Determining Target Aspect: the BDHI

The Target Aspect is a fundamental concept that appears very often. I discussed similar topics in this article but the TA deserves a more in-depth look, similarly to what I did with Collision and the TID.

Parentheses: Aspect Angle (AA) vs Target Aspect Angle (TA/TAA)

Before starting, a brief note about something that can be confusing, namely the difference between Aspect Angle and Target Aspect Angle. Those two values have similar meaning but they appear opposite. They are in fact supplementary angles “built” on the BFP, where it intersects the BB.
The AA is defined as: “the angle from the bearing line of the fighter to the tail of the target”.
The Target Aspect angle is defined as: “the angle from the bearing line of the fighter to the nose of the target”.


Some useful definitions, on top of the others already discussed in a different article, most of which will appear again later when the Intercept Geometry is discussed:

  • Cut: “The angle from fighter heading to bogey reciprocal.” (FH to BR);
  • DTG or HCA (Degrees To Go or Heading Cross Angle): “is the shortest number of degrees the fighter needs to turn to parallel the bogey’s flight path, or turn to the bogey’s heading.”.
  • DOP (Direction of Pass): “is the direction the bogey would pass from one side of the fighter’s flight path to the other.”.

The definition of Target Aspect should be by now well-known (if not, have a look at the link above). Geometrically, it is one of the angles of the triangle defined by the SR and the intersection of FFP, BFP. As DTG is the opposite angle in BFP and FFP and the sum of the internal angles of a triangle is 180, the Target Aspect can be immediately calculated as:

TA = 180° – DTG – ATA

Moreover, as DGT is the supplementary angle of the Cut, TA can also be calculated as:

TA = 180° – DTG – ATA
TA = Cut – ATA

A couple of notes

  • Approximation

    Since mental arithmetic is the means to process all the information and the avionics on the F-14 A/B is intrinsically imprecise, approximating the values is a welcomed practice. Therefore, 31L ATA can become 30L, and even the estimation of the DTG benefits from rounding. The procedures suggest the following rounding pattern:

    • 0, 1, 2 rounds to 0;
    • 3, 7 rounds to 5;
    • 8, 9 0 rounds to 0.

    On top of speeding up the calculations, rounding reduces the number of arithmetical errors (which are potentially greater than the imprecisions induced by rounding).

  • Left vs Right

    Using Left and Right, the RIO can describe the parameters of an intercept in a quickly and very simple manner. L/R applies to several concepts:

    • Antenna Train Angle: We are already familiar with how the RBRG displayed on the TID works, and how it can be used to immediacy know the ATA. Simply put, if BR < 180, then the ATA is Right (so the target is on the Right side of the F-14), if BR > 180 then is Left (therefore, the target is on the left of the F-14).
    • The Cut works similarly, being the angle created by the intersection of the FFP and the BFP, describes from what side the Bandit is coming from, starting from FH until reaching BR.
    • The Direction of Passage clarifies the direction from which the bandit approaches the fighter (note that it does not hint where the aircraft are along their flight paths).
      For example, if the fighter is heading North and the target is heading 155, the direction of the pass if Left-to-Right, vice versa, Right-to-Left.
    • Target Aspect also have a direction, relative to the target’s nose. A simple trick to understand the direction is comparing BR and BB. Starting from the BR, move towards BB. That direction indicate Target Aspect Left or Right:


The Bearing Distance Heading Indicator, is a display installed in both the front and the rear cockpit. Besides providing the Heading of the F-14 at a glance, it comes handy to monitor the TACAN (yardstick especially) and free other displays.

The simplest way to understand how the BDHI can be a very useful tool to determine the TA is by means of examples.

Practical Example I

In this scenario a hostile contact is being tracked by our AWG-9:

What can we say immediately about this contact, just by glancing at the TID AS? If you follow this site you probably got it already: it is placing us on a collision course and therefore has probably not really nice intentions! This because I inadvertently left the CAP behaviour turned on. It is not a particularly interesting information in this example but it is cool to see how previous concepts and discussions come together to increase the SA of the crew.

Back to the BDHI

This is a useful “mental picture” of the BDHI whilst we use it to calculate DTG and other values. It is much simpler than it looks but it requires a bit of practice (fun fact: when presenting a closely related topic, the documentation says, quoting: “this appears to be a mechanical approach at first, but after doing several hundred exercises, the spatial picture will come almost automatically.”. 🙂 ).

  • HDGF-14 is the value under the top mark on the ring of the BDHI. In this example, is 355°.
  • DGT is immediate, just find the HDGTGT and evaluate the angle between of the top mark and the HDG. The HDGTGT is 117°, and that value looks around 15° short from the 135° mark, let’s say that DGT is 120°.
  • Cut is the supplementary angle, so it is 180° – 120° = 60L. It can also be eyeballed using the same methodology used to determine the value of DTG.
  • ATA is displayed on the TID, in this case 339 → 31L (~30L).
Note how the mark on the ring of the BDHI points at 55° Left or 305° rather than 315°. Feature? Bug? At the end of the day is not a big deal, as long as you remember that that mark is different from the others.

Now onwards to the numbers!

HDGF14 = 355°
HDGTGT = 117°

BR = 297°
DTG = 120°

TA = 180° – DTG – ATA
TA = 180° – 120° – 30 = 30R
TA = Cut – ATA = 60 – 30 = 30R

The direction can be established by following the procedure described before: start from BR and move towards BB. As we follow towards the right, that’s the direction of the TA.

This is the scenario in TacView; which shows the Aspect Angle rather than the Target Aspect:

148R → 180 – 148 = 32, which is very close to what we determined by using the BDHI.

Practical Example II

Similar setup as the example above but with different directions. From the TID AS we can immediately tell that we are nowhere near close to collision. VC is also very low.

HDGF14 = 208°
HDGTGT = 262°
ATA = 343 → 17L
Let’s now have a look at the BDHI.

Instead of approximating the values, we can use the marks on the rotating compass as they clearly match 5° more than the 045°R notch of the outer ring.

DTG = ~50°
Cut = 180° – 50° = 130L

TA = 180° – DTG – ATA = 180° – 50° – 17L = 113R
TA = Cut – ATA = 130 – 17 = 113R

And a quick check with Tacview:

66R AA means 180° – 66R = 114R TA, which is quite close again to the 113R we estimated from the BDHI.

Closing Notes

As we have seen, by means of the BDHI, the RIO can easily estimate the Target Aspect of a contact.
Here are a couple of suggestions that hopefully will come handy:

  • Make it easy, make it fast

    As the contact’s TA changes over time (unless, as we know, when on Collision), there is little point in spending a lengthy amount of time calculating the Target Aspect. A good and prompt estimation usually works better. Don’t be afraid of rounding the values as discussed above!

  • Watch for the parallax!

    The parallax error is a problem familiar with anyone who has used an analogue tool such as, for example, an analogue voltmeter. Although a minor error, it would be better to avoid it by simply moving your head in front of the BDHI by means of the Track IR.

  • Snap view on BDHI

    A simple way to move the camera immediately in a specific position (in this case, right in front of the BDHI) is by creating a Snap View. These views are recalled by using [LWin] + [Num0 – Num9]. To save a snap view, just move your (virtual) head where you want it, press for instance [LWin] + [1] (the view will change to the correspondent snap view, not to the new one), then [RAlt] + [Num0] to save it. Next time you press [LWin] + [Num1], you will see your new Snap View.

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