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Need Help With Basic RF Telemetry

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bghansen2203

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Hey guys/gal's

As of the past year and a half I have become quite adept at building and designing high power rockets and motors as a hobby. My question to you is "What would be the best method/equipment for sending velocity data, GPS position, and altimeter data to the ground?". I have been doing some research online and have not yet found a solution to my problem. The intent of the experiment is to be able to efficiently transfer data during the rockets flight to a maximum altitude of 37,000 feet AGL. I have already gotten a stratologger flight computer/altimeter/stage timer chip that is RS232 compliant. I know from my very limited knowledge that in terms of RF communication that LOS is key to efficient transfer but I'm unsure about the power requirements of the RF transmitter, the types of antennas which will be best suited (omni directional for the rocket obviously) , and how the fresnel effect comes into play at high altitude. The baud rate of the device isn't crucial as 9600 should be more than enough to meet my data requirements. I have seen several 433 and 900 mhz options out there but they are relatively low power (500 - 100 mA). Space is an issue as the altimeter bay is 3.5" diameter but weight is not a factor, the rocket has 378lb maximum takeoff weight and currently weighs 36lbs loaded. Any help would be greatly appreciated.

Thanks,

-B
 

Maybe some rocketeers can offer a good advice. You can in principle use any low-power TX and a small RX for the amateur bands like 432 MHz, or, up to 2.45 GHz. Your modulation output then to be used to modulate the TX and then somehow to be demodulated in RX on the ground.
I would suggest to buy a low-cost 2.45 GHz "wireless camera" which transmits by FM at 2.45 GHz, usually accompanied by a good FM receiver with audio/video outputs. Your modulation can be applied to audio or video channel, and tested on the ground over >100 m for the best results.
The camera transmitter and receiver come with patch antennas; instead, you can use a dipole either inside or on the surface of your payload. For the best reception, the receiving antenna should be placed into a parabolic dish (you can use a satellite TV dish), and you or your team member should follow the rocket after launch by pointing the antenna after it. You can train all procedures beforehand by locating the transmitter (with the camera) on a balloon. This way you will learn the possible range , pointing, etc.

---------- Post added at 20:55 ---------- Previous post was at 20:53 ----------

I used a wireless camera like described, its cost usually is < $100.- The receiver really is very good for a distance between the patch antennas of up to 100 meters; with a higher gain of the dish, possibly well over a kilometer.
 

Thank you for the speedy reply and the very good advice. An FM camera would be a great addition to my project! I had considered possibly modifying an old 56K telecom modem to a basic stamp and then hooking the audio line out to an old gmrs radio and sending packets that way, I then had to scrap the idea because that is apparently a violation of certain FCC rules (they take the fun outa everything lol). I wonder if one could affix an rfid tracking tag to the rocket and build an auto guided antenna for the ground unit? My concerns with manual pointing is the fact that according to computer simulation the bird will reach a speed of over 640km/hr within the first 150 meters of the launch pad. Im thinking that no matter how many red bull's i have that day, I definatelty wont be able to move that quickly. I like the idea of the parabolic reflector, I have several old sat dishes on hand and even a couple old 900mhz directional antennas from when my internet was on a point to multipoint network.


Thanks again, and dont be suprised if im back with more questions lol

-B
 

I saw a tv program in the past year or two, which showed a rocket team do what you are describing. Telemetry with altitude readings, etc. The video (or one like it) may be available for viewing on a free tv website, or one of the science-oriented channel websites, etc.

So it is possible. I forget how high their rocket went. Since yours goes 37,000 ft then that's over 6 miles you want to broadcast. It will be a challenge to achieve with a transmitter that can fit in a 3.5 inch space.
 

As I wrote, I have not tested the 2.45 GHz wireless camera on a rocket :)) but its range on the ground was impressive.
An automatic pointing of the receiver antenna I think will be rather complicated. On the other hand, you can use two receiver systems- one with a smaller dish pointed manually to a rising rocket. Another system at the same channel can use a larger dish, pointed upwards to the space where your rocket enters seconds later. If the launch goes well, there will be not much "lateral" movement needed for the larger dish (also harder to move manually).
Again, you can train all with a balloon before assembling the rocket launch system.

Pointing an antenna automatically is quite complex, I know the conical scan on SCR-584 radar, and step-track system for 30-meter Intelsat antennas. Such attempt would be more expensive than anything you are planning. But maybe SPACEX or another rocket company has such equipment and can rent it.....
 

Thanks again for the quality info gents. I like the idea of dual antenna's tuned to the same frequency, this would be the most practical solution for my project. My previous idea was to use an automatic sat dish motor and modding the guidance chip inside to recognize only a single frequency. From what I understand the auto pointers base their positioning from both its ground gps position and the position of the satellite frequency and then extrapolate others from a list of known targets. I have in the past seen some inline RF amplifiers on the market up to 40Watt but as you have pointed out they would not fit into a 3.5" diameter space. Could one possibly hook the amp up to Rx rather than the Tx and still achieve a similar effect? The amps I have seen are all mostly in the 2.4ghz band and usually increase the Rx gain by about 25-30%. This wouldn't be such a problem because b/g routers are cheap and I have many at home with cracked firmware so that they function as repeaters for my media networking needs. I should have though about this a lot more during the R&D stages of the project and then could have incorporated certain spacial requirements into the design stage, but as Allanis Morssette said you live and you learn (Hate that song!!). This could definately put a kink into things when the project goes full power. 37,000ft AGL is on only a quarter engine load, the main idea was to try to get over 135,000 ft and actually make sub orbital altitude.

Thanks again everybody

-B
 

This may or may not be a sensible idea...

But you might consider installing transmitting equipment (or perhaps the final output stage) in a fin rather than in the payload bay near the nose. If the devices are more flat than cubical, that is. And if your fin construction is strong enough.

Then broadcasts would not be blocked by the body of the rocket (same as a wifi hookup is hampered by walls and furniture).

Furthermore a directional (or semi-directional) antenna in the rocket could be more feasible. Not necessarily a dish, but a lesser amount of directionality. It might be possible to fashion it into a nacelle (unless it creates a drag problem).

Some amount of broadcasting directionality would help you a lot. At all times the tail of the rocket is aimed generally in the direction of your tracking station. Thus the antenna would be aimed toward you as the rocket climbs (the portion of the flight that you are interested in). And even if the rocket goes horizontal, its antenna will be aimed more or less toward your tracking station.
 

My dear friend.

As for frequency: 70cm band is a good compromise between needed power, range and size. As long as you can maintain direct path (no obstacles) with your plane/rocket you can communicate (or at least receive data) with no problems from even 150km using simple and small yagi antenna (no high precision is required!)

Real time TV can be a problem because of doppler shift and power consumption needed. As for simple telemetry: you can receive 70cm signals from 0.5W transmitter with ease from even 25km.

As for mode: go for packet radio (APRS) if u want 100% reliable reports (but shorter range) or simply CW (more code) for longer ranges but with some errors (correctable by human).

As for weight: you can build simple microcontroler driven CW transmitter (with temperature, altitude and humidity information) with just few IC elements with total weight of about 30(sic!) grams and single 9v battery (about 45g).

Easiest way to send telemetry over VHF is by using APRS like solution. That what is designed for. There is no problem with implementing it on lets say PMR446/FRS/GMRS frequencies (license free).

This link should give you some overview how it works in practice (cross atlantic balloon).
**broken link removed**

ps. There are altimeter solutions build into single tiny chip.
 

Thanks again guys!

Hans, you are the man! I was seriously over thinking things in terms of my telemetry system. Live tv is not necessary in my application, just a fail safe in the event that things go bad and I end up loosing the camera in the rocket due to lost location/explosion. I'm very inexperienced in the world of radio communications and definately appreciate all the help that the people of this forum have provided. I like the idea of mounting the transmitter on the fins but am scared of the result at the moment of take off, the flame ball/heat of a launch is very intense and tends to leave a fairly substantial crater at the pad, I'm thinking the antennas will not survive this encounter very well not to mention the electro-magnetic flux field caused by the wearing of dissimilar metals in the convergent section of the nozzle coupled with the electro static buildup from high velocity particles in the exhaust gasses. The upper body of the rocket/payload section has been painted with a non conductive paint to help lessen this effect. I currently have a combined altimeter/gps/microcontroller board that is rs232 compliant and was made by a company called stratologger. By the look of things, I should be able to easily integrate it into a packet radio in the 70cm band.


Thanks again,

B
 

Thats stratologger looks very impressive. All that you need now is packet transmitter that will sent data from rs232.
Please browse the site: Byonics - TinyTrak3 GPS Position Encoder so you will get rough idea how it works and looks.

Of course major problem is that in theory you are not allowed to use default APRS frequency (144mhz) and you need to find an solution using PMR446/FRS/GMRS frequencies. That is becasue you are not legally allowed to transmit on amateur radio band (144mhz) without a license.
But there is many ready to use solutions like that: **broken link removed** (please look at the link, as its in fact almost THE thing that you need!).

Or you could just get a basic license (look for "amterur radio license") and go with standard and popular aprs hardware.

By using omidirectional antenna mounted on the top you could solve your fears about damaging the antenna.

Please look at this image: **broken link removed**
This is typical 70cm antenna connected to previously mentioned device. So now you have idea how small it can be.

In matter of fact this antenna could be nothing more than just pice of wire. Or even better you could mount it like that:
https://www.hamtv.com/giffiles/rocketantcamdet2.jpg in the middle of the rocket.

One more thing: its perfectly possible to make good antennas for very low cost (its just pice of wire!). So dont worry about damaging them at landing :)

ps. 70cm band is not much affected by ionized particles so rocket engine will not have much negative impact on transmission quality.

pps. This is exactly what you need:
index rocket transmitter
index earth recivier
Altus Metrum GPS Payload

Its kinda expensive IMO but whole design is open so you can build it yourself (or someone else) from basic components.
 
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Cool, a rocketry telemetry thread. Brings me back to my highschool days...

My gut tells me that something like 2.45GHz won't get you the range and reliability you need. You'll want a lower frequency like 433MHz or lower. Since you don't require high baud rates, that should be fine for bandwidth. But then finding a suitable antenna may be a challenge. What is your payload compartment made out of? Lots of metal nearby? You'll want to give the antenna a good amount of clearance from large conducting bodies.

Do you also want to receive data during descent? If so that probably makes things more challenging since it will drift for miles (I assume), and the orientation of the antenna will reverse, meaning a directional antenna would hurt you.
 

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