University of British Columbia | UBC Rocket Rocket
Jan. 5, 2020
12:45 p.m. PST

Spread spectrum radio frequency allocation (XBee modules)


We are the rocket team from The University of British Columbia. We are currently designing an SRAD GPS-tracking solution, and have run into concerns based on the recently released GPS Tracking Requirements document. Specifically pertaining to the frequency allocation rules.

Our design uses an independent GPS receiver module and an XBee radio module from Digi International. The radio module is a frequency-hopping spread spectrum module, transmitting in the 900 MHz band.

Spread spectrum transmitters utilize multiple frequencies during transmission and rapidly switch between them; because the energy is spread out over different channels, they do not introduce significant interference at any particular frequency. Furthermore, this means that our receiver is robust against interference on noisy channels. However, our module requires a minimum number of frequency channels (~50) to operate, and we understand that this conflicts with the request that each team transmits only on one pre-assigned frequency. We do have the capability to quickly restrict the specific channels that get used, to avoid introducing very minor interference on the most utilized frequencies.

We have had resounding success using this module at last year’s IREC, having maintained communications with our rocket until about 500 feet above ground. The success of the module in conjunction with our GPS receiver allowed us to quickly locate and recover the rocket. We have also verified that our module complies with FCC regulations for unlicensed usage (specifically, FCC Title 47, part 97).

Because spread spectrum transmitters present very little interference (spreading transmission over 50 channels equates to around 2% use of each channel) and are arguably more robust than transmitting only on a single channel, we think the GPS Tracking Requirements document should be amended to allow spread spectrum transmitters.

If possible, could we please receive some clarification on the use of spread spectrum radios such as our XBee modules?

Thank you very much for your time.
9 Replies

Andy moderator
Jan. 5, 2020
2:36 p.m. PST
Hello Team UBC and thank you for the note.

The founding principal for the new GPS tracking requirement document is the quick recovery of launched rockets. And that's it. We are standardizing on a set of transceivers which can be received and decoded at our MCC utilizing it's 30ft antenna mast.

Thank you also for the background on spread-spectrum, "frequency" hopping transmitters. We are quite familiar with it. One of the reasons we are not supporting it is that it requires a proprietary receiver which is tuned to the channels and timing. In essence, your team's receiver is the ONLY receiver capable of collecting and interpreting the coordinates from your XBee.

By following the GPS requirements document, you can track your rocket on your own handheld dongle antenna/Yagi using the assigned frequency AND ESRA can also track your rocket using a 30ft antenna mast. I trust you understand the significance and benefits of this additional receiving station.

You can continue to use your SRAD tracking solution, but you will still need to provide an ESRA compliant solution that meet the GPS Tracking requirements. I trust this clarifies your understanding even though it may not be the answer you were hoping for.

Hi Andy,

Thanks for the quick response and clarification. We are now searching for an ESRA compliant tracking solution.

Christopher Harris moderator
Jan. 6, 2020
8:35 a.m. PST
@UBC Rocket Team
A TeleGPS or 70cm Bigredbee are quick and easy answers to fulfill the requirement.

Rishi Chauhan
Feb. 19, 2020
7:28 a.m. PST
Respected Judges,
As per the IREC GPS Tracking and Recommendations document, the featherweight is an approved COTS solution to the GPS requirements and requires no HAM licence. Hence, our team has decided to opt for this solution.

However, upon further investigation into the Featherweight User Manual( ) we have discovered that the featherweight actually operates on LoRA spread spectrum technology.

So will the featherweight still be accepted?

Thanking you,

Rishi Chauhan
Feb. 19, 2020
7:30 a.m. PST
Forgot to tag the judges.

Aydin Nakhaee-zadeh Gutierrez
Feb. 25, 2020
9:08 a.m. PST
I am part of the COM team of CRANSEDS. I have the same question as @RishiChauhan, we were planning to use LoraWAN, wich uses 60+8+8 channels in USA from 900MHz - 930MHz. Not sure if the requirement stands only for FHSS or for any kind of spread spectrum technique like Chirp Spread Spectrum for LoraWan case.
Thank you in advance.
Kind Regards,
Aydin Nakhaee-zadeh Gutierrez

Christopher Harris moderator
Feb. 25, 2020
10:52 a.m. PST
The 900MHz band for the COTS GPS requirement is restricted to the Featherweight GPS Tracker. No other LoRa devices are approved for use as a COTS GPS solution.

Rishi Chauhan
Feb. 25, 2020
11:42 a.m. PST
@Christopher Harris
Thank you for the clarification!

Lev Dower
Feb. 1, 2021
8:50 a.m. PST
Hi...I have more than one end gadget and I need to establish that a discretionary sign to be sent over my xbee(the end devices).is it even conceivable ? how?
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