Skip to main content
43 votes

What are satellite time, GPS time, and UTC time?

Both GPS time and UTC are derived from the atomic time TAI, a reference used to define the time unit in SI, kept by 400 atomic clocks in the world. GPS time unit and UTC time unit are the TAI second. ...
mins's user avatar
  • 74.4k
36 votes

How did planes measure their speed relative to the ground before GPS?

The other answers seem to make a simple thing way too complicated. Yes there are DMEs or TACANs, that are sometimes used to get a ground speed, but it's only accurate if you're tracking exactly to or ...
falstro's user avatar
  • 11.8k
35 votes

In the rare event that the U.S. DOD shuts off the GPS, what's the alternative?

If the GPS is unavailable, it will be quite an impact to the aviation industry. All airliners in-flight will experience degraded RNAV performance, but they would make it to the destination using VORs,...
kevin's user avatar
  • 39.7k
34 votes

Why don't planes (mostly airliners) primarily use GPS for navigation?

Satellite-based navigation is not reliable. The operator can turn it off. The satellites can fail. The signal can be jammed or spoofed.
Anonymous Physicist's user avatar
28 votes
Accepted

If a GPS displays the correct time, can I trust the calculated position?

In general, if the time is correct, then the position is also correct. But the time needs to be correct to an accuracy of about 10 nanoseconds. You probably don't have access to an atomic clock, and ...
DeltaLima's user avatar
  • 83.5k
25 votes
Accepted

Do airlines use GPS for critical purposes? If so, why?

GPS, or to be more accurate, GNSS is a commonly used navigational aid in aviation, especially in smaller passenger planes. Larger airliners are more likely to be also equipped with inertial navigation ...
Jpe61's user avatar
  • 28.8k
23 votes

What's the minimum number of sensors for a hobby GPS waypoint-following UAV?

The absolute minimum for a generalized vehicle that needs to know its position and attitude (orientation) in space is one per degree of freedom. This can be reduced if we have information about the ...
AEhere supports Monica's user avatar
21 votes

What is the difference between GPS, GNSS and RNAV?

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 ...
Ralph J's user avatar
  • 51.8k
20 votes
Accepted

How does drone reliance on GPS make it 'semi-stealthy'?

For being remotely controlled, drones need to emit and receive a radio signal. If the outgoing signal contains a video feed, it will be high bandwidth and be like a beacon announcing the location of ...
Peter Kämpf's user avatar
19 votes

What is a blind encoder?

A blind encoder is an altimeter that encodes pressure altitude and outputs it on a databus. They are "blind" in that they do not display data to the pilot. Most general aviation units use a parallel ...
Gerry's user avatar
  • 19.9k
17 votes

How did planes measure their speed relative to the ground before GPS?

There's one method that has been successfully used since long-distance flights became available - you took out your map, and tried to match it to features under your plane. This allowed you to correct ...
Luaan's user avatar
  • 271
16 votes

Why don't planes (mostly airliners) primarily use GPS for navigation?

While the other answers are correct about why GPS (or similar GNSS, such as GLONASS, etc.) aren't used exclusively for aircraft navigation, I would say that, these days, the premise of this question ...
reirab's user avatar
  • 19.5k
15 votes

What's the minimum number of sensors for a hobby GPS waypoint-following UAV?

It's probably possible with only the GPS receiver, but it wouldn't be easy and you might have to make some compromises on the airframe design to achieve the necessary passive stability. The ...
pericynthion's user avatar
  • 4,640
15 votes
Accepted

Why is training regarding the loss of RAIM given so much more emphasis than training regarding the loss of SBAS?

From a safety perspective RAIM is more critical than SBAS. If you are flying using GPS, you always rely on RAIM to keep you safe, sometimes you need SBAS too. Receiver Autonomous Integrity Monitor is ...
DeltaLima's user avatar
  • 83.5k
14 votes
Accepted

Why did my phone's GPS receiver stop working when the plane was descending to land?

A GPS needs a line of sight to (at least) 4 satellites in order to compute a position. Given that you were able to get a position during the cruise, I assume that you were close to a window and had a ...
Quentin's user avatar
  • 436
14 votes

Can a pilot use their GPS/FMS to fly a VOR or NDB approach?

In the US they can (Effective: May 26, 2016). This change allows for the use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure (IAP) ...
JScarry's user avatar
  • 7,464
14 votes
Accepted

Are NDBs and VORs dying as a navigational method?

Yes, without a shadow of a doubt. Many airports now publish GNSS (Generic term for all types of satellite navigation) approaches, completly negating the need for those aids even during complex, ...
Dan's user avatar
  • 9,308
14 votes

How to navigate an aircraft without GPS?

The question you should be asking is rather, how did airliners navigate before the widespread adoption of satellite-based navigation systems such as civilian GPS? One can navigate by way of landmarks ...
user's user avatar
  • 6,984
13 votes

Are helicopter pilots required to use GPS navigation?

No, pilots are not required to use GPS maps or moving GPS while flying helicopters or fixed-wing aircraft. There are some approaches to landing that require GPS (called RNAV) approaches, but the ...
Ron Beyer's user avatar
  • 36.2k
13 votes

Do airlines use GPS for critical purposes? If so, why?

As a pilot I do not disagree with the informative comments above, and would like to add a few additional points to ease your fears. Pilots DO use GPS (satellite) for navigation - in concert with an ...
Rod Dewell's user avatar
12 votes

Are NDBs and VORs dying as a navigational method?

Yes, it is true. This FAA page has more info. The Very High Frequency Omni-directional Range (VOR) Minimum Operational Network (MON) provides a conventional navigation backup service in the event of ...
JScarry's user avatar
  • 7,464
12 votes

If a GPS displays the correct time, can I trust the calculated position?

If the time is correct, the position is likely (but not necessarily) correct. Consider the extreme example where your GPS device can only acquire one satellite. You would have a good idea of the time, ...
Eugene Styer's user avatar
  • 3,709
11 votes

How did planes measure their speed relative to the ground before GPS?

According to a story I read about the SR71, planes used to be able to ask ATC for a "ground speed check", i.e. ask "What's my speed on the ground?" ATC would measure it using their ...
ChrisW's user avatar
  • 3,094
11 votes

In the rare event that the U.S. DOD shuts off the GPS, what's the alternative?

There are going to be bigger issues. The shipping industry is GPS reliant, the communications industry heavily relies on GPS time signals, Routing for trains, and so on. The fact is that the chaos ...
mongo's user avatar
  • 17.8k
11 votes

What are the considerations for coding the GPS before engine start without external power?

15 minutes is a long time to be draining the battery, it's not just the G1000 that will be running but the electronic gyros, beacon, etc. If the airplane hasn't been recently flown and it's cold ...
GdD's user avatar
  • 54k
11 votes
Accepted

Why weren't SA-On GPS units updated to become SA-Aware?

As the linked ICAO article on GPS accuracy states, the issue is not just the navigation accuracy but the reported accuracy which is critical when used for ADS-B. From the report: 3.2.1 For GPS ...
Gerry's user avatar
  • 19.9k

Only top scored, non community-wiki answers of a minimum length are eligible