2dRMS is short hand for 2 × delta RMS. Delta indicates we are using the difference in data points. In this case the distance between individual intersections of perpendicular bisectors and the computed fix. RMS is short hand for Root Mean Square, and is a method of computing a useful average value. Assuming normal distribution of errors, the probability is 95% to 98% that the emergency transmitter will lie within the 2dRMS distance of the computed fix location.



CEP is short hand for Circular Error Probability

Cocked Hat

A term from navigation a "cocked hat" is the triangle that forms when three lines of position are plotted on a chart. It comes from the shape of the hat worn by naval officers at the time the phrase was coined. Two or more lines of position are needed to 'fix' the position in a single location. The size of the "cocked hat" gives an indication of the errors accumulated in the bearings and drawing the lines on the chart.

Emergency Transmitter

A radio transmitter activated either automatically or manually in the event of an emergency.

ELT - Emergency Locator Transmitter

An emergency transmitter used in aircraft. Certified under a number of Technical Servers Orders (TSO) C-91 and C-91A transmit an amplitude modulated swept tone on 121.5 MHz, C-126 transmit a short duration digital signal on 406 MHz and the same swept tone as C-91/C-91A units on 121.5 MHz

EPIRB - Emergency Position-Indicating Radio Beacon

An emergency transmitter used on ships, boats, life rafts and some life preservers and survival suits.

Line of Position

A line of position is a partial result of a navigation fix. One example is a bearing to a known location. The captain of a ship may take a compass bearing on a known lighthouse. From that the captain knows the ship is located somewhere along a line drawn from the lighthouse along the reciprocal of the bearing. The distance from the lighthouse may be estimated, but not with acceptable accuracy, so the captain only knows the ship is positioned somewhere along the line. 

PLB - Personal Locator Beacon

An emergency transmitter used when the use of an ELT or EPIRB is not appropriate or not allowed.


Search And Rescue Satellite. An electronic package carried on satellites designed to detect and locate emergency transmitters.


A radio frequency not deliberately created or transmitted, especially in a device which does normally create other frequencies.