I was looking at this converter.
The interest was because the Sandel SN3500 EHSI has a roll steering option, available on its FCS (analog autopilot control) outputs, but the feature works only with an ARINC 429 GPS – not with the KLN94 which outputs NMEA on RS232.
However I can’t see how this will work because the roll steering feature in the Sandel merely does what every other “roll steering converter” does: converts the ARINC 429 roll data into a roll angle (or something like that).
Is it really feasible to implement such a conversion? It ought to be (if the NMEA data contained the flight plan or at least the currently active leg and the one after it) but would anybody bother?
9/10 for an obscure topic
The ARINC 429 Label 121 is a very precise bank angle generated by the GPS as an autopilot command to follow the desired path. The format is:
121 02 HORIZONTAL COMMAND (TO AUTOPILOT) (BNR)
11-14 SPARES PAD ZERO
15-28 BINARY ANGLE (180 / 16,384)
29 SIGN FLY LEFT FLY RIGHT
32 PARITY (ODD)
This information is not available in any digital format from the KLN94. It is not equivalent to a CDI signal L/R as it is a bank angle. There is an analog signal that might be used to convert to this format, but others have tried and failed to get it to be responsive enough to work. The analog signals can be directly connected to the KFC225, but I wouldn’t waste my time as I have heard nothing positive about this.
Edit: The roll steering converters convert the 121 label bank angle to a heading error signal. The autopilot responds to the heading error signal by commanding a bank equal to the difference between the heading bug and the current heading (at least the relationship is proportional), so a 5 degree difference the autopilot commands a 5 degree bank, 10 degree difference a 10 degree bank, and so on, up to the autopilot bank limit. So the converter just converts the ARINC Label 21 bank angle to an equivalent heading error. In the case of the KFC225, it has an ARINC input and responds to the 121 label directly when the CDI source is GPS and Nav mode is selected on the autopilot, so it does not require a converter.
Does this mean the Label 121 data is generated with account of the required wind correction, aircraft speed, etc?
Indirectly yes. Without an airdata and heading input the GPS only has GS, track, and the course information to work with. The steering will generate a bank angle necessary to align track with DTK and along the desired course centerline, which in effect is wind correction, at least for straight line courses. As long as the autopilot maximums are not exceeded, the roll steering will correct to the course during a turn without overshoot or undershoot as the corrections are continuous around the turn. With the heading and airdata inputs, it can do a better job of anticipating the turns. For example, flying a hold the normal oval shape will be adjusted and not be oval shaped. Without the ADHD inputs, it will keep the oval shape, but adjust the size of the pattern based on groundspeed.