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I would like to know the fundamental difference between GPS, GNSS and RNAV. How are they related each other? Are all/some of them synonyms?

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As stated in the othe answers,

  • GPS is the US Global Positioning System
  • GNSS is an umbrella term that encompasses GPS as well as other nations' satellite systems that achieve essentially the same capability

RNAV is the aircraft capability that allows you to navigate from point to point, defined by Latitude/Longitude and independent of any ground-based system. There are various ways that the hardware in the aircraft maintains its position. It may use GPS sensors, it may use GLONASS, it may uses a combination of satellite systems, it may use DME/DME updates from ground-based navaids. But the point of RNAV is that for the most part the procedures are agnostic with regard to how the position info is obtained.

Thus, you can have RNAV routes and approaches, meaning that the airway or approach segments are defined by Lat/Lon waypoints, and as long as your system meets defined accuracy tolerances (usually 1.0 NM enroute and 0.3 NM for an approach in the US), you can fly that airway or approach. There are more stringent tolerances incorporated into RNAV(RNP) approaches, and these require that the system can monitor its accuracy and alert the pilot if the system isn't confident that its position is accurate to within a specified performance. This can be set to match whatever is required on the approach; 0.1 NM is common, although there are other values that can be specified.

In the US, the FAA typically distinguishes basic RNAV approaches by designating them RNAV(GPS) in the title, as opposed to RNAV(RNP). (The RNAV(RNP) approaches require a couple of additional things beyond basic RNAV, but those are beyond the scope of this question.) Internationally, "RNAV(GNSS)" is often used instead of "RNAV(GPS)" in the approach title, but functionally they're pretty well the same thing -- it's simply a nod internationally to the fact that if your FMC knows where it is, the approach designer doesn't care which satellite system it's getting its position data from.

In the "old days", it was possible to have RNAV operations that worked by taking radial/DME data from a single VOR and creating a pseudo-VOR essentially whereever you wanted, and the HSI or other display would act as if the VOR were in the new location. This was very early RNAV capability, in that it allowed you to fly a straight line course by keeping the HSI centered rather than flying VOR to VOR. These days, that capability is pretty much gone, in favor of the much greater capability of GPS.

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  • $\begingroup$ The last paragraph sounds like the pseudo-VOR mode of HSI is related to determining position from VORs, but those are two independent things. The HSI still behaves that way if displayed, just map display is used more often now instead. And most new systems probably still can fall back to determining position from VOR/DME distance and radial, just they prefer GPS when available. $\endgroup$ – Jan Hudec Jun 9 '16 at 11:59
  • $\begingroup$ @JanHudec The old equipment with the pseudo-VOR was not what we'd consider today as RNAV in that it did NOT use Lat/Lon -- it simply "displaced" the VOR (which you had to be receiving) a specified distance on a given radial, and your HSI still worked like a normal HSI for a VOR (course select, etc). The capability now to determine Lat/Lon using DME updates from several VORs is entirely different - real RNAV now, and an RNAV course now does NOT look like a traditional HSI -- no course selecting is available, the course is driven by the system. Two entirely different capabilities. $\endgroup$ – Ralph J Jun 9 '16 at 12:37
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Are you asking about the acronyms on their own or in relation to instrument approaches?

Generally speaking, GPS stands for Global Positioning System and is a satellite based navigation system built and maintained by the US government.

GNSS stands for Global Navigation Satellite System, which encompasses GPS as well as the Russian built and operated GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema/"Global Navigation Satellite System" in Russian) and Galileo (EU) or Beidou (China).

RNAV stands for Area Navigation and is one of the main components of the US National Airspace System.

All of these things are forms of navigation used by aircraft and sometimes boats too.

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    $\begingroup$ RNAV no longer indicates ground based stations like VORs and DMEs. It can certainly use that though the procedure (SID or STAR) may limit that. DME\DME RNP 0.3 NA. It is primarily based on satellite GNSS systems. In the past, it was used with pseudo VORs and IRS/INS systems. Many FMS systems today can fall back to using the older technology when GNSS accuracy is reduced. $\endgroup$ – wbeard52 Jun 8 '16 at 16:11
  • $\begingroup$ @wbeard52 I took that part of his answer to mean that the radio nav-aids were used as waypoints in the GNSS. $\endgroup$ – Ryan Mortensen Jun 8 '16 at 17:16
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    $\begingroup$ No, the statement "RNAV is generally based on the ground-based radio navigation beacons like VOR's" is simply wrong. The RNAV systems certainly CAN navigate to a waypoint that corresponds to a VOR or to a fix that is also defined off of a VOR, but RNAV routes & procedures are increasingly based off of points defined by Lat/Lon that have no correspondence to anything ground-based. Also, do boats use RNAV? Hadn't heard of that one before. $\endgroup$ – Ralph J Jun 8 '16 at 18:11
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    $\begingroup$ Not only is that wrong, but it is exact opposite of what it should be. Airways and procedures were in the past generally based on the ground-based radio navigation beacons and RNAV is the ability to use those that are not. $\endgroup$ – Jan Hudec Jun 9 '16 at 11:42
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Global Positioning System (GPS) is a U.S. initiated, monitored and maintained system.

As Dave pointed out, GLONASS is maintained by Russia, Galileo by Europe and BeiDou by China.

All of these systems and others comprise the Global Navigation Satellite System (GNSS). There is a subset of GNSS called satellite based augmentation system (SBAS). Under this subset is the U.S. wide area augmentation system (WAAS), EGNOS by Europe, MSAS by Japan and GAGAN by India.

"Random" Area Navigation (RNAV) allows an aircraft to choose any course within a network of ground based stations or GNSS. It is typically developed and flight checked (in the U.S.) and published as procedures. In the U.S. you will see approaches titled RNAV (GPS)... that use procedures developed by the FAA.

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