Altimeters vs. GPS?

[DaddyB]

Is there a common issue with barometric pressure altimeters (RRC3s in this case) registering lower altitudes than GPS trackers (Featherweight tracker/ground station)? Did a test flight, and the two RRC3s were within 7 feet of each other, while the Featherweight registered a bit over 100 feet higher...

Which would be more accurate? Could the RRC3s be registering lower due to vent holes a bit small, so pressure is not equalizing as rapidly as possible?

TIA

 

[Sooner Boomer]

Is there a common issue with barometric pressure altimeters (RRC3s in this case) registering lower altitudes than GPS trackers (Featherweight tracker/ground station)? Did a test flight, and the two RRC3s were within 7 feet of each other, while the Featherweight registered a bit over 100 feet higher...

Which would be more accurate? Could the RRC3s be registering lower due to vent holes a bit small, so pressure is not equalizing as rapidly as possible?

TIA

When do the barometric altimeters record the max altitude? Eggtimer records it one second past peak. I've seen a difference in measured altitude between an ET Apogee and a Featherweight tracker. As you state, the Featherweight reports a higher altitude.

 

[jderimig]

Is there a common issue with barometric pressure altimeters (RRC3s in this case) registering lower altitudes than GPS trackers

Yes, about half the time.

 

[DaddyB]

I'm fairly sure the RRC3 altimeter record max altitude.

 

[cerving]

When do the barometric altimeters record the max altitude? Eggtimer records it one second past peak. I've seen a difference in measured altitude between an ET Apogee and a Featherweight tracker. As you state, the Featherweight reports a higher altitude.

Actually, we record the actual apogee (because we're taking 20 samples per second), but we don't call it until nose-over. We want to make sure that you're not going any higher first. I have flown multiple altimeters in the same rocket, both Eggtimer and other manufacturers, and for the most part they agree within a pretty small margin, especially on flights over 5K.

 

[DaddyB]

That is very helpful, actually. We also hoped to use altitude data going up to rule out possible early separation, but for some reason we don't have data for 33 seconds -- we went from pad at 14:00:05:818 to 14:00:38:278, at which point it was 2316 feet high, crested at 2330 at 14:00:39:326, and 2313 at 14:00:40:287. Don't know why the gap appeared...

 

[Spacedog49Krell]

Comparing my Ublox M8N GPS to my MS5611 barometric sensor, the GPS altitude is 25-35 meters higher than the MS5611. Both have error bands and limitations, unless you have an unlocked GPS system. I prefer barometric data below 10km. Data from balloon flights show an increasing barometric altitude error compared to GPS above 13km.

 

[cls]

Baro is correct. GPS altitude is wrong, it can never get a solution to converge closer than any except the crudest baros. That's why the TRA altitude contest is broken, GPS is just wrong.

 

[rocket_troy]

Baro is correct. GPS altitude is wrong, it can never get a solution to converge closer than any except the crudest baros. That's why the TRA altitude contest is broken, GPS is just wrong.

That's quite a claim given there are so many flavours of GNSS receivers and so many flavours of Baro sensors. We've discussed in the past that accurate baro sensing is temperature dependant where GNSS isn't. I'm not suggesting it's wrong, but I would like to hear more rationale underlying it.
Assuming we're talking about apogee reporting here - not the extreme flight dynamics we associate with acsent.

TP

 

[Neutronium95]

My understanding is that most barometric altimeters don't use a temperature corrected algorithm for calculating altitude, because the temperature in the avbay is quite likely to not be the temperature of the surrounding air. This leads to minor inaccuracies in barometric altitude, but nothing too crazy.

 

[Art Upton]

Lets just go back to theodolites and not have to worry about all that

 

[kjhambrick]

@DaddyB --

I would believe the GPS ... but ...

One thing you might try is to calculate the Density Altitude from Launch Site Altitude and Temperature and your raw Barometric Altitude.

See Richard Nakka's site for details: Richard Nakka's Experimental Rocketry Web Site > Altimeter Correction to Account for Launch Site Temperature

I find that the density altitude ( raw barometric altitude adjusted for launch site temperature ) is almost always a little higher for my launch site temperatures than the raw barometric altitudes.

HTH

-- kjh

EDIT: Note that Richard has a handy .xlsx file you can plug your numbers into in case your eyes glaze over on the maths

 

[Adrian A]

My understanding is that most barometric altimeters don't use a temperature corrected algorithm for calculating altitude, because the temperature in the avbay is quite likely to not be the temperature of the surrounding air. This leads to minor inaccuracies in barometric altitude, but nothing too crazy.

On a hot day, the standard atmosphere model used for converting pressure to altitude will under-estimate altitude by 10%. Baro sensors are good enough for main chute deployment, but for apogee accuracy, GPS wins for flights over about 1000 feet.

 

[kjhambrick]

Lets just go back to theodolites and not have to worry about all that

Yes !

Every TRA and NAR club needs one of these !




-- kjh

 

[Art Upton]

Yes !

Every TRA and NAR club needs one of these !
View attachment 642571



-- kjh

Not quite the same, but many clubs had these, I think we borrowed them from another club for our 1974/75 regional contests.

 

[Buckeye]

Is there a common issue with barometric pressure altimeters (RRC3s in this case) registering lower altitudes than GPS trackers (Featherweight tracker/ground station)? Did a test flight, and the two RRC3s were within 7 feet of each other, while the Featherweight registered a bit over 100 feet higher...

Which would be more accurate? Could the RRC3s be registering lower due to vent holes a bit small, so pressure is not equalizing as rapidly as possible?

TIA

Did you launch in a hypothetical air column at mid latitude? Your baro altimeter thinks so. Baro altimeters use a Standard Atmosphere Model.

GPS is more accurate for apogee. You may lose lock during high speed ascent, which may explain the gap in your data.

The SAM model can be replaced with the nearest balloon sounding measurements from the University of Wyoming, and thus become more accurate and closer to GPS.

 

[RCBrust]

For those that are interested, below are about 800 data points gathered over many years of driving recording barometric altimeters, that my company (Soaring Circuits) used to make, through a local low and high point in my area. The difference in elevation is about 585 feet. The red data is the raw altitude and the blue data is temperature corrected. As mentioned in another post, the temperature is the ambient outside temperature, not the temperature where the altimeter is housed. The barometric pressure (at the time of measurement) can also be used to further compensate but the effect is much less than temperature.

Gives you a feel for what kind of accuracy you can get out of barometric altimeter, although this data is for altitudes much lower than many rocket flights.

Randy

 

[kjhambrick]

Back on topic ...

I don't have a GPS yet but I do see better agreement between axial inertial altitude ( axial acceleration integrated twice ) and density altitude than I do with raw pressure altitude.

These are my two most recent flights ( H180W and I161W ):


PAlt is raw pressure altitude, reported by the Blue Raven,
DAlt is density altitude calculated ala Richard Nakka's method.
IAlt is inertial Altitude from axial acceleration ( invalid after drogue deployment at apogee ).

The H180W flight was at Hutto, TX ( 640 ft ) and the site temperature was 75F.
The I161W flight was at Apache Pass, TX ( 410 ft ) and the site temperature was 75F.

Note that the density altitudes for 75F site temperatures are VERY close to the inertial altitudes.

Whee !

-- kjh

 

[Adrian A]

Did you launch in a hypothetical air column at mid latitude? Your baro altimeter thinks so. Baro altimeters use a Standard Atmosphere Model.

GPS is more accurate for apogee. You may lose lock during high speed ascent, which may explain the gap in your data.

The SAM model can be replaced with the nearest balloon sounding measurements from the University of Wyoming, and thus become more accurate and closer to GPS.

View attachment 642576

And in the weather balloon sounding instrument package, guess what the NWS uses for measuring altitude? Yep, GPS!

 

[Sooner Boomer]

And in the weather balloon sounding instrument package, guess what the NWS uses for measuring altitude? Yep, GPS!

Aaahhhhh....probably not. The balloon sondes are of a specific weight. The balloons are filled with a specific amount of helium. The rise rates of this configuration are well known. Time after release gives you a fairly accurate altitude (enough for meteorological work). Pibals (pilot balloons) are also used for spot measurement of winds* (a pibal has only a light, and is tracked using a theodolite on the ground, no instruments on balloon ). I was flying balloon sondes back when the Omega system was still on. I ha* a radar wind profilomoterted using the paper tapes to "program" the receiver used for the Vaisala sondes (which had a partial GPS engine on board). Now, they're programmed with by bluetooth or via USB prior to launch.

* now, a radar wind profile measuring device is used.

 

[cerving]

The problem with GPS altitude is that you WILL lose lock at launch, due to COCOM limitations (yeah, I know they're named something else now...) Most GPS chips will lock out at 4G and/or 515 m/s and/or 18 km It's the "and/or" that's the kicker.

On a typical HPR flight over 5,000' in altitude, you will generally lose the lock at launch, and get it back sometime around apogee when the rocket slows down and the GPS has some time to reacquire the lock. If you hang a datalogger on the GPS output you'll see it go quiet during that time. With extreme flights, you're going to be coasting for a long time... so you may get your satellites back well before apogee, which is why GPS works fine for those very high flights (30K or more).

For most HPR sport flights, though, you're better off with the baro altitude, flawed though it may be. The good news is that everybody's baro altitude is equally flawed... so if you Vendor A altimeter says you went to 8,153' then there's a pretty good chance that your Vendor B altimeter will read very close to that number, which is probably good enough for most flights.

 

[Adrian A]

Aaahhhhh....probably not. The balloon sondes are of a specific weight. The balloons are filled with a specific amount of helium. The rise rates of this configuration are well known. Time after release gives you a fairly accurate altitude (enough for meteorological work). Pibals (pilot balloons) are also used for spot measurement of winds* (a pibal has only a light, and is tracked using a theodolite on the ground, no instruments on balloon ). I was flying balloon sondes back when the Omega system was still on. I ha* a radar wind profilomoterted using the paper tapes to "program" the receiver used for the Vaisala sondes (which had a partial GPS engine on board). Now, they're programmed with by bluetooth or via USB prior to launch.

* now, a radar wind profile measuring device is used.

Cool that you used to be directly involved. It looks like the systems have evolved. Here's what it says now:

https://www.weather.gov/upperair/radiosonde
https://www.weather.gov/media/upperair/Documents/RS41-NG Datasheet B212114EN-A.pdf

Wind, height and pressure are derived from velocity and location measurements of the RS41 GPS receiver. Height and pressure are calculated from satellite ranging codes, combined with differential corrections from the MW41 ground station.

https://www.weather.gov/media/upperair/Documents/DB-DFM-17-EN_V01.06.pdf

 

[Sooner Boomer]

Cool that you used to be directly involved. It looks like the systems have evolved. Here's what it says now:

https://www.weather.gov/upperair/radiosonde
https://www.weather.gov/media/upperair/Documents/RS41-NG Datasheet B212114EN-A.pdf


https://www.weather.gov/media/upperair/Documents/DB-DFM-17-EN_V01.06.pdf

The NWS would not use those sondes. The ones listed only work in the 400 MHz range. That's fine for research. The sondes used by the Weather Service are launched twice a day (I assume this is automatic now), at 0:00Z and 12:00Z. The radiosondes transmit on 1680 MHz.
[edit] Oops, looks like I'm wrong about that. The NWS recently changed back to the 400 MHz range and abandon the 1680 frequency to avoid interference with satellite downlinks.

 

[Adrian A]

The problem with GPS altitude is that you WILL lose lock at launch, due to COCOM limitations (yeah, I know they're named something else now...) Most GPS chips will lock out at 4G and/or 515 m/s and/or 18 km It's the "and/or" that's the kicker.

On a typical HPR flight over 5,000' in altitude, you will generally lose the lock at launch, and get it back sometime around apogee when the rocket slows down and the GPS has some time to reacquire the lock. If you hang a datalogger on the GPS output you'll see it go quiet during that time. With extreme flights, you're going to be coasting for a long time... so you may get your satellites back well before apogee, which is why GPS works fine for those very high flights (30K or more).

For most HPR sport flights, though, you're better off with the baro altitude, flawed though it may be. The good news is that everybody's baro altitude is equally flawed... so if you Vendor A altimeter says you went to 8,153' then there's a pretty good chance that your Vendor B altimeter will read very close to that number, which is probably good enough for most flights.

My last flight had an initial boost with a maximum of 16 Gs that lasted for about 0.4 seconds, followed by 5 Gs for another 12 seconds or so. The GPS data receiver only had a problem during the first second of the flight . I think the COCOM limits only apply to maximum velocity and maximum altitude. But high accelerations are still a challenge for the GPS receivers, which are all designed to throw out spurious data. "What you you mean, the receiver was still on the ground and then 1000 feet higher one second later? That can't be right!" The fact that it was over 12 Gs for only about 0.4 seconds allowed the software filters to catch up quickly.

Each red dot is a separate measurement from the Featherweight GPS Tracker's Ublox 10 GPS receiver, which is outputting at 10 samples per second when it has a lock.

 

[mikec]

GPS altitude accuracy depends strongly on the specific geometry of the satellites you are locked onto.

 

[Spacedog49Krell]

The geometry and number of satellites determine location and altitude accuracy. My test location varies around a 10 meter radius. The altitude varies +/- 20 meters.

 

[WizardOfBoz]

So.... When I was a young engineer, I wrote a specification for a pressure transmitter, using a specific model number that I thought would work. My more senior colleague George told me
"It won't work"
"Why not George? The range is correct!"
"Won't work"
"But the material of construction is consistent with application."
"Won't work"
After several more rounds of me giving George reasons why it should work, and him telling me it wouldn't, I finally said
"George, you're going to have to give me a reason rather than a denial"
"Jim, I could go into great depth on why it won't work. Suffice to say that I invented that type of transmitter, I hold the patent (several actually) on it, and I have my reasons for saying 'it won't work'"
"Uhhhhh. Ok. I'll find another transmitter"
(He did later explain)

In any case, pressure transmitters generally have an error spec that is a "percent reading plus percent full scale". So if you have a 1.5% of reading and 1.5% of full scale, at 95 of full scale you have about 2.1% (if my log mean calc is correct) or so (its more complex, but the approximation is good enough for an explanation anyway. If you get down a reading that is only 16% of full scale you have an uncertainty of about 1.6%, or about 10% of the reading. That's the situation at about 50,000 feet (15000m or so).

So the question depends upon just how high that you are measuring altitude. At some altitude, GPS may be more accurate. At
http://www.gpsinformation.net/main/... on this,1.5 x Horizontal error specification.
they say that "standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP [Degree of Precision] of 1 for 95% confidence. " Barometric estimates seem to be about 25 feet at low altitude going up to 100 feet at higher altitude (but the altitudes weren't specfied - sigh). In any case, if you are lower than 15K feet, I would think barometric would be fine.

That said, I'm hoping to qualify for L1 next week and my Mad Cow Super DX3 (4") diameter, with a blue tube avbay will be motor deployment and will contain... an Eggtimer ION monofunction altimeter. So if someone has specific actual experience, it probably beats my general theory.

From an engineering standpoint, ideally you'd mix (fuse) your estimate from both signals, varying the barometric altitiude estimate less and the GPS more as your altitude increases. There's a paper on that here.

Using a Kalman filter theoretically gives you the best estimate from either source, and you can also combine the inputs. Not sure that I've seen a filter that variably weights the two sources as the altitude changes.

 

[lr64]

My last flight had an initial boost with a maximum of 16 Gs that lasted for about 0.4 seconds, followed by 5 Gs for another 12 seconds or so. snip

Wouldn't that get you in the neighborhood of 2,000 fps? Or was there a lot of deceleration between the first and second burn?
------------------------------------------
I wonder if anyone has used differential GPS for more accurate altitude measurements, and how much the fancier equipment costs.

 

[Adrian A]

Wouldn't that get you in the neighborhood of 2,000 fps? Or was there a lot of deceleration between the first and second burn?
------------------------------------------
I wonder if anyone has used differential GPS for more accurate altitude measurements, and how much the fancier equipment costs.

The velocity plot is upthread. Max velocity was about 1000 ft/sec. The acceleration ramped down as the velocity and drag increased.

 

[lr64]

So it wasn't really 5 g's for 12 seconds, which would have been surprising. I'd like to see what sort of grain could deliver enough rising thrust to do that.