TEST BENCH issue #001

Altimeter Accuracy and Precision
Although this initial issue of Test Bench deals with electronic altimeters, our example diagram is of a mechanical one. This aneroid pressure capsule device predates WWII. It works from below sea level, to 50,000' AGL (Above Ground Level). A tiny movement of the capsule is amplified by many sets of planetary gears, causing up to 50 rotations of the display needle. No electrical power is needed. These units still dominate the light plane field, and are available used/calibrated for about $100.

Accuracy and precision of electronic or mechanical measuring instruments are both important, but are sometimes confused.
 

ACCURACY: "trueness" of reading. If it says 1013 feet, but is actually 800 feet, that is bad accuracy.

PRECISION: "fineness" of reading. If it says 1013 feet, 3.14159265358 inches, that is high precision.

 
Some instrument makers provide greater display precision than is warranted, in order to give the illusion of better accuracy. For example, if a scale can only really resolve 0.1 ounces, but is graduated in micrograms, the user may believe that he can achieve microgram accuracy, but it will not be possible.

 

TYPES OF ALTIMETERS
Three types are in use: baro, accel, and GPS. Barometric altimeters use an absolute pressure sensor (has an internal vacuum reference cavity) to provide a voltage output proportional to pressure. As you go higher, pressure decreases. This is a logarithmic function, so some means must be provided to linearize the measurement. Anti-log amps, look-up tables, and compensation calculations are typical methods. Problems arise when rockets exceed about 700 mph...shock wave turbulence of supersonic flight can easily cause premature apogee deployment, unless some preventative means is employed; otherwise baro units always provide good data of true height. For best accuracy, temperature must be measured quickly, and worked into the altitude calculation.

Accelerometer altimeters measure the instantaneous g-force along the axis of the rocket. Integration of acceleration yields velocity, and integration of velocity yields displacement (altitude). The BIG problem is that unless the flight is exactly straight, errors result. The integrating accel was first used in the space launches of the 1950's... not for altitude determination, but to fire upper stages at the right time. Indeed, at least one company sells an accel unit with staging capability. For delayed staging, which increases altitude, Transolve offers the AIRSTART module (use with P6/3000 altimeter).

Sensing and logging of accel data is useful, but one must be aware of the errors and failure modes inherent with this method. A popular product made by COMPANY B, has both baro and accel sensors. A flight at LDRS, where a rocket turned parallel to the ground at 2000' while still under thrust, showed that they do not sense the baro sensor until the accel has picked off apogee...the accel profile DID NOT trigger the apogee device, and the rocket sailed over the horizon and crashed.

GPS would seem to be an ideal means of x, y, z position fixing. The problem is that the low cost devices (~$150) only sample once or twice per second, which creates coarse data. Military GPS modules sample at about 300 times per second, but cost about $5000, and may be classified items. Still, at least one model rocket GPS system is available at this time.

 

CONVERSION RESOLUTION (and patents)
The analog signal from an accel or pressure sensor must be converted to digital form in order to be processed by a µC (micro-controller). Usually, an a/d (analog to digital), or v/f (voltage to frequency) converter is employed. Many altimeters use an 8 bit a/d, and have a range of 25,000'. Lets look at the math:
8 bit a/d has 256 sample intervals.
Real resolution = 25,000' / 256 = 97.656'.
So, an 8 bit a/d with pressure sensor provides 100' precision. But, many of these low cost altimeters "beep-out" altitudes down to the foot. It is clear that the extra resolution cannot be valid.

For good results, accel type systems need greater than 8 bits...12 bit a/d with fast sampling is best.

A test baro system built at Transolve around an ADUC812 (8 bit µC with 12 bit a/d) was calibrated at an FAA certified repair station. Range was to 50,000', and it could resolve 3' at sea level, and 10' at 50,000'.

COMPANY A uses v/f with an anti-log amp, and claims 16 bit resolution. Several of these altimeters have been examined, tested, and repaired on our Test Bench. The anti-log amp circuit has proven to be drifty...old units sometimes fire apogee at liftoff. This circuit restricts precision to about +/-50'. Claiming 16 bit resolution looks good on paper, but in practice, the 100µV / foot scaled signal is swamped by sensor hysteresis and noise, and aerodynamic effects. The front end might be able to resolve 15', but the drifty anti-log amp adds significant distortion. The amazing thing is that it is supposed to be patented! Repeated attempts to obtain the patent # from the firm have failed.

Goto: uspto.gov to research patents. There are no rocket altimeter patents. There are some model rocket patents, however. Some experts say that if you are not a $10 million company, you can't afford to enforce any patent you get.

Back to COMPANY A. He claims that his $90-$160 altimeters read accurately to the foot. The master calibrator at the FAA lab costs $3000, has to be calibrated twice a year, and they say it is accurate to 3'. Use your own judgement to evaluate his claims!

 

CONCLUSION
Do some research before buying an altimeter. Ask the manufacturer how many bits the conversion system has, how it is linearized, and how long the battery lasts. Compare price, performance, and reputation. Watch out for marginal power sources...tiny batteries that charge a 2200µF capacitor every time the power is turned on can cause trouble (like crashing your rocket). Ask about assembly methods, cleaning, burn-in, and conformal coating. This coating helps protect it, but often prevents repair. Ask about the repair policy. It can range from the Transolve $5 base fee, to $80 minimum, to "throw it away". Find out how the calibration was done (traceable to the National Institute of Standards and Testing?).

Please keep safety at the top of your list. Ground test systems before flight. Use back-ups. Don't hook-up or install charges to live altimeters. Power-up on the pad! Keep gunpowder residue OFF circuit boards...it is conductive AND corrosive. Watch those vent holes...very high speed or large volume rockets (minimize bay volume) need extra venting. Remember to turn it on! The Transolve Flux Capaciter back-up unit turns itself on at take-off. There is no power switch to forget about.
 

Transolve...don't leave the ground without us!

 

© 2001 Transolve Corporation. All rights reserved.