Telemetry

Telemetry

In 2017, I plan to pass my L1 HPR certification. This outlook gives me roughly a year to build my high power rocket. I have this goal not only because it is cool to build powerful rockets, but I am looking for a platform to experiments and develop my own avionics. Something with a faster turn-around than a weather a balloon. There are many aspects to rocketry, so you may see my posts about this project scattered all over my G+ collections, depending on the core topic. Today, I will share about the first functional block of the telemetry sub-system. Telemetry allows us to measure and transmit metrics of interest to an aggregation or storage point. Obvious metrics of interest for a rocket are the altitude, linear and angular speeds, accelerations, temperatures, etc. Imagination is the limit! But the one metric that I consider to be critical is the location. Indeed, the last situation I want to do deal with is wandering for miles in the high desert searching for my rocket – or its debris if anything decided to go wrong! To avoid this, I plan to use a GPS module with a medium update rate (let’s say 10Hz or so – and no need to unlocked it as the one I used for the stratospheric balloon flight). To transmit the data – this is what this post is about –, I decided not to go with a HAM radio, but instead to experiment with the XBee-PRO 900 HP modules (902-928 MHz, 10 Kbps-200 Kbps). One in the rocket to transmit, and one on the ground attached to a laptop to receive the telemetry. With line-of-sight, which should not be a problem in this application, the range can reach up to 9 miles (14 km) @ 10 Kbps [4 miles (6.5 km) @ 200 Kbps]. I used XBee modules in the past to remotely measure power & energy consumption of smart power plugs with the Intel Energy Checker SDK, so I learned to appreciate them. Let’s see if 5-6 years later, I will still like them. What I already like much more is the latest XCTU configuration software. What a change! It is a delight to use it compared to the first revision. Something I was not aware of, is that the programmable version of the XBee-PRO 900 embeds a Freescale MC9S08QE32 Microcontroller. So, it would be possible to directly query the GPS module from the XBee and transmit the data without the need of a computer board. But since I plan to use the rocket as an avionics experimental platform, I will likely still have a dedicate an on board computer (maybe Intel Curie based) to run additional code. But I may as well segregate the GPS data read/transmission onto the XBee’s microcontroller for isolation and resilience. Nonetheless, I feel pretty good about this solution. Of course, there are many details that needs to be ironed out, and many of them may turn wrong, but it is promising.