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### Topic: Speed measurement for rockets (Read 370 times)previous topic - next topic

#### MarkT

#15
##### Aug 01, 2020, 02:11 pm
Altitude accuracy from GPS depends on having satellites acquired both high in the sky and close
to the horizon - being 1000m up in the air helps with the latter, but I doubt a GPS would acquire new
satellites in the timescale involved!

Not sure if many GPS's expose vertical error estimates like they do for CEP?  You could time the launch
when the vertical error estimate is lowest.

If you manage to get to hypersonic speeds (ie Mach 5 +), then you have to worry about shock-plasmas
killing the GPS signal

This is where I learnt my rocket science BTW - recommended! https://www.edx.org/course/hypersonics-from-shock-waves-to-scramjets-2
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### srnet

#16
##### Aug 01, 2020, 03:46 pm
Ublox GPSs can be configured for a range of 'platform' settings, and can for instance be set to work at up to 50,000m altitude.

However all the platform setting significantly restrict vertical velocity, the max setting varies from 20m/s for 'pedestrian' mode up to 100m/s maximum for 'airborne < 4g' mode.

#### MrMark

#17
##### Aug 01, 2020, 10:37 pm
Altitude accuracy from GPS depends on having satellites acquired both high in the sky and close
to the horizon - being 1000m up in the air helps with the latter, but I doubt a GPS would acquire new
satellites in the timescale involved!
The reason GPS altitude accuracy is worse than horizontal accuracy is because the accuracy is a function of the relative geometry of the visible GPS satellites (referred to as Geometric Dilution of Precision or "GDOP").  The optimum geometry in a particular dimension is achieved when there is a visible satellite in either direction along that axis.  For altitude that would mean a satellite directly above and directly below.  There will never be a visible satellite directly below in any normal scenario because there is a planet in the way.

#### SteveMann

#18
##### Aug 01, 2020, 10:49 pm
The US military, who build and own the GPS, has placed a speed and height limit so no receivers can be used in hostile guided weapons - good idea of course. Consumer GPS stop working when moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).

GPS altitude measurements are so inaccurate that aircraft flying a GPS approach use barometric altitude for decision heights.

Why not use an accelerometer? Take a reading once a second and then you can calculate speed, altitude and acceleration.
I am usually so far out of the box that most people don't know what I am talking about.

Please do not ask for help by PM. I will not respond.
If you need help, post a question on the appropriate forum.
Click on Add Karma if I helped you.

#### TomGeorge

#19
##### Aug 02, 2020, 12:20 pmLast Edit: Aug 02, 2020, 12:23 pm by TomGeorge
The US military, who build and own the GPS, has placed a speed and height limit so no receivers can be used in hostile guided weapons - good idea of course. Consumer GPS stop working when moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).

GPS altitude measurements are so inaccurate that aircraft flying a GPS approach use barometric altitude for decision heights.

Why not use an accelerometer? Take a reading once a second and then you can calculate speed, altitude and acceleration.

[sarcasm]

• UNO
• IMU MPU6050
• GPS
• 9V Battery
• sg90 servo activated control surfaces
• L298N motor drive...
• couple of hobby motors and props..
• HC-SR04 ultrasonic sensor to get nap of the land flying
• a model plane from one of many many internet sources...
• C4 might be hard, but this is the internet, somebody will sell me some.

[/sarcasm]
Or is it easier said than done?

Tom....
Everything runs on smoke, let the smoke out, it stops running....

#### GoForSmoke

#20
##### Aug 02, 2020, 01:45 pm
Fly the rocket at night with strobe lights and take long-exposure pictures from 3+ cameras with grids over the lenses.

How well you know where the cameras are and view angles is how well you can reconstruct the flight from the data.

2) http://gammon.com.au/serial <-- techniques howto
3) http://gammon.com.au/interrupts
Your sketch can sense ongoing process events in time.
Your sketch can make events to control it over time.

#21
[sarcasm]