Avionics specs for PSAS | Other subsystems | Forum

As discussed elsewhere, our contact at PSAS has suggested that the best way for CSTART and PSAS to cooperate on OHKLA would be for PSAS to focus on electronics and recovery and for CSTART (or another Friend of CSTART) to focus on the rocketry itself.  We seem to be finally starting to make some progress with the rocketry aspect, and now 3 directors have voiced their willingness to let PSAS help with the electronics.  So!  Let's come up with some rough specs we can hand to PSAS to see if they think they can help.

As for functionality, I think all we really have is:

• ESSENTIAL: ability to reliably record maximum altitude and/or data which can be processed post-flight to yield maximum altitude
• DESIRABLE: ability to take and store some cool photos and/or video from the flight

With regard to restrictions, all I can really think of is a maximum mass and a maximum volume.  5kg as an upper mass limit seems reasonable enough, as for volume, I can't imagine the diameter of the top of the rocket exceeding 200 mm, but perhaps we should wait until we have some harder numbers on that?

Your first point is essentially breaking "data which can be processed post-flight to yield maximum altitude" up into explicit requirements, which I guess makes sense.  We should be careful with our wording on the GPS issue.  We want to record them while in flight, too (I don't know how well it will work at high altitude or speed, but in terms of interpreting data, GPS coordinates will be the easiest thing to work with), and once we're on the ground I guess we can stop recording after a few seconds and then switch exclusively to trasmission.  I'm not sure we should relegate pressure to an "only desirable" status because it can probably help with recovering the trajectory data afterward as well, since pressure varies with altitude.  Temperature is certainly a desirable optional – it's not really essential to trajectory recovery, but it might be nice to get an idea of the extremes incurred so we can make sure all of our components are good for long-term use at those temperatures.

Here's a crack at a rigorous listing of our needs (essentials):

• Ability to record accelerometer data during flight
• Ability to record gyroscope data during flight
• Ability to record magnetometer data during flight
• Ability to record barometer data during flight
• Ability to record GPS data during flight
• Ability to transmit GPS data after landing

I guess there are basically only two states the system needs to be in, a "record everything" state and a "transmit GPS" state.  The system should be in "record everything" when booted up and then transition to the "transmit GPS" state once certain conditions are met (altitude and velocity both below some threshold) or, as a failsafe, once a certain length of time has elapsed since boot up.
The more I think about it, the more it makes sense to me to try to keep the video system, if we end up doing one, and the "actual avionics" as close to completely separate as possible, since they serve separate goals and have different levels of criticality.

joe.haydu said:

I just wanted to call those two items out specifically, since maximum altitude could be measured with an altimeter, and it seemed like trajectory data would be good to have. I think the list is pretty much spot on, I don't see that we'd need anything else, although you did mention on IRC that we might want to trigger the parachutes with the controller, so it seems like that should be added if that is the case. Do we have any ballpark estimates for how far we want the GPS signals to be transmitted?

Yep, for completeness we should mention the need for control channels.  Since we haven't done much work yet on figuring out exactly how nose/body separation and parachute deployment is going to work, we can't say with confidence how many channels we will need – it could be as low as just one.  This is probably something we should discuss.

Good question with the GPS range. This is a very preliminary figure, but the CSXT amateur space rocket had a planned landing point about 25 miles (40 km) downrange of it's launch point.  Allowing for differences, we probably shouldn't expect our rocket to land any more than 60 km away from it's launch point.  Of course, we don't necessarily need to be able to transmit the coordinates over 60 km: we should know roughly which direction to look in, relative to the launch site, so we can always drive in that direction with a receiver and start moving around in circles once we're in the approximate area – so maybe we could get away with as little as 10 km?  Even that feels like rather a lot to me, though, although I'm hardly a radio expert.  If 10 km is wildly unrealistic for the size and power we'll be working with we may have to give some more thought to exactly how we're going to find this thing.

If we could be sure of cell phone reception at the landing area that would make things a lot easier for us, but as it is we have no idea where we're likely to actually launch this thing.  Perhaps that is something else we should do some more research on.

Progress on this has stalled a bit.  Are you still around and interested, Joe?

One benefit of all the recent mass-modelling work from an avionics perspective has been that we can now get relatively decent upper bounds on the maximum acceleration (linear and angular) the avionics module will be subject to, which is an important design requirement – among other things, it will help us select appropriate accelerometers and gyroscopes.