ATIS Automatic Terminal Information Service (recorded voice message that provides weather and airport services information).
The ATIS is transmitted on the VHF aviation band (normally at the lower end) and gives operational runway/s, wind, temperature, visibility and other significent terminal information. Each update is given a alphabetic character designator i.e. ALPHA then BRAVO etc.

DME Distance Measuring Equipment (ground navigational aid that can provide display of distance to selected ground navigational radio transmitter).
With this Australian invention aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs - two pulses of fixed duration and separation. The ground stations are typically collocated with VORs. A typical DME ground transponder system for enroute or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.

ILS Instrument Landing System (uses precision LOCalizer and glide-slope or Glide Path radio transmitters near a runway to provide landing approach guidance).
The ILS uses the 108.10 MHz to 111.95 MHz VHF band. An ILS consists of two independent sub-systems, one providing lateral guidance (Localizer), the other vertical guidance (Glideslope or Glide Path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.

MLS Microwave Landing System.

NDB Non-directional Radio Beacon. NDBs may be operated on a frequency between 190 kHz and 1750 kHz although normally operate between 190 kHz and 535 kHz. Each NDB is identified by a one, two, or three-letter Morse code callsign.
Automatic Direction Finding (or ADF) equipment on the aircraft detects an NDB's signal and determines the direction to the NDB station relative to the aircraft. This may be displayed on a relative bearing indicator (RBI). The ADF can also locate transmitters in the standard AM mediumwave broadcast band (531 kHz to 1602 kHz).

VOR VHF OmniRange Navigation System (ground navigational aid that can provide display of aircraft position relative to course through selected ground navigational radio transmitter).
VORs are assigned radio channels between 108.0 MHz and 117.95 MHz in the VHF aviation range. The VOR system uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction.

ACARS Aircraft Communications Addressing and Reporting System is a digital datalink system for transmission of small messages between aircraft and ground stations via radio or satellite. These messages could be anything from Airways Clearence Request and delivery, Flight Management System messages to Aircraft Management System messages

ADS-B Automatic Dependent Surveillance Broadcast
An ADS-B-out equipped aircraft determines its own position using a global navigation satellite system and periodically broadcasts this position and other relevant information to potential ground stations and other aircraft with ADS-B equipment (a suitable Mode S 1090Mhz ES transponder, GPS etc.).

Mode S Type of secondary surveillance radar (SSR) equipment which provides Mode A and Mode C interrogations, discrete address (Mode S) interrogations from the ground or air, and a data link capability.