Reverse sensing with a GPS

Seems like there is a huge argument over semantics here.
 
Sac Arrow said:
Seems like there is a huge argument over semantics here.
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Shouldn't be. Here's the definition from the FAA's Instrument Flying Handbook:
"Reverse sensing. The VOR needle appearing to indicate the
reverse of normal operation."​
I vote we keep the term and reeducate those who misuse it.

dtuuri
 
Well I'll leave it to you guys to slug it out.
 
dtuuri said:
Shouldn't be. Here's the definition from the FAA's Instrument Flying Handbook:
"Reverse sensing. The VOR needle appearing to indicate the
reverse of normal operation."​
I vote we keep the term and reeducate those who misuse it.
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The only person to whom it would appear to indicate the reverse of normal operation is a person who was improperly trained to start with. Use of that term merely reinforces that improper training.
 
Sounds more like the educator is having a problem than the educatee.
 
Clip4 said:
Major barrier? Not hardly.
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My statement is based on my experience -- a couple of thousand hours of instrument training given, and I'll stand by it. It typically takes 4-8 hours of ground, sim, and flight training to overcome the damage done by instructors who taught CDI's as a "turn left/turn right" instrument rather than a "it's located east/west of you" instrument.
 
Ron Levy said:
My statement is based on my experience -- a couple of thousand hours of instrument training given, and I'll stand by it. It typically takes 4-8 hours of ground, sim, and flight training to overcome the damage done by instructors who taught CDI's as a "turn left/turn right" instrument rather than a "it's located east/west of you" instrument.
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That's one thing I make sure to do early on - pound the point home that "the CDI doesn't know or care which way the airplane is pointing." Not that the CDI is sentient or anything. Draw 4 quadrants: ToLeft, ToRight, FromLeft, FromRight. (because the CDI will show left or right of center)Then draw the airplane pointing all different directions within that quadrant - and they see the CDI never changes, and that they have to fly a direction (NESW, or combination) to get the needle to move to center, not left or right to get it to move.
 
EdFred said:
That's one thing I make sure to do early on - pound the point home that "the CDI doesn't know or care which way the airplane is pointing." Not that the CDI is sentient or anything. Draw 4 quadrants: ToLeft, ToRight, FromLeft, FromRight. (because the CDI will show left or right of center)Then draw the airplane pointing all different directions within that quadrant - and they see the CDI never changes, and that they have to fly a direction (NESW, or combination) to get the needle to move to center, not left or right to get it to move.
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Generally, we're in agreement, the only difference being I don't use or allow the use of the words "left" or "right" in the discussion of what the CDI tells you -- it's all cardinal directions. If the selected course is 360, I'll refer to the two sides of the CDI as the West/East sides, not the left/right sides. I find that helps drive home the point that fact that the needle is pointing left doesn't necessarily mean you need to turn left, and once you glean from the CDI that the courseline is to your West, you need to look at the HI to determine whether West is to your left or your right. Further, for intercepts, you look at the position of the needle to determine an appropriate intercept heading, but then you look at the HI to decide which way to turn to reach that intercept heading.

That said, if someone has had nothing but "needle means left/right" throughout their prior training, it takes several hours longer to get their minds right enough to quickly interpret the indications and take appropriate action. However, once that happens, their performance improves dramatically and it makes a tremendous difference in how fast they learn PT's, holding patterns, and approach procedures.
 
Ron Levy said:
Generally, we're in agreement, the only difference being I don't use or allow the use of the words "left" or "right" in the discussion of what the CDI tells you -- it's all cardinal directions. If the selected course is 360, I'll refer to the two sides of the CDI as the West/East sides, not the left/right sides. I find that helps drive home the point that fact that the needle is pointing left doesn't necessarily mean you need to turn left, and once you glean from the CDI that the courseline is to your West, you need to look at the HI to determine whether West is to your left or your right. Further, for intercepts, you look at the position of the needle to determine an appropriate intercept heading, but then you look at the HI to decide which way to turn to reach that intercept heading.

That said, if someone has had nothing but "needle means left/right" throughout their prior training, it takes several hours longer to get their minds right enough to quickly interpret the indications and take appropriate action. However, once that happens, their performance improves dramatically and it makes a tremendous difference in how fast they learn PT's, holding patterns, and approach procedures.
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Of course the easier way is just to make sure the radial on the CDI is within 90º of your heading. :D I know, not practical on a backcourse.
 
EdFred said:
Of course the easier way is just to make sure the radial on the CDI is within 90º of your heading. :D I know, not practical on a backcourse.
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If you're flying a localizer backwards (either on the front or back course -- doesn't matter), leaving the forward direction course at the top of the CDI makes it all much easier. You just look at the bottom of the instrument rather than the top to pick your intercept heading.

Example: You're flying the Runway 9 localizer backwards (either inbound on the back course or outbound on the front course). You've drifted south of course, i.e., the centerline is to your north. If you keep the OBS on 090, then the needle will be deflected to the north side of the case. If you were flying it forwards, you'd look up from where the needle is to the OBS ring and see the needle is aligned with, say, 070, and a turn to 070 would turn you toward the localizer tracking inbound (i.e., 070 is your intercept heading). If you're flying it backwards, look down to the ring, and you'll see the needle is aligned with 290, so turning to 290 will move you toward the localizer tracking inbound (i.e., 290 is your intercept heading). Either way (inbound or outbound, forwards or backwards), take that intercept heading from the CDI, look to the HI, and then turn in the appropriate direction to that intercept heading. Makes it much easier to track a localizer backwards.
 
coloradobluesky said:
OK. So you fly the G1000. Is it capable of "xxxxxxx xxxxxx"? or whatever you want to call it? I mean there is something going on there. I've even seen them "flip". And...I'm wondering if there isn't some Garmin/King difference in behavior. I know in my King, when I was in enroute mode it didn't matter where you turned the OBS knob. In OBS mode, it did matter. Not sure how Garmin handles that. And do the G1000's do it any differently than the 430's?
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Perhaps you should call it "reversed perception" as it's a human failing not an equipment one.

It may help to understand what VOR/LOC and in particular GPS "sensing" really provides. None of these systems by themselves detect or present anything about the direction your airplane is pointed. GPS raw data consists of nothing more than a 3 dimensional (actually 4 dimensional but we're generally less interested in the exact time while flying) position in space relative to an Earth based frame of reference (along with a velocity vector if carrier phase is sensed as well as signal arrival times). In most cases your direction and rate of travel is computed from a sequence of such fixes and you could just as easily (from a GPS sensing perspective) be flying backwards or sideways. VOR raw data only provides a measure of the angle between the line from your airplane to the station and a reference line (typically close to parallel with a line from the station to the Earth's magnetic N pole). LOC raw data is even more limited, it can only measure a similar angle relative to the signal's centerline and only within about +/- 15 degrees of that centerline.

There are various means of presenting this information to pilots. For a LOC there's rarely more than a simple somewhat linear inidication of the raw angle and more often than not this shows up on the CDI (needle) portion of a co-located OBS (Omni Bearing Selector) or HSI. Either way the OBS (or HSI course selector) setting has no effect on the angular deviation displayed on the CDI except that with a HSI turning the course selector physically rotates the CDI.

VOR indicators (OBS/CDI and HSI) are also the most common type of display for VOR data but there are other forms such as a DVOR, a numeric display of the raw angle in degrees) and the RMI, a display that consists of a needle which "points to the station" by electronically or mechanically subtracting the measured VOR signal and the aircraft heading.

GPS data is generally presented in a confusingly similar manner that is actually quite different behind the scenes. Since there are no "ground stations" through which a signal reference line exists, a GPS receiver has to "compute" a reference point and line to compare the GPS derived position with. For enroute guidance the comparison usually generates the distance from the reference line rather than the angle but an angular comparison is used on most final approach course guidance. GPS position is also often presented as a point on a "map" along with the computed reference line (and point). The reference line can be defined by a single point plus an angle from north mimicking the behavior of a OBS/CDI displaying VOR data or as a line (more accurately a great circle route) between two computed points.

If you've digested all that, it should be becoming obvious that "reverse sensing" is nothing more than a situation where the presentation of the same raw data is less straightforward than the more easily understood case where the CDI needle's displacement is in the same direction is the measured or computed course line. With a LOC displayed on a OBS/CDI this occurs when the aircraft's nose is pointed away from the ground station, or when displayed on a HSI if the course pointer is set to the "back course". With a VOR on a CDI that situation exists when the OBS setting is approximately opposite the airplane's heading.

To emphasize the inadequacy of calling this "reverse sensing" consider what happens when the aircraft heading is close to 90° from the VOR OBS setting or LOC course. In that case the display makes even less intuitive sense with the CDI's left/right deflection indicating something related to a fore/aft displacement between the aircraft and the LOC course or selected VOR radial. If the former is "reverse sensing" this ought to be called orthogonal sensing and I strongly suspect such a naming would cause even more confusion in the cockpit.

To make matters even worse with GPS the CDI deflection on a OBS/CDI or HSI can depend on the make/model of the GPS as well as the selected navigation mode in some cases. Some GPS installations feed the OBS selection or HSI course pointer angle to the GPS which then applies that to the raw data in order to create a CDI presentation that behaves similarly to a VOR display but some (e.g. Apollo IIRC) don't have that connection and appear to work more like a LOC display.
 
gismo said:
Perhaps you should call it "reversed perception"
.............................................................

If you've digested all that, it should be becoming obvious that "reverse sensing" is nothing more than a situation where the presentation of the same raw data is less straightforward than the more easily understood case where the CDI needle's displacement is in the same direction is the measured or computed course line. With a LOC displayed on a OBS/CDI this occurs when the aircraft's nose is pointed away from the ground station, or when displayed on a HSI if the course pointer is set to the "back course". With a VOR on a CDI that situation exists when the OBS setting is approximately opposite the airplane's heading.

To emphasize the inadequacy of calling this "reverse sensing" consider what happens when the aircraft heading is close to 90° from the VOR OBS setting or LOC course. In that case the display makes even less intuitive sense with the CDI's left/right deflection indicating something related to a fore/aft displacement between the aircraft and the LOC course or selected VOR radial. If the former is "reverse sensing" this ought to be called orthogonal sensing and I strongly suspect such a naming would cause even more confusion in the cockpit.
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I wonder if perhaps GPS navigation might tend to breed confusion with new pilots in this regard? Consider for example; on a full VOR approach with a procedure turn using the VOR the pilot has to manually twist the OBS to the correct bearing, if he neglects to change the OBS from the outbound bearing to the inbound during the procedure turn his error soon becomes obvious. If that doesn't prompt a correct understanding then flying outbound on a localizer should prompt some serious thought about just what the indicator is indicating. OTOH, the GPS does this twisting for him automatically, just select the desired approach and program to navigate via the IAF with PT and it'd be easy to perceive the CDI as a simple left/right indicator.
 
My understanding is the FAA made all the manufacturers of IFR GPS's have a connection to a VOR head (or similar CDI). It also required all IFR GPS's to have what is called an "OBS" mode. And when in OBS mode, all the mfg's devices are supposed to work the same. OBS mode suspends waypoint sequencing (your previous waypoints and courses stay on the screen) and the unit activates the OBS head so when you twist the knob, it changes the course. It really behaves more like a localizer than a VOR because it can "flip" and has no To/From flag.

In "leg" or "en route" mode, the FAA allows manufacturers to have a little more leeway in how they work. Function tends to vary more between units. I have seen the Garmin 430 and the King "flip" and do what the FAA and Jepp call "reverse sensing". But do it slightly differently, I'm not really sure. Some may behave more like an HSI.

I was hoping someone had a resource that detailed just how the different units actually work. Some magazine or agency that actually tested the different units and compare how it works. It matters.

One thing for sure, when you are in the clouds, flying a missed, you don't want to be twisting GPS knobs. You want it all in there already and set up and doing exactly what you expect. You don't want to have any suprises.

Does Microsoft flight simulator have different IFR GPS's portrayed accurately?
 
tehmightypirate said:
Keep in mind the G1000 uses an HSI so it DEFINITELY can't do "reverse sensing" or "reverse orientation" (or whatever term we want to use).
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I just want to mention for completeness that there is a G1000 failure mode (loss of magnetometer and/or AHRS, I assume) that will fail the digital HSI but allow a raw-data VOR display. In that case, the G1000 would exhibit all the normal behavior of a traditional VOR indicator. Unlikely in real life, I know.
 
coloradobluesky said:
My understanding is the FAA made all the manufacturers of IFR GPS's have a connection to a VOR head (or similar CDI). It also required all IFR GPS's to have what is called an "OBS" mode. And when in OBS mode, all the mfg's devices are supposed to work the same. OBS mode suspends waypoint sequencing (your previous waypoints and courses stay on the screen) and the unit activates the OBS head so when you twist the knob, it changes the course. It really behaves more like a localizer than a VOR because it can "flip" and has no To/From flag.
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The Apollo (II Morrow and Garmin AT) GX-50 GPS was an approach certified navigator that didn't necessarily have an OBS function controlled by the nav head. If the GX-50 was coupled to Apollo's companion SL-30 navcom that feature was included by virtue of a serial connection between the SL-30 and the GX-50 which passed the nav head OBS selection to the GPS. Without the SL-30 the GX-50 provided a pseudo OBS mode which required the user to manually set the desired course/bearing via a knob on the GPS. But AFaIK the SL-30 was not a required item in a GX-50 installation certified for IFR approaches.

And IMO this makes sense in that OBS mode is never used as part of any GPS IFR approach procedure.

The Northstar M2 might also have lacked a nav-head driven OBS mode as well but I'm less certain on that.
 
Yup -- poor training on how a CDI presents information. You seem to be thinking that a CDI gives you left/right information, but it doesn't. It only gives you cardinal directional information (i.e., N/S, E/W), and you need to transfer that information to your heading indicator to determine whether that direction is to your left or to your right.
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Ron is of course exactly correct on this point. It was hard for me to even understand the OP's question at first. Of course the CDI doesn't care about your heading.

I was confused by the initial question not because I'm particularly smart, rather I think that's because I'm old and learned IFR navigation before there was such a thing as a moving map. In those bad old days you had keep a God's eye view in your head, since the airplane didn't do it for you.

In any case, it sounds like the post #6 exercise would be a good simulator instruction scenario to help unconfuse these young skulls full of mush on how a CDI works. ;)
 
When teaching this sort of thing in ground school (pre-GPS), I would point out the headings of the little planes in the diagram were "distractors".

2-62.jpg


What I meant was since heading did not affect VOR indications, the student should ignore it entirely when approaching problems like the one above. Planes A and C have the same indication regardless of heading, as do the other two.

Same here. Mentally align yourself with the course set in the OBS and the indications will always make sense.

BTW, a trick when forced to deal with "reverse sensing" (LOC BC as the most common example) is to mentally shift gears and imagine the needle is your plane and the 4° circle tells you the direction to your course. IOW, needle left in the above example puts the 4° circle to the needle's right, so it means fly right.

Sounds confusing, but worked well for me and my students in practice.
 
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FastEddieB said:
When teaching this sort of thing in ground school (pre-GPS), I would point out the headings of the little planes in the diagram were "distractors".

2-62.jpg


What I meant was since heading did not affect VOR indications, the student should ignore it entirely when approaching problems like the one above. Planes A and C have the same indication regardless of heading, as do the other two.

Same here. Mentally align yourself with the course set in the OBS and the indications will always make sense.

BTW, a trick when forced to deal with "reverse sensing" (LOC BC as the most common example) is to mentally shift gears and imagine the needle is your plane and the 4° circle tells you the direction to your course. IOW, needle left in the above example puts the 4° circle to the needle's right, so it means fly right.

Sounds confusing, but worked well for me and my students in practice.
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I'm not a big fan of Eddie's technique. If the receiver in the picture above was tuned to the Rwy 36 LOC (which it isn't, since you see a FROM flag, but just imagine), and you were flying that LOC backwards (i.e., southbound), I would tell you to look at the bottom of the CDI where the needle is pointing (something around 210), and fly that heading to intercept the course. Then I'd tell you to look at your heading indicator to decide if you needed to turn right or left in order to turn to 210 from whatever your current heading was. No worrying about left/right or normal/reverse or front/back course when looking at the CDI, just whether I'm trying to fly north or south.

BTW, the headings of those planes in that figure aren't "distractors", since all four would have the same VOR CDI indications, and the only way to differentiate between them is to check the heading indicator (not shown in that picture). The point made by that question is that the VOR CDI doesn't care which way you're pointed, only where you are, and thus cannot be used in isolation to decide which way or how much to turn. And there are questions in the IR written question bank in which you do need to use heading as well as needle/flag indications to select the correct aircraft for the indications.
 
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Wow really? Now I'm MEGA confused. See, it all started when I had this VOR in my airplane and the stupid TO/FROM flag got all confused and would say FROM when I KNEW I was on a TO radial (I mean I KNOW where the VOR antennae is DUDE). Not only that, but when I get NEAR the VOR the stupid thing really goes haywire! (so I'm SURE it's broken) So I started doing ILS's instead. Then after flying the ILS, ya know what happened? The STUPID thing FLIPPED and read right instead of left TOO!! I'm not kidding!!!! So I took my plane back to my avionics guy and complained. He tested it and claimed it was OK. But I knew better. He said if I didn't like it, he could sell me one of those fancy IFR/GPS's so I sprung for that to get me outa this mess. Now I discover the blankety blank thing does the SAME THING (well almost). Anyway, IT can read right instead of left and flips now and then TOO! (guess i'm lucky the damn thing doesn't read UP instead of DOWN)

This whole thing is RUINING my training. I mean I was really HOPING I could be a Captain like Cap'n Ron and be 100000% right all the time and get all the good looking girls. But the instructors around here won't even go up with me anymore. The last one that would showed up with his PARACHUTE! (I mean, that's a bad sign right?)

But I have a solution. Instead of IFR training, I'm gonna switch to AEROBATICS! I figure maybe my stupid instruments will read correctly if I'm UPSIDE DOWN!!! ;)
 
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coloradobluesky said:
Wow really? Now I'm MEGA confused. See, it all started when I had this VOR in my airplane and the stupid TO/FROM flag got all confused and would say FROM when I KNEW I was on a TO radial (I mean I KNOW where the VOR antennae is DUDE). Not only that, but when I get NEAR the VOR the stupid thing really goes haywire! (so I'm SURE it's broken) So I started doing ILS's instead. Then after flying the ILS, ya know what happened? The STUPID thing FLIPPED and read right instead of left TOO!! I'm not kidding!!!! So I took my plane back to my avionics guy and complained. He tested it and claimed it was OK. But I knew better. He said if I didn't like it, he could sell me one of those fancy IFR/GPS's so I sprung for that to get me outa this mess. Now I discover the blankety blank thing does the SAME THING (well almost). Anyway, IT can read right instead of left and flips now and then TOO! (guess i'm lucky the damn thing doesn't read UP instead of DOWN)

This whole thing is RUINING my training. I mean I was really HOPING I could be a Captain like Cap'n Ron and be 100000% right all the time and get all the good looking girls. But the instructors around here won't even go up with me anymore. The last one that would showed up with his PARACHUTE! (I mean, that's a bad sign right?)

But I have a solution. Instead of IFR training, I'm gonna switch to AEROBATICS! I figure maybe my stupid instruments will read correctly if I'm UPSIDE DOWN!!! ;)
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Get an ADF, them things know which way the airplane's pointed.....well, at least relative to the NDB :D
 
Ron Levy said:
I'm not a big fan of Eddie's technique.
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Not for everyone.

Worked for me and my students, is all I'm saying, and its something to try if you find yourself chasing the needle the wrong way.

YMMV, or course!

And you're right, they're only distractor's regarding needle indication, not OBS setting.
 
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peerlesscowboy said:
Get an ADF, them things know which way the airplane's pointed.....well, at least relative to the NDB :D
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No, they don't. The only know where the NDB is located relative to the airplane's nose. Unless your ADF indicator is an RMI with its own built-in magnetic heading reference source (hard to find on the light singles most of us here are flying), they don't know which way the plane is pointed, or where the plane is located relative to the NDB, without also using a heading indicator.
 
Ron Levy said:
No, they don't. The only know where the NDB is located relative to the airplane's nose. Unless your ADF indicator is an RMI with its own built-in magnetic heading reference source (hard to find on the light singles most of us here are flying), they don't know which way the plane is pointed, or where the plane is located relative to the NDB, without also using a heading indicator.
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Wasn't that what I said, and in a lot simpler language :confused:

BTW, "400 feet down" from where?
 
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peerlesscowboy said:
Which way the airplane's pointed relative to the NDB, aka "relative bearing".
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That's not "relative bearing". From the Instrument Flying Handbook:
The ADF needle points to the NDB
ground station to determine the relative bearing (RB ) to the​
transmitting station. It is the number of degrees measured​
clockwise between the aircraft’s heading and the direction​
from which the bearing is taken.
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IOW, the normal ADF indicator tells us where the station is relative to the nose of the airplane, not "Which way the airplane's pointed relative to the NDB".
 
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FastEddieB said:
I thought it was.

adf_bearing.gif


Regardless, I got what John was saying.
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He's got it backwards, even if you got what he was saying. The relative bearing in that picture is about 225 degrees. However, the aircraft's heading relative to the station (which is what John called RB ) is about 135 degrees.
 
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peerlesscowboy said:
Get an ADF, them things know which way the airplane's pointed.....well, at least relative to the NDB :D
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Needle pointing to the tail, tells you the nose is pointed away from the station.

Needle pointing to the nose, tells you the nose is pointed towards the station.

All relative to the NDB, of course.

Ron, I really think you're overthinking this. His statement, replete with smilie*, is quite accurate.



*MEANING IT WAS A JOKE!!!
 
FastEddieB said:
Needle pointing to the tail, tells you the nose is pointed away from the station.

Needle pointing to the nose, tells you the nose is pointed towards the station.

All relative to the NDB, of course.

Ron, I really think you're overthinking this. His statement, replete with smilie*, is quite accurate.



*MEANING IT WAS A JOKE!!!
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Ron reminds me of my ex-wife, if he can't find something sensable to argue about he'll just invent something. Really, is that YOU, Arlene? ...........sometimes I wonder.............
BTW Ron, "400 feet down".......from where?
 
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FastEddieB said:
Needle pointing to the tail, tells you the nose is pointed away from the station.

Needle pointing to the nose, tells you the nose is pointed towards the station.

All relative to the NDB, of course.

Ron, I really think you're overthinking this. His statement, replete with smilie*, is quite accurate.



*MEANING IT WAS A JOKE!!!
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It's not accurate, and it will only confuse anyone who doesn't already understand ADF/NDB navigation. I teach this material for a living, and when people get misconceptions like "reverse sensing" and John's inaccurate description of "relative bearing", it only makes it harder for them to learn how to do radio navigation properly later on.

Even if it was intended as a joke.
 
The main thing to remember about the ADF needle is that the head of the needle can only fall.
 
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