Lockheed Martin calls it radar pilotage - a way to land in reduced visibility without help from ground systems.

John Wiley and Graham Warwick/ALBUQUERQUE

LOCKHEED MARTIN'S autonomous precision-approach and landing system (APALS) offers airlines the ability operate in reduced visibility free of any dependence on ground-based equipment. To see if the system's promise is being fulfilled, Flight International has evaluated a pre-production APALS installed in a Gulfstream II business jet.

Flight International's John Wiley was invited to fly the APALS-equipped GII at Albuquerque, New Mexico, where Lockheed Martin was conducting a series of approaches before a demonstration planned for early February, in Richmond, Virginia, when some 75 pilots from airlines, airframe manufacturers and airworthiness authorities will evaluate the system. The GII is the aircraft used for APALS proof-of-concept trials at Albuquerque in late 1994 (Flight International, 5-11 October, 1994, PP32-34).

The APALS uses an aircraft's radar to produce precise position-updates for an inertial-navigation system (INS), which then generates signals emulating the instrument landing-system (ILS) to drive a flight director, auto-pilot, or, eventually, a head-up display (HUD). The system uses the existing weather-radar antenna to produce high-resolution synthetic-aperture radar (SAR) images of the ground under the approach path. These are correlated with a database of SAR scenes for that approach, to update position.

The GII is equipped with a pre-production APALS unit, which includes a global-positioning system (GPS), inertial-measurement unit and radar and navigation processors. The system generates pseudo-ILS glide-slope and localiser signals for display on the GII's electronic flight-instrument system. This is representative of the initial APALS production configuration, which will drive a flight director, to allow hand-flown Category II approaches.

A separate APALS radar receiver/transmitter is installed linked to the antenna of the GII's elderly Honeywell Primus 650 weather radar. In APALS operation, control of the antenna is "stolen" from the weather radar at the end of each scan and the beam pointed at the ground, at 45¡ to the flight path, for 0.25s. This enables the APALS to collect a SAR image for correlation with the database of ground scenes (see box).

GPS is used for the initial position fix, to around 100m (330ft). This is accurate enough to "initialise" the APALS - to tell it where to look for its first ground scene. After five successive correlations APALS-updated INS position accuracy reaches 1.5m. This is maintained down to 100ft above ground level, when the radar becomes ineffective, and the aircraft then continues to touchdown on inertial guidance only.

FLYING APALS

The flight plan called for five to seven approaches, with at least one to be observed on the flight test consoles mounted in the cabin. The down- wind leg would be flown at 200kt (370km/h), with no flaps and with gear retracted. The base leg would be flown at 180kt, with 10¡ of flaps, and the intercept leg to final approach at 160kt, with 20¡ of flaps. When the glide-slope marker was within one dot of intercept, the landing gear would be extended and the flaps fully lowered. The GII weighed 25,400kg, so final approach speed was a zippy 155kt.

The pilot in command made a normal take-off and gave me control of the aircraft on the cross-wing leg. I leveled at 8,000ft and accelerated to 200kt. It took a few minutes to adjust to the GII's handling, a slight stagger in the throttle alignment and the significant airspeed effects of minor power changes adding modestly to workload.

To enable the APALS, I had only to move a toggle switch on the centre-console control panel. As the system had not been initialised, my only display was a red "X" across the electronic horizontal-situation indicator. As flown, the APALS did not initialise until the aircraft was within 10-15¡ of the runway heading. With this narrow window, I had to make a slight jog towards the runway before returning to the intercept heading given by air-traffic control.

Initialisation took just a second or two once the APALS had identified a cluster. The red "X" disappeared and glide-slope and localiser information came up on my display. Our pattern consisted of all right-hand turns so, after initialisation, the localiser bar showed full left deflection. The glide-slope bar showed full up deflection.

I met a shifting 10kt tailwind on my first approach, requiring numerous thrust changes to stay on glide-slope and localiser. With minor speed deviations, I was able to stay on the APALS approach down to 100ft. I drifted slightly high, but, even with this error, I was 100ft within Cat I limits hand flown.

The second approach was more relaxed, as I adjusted to the GII's handling characteristics and power requirements. I was able to cross-reference my approach display, based on the APALS, with that of the pilot, which used ILS information. Since the displays are electronic, there was no parallax. The greatest deviation occurred while still far from the runway, but even this discrepancy was no more than one-eighth of a dot.

Our third approach turned us over the outer marker, 11km (6nm) from the end of Albuquerque's runway 08. This simulated a short turn to final, or a visual approach with the APALS providing glide-slope information in lieu of a visual approach-slope indicator. With the narrow initialisation window, this did not allow sufficient time to align fully with the runway, highlighting the need for an earlier initialisation.

Since our flight, according to APALS programming manager Keith Loss, software changes have been made and initialisation is earlier. Apparently, the programmers had assumed that approaches would be flown straight in and had not considered earlier initialisation. Now, localiser information can be provided while on the downwind leg and glide-slope information when within 80¡ of the nominal runway heading.

Subsequent approaches, including an ILS approach to another runway, confirmed what I had seen earlier: that the APALS seems no different to the ILS. As flown, the system had a two-step arming process - first localiser then, after initialisation, glide-slope - making it possible for a pilot to forget the second step and miss capturing the glide-slope. Lockheed Martin says that the system will be modified to enable single-step arming, as with ILS.

While some airlines require all approaches, regardless of conditions, with the weather radar switched on, others do not. As the APALS requires the radar on, it was suggested that, when selecting the system, there should be an annunciation if the radar is not turned on, or the radar should be switched on automatically.

AIRLINE BENEFITS

For an airline, the $150,000 APALS, offers the capability for at least Cat I approaches to every runway used, regardless of ground equipment. Cat II operation is promised at airports with everything required for Cat I, except for the ILS. At those same airports, a $175,000 dual-channel APALS should allow at least Cat IIIa, and possibly Cat IIIb, approaches, Lockheed Martin says.

Lockheed Martin has a memorandum of understanding with US launch customer Lone Star Airlines, a regional carrier based in Fort Worth, Texas. Lone Star plans to equip its four Dornier 328 regional turboprops with Cat II systems driving the Honeywell flight-director and intends use the APALS to perform Cat I approaches to non-precision airports in Mexico and at Colorado ski resorts.

Discussions continue with two European carriers interested in equipping the same narrow-body-airliner type with the APALS, certificated for Cat III automatic landings. Lockheed Martin says that US Cat II certification is planned for mid-1997, with European Cat III certification to follow later in 1997. There is also military interest.

The company continues to work on a HUD version of the APALS, using a fixed-base simulator. The intention is to gain approval for hand-flown Cat III approaches to any airport using the HUD, with conformal-runway symbology being used in lieu of runway lighting. HUD certification is hoped for by mid-1998.

Enhancements are planned for the production APALS, including optional modules for an FM-immune ILS, flight-management system and VHF data-link. The system will meet the requirements established for an ARINC 756-standard GPS navigation and landing unit, or an ARINC 755 multi-mode (GPS/ILS) receiver, the company says.

No additional skills are required to fly the system as evaluated. The pilot will notice no difference between an ILS and the APALS, but the system promises to enable the pilot to take off confident of at least Cat I limits at the destination airport, without concerns about the existence or availability of ground equipment.

Source: Flight International