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Have got some existing low power 2mW XBee series 2 with an Explorer that I've used successfully to communicate between a PC with the XBee Explorer and a remote Arduino based project.

I'd now like to create a one way link to send telemetry data from a model rocket with an Arduino based telemetry board that has a ZigBee option. I'm thinking I'd be able to get away with the existing low power XBee I have plugged into the XBee explorer as the base station to receive the data and just purchase a high power XBee with an aerial to mount inside the rocket.

Does anyone have any experience or comments about combining a low and high power XBee to create this one way link from the rocket to a base ground station?

I realise the XBee must be of the same series (1 or 2/2.5) to be able to successfully communicate.

The ones I currently have are these www.sparkfun.com/products/8691 and I'm thinking if I bought something like this http://www.littlebirdelectronics.com/products/XBee-Pro-50mW-Series-2.5-RPSMA.html# I'd be able to create a reasonably good distance link for a smallish model rocket to send it's telemetry data through.

One issue I'm seeing is that the transmitter may need to be in some sort of broadcast or no sync/acknowledge mode as the receiver would be low power and not be able to transmit data/acknowledgements back to the rocket to acknowledge data being received. It also may (given a very good flight) go out of range and so coping with nobody listening/receiving needs to be configured into the transmitter end.

Second, what aerial or version of the 50mw XBee would make sense to use in the rocket? The link to the one I've shown above requires an external RSPMA aerial to be plugged in. Assuming the combination of high to low power sending only would work, would this be the right aerial type to use being mounted internally in a rocket?

Or am I just trying to make this all to complicated and should ignore the fact that I have low power XBee modules now and buy 2 high power ones that I won't be transmitting from one with?

Thanks.

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I guess you are not trying to get range >1km, right?

Are you really sure you need telemetry? Logging data into microSD card is way easier & more reliable. microsd card can survive even the most horrible crash.

If you really need to extend range, I would suggest directional antenna on ground receiver with servos to track rocket. That would be very fun and will extend the range much more than 50mW transmitter.

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  • \$\begingroup\$ Range is not expected to be more than 1km at this point, so that shouldn't be a problem. The Arduino will be logging to a microSD card as well. So all data will be captured in a higher resolution than intended to be transmitted. I also have thought about two servo motors on a directional antenna, but that will be a second stage for the project. More interested to understand whether it's possible and whether the range will still at least be acceptable up to the 50mW versions transmit power. \$\endgroup\$ – user2568 Jan 11 '11 at 6:24
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It's been a couple years since I worked with ZigBee, but I think that most commands require ACK sequences. This is the case for most (if not all) popular RF networking protocols.

It should be possible to implement your own protocol. This could be as simple as an uninterrupted stream of data if you're just logging it to a PC or laptop. You would get much higher data rates with such a system when comparing to ZigBee. A line encoding that gives you DC balance would help, as would clock synchronization, bounded disparity, and/or error correction. But, if it's not a big deal to loose a few data points every once in a while, you should be able to develop a simple protocol that wouldn't require replies.

Edits: Reading the datasheet confirms that ZigBee requires acknowledge packets. It even says that "It is possible in rare circumstances for the destination to receive a data packet, but for the source to not receive the network acknowledgment. In this case, the source will retransmit the data...." - This is exactly what you don't want. Transparent mode appears to be a system by which the XBee does the ZigBee configuration automatically-not a low-level version of the API. You'll want to check this. The last RF job I did used the Microchip MRF24J40, which took the opposite approach: The whole communication stack was on the host processor. It looks like you'll need custom firmware for your XBee to work with a custom protocol.

To understand why the communication will be unidirectional, you should know that the 2mW and 50mW specs refer to the transmit power only. The receiver sensitivity is almost the same for both at about -100dBm: The high-power version is actually a little bit less sensitive on the receiving end than the low power version, probably due to parasitics of the active antenna circuit. Since they're both operating on the same frequency, they should be perfectly able to communicate - There's no way of knowing whether an incoming signal is a low-power transmitter close by or a high-power transmitter far away.

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  • \$\begingroup\$ No problem with a simple protocol. As the data will all be logged to an SD card, the radio link is to provide some information to people on the ground as to what's happening. As such, losing some data isn't critical. It will also help with having a simplified GPS lat/long being sent to know last locations the rocket transmitted from and aid with its recovery. I'm more interested in whether the Zigbee will provide the range of a one way high to a low power receiver and that the two devices will communicate this way. \$\endgroup\$ – user2568 Jan 11 '11 at 23:20
  • \$\begingroup\$ Problem with simple protocol: After further reading, the XBee doesn't appear to allow it. \$\endgroup\$ – Kevin Vermeer Jan 12 '11 at 0:13
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The S1 modules allow use of 'broadcast mode' where ACKs are not sent. Using an S1 module in broadcast mode will work correctly with a low power module in the base station, presuming there is no need for the base station to transmit to the rocket. I do not believe this is possible with the S2 units - S1 is good for point-to-point, S2 is good for mesh.

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