RRC3 altitude reporting

First off, I have been very happy with my RRC3, so much so I bought a second one. However, I have a question.

Why does the altimeter beep out a higher altitude than the stored data shows (typically beeps report ~50 ft higher? Also, the recorded data always shows landing below the launch altitude, even when they are clearly near level, or even the landing spot is slightly elevated. Any explanation for this? Do other altimeters do this, and if so why?

Cl(VII) said:

First off, I have been very happy with my RRC3, so much so I bought a second one. However, I have a question.

Why does the altimeter beep out a higher altitude than the stored data shows (typically beeps report ~50 ft higher? Also, the recorded data always shows landing below the launch altitude, even when they are clearly near level, or even the landing spot is slightly elevated. Any explanation for this? Do other altimeters do this, and if so why?

Click to expand...

The altitude/pressure relationship is mathematically complicated, and it's hard to evaluate in an embedded system like an altimeter. Therefore, most altimeters use approximations to compute altitude for the beeps. Approximations are commonly (not always) linear or quadratic functions. In some ranges, they may be biased a bit high; in other ranges they may be biased a bit low. When you download your stored data to a computer, the full, complex expression may be used, and the altitudes may differ somewhat from the beeped-out figure. Evidently, your rockets tend to reach altitudes where the onboard approximation is biased high.

As for the launch altitude... not entirely sure. It's common for the launch site pressure (and hence altitude) to be computed as an average of several (~10) readings before launch. That's because the value is used in the computation of all other altitudes; it has to be as solid as possible. The altitude of the landing spot is a single value, which is likely to have more noise. Can't say that accounts for your observation.

Regards,
-Larry

CL(VII) I have two RRC3s as well, and noticed the same on both units. I chalked it up to pressure anomalies/differences of samples.
Mine have been a little bit closer in readings, around 15-20ft difference, but since I'm not using them for contests or anything, I'm fine with it.
Very reliable units so far. I also use the RRC2+ (3 on hand, and just ordered two more!)

Larry Curcio said:

The altitude/pressure relationship is mathematically complicated, and it's hard to evaluate in an embedded system like an altimeter. Therefore, most altimeters use approximations to compute altitude for the beeps. Approximations are commonly (not always) linear or quadratic functions. In some ranges, they may be biased a bit high; in other ranges they may be biased a bit low. When you download your stored data to a computer, the full, complex expression may be used, and the altitudes may differ somewhat from the beeped-out figure. Evidently, your rockets tend to reach altitudes where the onboard approximation is biased high.

As for the launch altitude... not entirely sure. It's common for the launch site pressure (and hence altitude) to be computed as an average of several (~10) readings before launch. That's because the value is used in the computation of all other altitudes; it has to be as solid as possible. The altitude of the landing spot is a single value, which is likely to have more noise. Can't say that accounts for your observation.

Regards,
-Larry

Click to expand...

Thanks for the detailed response. I would say that the landing spot issue isn't a single point as the trace of the decent passes through 0 altitude and continues on down, so it would have to be more of a systematic error, rather than a single point variance. I figured there had to be a difference in computing algorithm between the two methods of calculating and communicating altitude, but I don;t have any background in how the measurements are actually made, so your explanation has been quite helpful. Thanks again.

dgreger said:

CL(VII) I have two RRC3s as well, and noticed the same on both units. I chalked it up to pressure anomalies/differences of samples.
Mine have been a little bit closer in readings, around 15-20ft difference, but since I'm not using them for contests or anything, I'm fine with it.
Very reliable units so far. I also use the RRC2+ (3 on hand, and just ordered two more!)

Click to expand...

I wonder if the effect is different at given altitudes. Chances are you have flown yours higher than I have as I am new to High power and just getting a few rockets through 1000-2000 ft shakedown flights before I hit them harder. Thinking about it, I think that my highest flight with one (2500 ft) was probably the one with the least absolute difference i readings between the beeps and file. In any event, I'm with you that the things are very reliable and easy to set up, I'm not changing because of this. I'm not worried about the accuracy of altitude as much as I was/am curious to know why they vary. Thanks.

Cl(VII) said:

First off, I have been very happy with my RRC3, so much so I bought a second one. However, I have a question.

Why does the altimeter beep out a higher altitude than the stored data shows (typically beeps report ~50 ft higher? Also, the recorded data always shows landing below the launch altitude, even when they are clearly near level, or even the landing spot is slightly elevated. Any explanation for this? Do other altimeters do this, and if so why?

Click to expand...

I can answer your questions, but first:

1. I'm glad you like your RRC3 and have purchased a second altimeter.
2. When you refer to recorded data, my guess is you are referring to the uploaded 20Hz baro data evaluated independently via the mDACS Flight Viewer?
3. Would you mind posting the .rff file and/or Flight Viewer graph in question?

Small variances in launch pad altitude vs real ground altitude could be due to wind blowing across the electronics bay port. The more wind, the higher the altitude will appear to a barometric sensor. I presume a low pressure system in the area can also have the same effect by lowering barometric pressure. These pressure sensors are a little temperature sensitive as well.

Gerald

MWC said:

I can answer your questions, but first:

1. I'm glad you like your RRC3 and have purchased a second altimeter.
2. When you refer to recorded data, my guess is you are referring to the uploaded 20Hz baro data evaluated independently via the mDACS Flight Viewer?
3. Would you mind posting the .rff file and/or Flight Viewer graph in question?

Click to expand...

Thanks for the offer. I was going to upload the file last night, but my 6 year old got up from the dinner table and promptly fell into the corner of a different table ...spent the evening getting his head glued back together in the ER. Hopefully tonight will be less eventful.

Cl(VII) said:

Thanks for the offer. I was going to upload the file last night, but my 6 year old got up from the dinner table and promptly fell into the corner of a different table ...spent the evening getting his head glued back together in the ER. Hopefully tonight will be less eventful.

Click to expand...

Oh man. I hope he is better, soon.

Greg

GregGleason said:

Oh man. I hope he is better, soon.

Greg

Click to expand...

Thanks. He'll be fine; he got lucky in that he missed his eye by less than an inch. Some dermal adhesive and a few steri strips pulled it right together. It was the depth (I don't know how we weren't looking at skull) more than length that sent us to the doctor. I can't say enough how much I appreciate children's ERs; they not only know how to take care of small people, but they know how to handle and calm frightened/hurt small people.

A less eventful evening mercifully. Here is a representative rrf file. This one beeped out 1447 ft upon landing, but the file shows 1392 max alt. It also shows about a -60 ft landing. If any thing the landing spot was a few feet (<10) above the launch site.

View attachment Cher-I_AT_H148R_20140726.rff

Thanks for posting this file, and I'm glad the boy is OK and on the mend... having raised 3 boys myself I understand those times.
What was your arming/launch detect altitude set to for this flight? (this question gives me an idea for new a feature, BTW)

I believe it was 300 ft. I can check this evening if that has something important to do with it.

MWC said:

Thanks for posting this file, and I'm glad the boy is OK and on the mend... having raised 3 boys myself I understand those times.
What was your arming/launch detect altitude set to for this flight? (this question gives me an idea for new a feature, BTW)

Click to expand...

Here's what I believe to be happening:

1. In regard to the lower elevation at landing, the RRC3 has a 1-second "pre-launch" buffer, thus all data prior to launch detect (most likely 300 ft. as this is default) that's older than 1 second is not recorded... I've blown up the 1st second of your flight, and my guess is that your bird required more the 1 second to reach the 300 ft. launch detect trigger (as evidenced by the data I can see). You might consider dropping the launch detect threshold to a lower altitude for a lower/slower rocket like this.



2. In regard to the beeped and/or Flight Log recorded peak altitude vs. the mDACS derived peak altitude, there will always be slight differences. The altimeter performs its own independent filtering and pressure/altitude conversion in firmware from the 20Hz raw baro data, and that result is what's beeped and displayed from the Flight Log memory (i.e. your last flight summary). The mDACS desktop also reads the 20Hz raw baro data as well (from flash) and performs it's own independent filtering and conversion. The small differences in data precision and algorithmic implementation produces the different results between the two platforms.

Thanks for the feedback, very helpful.

The reason I have the launch detect at 300 ft is it is almost always windy in TX. I am concerned that even though my electronics bays are well ventilated that a stiff gust could cause an improper launch detect reading. However, I only use 300 out of an abundance of caution, and frankly ignorance of what a safe value would be. Do you have a recommendation on a minimum safe value for this?

Cl(VII) said:

Thanks for the feedback, very helpful.

The reason I have the launch detect at 300 ft is it is almost always windy in TX. I am concerned that even though my electronics bays are well ventilated that a stiff gust could cause an improper launch detect reading. However, I only use 300 out of an abundance of caution, and frankly ignorance of what a safe value would be. Do you have a recommendation on a minimum safe value for this?

Click to expand...

Caution is a good thing, and lowering the launch detect elevation does make the unit more susceptible to spoofing in windy conditions on the pad... for this bird I'd try 250' which should capture that additional data you are missing.

Thanks again, Jim.