Modern aircraft have a number of systems to their disposal to enhance flight safety. The common term is aircraft collision avoidance systems.
These range from radar, TCAS, portable CAS, terrain awareness system (TAWS), ground proximity warning system (GPWS), synthetic vision in EFIS and the obstacle collision avoidance system.
TCAS or Traffic Collision Avoidance System interrogate other transponders and process these replies from other aircraft enabling them to show these on the PFD, MFD displays or a special IVSI indicator.
Some even issue aural warnings to the pilot helping him to avoid a potential midair collision or even flight into terrain.
Mode S transponders even exchange data with them so that aircraft equiped with these systems know where you are and are able to remain at a safe distance from you and other traffic.
For the experimental homebuilt aircraft pilot these are usually quite expensive systems, except for the portable CAS and the good old see and be seen system (which has its flaws).
These are used to prevent controlled flight into terrain (CFIT) accidents. Based on radar altimeters in ground proximity warning system (GPWS) and GPS based terrain awareness systems (TAWS). Modern EFIS systems include a form of TAWS by using GPS data to create a synthetic vision, which is a 2 or 3D view of the landscape around the current position of the aircraft displayed on screen.
Automatic Collision Avoidance Technology (ACAT) is developed by the NASA Dryden Flight Research Center to be used in a smartphone as an assisted automatic ground collision avoidance system which steers the aircraft away from the ground or mountains by using GPS, accurate ground maps and a connection to the aircraft flight controls. The technology could help prevent controlled-flight-into-terrain accidents by general aviation and unmanned aircraft.
More information here in a YouTube film from Airboyd: ACAT from NASA
Terrain Awareness and Warning System is the common name of a system to avoid CFIT accidents. It is divided into classes A, B and C where A is the most sophisticated in terms of detection of, for example: imminent impact, reduced terrain clearance, very high rates of descent or negative climb rates after take-off and more.
Class A systems are able to indicate excessive closure rates, aircraft not in landing configuration during descent and ILS deviations, all of this is displayed to the pilots showing all relative data.
Sandel has expanded TAWS with their ST3400H HeliTAWS terrain avoidance system so that low flying pilots are aware of obstacles like high tension lines which are sometimes totally unvisible against the background. This problem already has caused serious accidents in the past where people have lost their lives and power delivery was disturbed.
A Ground Proximity Warning System (GPWS) is based on a radar altimeter system, where the altitude of the aircraft above terrain is presented to the pilot as an indication of absolute altitude of the aircraft above the terrain below (AGL - Above Ground Level).
The system transmits a radar pulse vertically downward and listens for the response, that time is translated into altitude and indicated on a display. The range is usually from 40 ft to 2500 ft. Voice call-outs are used during the final approach for landing.
Developed in the late 70s and 80s it is primarily used to enhance pilot situational awareness around the aircraft in relation to terrain and obstacles. The system can be connected to databases with geographical, terrain, aeronautical and other information sets to display that on the PFD or Multifunction display.
For position information a SVS uses inertia or GPS data, where GPS is used in low cost and entry level systems. As a navigation enhancement, so called highways in the sky (HITS) are pictured on the display to help the pilot navigate easily through purple rectangles and he/she can remain clear of any obstacles by flying inside these rectangles.
Data from forward looking radar can be added to the SVS to produce a more detailed picture regardless day or night. This way objects not in one of the databases but detected by radar are shown in real time on the displays.