GPS on Radar Opaque Airframes

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edwinshap1

edwinshap1

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How have people dealt with CF/metal airframes and GPS units? Has anyone run the wire outside the airframe and epoxied the wire to smooth it? Does the board itself need to be outside the airframe to pick up GPS?

Any info helps :)
 
The receiving antenna needs to be outside the airframe, not the board.
 
So does the Rf transmitting antenna if using a tracker. I've heard of copper foil being embedded on the surface of a CF airframe for a tracker transmission antenna. I suspect there are small GPS receiver antennas but I don't know if they would mount on the surface of an airframe. The GPS receive antenna is embedded many times on the square chipsets/boards. Actually it might have to be mounted outside on a totally CF airframe but that would be impractical.
I believe the only workaround (sans embedding a GPS receive antenna on the outside of a CF tube) is using a radio-lucent nosecone for nosecone mounting or putting the GPS tracker with a robust antenna on the sustainer/apogee shockcord. The shockcord treatment might not give an accurate apogee deployment altitude though, would depend how quick it achieves a lock after deployment.
I had the experience with a metallic painted rocket that shielded the Rf GPS tracker at altitude but the GPS had a lock on 6 to 8 satellites and stored the positions to onboard memory (BeelineGPS). The rocket fortunately came down within eyeshot and the valid positions were downloaded and recorded the flight perfectly. It was radio opaque for the 70cm transmitter but radio-lucent to the incoming GPS signals. I don't know, does CF block everything? If so, makes GPS reception tricky. Kurt
 
ksaves2 said:
So does the Rf transmitting antenna if using a tracker. I've heard of copper foil being embedded on the surface of a CF airframe for a tracker transmission antenna. I suspect there are small GPS receiver antennas but I don't know if they would mount on the surface of an airframe. The GPS receive antenna is embedded many times on the square chipsets/boards. Actually it might have to be mounted outside on a totally CF airframe but that would be impractical.
I believe the only workaround (sans embedding a GPS receive antenna on the outside of a CF tube) is using a radio-lucent nosecone for nosecone mounting or putting the GPS tracker with a robust antenna on the sustainer/apogee shockcord. The shockcord treatment might not give an accurate apogee deployment altitude though, would depend how quick it achieves a lock after deployment.
I had the experience with a metallic painted rocket that shielded the Rf GPS tracker at altitude but the GPS had a lock on 6 to 8 satellites and stored the positions to onboard memory (BeelineGPS). The rocket fortunately came down within eyeshot and the valid positions were downloaded and recorded the flight perfectly. It was radio opaque for the 70cm transmitter but radio-lucent to the incoming GPS signals. I don't know, does CF block everything? If so, makes GPS reception tricky. Kurt
Click to expand...

Interesting info. Never thought to find out if it was transparent to the GPS itself. Putting a wire outside the airframe is pretty easy, I'm just worried about the unit itself.

I've put units in the nosecone, but if the parts detach I loose the casing and altimeters.
 
Interesting thought that a carbon airframe might not be radio opaque at GPS frequencies.

I would be quite amazed by that. A good carbon airframe should be a good Faraday cage (Rf shield) at almost all radio frequencies.

Ken
 
KenBiba said:
Interesting thought that a carbon airframe might not be radio opaque at GPS frequencies.

I would be quite amazed by that. A good carbon airframe should be a good Faraday cage (Rf shield) at almost all radio frequencies.

Ken
Click to expand...

Hi Ken, To be clear, my experience occurred with a standard spiral wound glass tube that had been painted with Rustoleum rattle can metallic paint.
The Rf was sucked up by the paint job and I didn't receive any packets at altitude. The rocket recovered within sight and the BeelineGPS had a good .kml
file downloaded from it. To my surprise, it had lock on 4 to 7 satellites which suggest the paint job was radio-lucent to the GPS signals.

It would be easy to test in a CF airframe. Put a BLGPS actively recording positions in a sealed CF airframe and walk around on the ground. Download the .kml. If no satellites seen, there's a problem. I would have thought someone would have tried this already. I've been told that most do a nosecone mount in a non-CF nosecone for flights. Kurt
 
CF airframe might be VERY different than metallic paint. Could be very different RF environments.

The GPS receiver on board also different problem than the telemetry channel.

So little time so many problems.

But generally, avoid metallics paints and CF when thinking about GPS and RF telemetry.

K
 
KenBiba said:
CF airframe might be VERY different than metallic paint. Could be very different RF environments.

The GPS receiver on board also different problem than the telemetry channel.

So little time so many problems.

But generally, avoid metallics paints and CF when thinking about GPS and RF telemetry.

K
Click to expand...

Yup, could be that CF blocks everything and a radiolucent glass N/C is the best alternative. A compromise would be to use a "hardened"
GPS tracker on the sustainer/apogee shockcord to be ejected at altitude. Technique would be to have the tracker running to get a lock on
the current satellites, pack it in the rocket, launch and then hopefully it would get a lock on the way down and start transmitting position.

This wouldn't be useful if one was going for an altitude record but would be for tracking the downside for recovery purposes. The reason is the rocket might be on the way down a ways before the newly ejected tracker gets a lock and sends positions.

Kurt
 
For almost all amateur flights, CF is largely decorative not structurally required.

But for folks like me, when my ticket goes high it becomes invisible. Tracking is a requirement while CF is a decorative nice to have.

Ken
 
A standard solution is to attach 2 (and only 2) patch antennas on the surface of the airframe 180 degrees apart. This provides 4pi (speherical) GPS reception.
This works equally well with metal or CF rocket airframes. You may have design and manufacture your own antennas if you want them to be conformal antennas, and this is not trivial.

Bob
 
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