A question of altitude...

I have been waiting on the arrival of the Micropeak USB adapter to resolve the difference between the altitude it gave (381.7 feet) for the peak of Saturday's Payloader One flight versus the 429 feet measured by the Perfectflite Firefly. As I mentioned in my earlier post, the Micropeak altimeter slipped its retaining ring and jumped around inside the payload section, but that does not seem to be a good explanation for the huge 47 foot discrepancy between the two. Unlike the Firefly, the Micropeak is a recording altimeter, storing data at roughly 0.2 second intervals for the first 48 seconds of the flight. That data can be downloaded into a Mac, Linux, or Windows computer using the optional USB interface, which I also ordered.

But I did not have the USB interface...

Tracking showed it delivered to my building's apartment parcel locker on Saturday, but the postman mistakenly placed the key to said locker in the wrong mailbox. I had to wait until yesterday to catch him delivering the mail, and when we checked, my package was not there, taken by the person who erroneously received the key. I was disappointed, as I could not recoup my loss (the package was uninsured), and the postman apologized profusely for his mistake. Returning to my apartment, I resigned myself to having to order another when I could scrape up the funds; answering the question of the peak altitude of the Payloader One would have to be postponed until a later date.

But today, after work, my faith in humanity was restored when a neighbor delivered the package to me as I was walking in the door. She and I were coming home from work at the same time, and she noticed my apartment number. Telling me to wait, she fetched the USB adapter, still in the unopened box. I was most happy, and thanked her profusely. Honest folk still exist!

Micropeak USB adapter. The altimeter is placed upside down, with the LED on
top of the sensor in the middle of the adapter.

I wasted no time in unpacking the adapter and connecting it to my computer. Being so small, the Micropeak has no USB connector - it communicates with the adapter by flashing its LED at 9600 baud, which is pretty nifty. I fired up the software, selected the USB adapter from the menu, turned on the Micropeak altimeter, and set it on top of the adapter with the LED facing down. A scant few seconds later, the flight profile was displayed:

Payload One flight profile (Click to enlarge).

Two things were immediately obvious from the plot:

1. The altimeter flopping around the payload section introduced strong oscillations in the data, especially in the speed and acceleration profiles (Big surprise, eh?), and
2. It was not given adequate time to sense ground level pressure, as it has the rocket landing at -54 feet. Ground level is supposed to be at 0.

This then is the answer to the discrepancy - the actual altitude attained by the Payloader One is the 381.7 feet reported by the flashing LED + the 54 feet below zero, as that was the real ground level. 381.7 feet + 54 feet = 435.7 feet, which is in reasonable agreement with the 429 feet reported by the Firefly. I can live with a 6 foot difference.

This is why I like recording altimeters - I would have never known that the Micropeak had not sensed ground level and adjusted its zero without looking at the data. Well worth the additional money (The Firefly costs $25; the Micropeak $50).

The software also displays a nice summary of the flight, and presents the raw data in tabular format so it can be copied into other applications, like Excel. You can also export the data as a csv file for import into almost anything. It's a pretty nice piece of code and runs under Mac OS X, Windows, and Linux.

Payloader One flight summary as reported by the software (Click to enlarge).

Raw data display (Click to enlarge).

I can't wait to fly this altimeter again - it is very easy to use! Will have to work on a proper mounting scheme, though. These things are really small, unlike the older altimeters in my collection.