How to FDAI
The FDAI (Flight Director / Attitude Indicator) is a replacement for the NavBall created by alexustas. It provides more information and greater control than the stock NavBall. There are three components to the FDAI: The FDAI instrument itself, the FDAI GUID mode control panel, and the scale selector switches. Each are described below.
The FDAI contains a replacement NavBall, which should be familiar to KSP'ers. The NavBall in the FDAI functions in the same way as the stock NavBall, but without the glowing direction indicator icons. In addition to the NavBall, there are three Error Scale needles, three Rate indicator arrows, and one roll indicator chevron. An error flag extends when an invalid mode is selected, and a docking flag extends when the error needles are in docking port alignment mode.
Error scale needles provide feedback on aligning the front of the craft with the vector selected on the GUID mode panel (see below). There are three needles, one each for roll, yaw, and pitch. Under normal conditions, the roll needle (the top needle) is set to the neutral position. It is only engaged for docking port alignment. The pitch needle (on the right) and the yaw needle (on the bottom) provide feedback on which direction the craft needs to rotate to point at the selected direction.
There are six tick marks to each side of the zero position for the error needles. The value of the tick mark depends on the error scale setting (see below).
The rate indicator arrows provide feedback on which directions the craft is currently rotating, and how quickly. When all arrows are zeroed, the craft is not rotating at all. There are three arrows, one for each axis of rotation. The rate scales are labeled, with roll on top, pitch on the right, and yaw on the bottom. The rate arrows are on the same side of the FDAI as their corresponding error needles.
Each rate arrow's scale contains ten tick marks to each side of the zero position. The value of the tick mark depends on the rate scale settings (see below).
The roll indicator chevron displays the craft's current roll. When the chevron is pointed towards the top center of the display window, it indicates that the craft is in a "wings level" orientation. The chevron always points towards the north pole of the navball.
The mode control panel selects which vector the error needles display. In addition to the six cardinal orbital vectors (prograde, retrograde, radial in, radial out, normal, and anti-normal), the FDAI can be configured to show surface velocity prograde and retrograde, as well as the maneuver node vector, target direction, and target-relative prograde and retrograde. In addition, the GUID can be used to place the FDAI in docking port alignment mode, allowing the pilot to align the craft with a targeted docking port. Each of these modes is discussed below. There is a mode dial, a +/- selector switch, and a Sync to SAS switch on this panel.
The +/- selector switch controls whether the FDAI points towards a vector or towards the anti-vector. For instance, when the mode dial is set to ORBITAL, setting this switch to '+' indicates prograde; the '-' position shows retrograde. The one exception to this behavior is when the mode dial is set to TARGET. Setting the selector switch to '-' puts the FDAI in docking port alignment mode. However, this mode is only enabled when the craft is targeting a docking port, and a docking port is being used as the reference part on the craft.
The mode dial controls which vector the error needles will track. Going clockwise from the first position, these settings are:
- SURFACE - Directs the FDAI to display the vector towards surface-velocity prograde or retrograde (depending on the +/- switch setting).
- ORBITAL - Places the FDAI in orbital prograde (+) or retrograde (-) mode.
- RADIAL - Places the FDAI in radial out (+) or radial in (-) mode.
- NORMAL - Places the FDAI in normal (+) or anti-normal (-) mode.
- M.NODE - Places the FDAI in maneuver node mode.
- TARGET - Places the FDAI in target (+) or docking port alignment (-) mode.
- R.VEL - Places the FDAI in target-relative velocity prograde (+) or retrograde (-).
There are two additional modes for the FDAI. The first is "Sync to SAS", which is enabled by setting the Sync to SAS switch on the panel to "ON". The second mode is "Error", which occurs when an invalid mode is selected (Sync to SAS if SAS is off, M.NODE with no maneuver node, TARGET or R.VEL with no target, or docking port alignment mode when the requirements for DPA are not met). When in Error mode, the FDAI displays an error flag on the corner of the navball window, and all error needles return to zero.
When the FDAI is configured to TARGET -, it is in Docking Port Alignment mode. This mode is used to guide the spacecraft to a docking port on the target craft. In order for this mode to be enabled, the following requirements must be met:
- The vessel is targeting a docking port (not another vessel).
- The current reference transform of the vessel is a docking port.
If either of these conditions are false, the DPAI goes into error mode.
When in DPA mode, the FDAI provides yaw, pitch, and roll information on the error needles, allowing the pilot to align the craft with the targeted docking port. This instrument combined with the ARRT and X-Pointer allows for fast, accurate manual docking operations.
Sync to SAS tells the FDAI to automatically use the currently-selected SAS mode as its target vector. When SAS is disabled, set to Stability Assist, or set to an invalid mode (such as Maneuver Node when no node exists), the FDAI goes into error mode and the needles all go to zero.
Two switches control the scale of the error needles and rate arrows. These rate selectors allow the pilot to switch between coarse adjustments and fine-tuning.
The FDAI ERR SCALE switch can set the maximum range of the error needles to +/- 12 degrees, +/- 60 degrees, or +/- 120 degrees. Each tick mark next to the error needles counts of 2 degrees, 10 degrees, or 20 degrees, based on this switch's setting.
The FDAI RATE SCALE switch sets the maximum reported rate of the rate arrows to +/- 10 degrees, +/- 50 degrees, or +/- 100 degrees. Each tick mark next to the rate arrows correspondingly reports 1 degree/second, 5 degrees/second, or 10 degrees/second.
Here's a multi-step example of using the FDAI. We want to align our craft with prograde, so the GUID is set to ORBITAL +.
We've set the FDAI ERR SCALE switch to 60, and the FDAI RATE SCALE to 12, since we know we're well off the desired direction, but there's no urgent hurry to turn the craft.
At the start of the maneuver, the FDAI looks like this:
Prograde is to the right and slightly above our current facing, and we're almost "upside down" relative to the surface below us. The yaw error needle is all the way to its right-hand limit. Since the error scale is set to 60, that means that prograde is at least 60 degrees to the right of our current facing. The pitch needle is up two ticks from zero. Since an error scale of 60 means 10 degrees per tick, pitch is about 20 degrees above our current facing.
The first step is to align yaw. Because reported pitch is limited to +/- 90 degrees, pitch can report the same values when pointed away from the target vector (yeah, I'm not explaining that clearly). The best practice when the needles are far from the zero position is to align yaw first, and then align pitch. If the needles are fairly close to the zero, you can align them in whichever order you prefer.
Once we yaw to the right, we get something similar to this.
Yaw is now zeroed, so we're partially aligned. Next step is to adjust pitch, which is still about 20 degrees off based on the error needle and scale (scale = 60, so each yellow tick mark on the needle's scale indicator is 10 degrees).
Pitch is now aligned, but the craft is still pitching up: the Pitch Rate arrow is pointing at the 5th white tick mark above zero. Since our rate scale is set to 10, each tick mark is 1 degree, so we're still pitching 5 degrees per second. We'll need to make some more pitch adjustments to zero the rate and correct for the overshoot.
And here, finally, we've rolled the craft so the crew's heads are pointed "up" relative to the surface of the planet. There's no real need to do so, of course.
(TODO)