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GHA 15 Radar Altimeter

O

outnumbered

Member
Anyone flying with the GHA-15 ? Where did you mount it ? I see some builders are mounting it in the tunnel by the fuel valve (this puts it in the center of the fuselage and close to being aligned with the main landing gear. Others are mounting it aft of the baggage area (close to the stock battery tray). Thoughts ???
 
Under the co-pilot seat as far forward and inboard as possible. Work great and an easy spot to work / splice into the can bus.
 
DerekS said:
Under the co-pilot seat as far forward and inboard as possible. Work great and an easy spot to work / splice into the can bus.
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I like that spot also for the reasons you mentioned. It does seem to not 100% meet the following : “ The location chosen for the GHA 15 must be free of aircraft projections (landing gear, flaps, drainage vents, etc.…) within a 120° cone from the GHA 15 downward to prevent locking onto false altitude returns. “ as the cone will hit the right tire/wheel pant but only the outer 5 degrees or so of the cone at that location. Good to hear yours operates fine there. Do you have a pic of your install ?
 
I created a 3D print of the unit with the cone and did some testing. If you keep it centered the gear is definitely clear of the 120-deg cone. My plan is to install it just forward of the fuel valve (where the fuel filter bracket would go; I'm using wingroot fuel filters).
 
kirkbauer said:
I created a 3D print of the unit with the cone and did some testing. If you keep it centered the gear is definitely clear of the 120-deg cone. My plan is to install it just forward of the fuel valve (where the fuel filter bracket would go; I'm using wingroot fuel filters).
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I was thinking of similar placement. I think Damazio did this as well, and I liked his idea.

Hopefully you can post some pics when you create your doubler and install it.

Thanks,
Brad
 
It's going to be a while as that area is under my fuselage cart so I think it will have to happen when it is up on the gear... but I will update when I can. I did already create a simple doubler plate, again using a 3D-printed template. I don't actually have the avionics yet.
 
I mounted mine just aft of my fuel selector valve in the tunnel. This was a pretty easy location, but also has it very close to the center of pitch and roll with it just forward of the spar while providing the 20cm clearance needed from feet and seats along with the 120º cone clearance.
 
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mburch said:
From the installation manual:
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So, there's a concern with the radiated energy being a human body health issue??

That's hard to believe, given the mounting underneath the metal bottom of the plane....and the parts - butt and or feet - being the target, doubt them being pulse microwaved can amount to anything. What am I missing?

And for what it's worth, one data point, mine is mounted violating the 120 degrees by a couple degrees, and it works perfectly - never an erroneous measurement. Discussion with G3X folks when I installed it, they seemed to think the 120 degrees was fairly conservative.

One interesting thing is that the firmware, while not announcing altitude on normal takeoffs, will announce altitude on a touch and go climb out!
 
Mounted mine in the tunnel, just aft of the fuel pumps - it just barely misses the landing gear in the detection angle.

If I didn't have a TAS antenna behind the center spar, I'd have installed it there just below the flap motor, though, that'd be much cleaner.

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outnumbered said:
I like that spot also for the reasons you mentioned. It does seem to not 100% meet the following : “ The location chosen for the GHA 15 must be free of aircraft projections (landing gear, flaps, drainage vents, etc.…) within a 120° cone from the GHA 15 downward to prevent locking onto false altitude returns. “ as the cone will hit the right tire/wheel pant but only the outer 5 degrees or so of the cone at that location. Good to hear yours operates fine there. Do you have a pic of your install ?
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I used this location as well. The right wheel pant does sit within the 120* cone but has absolutely no negative effect. 90* cone is probably acceptable. Under the seat rather than the tunnel makes access much easier.
 
Regarding separation from other antennas, one of the G3X engineers said that the GHA 15 operates in the Gigahertz range, whereas the Com, Nav, Transponder, etc. operate in the Megahertz range, so they should not interfere with each other.

If you have to choose between clearance from structure, such as landing gear, versus clearance from antennas, I would choose clearance from structure, placing a priority on being near as possible to the aircraft's center of pitch and roll.
 
outnumbered said:
I like that spot also for the reasons you mentioned. It does seem to not 100% meet the following : “ The location chosen for the GHA 15 must be free of aircraft projections (landing gear, flaps, drainage vents, etc.…) within a 120° cone from the GHA 15 downward to prevent locking onto false altitude returns. “ as the cone will hit the right tire/wheel pant but only the outer 5 degrees or so of the cone at that location. Good to hear yours operates fine there. Do you have a pic of your install ?
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So... Took a look under the plane today, I'm actually out further than I thought but have no performance issues. The rivet lines left and right are the fluid line supports under the seats. This is looking backwards under the co-pilot seat.

The wheel does intrude on the 120 degrees. I do remember making a plan for what if this doesn't work, when I picked the location. I was splitting the difference as there is also a belly com antenna just out of the picture in the tunnel.

Looking at my logs, it's been flawless for my purposes (and my home pattern is right hand), but I haven't done proper testing of right hand steep turns (which should be the bad case for this install) to see what it does.

Take it as an an example of how far you can push it rather than the best spot :D.

Derek2025-01-26_19-11-24.jpg
 
rongawer said:
Regarding separation from other antennas, one of the G3X engineers said that the GHA 15 operates in the Gigahertz range, whereas the Com, Nav, Transponder, etc. operate in the Megahertz range, so they should not interfere with each other.

If you have to choose between clearance from structure, such as landing gear, versus clearance from antennas, I would choose clearance from structure, placing a priority on being near as possible to the aircraft's center of pitch and roll.
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It's 24GHz - transponder is 1.09GHz, so pretty far from each other's harmonics (closest is the transponder's 22nd harmonic, so not a factor).
It transmits with about 18mW, which suggests that the receiver is probably very sensitive, so I still kept 2ft from my other antenna, which transmits with 40W at 1.03GHz (and some TAS systems transmit with 250W!). COM radios usually transmit at 10W, which is still about 500 times as much power as the GHA15.

rongawer said:
near as possible to the aircraft's center of pitch and roll
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a.k.a. center of gravity :)
My rationale for having it near the landing gear (which also happens to be near the CG anyway) is so it's accurate during flare/landing - the height of the tire to the ground will vary the least with pitch and roll.

DerekS said:
testing of right hand steep turns
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Hopefully we're never relying on a radar altimeter during a steep turn anyway :)
 
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