and loss of navigation for airplanes flying within miles of the transmitters. Garmin is among those expressing concern to the FCC about the new network. The FAA is also onboard in seeking to prevent possible GPS interference.
But there is also potential GPS navigation interference already out there from some unknown number of GPS jammers that are sold mostly over the Internet. These devices are designed to temporarily disable a GPS device that may be in your car or truck and is transmitting your movements to others. If you don’t want your spouse to know where you are, or don’t want the boss to know how fast you’re driving (or not driving), these jammers can block reception of GPS signals by a receiver in your vehicle.
Using a GPS device to record, then transmit, the location and movement of a vehicle has become very common. For example, trucking companies want to know where their trucks are located and how well they are progressing toward the destination, and a GPS device can automatically send that information. You can also imagine why people would be curious about the movement and location of others and could “plant” one of these small GPS devices in a car to report its activity. Let’s just say these devices create a privacy issue.
The jammers that can prevent GPS reception cost only a few hundred bucks. The jammers are in a legal gray area at best, but they are proliferating. The effective range of most jammers is advertised to be in the 10s of meters, with some I have seen claiming to disable a GPS and cell phone out to 40 meters. That’s no issue for airplanes at cruise altitude, but what about nearing the decision point of only 300 or so feet above the runway on an LPV approach? The jammer would have to disrupt the signal for only a couple seconds to create a problem at the end of a GPS approach.
All of us should be concerned about the possible interference of new networks such as LightSquared. But those transmitters are regulated. If public pressure is kept on the FCC, as I know it will be, testing and modification if necessary can protect GPS operation interference from legal and authorized broadcasts.
That is not true for the many small portable devices designed specifically to interrupt GPS reception. That is a cat and mouse game in which those who want the information GPS can provide will continually search for ways to defeat the jammers. Those who want privacy from GPS reporting on their activities will pay more and more for powerful devices that provide privacy. It’s a GPS issue not envisioned when the system was designed by the Air Force for military navigation and weapons guidance beginning about 40 years ago.
Newly designed GPS satellites will contain advanced technology to help prevent jamming, but those satellites will take years to deploy. And the people designing the jammers won’t stand still in their search to find more effective ways to block GPS reception.