grouch
Well-Known Member
There seems to be no shortage this day and age in really good GPS transmitters and flight even controllers. I have witnessed several fantastic recoveries with these devices and must admit, the technology looks impressive. Being able to see the X marks the spot on a satellite image of your flying field is pretty cool, no doubt. So why would anyone worth an ounce of AP want to fly with a Radio Direction Finding unit? I have read so many comments like “they are frustrating” or “they take a lot of practice” to “GPS gets me there so much quicker”. Even with all the excellent tomes written about RDF, there seems to be a little more room for additional tips and techniques.
First my background. I have been using RDF in my rockets for a little over a year now. Not long compared to others out there but I do have background knowledge from other fields that is directly transferred. I am a HAM and an instrument rated pilot. The HAM thing taught me about what radio waves do and what’s going on but the instrument rating taught me how to use radio and this is what I want to share. I don’t claim to be an expert and I am sure the techies will pick me apart but I want to try to share with you what I do and how I understand it.
Since I am not able to draw or show diagrams, I will rely on descriptions so hopefully this will translate well enough. To understand how to get the most out of RDF and drastically cut down on the learning curve, you have to understand what the RX is doing out there in Farmer Bill’s crops. Your RX is sending out omni directional signals. This is easily misunderstood and I think it the sticking point for most people that have issues with RDF. The TX is not sending out a pencil fine beam straight to wherever you are, nor is it sending out an infinite number of pencil fine beams radiating from the center in all directions even though you think that is what omnidirectional means. What it is doing is sending out a narrow slice of pies that widens as they get farther away from the source. This means while your transmitter is out in a field of corn or alfalfa, your receiver is not going to point straight at it. It only points roughly in the direction where it is and the farther you are from it, the rougher the general direction. So you are probably thinking, yea dumb dumb, this is why I fly GPS because those RDF’s aren’t exact.
Now here is the way to actually look at what is going on and how to use it to your advantage. Remember pie. Not the numerical pie but a slice of pie. The beam the TX sends out gets wider the farther it travels from it’s source. While a slice of pie is a bit of an exaggeration, it is closer to the truth than a precise narrow beam of myth. Look at any aviation book where they discuss an ILS (instrument landing system). As the pilot is far from the end of the runway, the ILS is wide and sloppy. Your nose can drift a bit left and right and you will still show that you are right in line with the approach. As you get closer, the precision picks up to the point that as soon as your heading drifts even the slightest, you will notice you are out of line with the runway. An ILS is nothing more than a RDF system, same principle.
So what does that mean and how do you get your junk back when it’s out in the center of who knows where? Here is what I do. I rarely turn on my receiver while my rocket is in flight. If I were to be flying super high I might change this practice but I mostly stick to the 3-4,000’ range single deploy. Anyway, once the rack is off and the RSO lets people out, I turn on my receiver and point it way off the direction I think it is. I mean a good 45 degrees out or so. Then I hold my receiver level about face high and slowly make a sweeping arc towards the direction I think and past it. I am doing several things while I am doing this. I am listening for the increase in volume and I am keeping an eye on the S meter on the receiver. The volume will continue to pick up until it levels and begins to drop off. I make note where the drop off begins and mark a distant tree or group of trees in my mind while I slowly scan back to where I started, again noting where the volume begins to drop off. When I get a good idea where the drop offs are, I split the difference and that is the general direction on my rocket. Then I rotate my receiver antenna 90 degrees and do it again. This is because I have no idea in what orientation the transmitter antenna is and the receiver will pick it up better someways that others. If noting else, the second sweep confirms the first. Then I start walking off in the direction I averaged. This is where the slice of pie analogy helps. Imagine yourself standing on the crust and have no idea where the “first bite” portion is. A slow sweep will show you the edges on each side of your slice and if you split the difference, you get the center line.
So now I walk a little bit or a lot depending on how loud the signal is, stop and do it all over again. Each time you average the edges, you will find the edges get closer to each other. Walk stop and sweep, repeat. There will come a point where you get so close the sweeping becomes ineffective because the signal strength is so strong when you get close, it overloads the S meter and the volume of the beeping will not drop off no matter where you point your directional antenna. This means you are near ground zero and your rocket is close at hand. If you have a screamer, you probably already hear it, if not then you have to use some sort of attenuator to filter out some of the signal strength. After you attenuated the signal you have to get your edges back. This is where you really need to know your system. My Marshall receiver has a “Far, Medium and Near” setting. Far is far. Medium is generally about 10 yards, so when I can start picking the signal up on the medium bit, I am very close. If I can’t pick it up on the Medium setting yet, I keep walking in the known direction until I can. I keep walking until I can get it on the Near setting which by that time I have usually already found it
Now why take the time to learn all this? First of all, it’s not difficult and not time consuming. If you understand what the radio beam is actually doing, it’s as easy as walking right to it and only adds a little more time to actually just walking straight to it as if you knew exactly where it is. Another reason are the limits of the alternatives. When I bought my tracker, I was reading about the most recent Balls launch where a lot of people where having difficulties with losing GPS locks on the ascent and not being regain locks on the way down. I have to point out the obvious here, without lock, a GPS system is just ballast. GPS units also don’t get you to the exact location, there is some built in variances that allow for some play, say 10 meters or so. I can tell you for a fact, sometimes that is simply not close enough unless you are flying really large rockets with a large foot print and a big old parachute flapping around in the veggies. Little rockets are easily lost inside 10 meters. Also, the GPS units generally require a dedicated installation either as a part of the altimeter in the Alt-bay or a dedicated nosecone set up where as in most cases the RDF transmitter can be simply tapped to a shock cord and moved from rocket to rocket with little thought and difficulty. Lastly and most importantly, the RDF process is extremely enjoyable. Being able to find little rockets in chest high veggies is incredible. I honestly feel like I discovered hidden treasure when I see my rocket for the first time since it left the pad. Words can not explain it. Lastly being able to launch worry free allows you time to actually watch and enjoy the flight and not getting all puckered up when it starts drifting is a fantastic feeling.
If anyone else out there has any good nuggets of wisdom, please add. I just wanted to share with you what is going through my head and how I understand it all. And before any smart allec ask, these ARE the Cliff Notes.
First my background. I have been using RDF in my rockets for a little over a year now. Not long compared to others out there but I do have background knowledge from other fields that is directly transferred. I am a HAM and an instrument rated pilot. The HAM thing taught me about what radio waves do and what’s going on but the instrument rating taught me how to use radio and this is what I want to share. I don’t claim to be an expert and I am sure the techies will pick me apart but I want to try to share with you what I do and how I understand it.
Since I am not able to draw or show diagrams, I will rely on descriptions so hopefully this will translate well enough. To understand how to get the most out of RDF and drastically cut down on the learning curve, you have to understand what the RX is doing out there in Farmer Bill’s crops. Your RX is sending out omni directional signals. This is easily misunderstood and I think it the sticking point for most people that have issues with RDF. The TX is not sending out a pencil fine beam straight to wherever you are, nor is it sending out an infinite number of pencil fine beams radiating from the center in all directions even though you think that is what omnidirectional means. What it is doing is sending out a narrow slice of pies that widens as they get farther away from the source. This means while your transmitter is out in a field of corn or alfalfa, your receiver is not going to point straight at it. It only points roughly in the direction where it is and the farther you are from it, the rougher the general direction. So you are probably thinking, yea dumb dumb, this is why I fly GPS because those RDF’s aren’t exact.
Now here is the way to actually look at what is going on and how to use it to your advantage. Remember pie. Not the numerical pie but a slice of pie. The beam the TX sends out gets wider the farther it travels from it’s source. While a slice of pie is a bit of an exaggeration, it is closer to the truth than a precise narrow beam of myth. Look at any aviation book where they discuss an ILS (instrument landing system). As the pilot is far from the end of the runway, the ILS is wide and sloppy. Your nose can drift a bit left and right and you will still show that you are right in line with the approach. As you get closer, the precision picks up to the point that as soon as your heading drifts even the slightest, you will notice you are out of line with the runway. An ILS is nothing more than a RDF system, same principle.
So what does that mean and how do you get your junk back when it’s out in the center of who knows where? Here is what I do. I rarely turn on my receiver while my rocket is in flight. If I were to be flying super high I might change this practice but I mostly stick to the 3-4,000’ range single deploy. Anyway, once the rack is off and the RSO lets people out, I turn on my receiver and point it way off the direction I think it is. I mean a good 45 degrees out or so. Then I hold my receiver level about face high and slowly make a sweeping arc towards the direction I think and past it. I am doing several things while I am doing this. I am listening for the increase in volume and I am keeping an eye on the S meter on the receiver. The volume will continue to pick up until it levels and begins to drop off. I make note where the drop off begins and mark a distant tree or group of trees in my mind while I slowly scan back to where I started, again noting where the volume begins to drop off. When I get a good idea where the drop offs are, I split the difference and that is the general direction on my rocket. Then I rotate my receiver antenna 90 degrees and do it again. This is because I have no idea in what orientation the transmitter antenna is and the receiver will pick it up better someways that others. If noting else, the second sweep confirms the first. Then I start walking off in the direction I averaged. This is where the slice of pie analogy helps. Imagine yourself standing on the crust and have no idea where the “first bite” portion is. A slow sweep will show you the edges on each side of your slice and if you split the difference, you get the center line.
So now I walk a little bit or a lot depending on how loud the signal is, stop and do it all over again. Each time you average the edges, you will find the edges get closer to each other. Walk stop and sweep, repeat. There will come a point where you get so close the sweeping becomes ineffective because the signal strength is so strong when you get close, it overloads the S meter and the volume of the beeping will not drop off no matter where you point your directional antenna. This means you are near ground zero and your rocket is close at hand. If you have a screamer, you probably already hear it, if not then you have to use some sort of attenuator to filter out some of the signal strength. After you attenuated the signal you have to get your edges back. This is where you really need to know your system. My Marshall receiver has a “Far, Medium and Near” setting. Far is far. Medium is generally about 10 yards, so when I can start picking the signal up on the medium bit, I am very close. If I can’t pick it up on the Medium setting yet, I keep walking in the known direction until I can. I keep walking until I can get it on the Near setting which by that time I have usually already found it
Now why take the time to learn all this? First of all, it’s not difficult and not time consuming. If you understand what the radio beam is actually doing, it’s as easy as walking right to it and only adds a little more time to actually just walking straight to it as if you knew exactly where it is. Another reason are the limits of the alternatives. When I bought my tracker, I was reading about the most recent Balls launch where a lot of people where having difficulties with losing GPS locks on the ascent and not being regain locks on the way down. I have to point out the obvious here, without lock, a GPS system is just ballast. GPS units also don’t get you to the exact location, there is some built in variances that allow for some play, say 10 meters or so. I can tell you for a fact, sometimes that is simply not close enough unless you are flying really large rockets with a large foot print and a big old parachute flapping around in the veggies. Little rockets are easily lost inside 10 meters. Also, the GPS units generally require a dedicated installation either as a part of the altimeter in the Alt-bay or a dedicated nosecone set up where as in most cases the RDF transmitter can be simply tapped to a shock cord and moved from rocket to rocket with little thought and difficulty. Lastly and most importantly, the RDF process is extremely enjoyable. Being able to find little rockets in chest high veggies is incredible. I honestly feel like I discovered hidden treasure when I see my rocket for the first time since it left the pad. Words can not explain it. Lastly being able to launch worry free allows you time to actually watch and enjoy the flight and not getting all puckered up when it starts drifting is a fantastic feeling.
If anyone else out there has any good nuggets of wisdom, please add. I just wanted to share with you what is going through my head and how I understand it all. And before any smart allec ask, these ARE the Cliff Notes.
