SparkFun Forums

[BudgetEngineer]

(I am new to the SparkFun forum, this is my first post)

I was planning for my High Altitude Balloon Project that I hope to launch this coming summer and got stuck on how to communicate with the payload when it would be 20 miles up.

I read about HAM radios setup with a cool device called a TinyTrak, which converts GPS data into a signal that can be sent over the radio, but didn't want to get a license for this one project, or spend money on equipment that wouldn't be used all that often.

Further research revealed some groups had used a smartphone that would upload its position to a website, but cell coverage is non-existent or spotty in rural areas and at high altitudes.

Also, 900 Mhz radios like the one found here (http://www.sparkfun.com/products/9411)could be used, but they are very expensive.

A proverbial LED lit in my mind one evening and I thought:

Why can't I just use a two-way radio and to act as a transciever? I could send information bit by bit from the payload, even send commands from the chase vehicle to request info. They make some 40-50 mile radios don't they?

After some research, this proved to be a very affordable option. The problem, and the reason for my posting, is that there are several unknowns I hope the more experienced members can assist me with.

The information I will be transmitting is GPS coordinates, sensor data, battery life, and other numerical data.

For starters, would I be able to get good reception, transmitting from the upper atmosphere, and on the ground? Do clouds have affect the signal? The launch site is set to be smack in the middle of Kansas, so there will be flat fields for miles around.

Digging deeper, in what manner should the data be sent? I'm thinking something like Morse code, a pulse for a one, silence for zero. Or, would it be simpler to have the flight computer store audio recordings of the numbers 0-9, and play the appropriate recording for each number in the data string?

Finally, would this type of setup be reliable? I don't want to lose my precious payload!

Thanks in advance for your advice and help!

[wiggles]

You could easily run a standard serial stream over it, just hookup the serial connection to the mic/ heaphone jack (make sure to use TTL level voltages to avoid damaging the radios). You may also want to rig up some directional antennas, since it should be line of sight the whole way up. Directional antennas will also help finding it once it hits the ground.

[stevech]

The 100mW or 1Watt 900MHz data radios are what you want. Anything less, including low power 2.4GHz 802.15.4 like XBee, won't have the range you need. There's some simple math you can do to calculate the RF attenuation in line of sight for distance x.

You also need the robust modulation method in such a radio, for paths of several miles.
And a good terrestrial antenna. And error correction.

[wiggles]

by "it" I mean an off the shelf 2 way radio (preferably a 40-50 mile one), would have edited the previous post but it wont let me.

[BudgetEngineer]

Thanks wiggles! The best radios I can find without going overboard is a decent 36 mile Midland model for about $50 on Amazon.

The only problem is, using Pythagoras' theorem (A^2 + B^2 = C^2), I've worked out that the maximum distance the chase car can be behind the balloon in terms of longitude and latitude coordinates is only 1 - 1.5 miles at best. This means that we could lose contact for part of the journey.

If you know of any 40-50 mile radios, preferably around or under $100, I'm all ears.

Thanks again!

[wiggles]

After a little research it seems its illegal to modify the antennas on those radios. I looked around and couldn't find any under 100 either, hmmm. Looking around you can find 2 meter handy talkies for under $100 but that would require someone have a HAM license. If you dont want to get your license you may try to find a local HAM radio club, I'm sure someone there would be more than happy to help out. I'm also looking to launch a weather balloon so I'll let you know what I find.

[itikhonov]

Digging deeper, in what manner should the data be sent? I'm thinking something like Morse code, a pulse for a one, silence for zero. Or, would it be simpler to have the flight computer store audio recordings of the numbers 0-9, and play the appropriate recording for each number in the data string?

It's better to use something like DTMF (like in push-button telephones). It will perform better in presence of noise, packs 4 bits into single pulse and give some error-protection.

There is plenty information and code about DTMF.

[itikhonov]

Thanks wiggles! The best radios I can find without going overboard is a decent 36 mile Midland model for about $50 on Amazon.

The only problem is

Probably 36 miles specified is for land-to-land communications. Find out local HAM club they will give you all sort of advice you need.

[BudgetEngineer]

Probably 36 miles specified is for land-to-land communications. Find out local HAM club they will give you all sort of advice you need.

So this would mean that the range is different, hopefully better, in the air?

[itikhonov]

Probably 36 miles specified is for land-to-land communications. Find out local HAM club they will give you all sort of advice you need.

So this would mean that the range is different, hopefully better, in the air?

Yep. Major source of signal weakening is due to various material obstacles and you can use REALLY BIG antenna (or parabolic one) on receiver (it's should be ok to modify receivers even in US) to mitigate signal weakening due to distance.

Link for inspiration: http://pasadena.net/shootout05/ (yeah, it's 125 miles with 300 milliwatt)

[itikhonov]

By the way looks like altitude of your balloon will be around 36 miles so you should read this: http://en.wikipedia.org/wiki/GPS#Restri ... vilian_use

TLDR: be careful in choosing GPS module as some will not work above 11 miles

[wiggles]

Probably 36 miles specified is for land-to-land communications. Find out local HAM club they will give you all sort of advice you need.

So this would mean that the range is different, hopefully better, in the air?

Yep. Major source of signal weakening is due to various material obstacles and you can use REALLY BIG antenna (or parabolic one) on receiver (it's should be ok to modify receivers even in US) to mitigate signal weakening due to distance.

Link for inspiration: http://pasadena.net/shootout05/ (yeah, it's 125 miles with 300 milliwatt)

According to the FCC: 95.194 (FRS Rule 4) FRS units. section (C)"You may not attach any antenna, power amplifier, or other apparatus to an FRS unit that has not been FCC certified as part of that FRS unit. There are no exceptions to this rule and attaching any such apparatus to a FRS unit cancels the FCC certification and voids
everyone's authority to operate the unit in the FRS."

[wiggles]

how about MURS? http://en.wikipedia.org/wiki/Multi-Use_Radio_Service
allows up to 2 watts output and unlimited gain antennas, and handheld units are available for around $100.

[stevech]

You could easily run a standard serial stream over it, just hookup the serial connection to the mic/ heaphone jack (make sure to use TTL level voltages to avoid damaging the radios). You may also want to rig up some directional antennas, since it should be line of sight the whole way up. Directional antennas will also help finding it once it hits the ground.

Nope

[stevech]

how about MURS? http://en.wikipedia.org/wiki/Multi-Use_Radio_Service
allows up to 2 watts output and unlimited gain antennas, and handheld units are available for around $100.

Good choice, but let's do the math on 2 Watts, 150MHz, and low antenna gains.