BVR, WVR & Intercept: Recurring Concepts (TA, ATA, Drift, Collision)

Thanks Scottie and SkillyJ from Sabre Squadron for checking this article for grammar and conceptual errors.

The following concepts are a fundamental part of both Intercept and the Within Visual Range arena. They also help the RIO to build more thorough SA and picture by better understanding the situation displayed on the TID.
As these concepts appear very often, and they are somehow connected, I decided to put them in a single article, excluding the topics strictly related to a specific context (e.g. definitions such as the HCA for the Intercept Geometry).

Table of Contents

Note

As usual, this is my understanding of the Navy material and other sources. Feel free to point out any mistake or inaccuracy, I’ll be happy to correct myself.

Please note that when I refer to TA = ATA, the absolute value is considered, as the L/R direction is usually reversed.

Basic Definitions

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Most of the following definitions are from the usual CNATRA P-825, 5-1. I added sketches and drawings to make the concepts more clear.

wvr-1-definitions-ta-ata-bh-br-bb-fh-ffp-bfp
  • FH, Fighter Heading. “Heading of the fighter which, if extended through space, defines the fighter’s flight path”.
  • FFP, Fighter Flight Path, “The logical extension of the direction of the fighter’s travel through space on its current FH”.
  • BH, Bandit Heading. “Heading of the intercepted aircraft”.
  • BFP, Bandit Flight Path, “the logical extension of the bandit’s travel through space on the current BH”.
  • BR, Bandit Reciprocal Heading or Bandit Recip, it “is 180 degrees in the opposite direction from BH”.
  • BB, Bandit Bearing. “Line of sight (LOS) bearing between the fighter’s position and the bandit’s position. BB is independent of both FH and BH”.
  • SR, Slant Range. “Direct LOS distance between fighter and bandit”.
  • Rate of Closure (VC), “Sum of the components of fighter and bandit velocities that contribute to downrange travel”.
  • Lateral Separation “is the horizontal distance from the fighter to the BFP”. It is expressed in thousands of feet and is calculated as Target Aspect * Slant Range * 100.
  • Vertical Separation (VD), “is the perpendicular distance the fighter is located above or below the bandit’s flight path”. It is calculated as the LS. VD = Elevation * SR * 100.

Antenna Train Angle (ATA)

It is defined as “the position left or right of the fighter’s nose on the radar attack display”. For instance, if the target is 330° from the nose of the F-14B (RBRG), then the ATA is 30L (degrees are usually omitted).
This is not a new topic as we ran into it whilst discussing the Crank manoeuvre (which should put the target at minimum 50ATA).

Target Aspect (TA)

The Target Aspect is defined as “The bandit’s perspective pertinent to the fighter”.

bvr-5-target-aspect

Note that the point of view doesn’t really matter as long as the Target’s heading doesn’t change, and that varying the F-14’s heading doesn’t affect the TA immediately, whereas it has an immediate effect on the ATA.

wvr-1-target-aspect-ata

Determining Target Aspect and Antenna Train Angle by means of the TID

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The ATA is the simplest information obtainable by means of the TID, since it matches the BR reading displayed on the monitor. As discussed before, that Bearing is Relative to the nose of the F-14, therefore calculating the ATA is done instantly. In the picture below, the BR is 335°, therefore the ATA is 360°-335°=25L.

When determining TA from the TID, make sure to use the “correct” representation: the following is the same contact, seen in Aircraft Stabilized mode (left) and shortly after in Ground Stabilized mode (right).

bvr-5-geometry-TID-AS-GS

As we know, the TID in AS mode shows vectors relative to the ΔV of the aircraft, so eyeballing the TA in this mode may not be immediately apparent. If you can do it, by all means go for it, but for new players especially, working in GS is much easier.
Using the Launch Zone option (TID Control Panel) helps even more because the displayed vector is usually longer, making the eyeballing process much easier.

Contact Drift

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With Drift, we mean the movement of the target on the TID. There are different types of Drift:

Turn Drift

wvr-1-drift-manoeuvring
Turn drift is induced by a change of heading of the Section: if the aircraft turns in a direction, the contact will drift in the opposite.
This is an immediate concept and happens, for instance, every time you are driving your car and turn at an intersection: if the road you were coming from was straight, you will see it drifting in the opposite direction whilst you turn.

The sketch on the right shows the same concept seen by means of the TID in Ground Stabilized mode: as the F-14 turns to the left, the target drifts to the right. Note how the target’s vector does not change in this mode. In Aircraft Stabilized instead we will see something like this (it is an example, it depends on the speed of the aircraft and the target although I maintained the Target’s original heading):

wvr-1-drift-manoeuvring-aircraft-stabilized
Example in Aircraft Stabilized mode.

Despite being such a simple concept, understanding it is a fundamental skill for the crew.

Intercept Drift

This drift is caused by the movement of both the aircraft and the targets. It seems an elementary concept because understanding that two (non-hovering) aircraft move in space, and depending on their speed and heading they appear in different ways the more they move is quite straight-forward. On the other hand, understanding how and if a target is drifting is a different story if you are new to it.

The sketches below try to clarify the situation, resembling the view in TID GS (please note that there is a certain degree of approximation in these sketches).

Example I: Drift

The target below (red) is heading ~150°. The F-14 is heading North. The target is slightly faster and this is what happens as they approach each other.

wvr-1-drift-2-A-targets-moving

This situation is even more clear if we add some “dynamism” to it: on the left, the images superimposed. On the right, the view from the TID.

wvr-1-drift-2-B-targets-moving-plus-TID

The grey dashed line represents the RBRG (relative bearing), or the ATA. The progressive changing of the ATA is caused by the drift. In this case, the target is drifting towards the left. This means that we are not following a Collision Course and the RIO needs to intervene on the course and/or the speed of the F-14 if that was the objective.

Example II: No Drift

This situation is slightly different: the target is now matching the speed of the F-14 and the two aircraft are on a Collision Course.

wvr-1-drift-2-C-collision-course-targets-moving

The difference between this scenario and the previous looks marginal but it is obvious when we add “dynamism” to the situation:

wvr-1-drift-2-D-collision-course-targets-moving-TID

Besides marginal differences caused by inaccuracies when drawing the sketches, it is clear how the ATA did not change as the distance was decreasing.

Displayed Drift

This is the sum of intercept and turn drift, as perceived by the crew on the display.

Understanding the drift is a crucial part of the Intercept Geometry.

Collision Course

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“[..] is when TA and ATA are equal in magnitude, but opposite in direction, for two co-speed, co-altitude aircraft. If two aircraft are co-altitude and on collision course, there is a high likelihood they will collide if neither makes a change to heading, altitude, or airspeed.”.
P-825 6-2 also offers the mathematical demonstration and shows how to calculate different values, such as the Distance from the point of intercept (the distance at which the two aircraft collide – for once I don’t have to do the maths myself! 😛 ).
Collision course maximises the closure rate, which decreases the time necessary to complete the manoeuvre and therefore minimizes the TTK (time to kill).
The reason why co-speed, co-altitude profiles are used is to simplify the procedure by cutting the Math required in the cockpit. Two aircraft can meet at a collision point starting by pretty much any aspect or speed, but this causes a lot of calculations and wastes a lot time.

If this is not making much sense now, trust me that it will later when the Intercept Geometry is discussed (or, if you don’t want to wait an eternity, just grab the P-825).

Collision course in Aircraft Stabilized mode

wvr-1-collision-course-TID-aircraft-stabilized-vectors
The discussion about the drift included a simple representation of the TID in Ground Stabilized mode. The Vector originated by a contact in this GS mode represents its speed and heading.
In Aircraft Stabilized mode instead, the displayed Vector is relative to the aircraft. This mode is usually slightly more complex to use, especially for new RIOs, but it has the huge benefit or telling immediately if a target is on collision course or not. In this case, in fact, the Vector originating from the target points straight towards the F-14. Knowing this, monitoring the collision becomes extremely simple and immediate, since it does not require constant checking of the ATA or eyeballing the flight paths.
Why does this happen? The answer is again hidden in basic Euclidean geometry and we’ll get there by means of a couple of examples.

Example I: Co-speed, Co-altitude

The following example shows the F-14 HDG 360 and a contact HDG 105; co-speed, co-altitude, using the familiar Ground Stabilized mode.

wvr-1-collision-course-TID-aircraft-stabilized-situation

The situation is different when switching to Aircraft Stabilized mode, since the Vector originated by the TID in AS is the Euclidean subtraction of the two Velocity Vectors. Norm and direction of the vector can be calculated very quickly by means of geometry:

wvr-1-collision-course-TID-aircraft-stabilized-triangles

This is a sketch representing the scenario described above on the TID in Aircraft Stabilized mode.

wvr-1-collision-course-TID-aircraft-stabilized-situation-2

Note how the contact’s Vector points directly towards the aircraft. This happens when the angles of the triangle created by the Headings and the SR of the aircraft equal the angles of the triangle created by the Velocity Vectors, which is the Vector displayed by the TID in AS (triangles with equal angles are similar).

wvr-1-collision-course-TID-aircraft-stabilized-triangles-2

If the triangles abc and a’b’c’ are geometrically similar, then the target is on Collision Course for co-speed, co-altitude aircraft (→ α = α’; δ = δ’; a~a’; b~b’; c~c’).

Calculating the absolute values of TA and ATA is immediate. α can be calculated as the difference between the heading of the aircraft and the contact. In this case it is 105° since the F-14 is flying due North. ATA and TA can be calculated immediately as:

δ = (180 – α) / 2 = (180° – 105°) / 2 = 37.5°

As long as δ is constant, the aircraft and the contact are flying paths that eventually will result in a collision. Since δ = ATA, as long as the contact is Relative Bearing ~323°, then the collision course is still valid.

Example II: Correcting by manoeuvring

This is a similar situation but the two triangles are not similar. How to deal with this situation is a topic that will be faced later on. At the moment, the simplest solution is trying to recreate the situation discussed in the previous example.

wvr-1-collision-course-TID-aircraft-plate-1

Image I and II show the situation in Ground Stabilized mode. Despite being co-speed and co-altitude, the aircraft and the contact are not on a Collision Course. The triangle created by plotting the FFP, the BFP and the SR is not isosceles as a ≠ b.

wvr-1-collision-course-TID-aircraft-plate-2

Now the same situation in Aircraft Stabilized mode. I approximated the Vector geometrically and extended it (III).
The dashed lines in Image IV are the same triangle displayed previously. I also added the triangle used to draw the Vector in AS mode (green). The triangles look similar but they are not geometrically similar.

wvr-1-collision-course-TID-aircraft-plate-3

I scaled the green triangle and moved the contact to match the new position. Two things can be noticed:

  • Image V, the Vector in AS now points again straight towards the F-14;
  • Image VI, in GS, a and b are now equal (barring the usual imprecisions). The triangle is now isosceles.

The aircraft and the target are now on Collision Course.

Example III: Non co-speed scenario

Although manoeuvring to match a co-speed, co-altitude profile makes setting up a Collision Course much simpler, it is not a strict requirement.
The required TA and ATA to set up the Collision Course can be calculated or eyeballed via the TID AS, as the Vector shows the position of the point at which BFP and FFP intercept each other. This does not mean that the two aircraft will collide, unless such point overlaps with the position of the F-14 on the TID. In simple words, the Vector shows where the contact should look to see the aircraft. Since in this scenario we want the contact to look at the F-14, we want to position this imaginary line of sight over the F-14. Viceversa, if our goal was to break the collision, we would manouvre to place the Vector away from the aircraft.

wvr-1-collision-course-TID-aircraft-stabilized-ex3-demo1

If ratio of the distance DA-A’ (between aircraft A and the intersection of FFP and BFP, A’ – the Collision point) and the distance DA’-B (between the intersection A’ and the contact B) is equal to the ratio of the norm of the velocity vectors, then there is collision course. The demonstration is quite immediate as the FFP and the VVF-14 are parallel and cut by a line so the corresponding angles are equal. As two angles are equals, the triangles are similar. Even without the complete mathematical proof, it is intuitive how the necessary condition for this scenario to work is having the TID AS Vector pointing towards the aircraft.

wvr-1-collision-course-TID-aircraft-stabilized-ex3-sketch

Understanding and Correcting Drift Excess

The following considerations vary depending on the spatial positioning of the aircraft and the contact. For the sake of simplicity, a situation similar to the one described in the example above should be considered.

Understanding how a contact is displayed on the TID helps to correct a situation where the contact is drifting (if deemed necessary, as this is not always the case) but also helps general SA.

The first step is understanding where the contact is drifting. Intercept drift is usually caused by lack of co-speed or different TA and ATA. The drifting can be assessed by looking at the direction of the Vector on the TID AS or by monitoring the BR reading on the TID.

A non corrected drift may cause the contact to fly in front of the aircraft, and this is usually not a bad thing. It can actually be the intended outcome. The opposite instead should be avoided as it places the F-14 in a very vulnerable position.

Visualizing Intercept Drift and Collision Course

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The concepts discussed above may sound very abstract. Visualizing them on the TID definitely helps:

The immediate observation is that as the SR decreases, the more TA and ATA fluctuate unless the contact is not perfectly on collision course.

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