The Joint Electronic Designation System is the series of characters preceding an alphanumeric on the front of all military specification, (mil-spec,) electronics. This does not apply to ordinance, vehicles, or aircraft which follow completely different designation systems. Designations are broken down into three letters, each of which indicate specific parts of the unit's purpose. The number following the three letter prefix constitutes the design number, and if applicable which variant of the system the unit is.

In other words, by the time you get done reading this you will be able to determine what this bit of blather means: AN/APS-124B

Letters and meanings:

1st. Position-
A= Piloted aircraft
B= Underwater, mobile, submarine
D= Pilotless (drone) carrier
F= Fixed ground
G= General ground use
K= Amphibious
M= Ground, mobile
P= Portable
S= Water
T= Ground, transportable
U= General utility
V= Ground, vehicular
W= Water, surface and underwater
Z= Pilot and pilotless (drone) airborne vehicle combination

2nd. Position-
A= Invisible light, heat radiation
C= Carrier
D= Radiac
G= Telegraph-teletype
I= Interphone and public address
J= Electromechanical or inertial wire covered
K= Telemetering
L= Countermeasures
M= Meteorological
N= Sound in air
P= Radar
Q= Sonar and underwater sound
R= Radio
S= Special types
T= Telephone (wire)
V= Visual and visible light
W= Armament
X= Facsimile or Television
Y= Data Processing

3rd. Position-
B= Bombing
C= Communications
D= Direction finding, reconnaissance, surveillance
E= Ejection, release
G= Fire control
H= Record-reproduce
K= Computing
M= Maintenance/Test
N= Navigation aids
Q= Special purposes
R= Receiving
S= Detect/range, bearing, search
T= Transmitting
W= Automatic flight or remote control
X= Identification and recognition
Y= Surveillance and control

Essentially what all of this enables a technician to do is figure out what they are looking at if they're say rooting through a closet and find some old crusty thing with connectors on the front. (This has happened in the past.) Therefore, by breaking down the example given earlier we can determine the following from AN/APS-124B:
AN/= Standard prefix. (This actually means 'Army/Navy.' This will precede all three digit type codes.)
A= Piloted aircraft
P= Radar
S= Detect/range, bearing, search
124B= Somewhat arbitrary number used to keep track of what it is. These numbers are by no means sequential. The 'B' at the end of the number indicates that in the 124 series search radars mounted on piloted aircraft it is the second variant.

Sources: AVA(A1) Avionics School class materials, and Navy Electronics Education Training Series modules. Public domain, available through the Government Printing Office.

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