Operations Research: INtercept DIAgrams model (INDIA) – 1996


INtercept DIAgrams model (INDIA) – 1996

At TNO-FEL, air defence studies are performed to support the armed forces during the procurement, upgrade and deployment of air defence systems. Various tools and models have been developed for quantitative analysis relevant to those studies. One of those models is INDIA (the INtercept DIAgrams model), developed to support deployment analysis. 

INDIA’s main shell combining four separate modules. (1990's figure capture)
INDIA’s main shell combines four separate modules. (1990’s figure capture)

Generally, the air defence planning process is based on several well-known standard factors. The most important factors are:

  • mission,
  • enemy, and
  • terrain.

The factor mission describes air tasks to be performed by the defence, for instance, the protection of assets. The importance of the assets, being point targets or areas to be defended, determines the required strength of the defence.

The factor enemy describes the types and numbers of attackers that can be expected to attack the asset. An expected attack direction might also be known. The type of weapons that can be launched and the expected release distances might be known as well. This information can be transformed into a planning line around the assets, often referred to as the Estimated Weapon Release Line (EWRL). The air defence must then attempt to engage (intercept) the enemy before this EWRL to prevent weapon release.

The factor of terrain determines the natural conditions that restrict the deployment options in two ways. It makes some positions unsuitable to be used for the deployment of air defence systems because the positions cannot be reached by road and it restricts the Line-of-Sight or engagement possibilities from chosen positions.

Apart from these factors, the air defence planner also has to take into account things like expected weather conditions, countermeasures and redeployment or mobility requirements. Based on the factors described above the air defence planner has to determine which types and numbers of air defence systems are to be deployed at which position. Training and experience of the planner play an important role in this process, for example concerning the knowledge of air defence system performance characteristics and interpretation of terrain suitability from military maps.

If enough time is available the planner may send out reconnaissance (Recce) teams to find suitable locations to deploy air defence systems. These Recce teams can also measure the Line-Of-Sight restrictions from those locations. Such measurements are then translated into coverage diagrams. The coverage diagrams can be used as overlays on the area map with the EWRL around the asset to find out if the examined locations are acceptable.

An air defence planning problem is that the coverage does not enable the air defence planner to evaluate whether or not the chosen deployment will be able to engage (intercept) attackers outside the EWRL. The coverage only shows the Line-Of-Sight conditions for the deployment. This means that the planning process heavily depends on the air defence planner to make an estimation (based on his training and experience) of the intercept capability of the deployment versus the EWRL. Making in a limited amount of time, a good estimation for the deployment of several air defence systems (maybe even different types) against an enemy that may approach from different directions, with terrain conditions that vary in all directions, must be considered as an (almost) impossible, and certainly subjective, task. For these reasons, we have decided to develop a model called INDIA that can objectively perform this estimation.

INDIA is a model calculating and drawing different types of intercept diagrams based on digitised terrain databases. Those intercept diagrams show the intercept capability of the deployment graphically around the protected asset. In addition, INDIA provides quantitative measurements of the defensive quality against air defence guidelines. By quantifying the degree to which the deployment achieves the aim of the guideline, it will be possible to rank the various deployment options concerning six basic guidelines. Together with the intercept diagrams, this should give the air defence planner enough information to make an objective deployment choice, depending on his air defence mission, which determines the relative importance of the six guidelines.

Several factors influence the position of an intercept. These factors are:

  1. Terrain, determining the unmask and re-mask moments
  2. Air defence system:
    • command cell
    • sensor characteristics
    • weapon characteristics
    • locations of system elements (sensors and launchers)
    • engagement logic, including time delays for various phases of the engagement
  3. Target or attacker including altitude, speed and signatures

INDIA uses a digital terrain database (Digital Land Mass System (DLMS) from the US Digital Mapping Agency (DMA)) that contains both terrain elevation data and data of features (built-up areas, woods, etc.) on top of the terrain. Based on this database and the target altitude the mask and unmask positions can be determined for every possible target track.
The description of the air defence system contains the data (sensor envelopes and time delays) for the main mode of operations. A simulation model is built into INDIA to calculate, for every possible single target track, the intercept positions. This simulation includes a simple fly-out model. The fly-out model consists of a guiding principle and flight trajectory calculation method (for example F&F guidance using a pure following/pursuit trajectory).
Another feature of INDIA is the possibility to take into account the individual positions of the different sensor systems and the weapon launchers. This enables, for example, the examination of distributed architectures, where search or surveillance sensors, tracking sensors, guidance sensors and some weapon launchers are all positioned at different locations with different Line-Of-Sight restrictions.

The simulation model follows the basic timeline: unmask, acquisition, identification 1, target prioritization, target tracking, identification 2, target/weapon allocation, launch decision guidance and fly-out, intercept, and kill assessment. The simulation can go forward to the next phase on this timeline when conditions are met like target in range and in the sector, time-delay passed, and Line-Of-Sight provided. Similarly, the simulation can go back to one or more phases when such conditions are not met. Using this approach the intercept position can be calculated on every possible target track.

INDIA makes it possible to determine the positions of the first, second, n-th and last intercept. Based on the intercept position calculations, intercept diagrams can be produced. For this purpose, two threat options are distinguished: a unidirectional threat and an omnidirectional threat.

The unidirectional intercept diagram (image made in the 90's)
The unidirectional intercept diagram (image made in the 1990s)

For a unidirectional threat, equidistant and parallel tracks are defined over the width of the deployment. Intercept positions are calculated for each of those tracks. An intercept line (first, Nth or last) is then created by connecting these points. See figure.

The omnidirectional intercept diagram (image made in the 90's)
The omnidirectional intercept diagram (image made in the 1990s)

Omnidirectional threat tracks are defined through the centre or reference point (for example the defended asset) of the deployment from all directions.
In a similar way as with the unidirectional threat, the intercept lines can be determined (see figure). The unidirectional intercept diagram makes it possible to evaluate a deployment for area defence against a threat coming from one direction. The omnidirectional intercept diagram is intended to represent a point defence situation.

Measures Of Effectiveness are developed for the following six guidelines:

  • balanced fires
    equal firepower in all directions
  • weighted coverage
    concentrated firepower in the most likely attack direction
  • early engagement
    engaging the enemy before ordnance release
  • defence in depth
    subjecting the attacking aircraft to an ever-increasing volume of fire as it nears the defended asset
  • overlapping fire zones
  • overlapping engagement zones of the air defence units
  • mutual support
    protecting neighbouring air defence systems.

An example showing how a guideline can be developed into a quantitative MOE is given here.
The guideline of early engagement is used for this example. Early engagement aims to position and orient fire units so that they are capable of engaging aircraft before weapon release. To get a quantitative measurement of the degree of early engagement, the EWRL around the deployment must be defined.

The moment of the first intercept is determined, for each track over the deployment area, several seconds before the EWRL. The MOE is the average value over all tracks. The higher the value of this MOE, the better the deployment achieves early engagement.
Additional MOEs for this guideline can be the standard deviation of the MOE and the percentage of tracks where engagement before EWRL is not possible.
Note that for these MOEs it does not matter whether a unidirectional or an omnidirectional intercept diagram is used for the calculations. Similarly, MOEs have been defined for the other basic air defence guidelines.

Example of measure of effectiveness for early engagement
Example of measure of effectiveness for early engagement

In November 2003, INDIA was sold to BAE Systems (Defense Systems) Ltd for integration into BAE’s Tactical Command Software (TCS) for the Ground-Based Air Defense System (GBADS) of the South African National Defense Forces (SANDF).