FLY 'EM BETTER -- The GPS/Autopilot Interface
Jim Irwin, Vice President - Products
All things being identical, most autopilot radio couplers and trackers will track VOR signals better than they will GPS outputs. What? The heck you say ! Everybody knows GPS signal outputs to the autopilot are higher quality.
Right on ! And there in lies the problem - but there is an easy solution. Autopilot radio couplers and trackers were designed to perform in conjunction with typical VOR signal output quality and damping (filtering) time constants. These signals often vary in voltage as a result of internal receiver interference and/or anomalies in the ground station output. This problem is usually described as "Noise" or even as "an A-C component" on the DC signals. The resulting signal must be processed by the radio coupler or tracker which must filter out, or clean up, the incoming signal variations so that they do not cause the roll channel of the autopilot to respond with an Aileron input to each variation.
A number of techniques have been used to accomplish this feat, including active filtering of the signal and gain reductions of the signal after the intercept of the course. S-TEC, like most other autopilot manufacturers, uses both techniques.
In our case, we provide three gain and mixing combinations in the radio coupler or tracker which we refer to as: "Capture", which is the highest gain level with the least filtering and is used to turn on to and capture the course; "Capture-Soft", a lower gain level used for approximately 60 seconds to establish a cross wind correction, and "Soft", the lowest gain level which also removes the heading input when low cross wind correction angles are involved.
GPS and Loran C receiver units produce a steering signal that is the result of computer computations of the receivers position as opposed to being a derivative of the relative positions of RF signals and OBS directed courses. These computer outputs are very high quality signals that are not "noisy" and therefore do not need the filtering or gain changes provided in the autopilot for intercepting/tracking VOR/ILS signals. In fact, these signals are so clean that many autopilots, including S-TEC systems, will not track them properly for long distances in "Soft", or the lowest gain mode.
The solution is to select "Approach" mode on those systems having an "APR" mode switch. This will keep the radio coupler in "Cap-Soft" gain, providing the sensitivity needed to track the GPS/Loran C signal. For operations of S-TEC System 60-1/60-2 and 65 Systems with automatic "Approach" modes, a minor modification can be made to accomplish this task and provide high quality GPS tracking. Refer to the "Technicians Bench" column of this issue for additional information.
One other problem sometimes comes up with GPS interfaces. Most GPS receivers allow the pilot to select the display sensitivity in miles/portion of a mile per DOT. Some non TSO-C129 systems offer an extremely broad selection, such as .1 mile/DOT to 99.0 miles/DOT. Obviously if you selected 99.0 miles/DOT, or a 500 mile wide course width, the needle sensitivity to small course changes would be so slight that the radio coupler would not recognize course changes. The result would be serious wandering around in the sky.
In general, most existing autopilots with couplers designed for VOR/ILS inputs begin to lose some tracking performance with course sensitivities over 1 NM/DOT. In fact, it is not unusual for IFR systems in the corporate fleet, turbo props and jets, with high dollar GPS and autopilot systems, to have Flight Manual Supplement limitations to use "not more than 1 NM/DOT sensitivities for autopilot coupling".
From my experience with a variety of autopilots, the higher the sensitivity the better the tracking performance. The new TSO-C129 GPS systems which are IFR certified for non-precision approaches, will automatically transition the full scale cross track deviation sensitivity from 5NM (enroute) to 1NM (terminal) and .3NM (approach). These systems also have the ability to manually select the sensitivity. To have the best tracking performance, manually select the 1NM or .3NM per dot throughout the flight, and your autopilot will love you and you'll love the tracking performance.
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