Cool

What I'd like to know is the specifics of certification. Despite asking, I have no idea what tests are performed. Frankly, I don't trust any altimeter to be accurate, precise or repeatable based on my experience. The Parrot has a lot of potential to be better than others given what I've seen. I'm curious what actual testing they do to validate an acceptable altimeter.

Warren

Warren,

As far as I know, the only certification test performed by Tripoli was to review my published specs and make sure that I really was selling them as a commercial venture, and not just between friends. I offered to provide an altimeter for testing, but Tom never took me up on that.

But I've done a lot of testing and verification of the calibrations on my own and most of the results of that can be found toward the back of the user's manual, which is on my website at www.featherweightaltimeters.com.

Just wondering.
Im assuming these altimeters are after AGL accuracy as opposed to MSL accuracy, with no adjustment for barometric pressure. The other day sitting at the table, the alt. told me 4700 ft. The Longmont AP runway is at 5050. And my house is 75-100ft higher then that. 400-500 ft differance. I just assume there is no adjustment. Not to be splitting hairs, weather fronts are sloping and somtimes a couple of hudred miles from the front on the ground to the front at 30,000ft. Altimiters messuring stricly Baro pressure on a high flying HP rocket could be 100s maybe even over 1000 ft. or more off if it flew though one of these fronts.
Im am in no way an expert, since I didnt even know what an accelerometer was, untill last year. (A guy at my EAA club was showing us some stuff he was tinkering with) It seams to me measuring Adrians parrot against other so called certified altimeters isnt apples to apples.
And like Warren said "What are the perameters of certification".
Adrians parrot could be more accurate! I dont know, how they are all built. 🙄

Scotte

Scott,

The Parrot has both a barometric sensor and an accelerometer. The barometric sensor, despite the issues you bring up, is going to be the most accurate, since it's not dependent on the flight path angle from weathercocking. Also, the accelerometers have a harder job because they may be trying to measure accelerations of 50Gs based on an extrapolation from calibration at +/- 1G. I have also noticed a drift of up to 0.1 Gee over the weeks since a calibration, besides its temperature sensitivity.

Changes in the weather can cause barometric changes equivalent to 300-400 feet or more. You have a good question about whether those sorts of pressure effects can vary with altitude above one location when a front is coming through. It would possible, though perhaps to a lesser degree. But I'm not an expert.

Adrian, that is wonderful news for you. I have a friend who had a TRA record disallowed because the altimeter used was "not on the list". That would be an obvious downer.

I hope to attempt a Tripoli record for the M next year. While I don't have a lot of room or weight to mess with, I would like to send another unit along to verify some of the data. I'm glad you are on the list.

For what it is worth, the Pico altimeters are not OK'd by TRM. I'm told this has something to do with the fact that they don't compensate for temperature at altitude, but I'm an idiot on this sort of thing so I'll shut up now.

JW

Well I want dibs on a production unit Adrian. Any possibility of getting one within the week? I have some F and G motors that go out of cert on 12/31 and I need to make sure I take those altitude records before those motors are gone. (no one makes a 100% F or 98% G in 24mm anymore)

Warren

Scott,

The Parrot has both a barometric sensor and an accelerometer. The barometric sensor, despite the issues you bring up, is going to be the most accurate, since it's not dependent on the flight path angle from weathercocking. Also, the accelerometers have a harder job because they may be trying to measure accelerations of 50Gs based on an extrapolation from calibration at +/- 1G. I have also noticed a drift of up to 0.1 Gee over the weeks since a calibration, besides its temperature sensitivity.

Changes in the weather can cause barometric changes equivalent to 300-400 feet or more. You have a good question about whether those sorts of pressure effects can vary with altitude above one location when a front is coming through. It would possible, though perhaps to a lesser degree. But I'm not an expert.

Do you actually get data back from the accelerameters? 🙄

Do you actually get data back from the accelerameters?

Yes, the axial and lateral accelerations are recorded 200 times per second, and the pressure, battery voltage, temperature, and sensor reference voltage are recorded at 50 times per second. All the above are recorded at those rates for the first 5 minutes of the flight, plus about .4-.8 seconds before the flight. Then for the next 25 minutes it's all recorded twice per second. So, you would get lots and lots of data back, which you download via Hyperterminal and then play with in Excel or another spreadsheet program. I have an Excel template up on the site now to help get started.

You can use the accel data to help estimate how much the rocket weathercocked, how fast it was moving horizontally at apogee, what's the coefficient of drag, and how fast the rocket pieces moved apart during the deployment.

Warren, production Parrots are definitely in stock, and if you can order one via the website I should be able to mail it out tomorrow. If you want a loaner for the Parrot Altitude challenge, we can work something out. If I remember correctly, I offered to provide loaners at the club launches, but if you want one for a special end-of-year launch, just send PM me with your mailing address and I'll mail you one. And then either return it, win it, or buy it by the January club meeting?

Adrian
This maybe a little off topic maybe not. I dont totaly have a clear picture of accelorometers. The fellow I was talking about earlier Used them to measure a flight from Boulder Airport and track Roll, pitch, direction of travel and location based on accelerometers. I think he then put the data into another map data base (has terrain data ect.) I think he got the map data from the internet free, or maybe not. (A interesting note: One of the old Ex Commecial pilots said before GPS systems, the airlines used Million doller Accelerometer navigation systems. So accurate,
they would read their taxi speed from them, and keep the Tower guys happy)

Anyway, wouldnt a guy have to enter initial location data? And could you not enter this data into one of those terrain maps and get a visual on a flight in a 3D map. Maybe even radio the data back to a 3D terrain map on the ground? Sorry, just thing out loud sorta.

Scotte

This maybe a little off topic maybe not. I dont totaly have a clear picture of accelorometers. The fellow I was talking about earlier Used them to measure a flight from Boulder Airport and track Roll, pitch, direction of travel and location based on accelerometers

I think what you're describing is an inertial measurement unit, which would contain 3 accelerometers to cover the accelerations in the 3 dimensions, plus 3 gyros to measure rotational rates in the three dimensions. Accelerometers by themselves don't know which way they are pointing relative to the ground; an axial accelerometer by itself would put out the same readings for a motor burn whether the rocket is pointed up or down.

As an aside, I think I have seen in the club rules that a 3-axis accel might be acceptable for altitude contests. Without 3 axes of gyros, a 3-axis accelerometer would have the same problem detecting weathercocking that a 1-axis accel would.

Inertial Navigation yes, thats what Dad said the old F-4 use to use. A lot more primative when he was in the Airforce then more recent ones like described before that the airlines used. Dad said it would get you close enough within 20-30 miles of the ship (Since the F-4 was originlly a Navy plane) to ride in on a radio beacon.
Gyros, thats your next project Adrian!
😉

The biggest problem with solid-state gyros (the only kind we can afford as hobbyists) is that they are rate-gyros, not absolute position sensing gyros. They'll tell you the rate of rotation, but drift is horrendous - like ended up 180 degrees out within a minute or less. For truly stable IMU measurements, you need either a mechanical gyro such as were used in Apollo and most aircraft up until the 70's or laser ring gyros or their fiber optic descendents.

Another issue with the solid state devices is that they typically have very low g-tolerances and ability to track rates higher than relatively few degrees per second.

Warren

The biggest problem with solid-state gyros (the only kind we can afford as hobbyists) is that they are rate-gyros, not absolute position sensing gyros. They'll tell you the rate of rotation, but drift is horrendous - like ended up 180 degrees out within a minute or less. For truly stable IMU measurements, you need either a mechanical gyro such as were used in Apollo and most aircraft up until the 70's or laser ring gyros or their fiber optic descendents.

Another issue with the solid state devices is that they typically have very low g-tolerances and ability to track rates higher than relatively few degrees per second.

Warren

Yea, Im beginning to remember some of the conversation, from the demonstration a couple years ago. The cost of good parts copared to what he was using, and rocket aplications would be way more demanding. Hence the million dollar price tag for the airlines. But if before GPSs it kept you from wandering over USSR or North Korea and attracting a stray missle (it happened) it was worth it.