Editing True-range multilateration (section)

== Obtaining ranges ==
{{main|Ranging}}

For similar ranges and measurement errors, a navigation and surveillance system based on true-range multilateration provide service to a significantly larger 2-D area or 3-D volume than systems based on pseudo-range [[multilateration]]. However, it is often more difficult or costly to measure true-ranges than it is to measure pseudo ranges. For distances up to a few miles and fixed locations, true-range can be measured manually. This has been done in [[surveying]] for several thousand years {{snd}} e.g., using ropes and chains.

For longer distances and/or moving vehicles, a radio/radar system is generally needed. This technology was first developed circa 1940 in conjunction with radar. Since then, three methods have been employed:
* Two-way range measurement, one party active {{snd}} This is the method used by traditional radars (sometimes termed ''primary'' radars) to determine the range of a non-cooperative target, and now used by [[laser rangefinder]]s. Its major limitations are that: (a) the target does not identify itself, and in a multiple target situation, mis-assignment of a return can occur; (b) the return signal is attenuated (relative to the transmitted signal) by the fourth power of the vehicle-station range (thus, for distances of tens of miles or more, stations generally require high-power transmitters and/or large/sensitive antennas); and (c) many systems utilize line-of-sight propagation, which limits their ranges to less than 20 miles when both parties are at similar heights above sea level.
* Two-way range measurement, both parties active {{snd}} This method was reportedly first used for navigation by the [[Gee-H (navigation)|Y-Gerät]] aircraft guidance system fielded in 1941 by the Luftwaffe. It is now used globally in air traffic control – e.g., ''secondary'' radar surveillance and DME/DME navigation. It requires that both parties have both transmitters and receivers, and may require that interference issues be addressed.
* One-way range measurement {{snd}} The [[time of flight]] (TOF) of electromagnetic energy between multiple stations and the vehicle is measured based on transmission by one party and reception by the other. This is the most recently developed method, and was enabled by the development of [[atomic clocks]]; it requires that the vehicle (user) and stations having synchronized clocks. It has been successfully demonstrated (experimentally) with Loran-C and GPS.<ref name="Grant">"[https://journals.lib.unb.ca/index.php/ihr/article/download/23824/27609 Rho-Rho Loran-C Combined with Satellite Navigation for Offshore Surveys]". S.T. Grant, ''International Hydrographic Review'', undated</ref><ref name="Zhang" />