APPROACHES & LANDINGS

GLS grows in popularity

Ground-based augmentation system isn’t ILS or WAAS but it does provide precision approach at low cost.

By Don Witt
ATP. Learjet series, Airbus A320, Boeing 737, Boeing 757/767


GLS technology will permit precision approaches for relatively low investment in locations that do not have WAAS coverage, such as the airports of Micronesia.

First, some definitions. A GLS is a GPS landing system. More precisely, the letters stand for “GBAS landing system,” with GBAS standing for “ground-based augmentation system.”

A GLS approach is not the same as a WAAS approach, because WAAS is a satellite-based augmentation system (SBAS). Being a ground-based system, GLS uses a ground station, not satellites, to relay corrections.

WAAS, the satellite system, uses a lot of ground stations scattered around the US. There are nearly 40 of them, and each one’s position is surveyed and known precisely. The station has GPS receivers that work just like an aircraft’s to determine its “GPS position.”

The difference between each station’s GPS position and its known surveyed position is the GPS position error. Those errors, or rather their corrections, are transmitted to a WAAS equipped aircraft—but how? Here’s where the “S” in SBAS comes in.

WAAS corrections are transmitted to the aircraft via 2 satellites in geosynchronous orbit. The satellites’ orbits are so high that their orbital angular velocity matches Earth’s angular velocity (ie, rate of rotation).

Because of this relay by high stationary satellite(s), the WAAS-equipped aircraft gets the corrections no matter where it is in the WAAS coverage area, which is just about all of North America, plus Hawaii. And here’s the basic difference between GLS and WAAS. GLS has the same sort of ground station calculating corrections the same way—in this case one station for each airport.

However, with GLS the corrections are sent to the aircraft directly, line of sight, by a VHF transmitter which is part of the ground station. With GLS, corrections can be very precise because they are just for one small geographical area.

The GLS aircraft has a VHF receiver for this correction info which is part of the airplane’s GLS package. That receiver is probably a “multimode receiver” that can also handle WAAS and ILS approaches.

One good thing about GLS as it exists today is that the approach will require very little training for pilots. Boeing worked to keep displays in the 737NG and 787 similar to ILS presentations so that no simulator training would be necessary to learn GLS. (See upper photo on p 3.)

Universal Navigation’s GLS 1250 is in operation today in Norway. The operators manual describes the GLS approach procedure as follows. After selecting the GLS approach from the FMS database in the usual way, the pilot merely needs to confirm 2 green annunciations of “DGPS” (1 for each of the 2 receivers required for redundancy) and then fly the EFIS guidance exactly as one would for an ILS.

The question is this. If we already have a fine WAAS system providing approaches with minimums sometimes as low as 200 ft, and it’s available all over the US already, then who needs GLS? The answer is that GLS already has one big advantage over WAAS—and a second one that will arrive in the near future.

Current advantage

Today’s advantage is that GLS can provide precision approaches anywhere in the world. At present, WAAS covers the US (including Hawaii and Alaska), Canada, Mexico and Puerto Rico. MSAS, the Japanese SBAS system, covers operations in that nation.

Boeing GLS display. Other than the annunciation “GLS,” guidance for a GLS approach is essentially identical to that of an ILS, and procedures are very similar.

Europe is developing its own version of WAAS/SBAS, called EGNOS, but it’s not yet operational. And India is working on an SBAS system called GAGAN for the future—probably 2011 or later.

There’s even more stuff in the oven, but today, as widely available as WAAS is in North America and Japan, it’s not available anywhere else. Continental Airlines has long operated in Micronesia, where there is a need for low-cost precision approaches at the many island destinations served.

However, there is no SBAS. The airline has equipped the 9 Boeing 737-800s it operates there with Rockwell Collins multimode GLS receivers for approaches at GUM (Intl, Guam), and it plans to equip all its new 737NGs with GLS in anticipation of using the system at other airports around the world.

Since the ground GLS station at GUM is a Honeywell Beta 3000, which is not a certified system, all ap­proaches must be flown in VFR conditions. Nevertheless, the constant daily flow of Continental’s 737s into and out of GUM has meant rapid collection of a lot of data on GLS reliability and accuracy.

GLS has big ad­vantages for an operation like Micronesia. For one, it can be—and was—installed quickly. Continental Micronesia Dir Ops Capt Ralph Freeman recalls that the components of the GLS ground station arrived in Guam on a Friday and that he flew approaches with the system, including an autoland, on Sunday! As quickly as GLS can be installed, it can be removed even faster.

View of Newark Bay and Newark Bay Bridge on the approach to Rwy 29 at EWR. The structure of the bridge would interfere with ILS signals to this runway.

This is important in Micronesia. The whole system can be removed and stored safely away from damaging winds in a couple of hours. Typhoons visit Guam regularly. In 2002 one destroyed both the ILS and the VOR at Guam—neither of which is easy to dismantle and hide.

While Continental’s original intention was to expand an IFR certified network of GLS approaches throughout the Micronesian Islands, this has not happened. Someone has to pay for ground stations, and in Micronesia neither the airports nor the government wanted to pay.

Continental has refocused its GLS plans and its own financial re­sources on EWR (Newark NJ), whose Runway 29 has no ILS. Newark Bay Bridge, a high steel through-arch bridge (see photo on this page) lies under the approach path to Rwy 29 at EWR, preventing the installation of ILS there (since ILS signals would reflect off the bridge’s steel superstructure).

Instead, the runway has 2 RNP ap­proach­es (one from the north, one from the south) to supplement other nonprecision approaches (NPAs) to Rwy 29, but these have minimums of 403 ft AGL.

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