FLY 'EM BETTER Trackers vs. Couplers - Round 2 by Jim Irwin, Vice President, Products This article was originally written to describe the two types of radio course-tracking systems used in the general aviation fleet. There will shortly be three types of tracking systems. This revised article starts by talking about the new kid on the block, GPSS by S-TEC - or GPS Steering. GPSS International Navigation Systems such as VLF-OMEGA and Multi-sensor Navigation Management Systems have for years provided a computer generated "Roll Steering" signal that is output for use by the autopilot. This signal is received by the autopilot as a HDG error signal, identical to that produced by the HDG Bug. We all know how authoritative HDG mode is - full turn command capability with maximum autopilot banks, yet smooth with excellent resolution of even small HDG errors - flies all day with the "bug" in the middle, etc. Well, Roll Steering is coming to the piston-engine segment of GA in many of the new GPS Navigators, and S-TEC is adding GPSS modes to our new or upgraded autopilots and is introducing a GPSS Converter, available for earlier systems so no one will be left out. What does it do? Glad you asked. With a GPS Navigator with steering and an S-TEC autopilot with GPSS mode, multiple waypoint courses and complex steering maneuvers defined by waypoints can be accomplished completely automatically. The GPS Navigator switches from waypoint to waypoint, establishing the new course direction each time and simultaneously computes the turn rate and turn starting point, required for the autopilot to turn directly onto the new course centerline - without overshoot or scalloping, etc. The autopilot accepts this steering input and turns the aircraft to the new course with both precision and smoothness. In the beginning, the navigation databases will be used mostly in the cross-country phase, linking the waypoints for automatic directional changes without autopilot mode changes or intermittent manual operations. As the databases improve, GPS overlays of DME Arc approaches will be added (some of these exist) as well as partial or full procedure turns, and ultimately, probably, published holding patterns and missed approach maneuvering. Keep in mind that this will take time and a lot of software, but it is coming. When it all comes together, you will be able to identify the destination airport, the approach procedure, the initial approach fix (IAF) and the procedure turn, and then do the entire horizontal navigation element automatically. You will still have to make altitude changes and adjust power and configure the airplane, but - What the hey! We gotta have something to do! All kidding aside, it is a real thrill and a safety enhancer to have the system fly you to the procedure turn, and fly you from the procedure turn to the airport runway with database commands and precision rivaling a localizer. Meanwhile, you are in the loop conducting the vertical element of the approach with maximum accuracy and safety because you can concentrate more effectively on those elements while monitoring the progress of the approach. That's GPSS - pretty slick. Thank you technology, and thank you S-TEC. Tracking There are two types of radio signal tracking systems used in General Aviation Autopilots Ð trackers and couplers. Confusion about the difference in how they fly the airplane often leads to pilots being unhappy with their performance. The basic difference is that a coupler can intercept a course and then track it. The pilot, either with manual or autopilot commands must put a tracker on the course before the tracker can track the signal. Intercepting a course is a function of the Directional Gyro (DG), part of a system, which has a coupler. Systems without Heading Bug equipped DGs are trackers. Radio Couplers In order to intercept a navigation course (VOR/LOC/GPS) line of position you must know two things: (1) the direction of the desired course and (2) direction of the aircraft relative to that course. Private pilot VOR orientation instruction covers it best. To intercept and navigate a specific course, such as an airway to the station, move the OBS to select the course direction with a To indication, then turn the aircraft to that heading and observe whether the CDI needle is displaced Left or Right. Turn toward the displaced needle 45°, the intercept angle. As the needle centers reduce the intercept angle so that the heading of the aircraft and the course are the same with a centered needle. With the bearing (course) and aircraft heading information you can intercept and then track the course. An autopilot with a radio coupler uses a Directional Gyro in the same way. The HDG information is provided by the position of the HDG Bug, relative to the Lubber Line on the DG. The autopilot determines the position of the Bug by virtue of the voltage amplitude, or sometimes the phase relationship of the Bug position to the zero point, or Lubber Line position. The signal is mixed with the L-R signal in a way that causes the intercept angle to diminish to zero as the aircraft intercepts the desired course. After course intercept is complete, cross wind correction logic is employed to change the aircraft heading, in small increments, to correct any tendency to drift caused by a cross wind. Once a coupler-equipped system has intercepted the course line and the gain has automatically reduced to Cap-Soft and Soft conditions, they become trackers with the HDG information washed out. If the crosswind is very strong and the correction angle increases to approximately 31°, the DG signal is reinstated and a re-intercept made. Rapid yaw inputs will also reinstate the HDG component for better corrections until it is again washed out over a time period. With a DG equipped system, the HDG Bug thus becomes the course datum input to the autopilot while the L-R radio information indicates our position relative to the desired course line. With an HSI (Horizontal Situation Indicator) the course datum information for the autopilot is typically provided by a synchro or potentiometer output connected to the OBS needle instead of the HDG Bug. Because of the separate OBS output, the Bug is only used for HDG in an HSI, which allows for selected angle intercepts, i.e. using the Heading Bug for the intercept angle until the on-course turn is made. Radio Trackers A radio tracker is part of an autopilot system that does not use a Directional Gyro HDG Bug input to solve the navigation problem. Usually this is a system that does not require a DG for operation, such as an S-TEC System 40/50 or System Twenty/Thirty. Without the HDG Bug input, it is not possible to set up an intercept since no angular information exists. The system can only track the course once established on it. With a tracker, it is necessary to position the aircraft on the desired course centerline with the aircraft going in the same direction. With the intercept complete all that remains is the tracking function - hence the name Tracker. If a Tracker is engaged with too large an offset between the aircraft location and the course, such as when displaced by a crosswind, the autopilot will command a turn toward the needle with too large an intercept angle to remove as it joins the course. This will happen if the CDI needle is more than 1 to 2 needle widths from center or if there is over 5° difference between the aircraft HDG and the course. The result is an S turn across the course or, if the intercept angle is large enough, a divergent S turn where every pass across the course is at an increasingly larger angle. When this occurs, the pilot should disengage the tracking mode, reestablish the aircraft on the course, and then engage the tracker again. Trackers typically will track a radial or course as accurately as a radio coupler, they simply cannot perform the intercept. In S-TEC systems, for instance, the radio tracker in the System 40/50 has two gain levels, NAV for long-range tracking and APR (approach) for tighter close in VOR, Localizer or GPS tracking. Roll Centering The roll centering adjustment, which adjusts the system for the zero position of the turn coordinator gyro, is very important to being able to track in the SOFT condition in coupler equipped systems, or in NAV mode with System 40/50 trackers. We hope this new discussion on GPSS and couplers/trackers is helpful information, which you can pass along to your customers. If there are specific questions, give us a call or drop us a note, and we will address it in the next issue.