Updated: 2023-08-11 --- # Radar Principles RADAR or radio detection and ranging is the primary system used by military units to detect entities at long range. A common type of radar used by military systems is the pulse radar or a variation called a pulse-doppler (PD) radar. In these systems, pulses of radio energy are sent out, these pulses bounce off of objects and back toward the transmitter/receiver. These “echos” are interpreted by the system and show up as radar returns on the scope. The system calculates the distance of the object based on the time it took for the radar energy to travel to the object and back, and it calculates the azimuth of the object based on what direction the antenna was pointing when it received the return. Pulse doppler systems can also use the doppler shift of the return to determine the relative velocity of the object to the radar, which is known as “Closure” ![[Pasted image 20230811235845.png]] ![[radar-love-search.gif]] # Range and Azimuth Resolution Radars have limitations in their ability to differentiate contacts based on the proximity of those objects and their distance from the radar. The radar’s range resolution is the distance in range between two contact relative to the radar where both contacts can be resolved. If contacts are close enough together they will appear as a single contact. Range resolution does not change with the range to the contacts. The radar’s azimuth resolution is the distance in azimuth between two contacts relative to the radar where both contacts can be resolved. If two contacts are close enough together they appear as a single contact, azimuth resolution increases with range, i. e. It gets harder to resolve multiple contacts as they get farther away. ![[Pasted image 20230812000012.png|400]] ![[Pasted image 20230812000016.png|400]] # Radar Horizon Radar horizon is the limit of the radars ability to see around the curvature of the earth. It is a function of the altitude of both the radar’s and target’s altitude/elevation. The higher either the radar or target is, the farther the target can be seen by the radar. A decent rule of thumb is that an aircraft at 15,000ft has a radar horizon for surface contacts of 150 nm. From this number, each 1000 ft of altitude changes the radar horizon by 5 nm. E.g. A Radar at 10,000 would have a surface horizon of 125 nm. If the target is above the surface, do the same calculation then add the ranges together and subtract 25 nm. E.g. A Radar at 25,000 ft could detect a target at 5,000 ft at ~275 nm (200 nm + 100 nm - 25 nm) ![[Pasted image 20230812000101.png]] ![[Pasted image 20230812000106.png]] # Clutter Radar energy does not know the difference between an airplane, a ship, the ground, or a particularly dense cloud, it bounces off all these things and returns them all to the radar. Objects that are not desired radar returns are called clutter. There are two primary types of clutter. Mainlobe clutter is present in the primary direction the radar is looking and usually consists mostly of the ground returns. Mainlobe clutter can be filtered out, but doing so usually creates radar “blind spots” where aircraft with certain closure rates are filtered out. Sidelobe clutter is energy that “splashes” out in different directions than intended. It must be filtered out to create a usable radar picture, and this process can also create “blind spots”. ![[Pasted image 20230812000136.png]] # Refresh Rate and Track Files All radars have a refresh rate, this is the rate which the position of the returns are updated. This can vary from milliseconds in the case of single target tracking radars to seconds in the case of search radars. The refresh rate is usually primarily influence by how often the target is illuminated by the radar. In order to provide target data between radar refreshes, mission computers build track files. A track file is a record of the position of a contact over multiple refreshes. The positions are compared and along with closure information are used to create a course and speed for the contact. This information is used to dead-reckon the position of the object between refreshes. # Moving Target Indicator (MTI) Moving Target Indicators are a special capability of some search radars. It uses the phase change of multiple pulses on a contact in order to locate moving targets against background clutter.  MTI enables any moving ground targets to be detected despite the presence of ground clutter, most MTIs have adjustable thresholds and can detect objects moving even very slowly. One less well known capability of the MTI is that even rotating radar dishes provide enough movement to be detected, any rotating search radar will be located immediately after it begins rotating.  ![[Pasted image 20230812000208.png]] # Non-Cooperative Target Recognition (NCTR) NCTR is a special capability of some radars, it uses reflections from radar waves that travel down the intakes of aircraft and reflect the turbine blades. These reflections are distinctive enough that aircraft types can be determined by these reflections. NCTR requires the radar to be able to see down the intakes, so it requires aircraft to have a front/front quartering aspect in order to function NCTR is only available on some high end radars. # SAR Synthetic aperture radar or SAR Radar is a technology where certain radars can take many images overtime from different angles and stitch them together to form imagery of stationary objects. SAR imagery can be very high resolution, and provide significant detail about potential targets. SAR imagery takes time to generate, minutes to tens of minutes, it requires aircraft in certain parameters and has a range in the 10s of miles from the aircraft. Higher altitude aircraft can SAR more distant targets. SAR imagery cannot image moving targets. ![[Pasted image 20230812000244.png|300]] ![[Pasted image 20230812000253.png|300]] # ISAR Inverse synthetic aperture radar or ISAR, is a technology where the motion of an object is used to image it from multiple angles in order to create a usable image of an object. ISAR imagery can be used only on moving objects (although pitch and roll from sea state can be enough motion to generate useful imagery.) ISAR imagery requires the altitude difference between the radar and the object being imaged be within a narrow range. ISAR works best from a front quartering aspect. ISAR imagery requires a skilled operator to interpret what is being detected. ![[Pasted image 20230812000325.png]] ![[Pasted image 20230812000329.png|300]]