Help me understand GPSS

[TimRF79]

My plane has a GTN750, normal HSI and a KAP140.
When i put in a flight plane and put the KAP140 in NAV mode it makes the turns automatically.

What is the added benefit of GPSS?

 

[luvflyin]

My plane has a GTN750, normal HSI and a KAP140.
When i put in a flight plane and put the KAP140 in NAV mode it makes the turns automatically.

What is the added benefit of GPSS?

Sounds like GPSS to me. You do mean it makes the turns at bends in the flight plan don't you, as the GTN750 sequences from one leg to the next?

 

[455 Bravo Uniform]

GPSS is working. That means you don’t have to manually turn the course knob on the HSI. If you don’t have GPSS, you dial in the course that the GPS tells you to.

 

[jrcox19]

GPSS is working. That means you don’t have to manually turn the course knob on the HSI. If you don’t have GPSS, you dial in the course that the GPS tells you to.

OP said he was in NAV mode. If you're not in HDG mode, it's not GPSS doing the work.

 

[Hang 4]

Not familiar with all your equipment, but the big difference with GPSS is turn anticipation. An AP can follow a flight plan from say a GTN 750, but the turns will start at the waypoint and go past it. With GPSS the turn will begin before the waypoint (assuming its not a flyover WP) and join the new course past the waypoint. This will matter more when using it for holds and procedure turns in an approach or if you are using a waypoint to stay outside of airspace.

 

[Ravioli]

Even a GNS 430W will display and fly the turn anticipation.

I think OP has a valid question, since I've flown a plane that had a designated GPSS switch, and currently have no so such switch, but my AP does follow the plan and such.

[Reference point: I have Dynon D100 and use its autopilot for pitch and roll, the navigation is fed by either the 430W or an SL30]

 

[asicer]

Not familiar with all your equipment, but the big difference with GPSS is turn anticipation. An AP can follow a flight plan from say a GTN 750, but the turns will start at the waypoint and go past it. With GPSS the turn will begin before the waypoint (assuming its not a flyover WP) and join the new course past the waypoint. This will matter more when using it for holds and procedure turns in an approach or if you are using a waypoint to stay outside of airspace.

Yup, NAV mode, it's just left/right deviation from the course line. It's been a while since I've flown a KAP140, but as I recall, you fly it in heading mode with NAV mode armed and when the course deviation starts coming in it switches over to NAV mode and then centers the course. When you get to a waypoint, the course line flips but he KAP still keeps going until it sees the deviation and then tries to recenter it, like a student pilot fixated on chasing a CDI needle.

With GPSS, you fly the KAP140 in heading mode. When you get close to a waypoint, the GPS starts feeding the KAP140 different headings to keep the turn inside of the waypoint instead of blowing past it like in NAV mode. As far as holds and procedure turns, NAV mode only gets the inbound course for holds and PT's like what you see on your CDI needle so there's not much for the KAP140 to follow. With GPSS the GPS feeds headings for the KAP140 to fly, sort of like ATC giving a continuous stream of vectors to a pilot.

 

[BrianNC]

With GPSS, you fly the KAP140 in heading mode. When you get close to a waypoint, the GPS starts feeding the KAP140 different headings to keep the turn inside of the waypoint instead of blowing past it like in NAV mode.

So is this with a KAP140 only? In our club 172 with 2 G5s, GPSS, and an STEC 30 autopilot, heading mode activates the heading but and you basically manually fly with the heading bug. Nav mode deactivates the heading bug and makes it useless and it follows what's programmed into the GPS. Basically the opposite of what you seem to be saying unless I'm misunderstanding you.

 

[GRG55]

So is this with a KAP140 only? In our club 172 with 2 G5s, GPSS, and an STEC 30 autopilot, heading mode activates the heading but and you basically manually fly with the heading bug. Nav mode deactivates the heading bug and makes it useless and it follows what's programmed into the GPS. Basically the opposite of what you seem to be saying unless I'm misunderstanding you.

Depends on the autopilot. On some if you are in Heading mode and switch to Nav mode when the route from the NAV instrument (VOR or GPS) is too far away for the autopilot to figure out a route intercept it won't switch. It "arms" NAV mode instead, until it can figure out where the course is from the CDI starting to come in, so it can center on the NAV route.

 

[asicer]

So is this with a KAP140 only? In our club 172 with 2 G5s, GPSS, and an STEC 30 autopilot, heading mode activates the heading but and you basically manually fly with the heading bug. Nav mode deactivates the heading bug and makes it useless and it follows what's programmed into the GPS. Basically the opposite of what you seem to be saying unless I'm misunderstanding you.

It's been even longer since I've flown an STEC. However, things work differently when you throw a G5 into the mix. The G5 menu should have something that turns GPSS on and off. When GPSS is off, then you move the heading bug and the autopilot follows the heading bug. When GPSS is on, the STEC is disconnected from the displayed heading bug and instead the GPS feeds headings directly to the STEC (i.e. like ATC feeding vectors once or twice per second).

 

[BrianNC]

It's been even longer since I've flown an STEC. However, things work differently when you throw a G5 into the mix. The G5 menu should have something that turns GPSS on and off. When GPSS is off, then you move the heading bug and the autopilot follows the heading bug. When GPSS is on, the STEC is disconnected from the displayed heading bug and instead the GPS feeds headings directly to the STEC (i.e. like ATC feeding vectors once or twice per second).

Exactly how it works. When you turn the GPSS on it even has a little heading bug icon with an 'X' through it to let you know it's not functional.

 

[RussR]

My plane has a GTN750, normal HSI and a KAP140.
When i put in a flight plane and put the KAP140 in NAV mode it makes the turns automatically.

What is the added benefit of GPSS?

Some autopilots (I believe the KAP 140 is one of them) will do a pretty reasonable job in NAV mode, even as far as changing courses go. They do tend to wander more, and take longer to get established exactly on course than a unit with GPSS would, but overall do pretty well, except perhaps with a strong crosswind.

However, the real test is a holding pattern. Without GPSS, your autopilot likely won't be able to fly a holding pattern, primarily because the inbound course, and therefore the CDI, doesn't change throughout the entry and then the hold. So the autopilot doesn't know where to go. With GPSS it will fly the entry correctly, the turns, and the outbound legs just fine.

 

[Hang 4]

STEC 30 does not have a "NAV" mode. There are four; 1. ST which is basically a wing leveler, 2. HD (or Heading) which follows the heading bug or is used for GPSS if you have something that provides GPSS. 3&4, TRK LO and TRK HI, which are for VORs (LO) and LOC/ILS (HI).
For GPSS, if you have a unit that supports it, HD mode will follow it if GPSS is enabled. G5 and G500 have built in GPSS and a switch that will switch between following the heading bug and the GPS course (with turn anticipation). STEC has an adapter that will provide a GPSS signal as well and uses a switch to change modes.

As an aside, it will follow a GPS signal in TRK mode, but it will blow by waypoints until it realizes that it's "off course" and then will do a turn to the new course well past the course line. No big deal most of the time, but as others have pointed out, it will not do well in a hold or procedure turn. It's basically following the horizontal needle on the CDI and it doesn't "anticipate" turns.

 

[asicer]

However, the real test is a holding pattern.

Another would be if it only does fly-over waypoints rather than fly-by waypoints. Also if it can fly an RF leg or DME arc.

STEC 30 does not have a "NAV" mode. There are four; 1. ST which is basically a wing leveler, 2. HD (or Heading) which follows the heading bug or is used for GPSS if you have something that provides GPSS. 3&4, TRK LO and TRK HI, which are for VORs (LO) and LOC/ILS (HI).

NAV mode on the KAP140 corresponds to TRK LO on the STEC. Similarly APR mode and TRK HI, although APR might also follow the glide slope.

 

[TimRF79]

Some autopilots (I believe the KAP 140 is one of them) will do a pretty reasonable job in NAV mode, even as far as changing courses go. They do tend to wander more, and take longer to get established exactly on course than a unit with GPSS would, but overall do pretty well, except perhaps with a strong crosswind.

However, the real test is a holding pattern. Without GPSS, your autopilot likely won't be able to fly a holding pattern, primarily because the inbound course, and therefore the CDI, doesn't change throughout the entry and then the hold. So the autopilot doesn't know where to go. With GPSS it will fly the entry correctly, the turns, and the outbound legs just fine.

That makes sense, though I wonder how the KAP140 would do with a hold depicted on the GTN750?

In synopsis, GPSS makes the turn BEFORE the waypoint.
W/O GPSS the turn is when the NAV switches the magenta line and you MAY go past the turn.

 

[asicer]

That makes sense, though I wonder how the KAP140 would do with a hold depicted on the GTN750?

What does your HSI display as you are flying the hold? As I said before, the KAP140 in NAV mode is like a bad instrument student fixated on the CDI needle.

 

[chemgeek]

GPSS is able to intercept and capture course legs, fly hold entries, and anticipate turns during heading changes in the programmed flight plan. Without GPSS, your autopilot may require your position to be within a certain distance of the desired track and with a heading aligned within a certain tolerance in order to capture a course leg. Essentially, GPSS can anticipate turns and calculate course intercepts from any position relative to the desired course, while tracking is reactive to CDI deviation and may or may not be able to capture a course leg unless it is already relatively close to the desired position and track.

For my STEC AP, track mode requires you to be within a couple of dots of the desired track with a heading not more than 10 degrees different than the desired track to reliably capture the course leg. When flying past a waypoint with a course change, track mode will not usually capture the outbound leg unless the course change is relatively small. In almost all cases in this situation, track mode will overfly the waypoint then scallop during capture of the outbound leg, eventually damping out if capture is possible. GPSS (which is, for example, emulated in heading mode using G5s using heading commands supplied by a GPS navigator) will figure out an acceptable intercept and anticipate the turn and capture of the desired track without wandering back and forth.

 

[midlifeflyer]

Some autopilots simply follow GPS inputs in NAV mode. For others, but not all - some just don't have the capability, GPSS requires added hardware which converts GPS into a heading signal. With those GPSS mode is a manual switch and putting the autopilot in HDG mode.

 

[John Collins]

GPSS or roll steering is a totally different method of interfacing a navigator to an autopilot where the navigator provides a precise bank angle to guide the autopilot. Most legacy autopilots don't support this capability directly, although the Stec 55X and KFC 225 do. Modern autopilots such as the GFC 500/600 support GPSS or roll steering input. Since legacy autopilots don't support GPSS or roll steering input, converters were designed (originally by Stec) that take advantage of how the autopilot's heading mode works. The GPSS signal is a bank angle. The heading error signal is an input to the autopilot generated by the difference between the heading bug setting and the current heading on the DG/HSI. The autopilot takes this signal and responds by correcting back to the desired heading bug setting by commanding a bank angle that is proportional to the difference. So with a 5 degree difference in "heading to bug" will cause the autopilot to command a 5 degree bank and a 10 degree difference in "heading to bug" will cause the autopilot to command a 10 degree bank, and so on up to the maximum bank angle limit. In other words, setting the heading bug causes the autopilot to command a bank angle to correct the heading to match the bug. So the GPSS converter converts from a bank angle to a "heading error" signal which in turn gets the autopilot to command a bank angle. Bank angle is a much more flexible method of commanding an autopilot because it is not limited to flying straight line courses and therefore can fly procedure turns, holds, hold entries, DME arcs, RF (radius to fix) legs, turn anticipation, and near instantaneous wind correction. It does not need the course on the HSI or CDI to be set to track a course nor does it need the heading bug to establish an intercept and the CDI lateral deviation indicator is irrelevant. GPSS converters are ubiquitous and integrated into most all EHSI such as Aspen, G5, G500/600, GI275, ... To take advantage of the GPSS, the autopilot is set to heading mode rather than a navigation mode.

 

[BrianNC]

@TimRF79 Here is a good video I found explaining GPSS from American Bonanza Society. Speaking of NAV mode from your OP, he says once you have GPSS you will never use NAV mode again and tells why. As @John Collins said above, "To take advantage of the GPSS, the autopilot is set to heading mode rather than a navigation mode."

Basic autopilot info starts at 9:30, and the specific GPSS info starts at 36:15.

https://www.absweb.org/27.html

 

[TimRF79]

@TimRF79 Here is a good video I found explaining GPSS from American Bonanza Society. Speaking of NAV mode from your OP, he says once you have GPSS you will never use NAV mode again and tells why. As @John Collins said above, "To take advantage of the GPSS, the autopilot is set to heading mode rather than a navigation mode."

Basic autopilot info starts at 9:30, and the specific GPSS info starts at 36:15.

https://www.absweb.org/27.html

That was a good video, thank you.
... Now I want to see the main difference of a KPA140 on a GTN750; versus the GFC500....

 

[samlowe54]

I am not sure the KAP140 on a GTNXXX vs a GFC500 on a G5 is really a fair comparison. The older analog autopilots are good at what they do, but they can't really be compared to the newer digital units. The you have the attitude-based vs. rate-based comparisons as well. I have a S-TEC 60-2 in my Cardinal and it was a big improvement of my C300/S-TEC PSS but I could not say it was earth shattering. It did capture the holds and approaches and that was a nice upgrade, but at the end of the day it is still analog and rate-based. I do have GPSS and that is a great improvement, especially in turn anticipation. We do still use the heading bug for weather deviations so I can't say we would never turn off GPSS. The system is pretty much always in heading mode, the GPSS switch just determines whether the GPS or heading bug provides the roll input.

 

[John Collins]

I have an Stec 60-2 with a Stec GPSS converter. I expect to use heading mode for vectors or when I want to control the heading and GPSS mode otherwise. What I don't use is Nav Mode on the 60-2 except when on approaches. Nav mode for enroute navigation is not useful and I never use it in Lieu of using heading mode with GPSS.