During your career as a pilot you will see more advanced navigation systems in the airplanes you fly. Systems like Radio Magnetic Indicators, Horizontal Situation Indicators and navigation systems like RNAV, LORAN (now obsolete) and satellite navigation systems as GPS.
We will touch the surfaces on each of these systems and discuss how they work basically. As each manufacturer creates his own system it is almost impossible to discuss every system in depth on the market today.
GPS, Global Positioning System, is a space satellite based navigation system, in contrary to LORAN and RNAV which are terrestrial / Earth based. It provides highly accurate time, positioning and speed information to its virtually unlimited users. The system is online 24/7/365 and not affect by weather whereas LORAN and VORs can be. Its navigation is based on a worldwide grid reference system (WGS84), constant knowledge of the spatial position of the satellites and a very accurate time system.
At this time there are several GPS based navigation systems active or in the process of being commissioned: US - NAVSTAR (most commonly known), EU - Galileo, Russia - GLONASS and China - Beidou. The last three systems do not have dedicated devices as the US system has but for the user this means choice. And having a navigation system capable of receiving more than one system means more accuracy and a much higher reliability.
Most modern GPS capable mobile devices and dedicated aircraft navigation systems these days receive more than one system, mostly NAVSTAR and GLONASS.
The GPS receiver has a number of channels (8, 12 up to 20 or more) to receive and track satellites in its view. The civilian use frequency is 1575,42 MHz (L1) and 1227,60 MHz (L2). The satellites orbit the Earth in 90 minutes and there are at least 24 satellites active/online. These signals are (with triangulation) translated into a two or three dimensional position fix (three satellites, with four it can calculate altitude and with five it can single out errors). Accuracy for civil users is around 328 feet/100 meters, although this is improving it is enough to find an airport. Military users have, of course, a much higher accuracy. For those of you wishing more detailed information, visit the GPS.gov website.
Satellite based navigation once started as a backup to good the old VFR / radio navigation systems (although some early users used it as their primary system) and it has evolved into an IFR certified navigation system serving direct navigation, enroute and GPS approaches to runways.
Garmin has a number of aviator GPS capable watches (D2 series), which are ideal to be used as a navigation backup for flight instructors as they sometimes hop from plane to plane (I have one too!). Worthwhile to check out!
The RMI, Radio Magnetic Indicator, is a combination of the directional gyro (heading indicator) with two bearing pointers. The double needle is the ADF/NDB and the single needle is the VOR or sometimes another ADF, depending on configuration. Usually the RMI compass card is a slaved one referenced to magnetic north.
A HSI, Horizontal Situation Indicator, combines a heading indicator with a VOR indicator with deviation dots in one display. Included is also a glide slope indicator for ILS approaches and a TO/FROM flag.
The advantage of these indicators is a very high situational/navigational awareness regarding your position. The chance of getting lost or being unsure of your position is very low.
RNAV is a system that uses a VOR/DME or a VORTAC (VOR and Military TACAN) in combination with a course line computer (CLC). This CLC is able to create phantom VORs, it electronically relocates the VOR from the original position in such a way that this phantom VOR becomes a way point along your route. This makes it possible to fly in a straight line, ie following (zigzagging to) VORs, to your destination. Hover the mouse over the picture to the right.
An advantage of RNAV is that you can navigate with the VOR indicator. Needle deflection indicate course corrections but the deviation scale (dots) is in nautical miles i.e. degrees. The DME part will show the correct distance to the way point, ground speed can differ. That depends on what the manufacturer of the CLC has set.
RNAV provides course and distance information and gives you the ability to fly direct routes thereby saving fuel and time lowering your aircraft expenses.
LORAN-C is a LOnge RAnge Navigation system type C. It is based on a number of worldwide located high power low frequency (LF: 90 - 110 kHz) stations transmitting a set of coded pulses from a master station and a secondary station transmitting after the master pulse.
The future of LORAN has been quite uncertain as GPS was slowly taking over as the main navigation system in the 90's. Personally I think it would be a good backup to any satellite based navigation as LORAN signals are strong and difficult to jam and can be received 24/7/365.
The US Coast Guard Navigation Center, which used to operate LORAN-C, has a web page dedicated to this system. It was decommissioned in 2010 because it was deemed to be unnecessary due to technological advancements. But hey, who needs a backup?
2019: From the Stanford ENGINEERING GPS Lab:
With the perceived vulnerability of GNSS systems, and their own propagation and reception limitations, renewed interest in LORAN applications and development has appeared. Enhanced LORAN, also known as eLORAN or E-LORAN, comprises advancement in receiver design and transmission characteristics, which increase the accuracy and usefulness of traditional LORAN. With reported accuracy as good as +/-8 meters, the system becomes competitive with unenhanced GPS. eLORAN also includes additional pulses, which can transmit auxiliary data such as DGPS corrections. These enhancements in LORAN make it a possible substitute for scenarios where GPS is unavailable or degraded.
So I was right after all those years. They need a backup just in case... Read more about this in this next PDF file for using eLORAN as a GNSS backup. A bit dated but it explains the LORAN system perfectly