GPS repeater for Twin Otter

[bdrake529 0]

Does anyone here know anything about obtaining and installing a GPS repeater in a Twin Otter for use with GPS freefall recorders?

I've recently begun jumping with a Garmin Foretrex and have been frustrated by the difficulty of getting a consistent satellite link. I can get a signal if I sit in the co-pilot's seat, directly behind the cockpit, or right by the door. Other seats = no signal = no jump data.

The pilot/dzo said he'd let me install a repeater so now I'm on the hunt for info. Where do I get one, how much do they cost, what is involved with installation, tips for consistent results, etc...

I've heard that Tracking Derby brings repeaters to their race events. I've sent an email to them via the contact form. Does anyone here have any personal experience with the technology they use? I like the idea of a portable solution so I can take it with me to other DZs.

Thanks in advance,

Brian

Brian Drake

[KrisFlyZ 0]

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I like the idea of a portable solution so I can take it with me to other DZs.

Thanks in advance,

Brian

Sorry if this post is not helpful. Search for Scott Campos's posts on this subject. LouDiamond on here.

I jumped a season with a Foretrex 201(still have it). In theory because of DGPS (WAAS/ENOS capability) these units are supposed to be extremely accurate....but as you mentioned they hardly acquire and maintain a signal in the airplane (other than in the co-pilots seat)...that makes them almost useless.

I use an Edge 205 and an Edge 305(with the new SiRF chip) and even though they don't have DGPS....they maintain a strong signal from 8-9 satelites(about 20-30 ft accuracy) even inside a Beaver. Ideally a GPSMAP 60 and 76 Cx with DGPS and the SiRF chip sounds like a good choice but it is a pain to mount.

Kris.

[bdrake529 0]

I searched Scott's posts and all I could glean was that I'm looking for a "re-radiating antenna". No specifics on a recommended product or instructions on use.

I did a Google search and found this

For about $50, I'm tempted to just buy it and see if it works. However, I don't like to reinvent the wheel when there are others out there with experience to share.

Thanks,

Brian

Brian Drake

[LouDiamond 1]

I am on the road right now so I can't give you the specific site info on where to look, however, you are on the right track. Repeaters are available but they can be pricey. The link you posted is one of several low cost options for re-raidiating antennas that are either battery operated or plug into a cigarette adapter. The higher dollar repeaters can be installed inside buildings or inside aircraft relatively easy but they are outside what most people are willing to invest. I am assuming you don't want to invest a lot of money?

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[ZigZagMarquis 8]

http://www.gpssource.com/index.php

[bdrake529 0]

I'm looking for a solution that allows satellite reception from any seat in the Otter on the ride to altitude. Ideally for multiple GPS receivers. Will the low cost option provide this?

If so, then of course I don't want to "invest a lot of money".

But if not, what are the "pricey" options and how "pricey" are we talking? I was originally thinking of paying for this myself, but if a costly solution is the only one that truly works, I may have to solicit financial cooperation with other jumpers (trackers and wingsuit fliers) at my DZ.

Brian Drake

[piisfish 135]

did you write to Claude from Trackingderby ? He might be able to give you some good info.

scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

[Eule 0]

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Does anyone here know anything about obtaining and installing a GPS repeater in a Twin Otter for use with GPS freefall recorders?

Not very much, but here's what I think I know.

As far as installing things in the airplane in general, I believe it's both easier, cheaper, and less hassle (FAA) if it's something you can just run on batteries (or maybe on the cigarette lighter; Cessnas have them but I don't know if Otters do) and is portable - nothing bolted to the airplane or permanently wired to the airplane. (Check with your friendly local airplane mechanic (A&P) for better information.) For instance, lay the receive antenna and battery box on top of the instrument panel, run the cable back into the cabin, and hang the transmit antenna from the ceiling in the middle of the cabin. The optimal place for the receive antenna is probably on top of the airplane, and you can buy active receive antennas that are designed to go there, but they start at maybe $350 and go up from there, professional installation at $100/hr not included.

Looking at a couple of reviews of using the ~$50 active repeaters in cars, it looks like they _might_ work in this application. The most important thing is to install the cable and the antennae such that they can't be tripped over, get tangled with the aircraft controls, get snagged by a jumper and ripped off the wall, snag somebody's handles, etc. After that, the next thing is probably getting the receive antenna to where it has a good view of the sky. I suspect that in an Otter the antenna will be able to see slightly more sky from the cockpit than from next to the door in the back, but either place may work. Then you have to get the transmit antenna in a good location inside the cabin. "Good" in this case probably means "up high", but spacing it a bit away from the ceiling may be better than having it right on the ceiling.

You could conceivably test the receive antenna locations on the ground with the engines shut down (i.e., cheaply), as long as your handheld GPS will give you a status page that shows how many satellites it can see. Park the airplane outside and pointed the same direction it usually is pointed on jump run and try your handheld GPS from both the cockpit and the door in the back. Put the handheld GPS as close as you can get it to where you might mount the receive antenna, and see how many satellites it can pick up. You might try it at different times of the day. If jump run can be lots of different directions, it might be harder to cover all the possibilities.

For some background on what's going on:

An antenna is basically a chunk of wire. By changing the length of the wire, you can make it so the antenna picks up some frequencies better than others. Antennas work both ways, so an antenna that is set up to receive a particular frequency well can also be used to transmit on that frequency fairly efficiently. So, you can make a simple repeater with two antennae - one outside the plane and one inside it - connected together. The GPS signals hit the outside antenna, go through a cable, and come back out on the inside antenna. This is usually called a "passive repeater", since there are no amplifiers involved.

The signal picked up by a chunk-of-wire antenna is not very large. If you want to pipe it through a cable, as-is, to a GPS receiver, you need some pretty high-quality cable (read thick and expensive), and the cable usually can't be very long. To get around that, you can put an amplifier right next to the chunk-of-wire antenna to boost the signal. This boosted signal can then be fed through cheap, thin cable for a long distance, while still remaining usable at the GPS receiver. An antenna together with an amplifier in the same box is usually called an "active antenna". If you take an active antenna and connect a chunk-of-wire antenna to its output, the boosted signal will get re-radiated through the chunk-of-wire antenna and you have an "active repeater". This is what the $50 device you linked to appears to be doing.

Neither of the above devices really "care" about what's going through them - they just pass along whatever happens to be in a certain band of radio frequencies. I _think_ the more-expensive repeaters that LouDiamond is talking about actually have GPS receivers in them. They receive and decode the GPS signal, then re-encode it and re-transmit it - they have a microprocessor looking at the signal, rather than a simple amplifier.

As an analogy, imagine you are reading dropzone.com at work but you don't want your boss to know. You could put up a couple of mirrors so that you can sit at your desk but still see if your boss is coming down the hall - that's a passive repeater. You could point a video camera down the hall and watch the output of the camera on a small monitor at your desk - that's an active repeater. With both of these, you see everything that's happening in the hall, whether you care about it or not. Finally, you could pay a kid to stand by your door and tell you when he sees the boss coming - that's the expensive repeater, and with that one, you only get told about the one thing you really care about - is the boss coming or not.

I hope this helps!

Eule

PLF does not stand for Please Land on Face.

[phoenixlpr 0]

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They receive and decode the GPS signal, then re-encode it and re-transmit it - they have a microprocessor looking at the signal, rather than a simple amplifier.

I doubt that. You can not make/code valid GPS signals by yourself.

[LouDiamond 1]

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I'm looking for a solution that allows satellite reception from any seat in the Otter on the ride to altitude

Some of the portable ones will allow for this. The important part is read the spec sheet as some require the receiver to be in contact or very close to the re-radiating element to work. They are also easier to install as they are not permanent and don't require any aircraft modifications,additions, etc.

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what are the "pricey" options and how "pricey" are we talking?

Take a look here to see an example. The other thing you also have to keep in mind is the required forms you are suposed to apply for(see site fine print). I am unaware of this ever being a problem and equate it to the same forms one is "supposed" to fill out when they buy high power 2 way walkie talkies. If your aircraft already has an onboard GPS reciever, it may be easier and cheaper to use that receivers antenna and simply install a in line amplifier. I believe the web page offers kits to do just that.

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[bdrake529 0]

I received the following email from the PC-Mobile, the company that sells the "cheap solution" - http://www.pc-mobile.net/gpsant.htm

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Our re-radiating antenns is designed to be put on top of the GPS receiver so it does not suit your situation.

There is such thing as "repeater" for consumers which re-radiates signal up to around 1-2 metres. We do not have information on this but you can make a search on the internet. It has not been very popular so you will find it available in very few places.

So it looks like I'll have to pursue the "pricey" options.

I'm still on the look out for someone who has first hand experience doing this (or knows someone they can refer me to) so that I don't have to reinvent the wheel. I've already contacted Tracking Derby via the form on their site but haven't heard back from them. Does anyone know an alternate method of contacting them?

Thanks,

Brian

Brian Drake

[LouDiamond 1]

Take a look HERE and Here.
If your A/C doesn't have a cigarette plug in the cockpit you can cut it off and splice on alligator clips and use a suitable battery/power source, making the whole thing portable.

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[bdrake529 0]

Scott,

Thanks for the recommendation (and all your help).

I just ordered a low-cost setup similar to what you posted (with an additional recharging battery pack). It's got a re-transmitting range of 2m which I hope will be enough for my purposes. If that's not quite far enough to cover all the seats, I can always get 2 to cover each half of the plane. Or I could install the one within range of the door and then rig up a basket/pouch or something for jumpers to put their receivers in on the ride to altitude and then retrieve on their way to the door.

I'll be able to test what I bought in a few weeks and will post my findings.

One concern I have is that since my GPS receiver will be getting its multi-satellite signal from the repeater (singular location), will it maintain the signal upon exit, or will it lose the connection and have to reacquire in the air? Unfounded concern or unfortunate reality? Anyone know?

Brian

Brian Drake

[billvon 2,417]

>will it maintain the signal upon exit . . .

Here in the lab we have no problems going from the repeater in the lab to the outside. (On the other hand, when we use the GPS simulator in the lab and then take the units outside, it gives them fits.)

[Eule 0]

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If your A/C doesn't have a cigarette plug in the cockpit you can cut it off and splice on alligator clips and use a suitable battery/power source, making the whole thing portable.

If you do this, please make sure there is a fuse involved somewhere. A battery of 8 fresh alkaline AA cells can dump over 100 watts into a dead short, at least for a brief time. 10 freshly-charged NiCd AA cells might approach 200 watts. Fires usually suck, but fires in airplanes really suck.

Eule

PLF does not stand for Please Land on Face.

[antonija 0]

I just got response from Claude from trackingderby about the GPS repeater they use in the planes. He says it's a 1500USD worth of equipment but it's portable and doesn't need any modification to the plane.

I also contacted organizers of wingsuit boogie that was in Hungary last weekend and his reply was that he just used what they sell in any GPS store but he doesn't know the manufacturer/model or the technical specs, so I'm assuming they used GPS repeaters you can buy for in-car use with up to 2m radius. They were operating Turbolet L-410 which can take at least 18 skydivers and apparently it worked.

P.S.: I know this thread is old but I want one of these at our DZ too

I understand the need for conformity. Without a concise set of rules to follow we would probably all have to resort to common sense. -David Thorne

[nigel99 144]

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They receive and decode the GPS signal, then re-encode it and re-transmit it - they have a microprocessor looking at the signal, rather than a simple amplifier.

I doubt that. You can not make/code valid GPS signals by yourself.

Yes you can and yes that is exactly what expensive "re-radiators" do (demodulate and then remodulate). - actually it is fairly simple to do.

More of a concern to me would be the legality. In the EU re-radiating units are not legal unless they are used in screened room. I imagine FCC Part 15 has the same restrictions, because the GPS frequencies are effectively licensed by the US government. By re-radiating you WILL affect any GPS instrumentation on the aircraft and there is a chance that if your equipment fails it could block the aircraft from receiving GPS.

As someone already mentioned you might get away with using 2 GPS antenna's back to back (make sure they are passive antenna's). You could possibly rig up a simple system for less than $25 doing this. You might be able to couple enough signal to get maintain lock. In acquisition mode GPS needs a much higher signal level than tracking or locked mode. If you want details or part numbers to try send me a pm.

Experienced jumper - someone who has made mistakes more often than I have and lived.

[antonija 0]

Can you explain more about legal issues? I'm asking because you can buy GPS repeater for in-car use with no legal restrictions (and it also rebroadcasts GPS signal but on a very small area, just like the one in the airplane should).

Also how can repeater affect GPS on the aircraft? Apart from giving it a second set of the same data it gets from satellites...

I understand the need for conformity. Without a concise set of rules to follow we would probably all have to resort to common sense. -David Thorne

[nigel99 144]

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Can you explain more about legal issues? I'm asking because you can buy GPS repeater for in-car use with no legal restrictions (and it also rebroadcasts GPS signal but on a very small area, just like the one in the airplane should).

Also how can repeater affect GPS on the aircraft? Apart from giving it a second set of the same data it gets from satellites...

I think you will find that in the user manuals of the device if they are truly legal it tells you to ensure that the re-radiating element is only used in a screened or shielded room. Another "trick" that they do is to put in the user manual that you should not operate the equipment within the country. Example wording "It is illegal to use this equipment with the UK, ensure that you comply with local laws when using this equipment". If you want to know more details then CFR47 Part 15.205a details restricted operating frequencies and GPS (1575MHz) is one of them. The wording is

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Except as shown in paragraph (d) of this section, only spurious emissions are permitted in any of the frequency bands listed below:

Re-radiating is clearly not a spurious emission but an intentional radiator, so I do not see how any unit can be legal in the US without being in shielded aenvironment.

I don't know how accurate GPS on aircraft are but when you re-radiate you are providing the co-ordinates of the receive antenna of your re-radiator. At the very least the distance between your antenna and the aircrafts will come into effect so the aircraft could report that it is 20-30ft from where it actually is. But more importantly if your re-radiator has poor signal quality or can't get lock and you re-radiate that signal neither will the aircraft be able to get lock.

Experienced jumper - someone who has made mistakes more often than I have and lived.

[antonija 0]

Thank for the info. I hope I get a chance to talk to pilot that flew during wingsuit boogie in Hungary and ask him what type of equipment they used and did he have any problems with his GPS due to antenna on board.

Back to legal issue: Would airplane hull be considered a "shielded room"? It's impossible to get GPS lock in there unless you're in the cockpit next to window, so I'm guessing the hull is equally capable of shielding reradiated GPS signal from leaving the aircraft (especially with those 2m radius antennas).

BTW, I was thinking of getting this thing for our DZ. Any thoughts?

I understand the need for conformity. Without a concise set of rules to follow we would probably all have to resort to common sense. -David Thorne

[nigel99 144]

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Thank for the info. I hope I get a chance to talk to pilot that flew during wingsuit boogie in Hungary and ask him what type of equipment they used and did he have any problems with his GPS due to antenna on board.

Back to legal issue: Would airplane hull be considered a "shielded room"? It's impossible to get GPS lock in there unless you're in the cockpit next to window, so I'm guessing the hull is equally capable of shielding reradiated GPS signal from leaving the aircraft (especially with those 2m radius antennas).

BTW, I was thinking of getting this thing for our DZ. Any thoughts?

In all honesty I doubt it would meet the legal requirements. I would be especially cautious on aircraft as even if there is no effect if there is an accident/problem it would probably come to light. It "might" be possible to do a site survey with the equipment where you essentially claim that the unit is fitted to that model of aircraft following specific guidelines and providing documentary proof that the re-radiation is contained.

In all honesty I doubt that a pilot would notice any effect with a working unit. Bearing in mind the "offset" error on a working system is at most the length of the aircraft.

One point worth noting is that from the FCC point of view it is the normally manufacturer or reseller who is liable and not the user. Since I know nothing about aviation laws or rules that may not apply to its use on aircraft. Certainly in the EU the regulators would pursue the manufacturer/importer not the end-user (again from a Radio and not Aviation perspective).

Experienced jumper - someone who has made mistakes more often than I have and lived.

[jumpwally 0]

What exactly are you trying to accomplish ?

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