Altimeters that use GPS for deployment??

[JimJarvis50]

I'm trying to identify altimeters that use GPS for apogee deployment. I am aware of Kate and the AIM XTRA. Are there any others available or soon to be available?

Jim

 

[Dave S.]

You may want to look at this:

http://rocketsetc.com/altimeter-comparison/

Not sure how out of date that chart is, but in another newer GPS altimeter is the Eggtimer Quasar.

 

[Handeman]

I wasn't aware of any altimeters that relied only on GPS for altitude. Kate has a 100,000 ft barometric altimeter so I assume it only used GPS above 100K ft.
"The XTRA includes a pressure sensor capable up to 100k feet. This allows for very accurate altitude readings." so the AIM XTRA seems to also use a barometric altimeter for altitude measurements below 100K ft.

I have found that using dual altimeters and a GPS unit, that the two altimeters are very close together, but the GPS apogee reading is usually different by about 25% or more.

Considering the slow rate of GPS fixes and inaccuracy of altitude readings in fast vertically moving rockets, I wouldn't trust a GPS unit to accurately determine apogee for use in deployment. I certainly wouldn't trust it for main deployment, although considering the slower decent rate, that might be accurate enough if you leave enough margin.

I would like to hear about any GPS unit that can accurately determine apogee fast enough to be used for deployments.

 

[Handeman]

You may want to look at this:

http://rocketsetc.com/altimeter-comparison/

Not sure how out of date that chart is, but in another newer GPS altimeter is the Eggtimer Quasar.

The chart may be a little out of date, but what I find interesting is that the only item listed without a Barometric altimeter, does NOT have pyro outputs.

BTW, The Quasar is a combination 3-output recording altimeter and GPS tracker. Although they don't state specifically, I believe it uses a baro or accelerometer for altitude data and pyro control, not GPS.

 

[plugger]

I wasn't aware of any altimeters that relied only on GPS for altitude. Kate has a 100,000 ft barometric altimeter so I assume it only used GPS above 100K ft.

I don't think any rely solely on GPS, but as Jim mentioned Kate and AIM XTRA will use GPS data as part of the logic for events, assuming it's sane.

I have found that using dual altimeters and a GPS unit, that the two altimeters are very close together, but the GPS apogee reading is usually different by about 25% or more.

That sounds like a chipset issue to me and if I had to guess I'd say you're using a MediaTek 3333/3339/Quectel L80 GPS chipset. I've tested those in sport rockets and found the GPS get very confused post boost. They're good enough to get final coordinates (at least the MT3339 is) but they're not great for apogee verification. I expect this is why they're not on the Tripoli approved GPS list for Apogee verification.

 

[cerving]

FYI, the Eggtimer Quasar uses baro for deployments... they're highly reliable up to around 60K. If you're going much higher than that, you probably want to use an altimeter with a full 6-DOF IMU to determine apogee. (Kate, Telemega, Blue Raven) Note that deploying a drogue much above 60K is not going to have a significant aerodynamic effect, there's very little air for it to catch, and it's likely that the shroud lines would tangle with the rocket too if there's any pitch/yaw motion at all (which is fairly likely at high altitudes).

Using GPS for deployments would be a bit tricky... you'd probably want a really good helical antenna to counteract any signal losses due to X-Y-Z motion.

 

[Grog6]

Any Laser or radar versions? seems like that would work below 10k or so.

 

[VernK]

I wasn't aware of any altimeters that relied only on GPS for altitude. Kate has a 100,000 ft barometric altimeter so I assume it only used GPS above 100K ft.

I would like to hear about any GPS unit that can accurately determine apogee fast enough to be used for deployments.

Kate uses GPS as the primary sensor for both apogee and main chute deployments regardless of altitude. The GPS always recovers within just a couple seconds after motor burnout so it is valid and accurate long before apogee. However, if for some reason the GPS should happen to not have a valid fix, then the accelerometer is automatically used as a secondary backup sensor for apogee deployment. Likewise the baro sensor is used as a backup sensor for main chute deployment.

 

[JimJarvis50]

FYI, the Eggtimer Quasar uses baro for deployments... they're highly reliable up to around 60K. If you're going much higher than that, you probably want to use an altimeter with a full 6-DOF IMU to determine apogee. (Kate, Telemega, Blue Raven) Note that deploying a drogue much above 60K is not going to have a significant aerodynamic effect, there's very little air for it to catch, and it's likely that the shroud lines would tangle with the rocket too if there's any pitch/yaw motion at all (which is fairly likely at high altitudes).

Using GPS for deployments would be a bit tricky... you'd probably want a really good helical antenna to counteract any signal losses due to X-Y-Z motion.

Are you talking about using a horizontal orientation to determine apogee? I would be concerned about accelerometer drift on a long flight, but perhaps that would be minor in comparison to a 90° angle change? I also would place the altitude of "no aerodynamic effect" closer to 160K or so, so if tumbling occurred at 120K, there would certainly be a loss of apogee altitude, but perhaps not any damage.

Jim

 

[cerving]

An IMU can determine apogee by sensing when the rocket noses over. It requires a 3-axis accelerometer and a 3-axis gyro to do that properly, and a bunch of math. You need all that if you want to account for any tumbling or rotation. That's the advantage of a baro, assuming that the air isn't too thin to use one... it's a direct altitude measurement and can easily be done by small 8-bit processors like the one in the Quark.

 

[plugger]

Using GPS for deployments would be a bit tricky... you'd probably want a really good helical antenna to counteract any signal losses due to X-Y-Z motion.

huh, I didn't realise GPS satellites are RHCP. Thanks for that!

 

[VernK]

An IMU can determine apogee by sensing when the rocket noses over. It requires a 3-axis accelerometer and a 3-axis gyro to do that properly, and a bunch of math. You need all that if you want to account for any tumbling or rotation.

In theory yes. In practice, it gets very difficult to do if the rocket is rolling because it requires a very high accuracy roll gyro. If a rocket is rolling at say the modest rate of 1000 deg/sec (2.8 rev/sec) then after just 18 seconds it has rolled 18,000 degrees. If the roll gyro is only accurate to 1% then after those 18 seconds it will be in error by 180 degrees of rotation. Which means any pitch or yaw angle changes are now being accumulated in the opposite direction from reality. Which means keeping track of the orientation is no longer working.

High altitude flights take a lot longer than 18 seconds to reach apogee and can also potentalliy roll faster than 1000 deg/sec, so the accuracy demands on the roll gyro get to be very difficult to achieve if you really want to know the true orientation of the rocket the whole way.

 

[jderimig]

An IMU can determine apogee by sensing when the rocket noses over. It requires a 3-axis accelerometer and a 3-axis gyro to do that properly, and a bunch of math. You need all that if you want to account for any tumbling or rotation. That's the advantage of a baro, assuming that the air isn't too thin to use one... it's a direct altitude measurement and can easily be done by small 8-bit processors like the one in the Quark.

That could be an input for apogee detection but for very high flights an unreliable method because rockets can tumble in very light atmosphere well before apogee.

 

[jderimig]

In theory yes. In practice, it gets very difficult to do if the rocket is rolling because it requires a very high accuracy roll gyro. If a rocket is rolling at say the modest rate of 1000 deg/sec (2.8 rev/sec) then after just 18 seconds it has rolled 18,000 degrees. If the roll gyro is only accurate to 1% then after those 18 seconds it will be in error by 180 degrees of rotation. Which means any pitch or yaw angle changes are now being accumulated in the opposite direction from reality. Which means keeping track of the orientation is no longer working.

Gyro roll axis accumulated error can be corrected with the magnetometer data. One of the very few practicel uses of a magnetometer on a rocket.

 

[Buckeye]

I have found that using dual altimeters and a GPS unit, that the two altimeters are very close together, but the GPS apogee reading is usually different by about 25% or more.

Considering the slow rate of GPS fixes and inaccuracy of altitude readings in fast vertically moving rockets, I wouldn't trust a GPS unit to accurately determine apogee for use in deployment.

I would like to hear about any GPS unit that can accurately determine apogee fast enough to be used for deployments.

25% is a lot. The baro altimeters are more wrong than the GPS. Still, something else must be awry. Don't confuse MSL with AGL, for example.

Rocketry GPS units can capture apogee very well, as the rocket is moving very slowly. Most have a resolution of 1 Hz (Featherweight GPS is 10 Hz). Baro altimeters have logic that says something like "wait 1 second after apogee detection before firing deployment event." So, GPS is similarly fast enough. Sure, GPS lock is lost during boost, but apogee and descent are recorded good enough for deployments, imo.



I have had good success in processing log files from BRB900 and FW and plotting them against baro altimeters for apogee comparison. Once the Standard Atmosphere Model is undone from the alts and corrected with actual local sounding measurements, the absolute agreement is pretty good.

 

[cerving]

My experience with several GPS's flying in tandem (including uBlox) has been that apogee detection works best at higher altitudes and longer ascents. For many SPORT flights, it doesn't necessarily have enough time to recover after the LOS at boost... for flights over 10K, it generally does. YMMV...

 

[JimJarvis50]

I've had Kate flights as low as 3K and as high as 142K (and a flight not shown to 42K). Looks like the various sensors were in good agreement.

I'm also interested in the accuracy of inertial apogee. For the Kate flights above, it looks pretty good, even with tumbling on the higher flight. Some units use a "calibration" at 1G and -1G. I would speculate that this is to improve the accuracy of the accelerometer at low acceleration values?

Jim

 

[cerving]

Kate has a much better GPS antenna than other GPS trackers, so it's not surprising that it picks up apogee at lower altitudes.