GPS Module for Altitude Measurement of Rocket Recovery System - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: January 19, 2018, 08:34:54 pm
content: Hi everyone,I am currently working on a rocket as part of a student organization project. We would like to trigger the recovery system of the rocket based on GPS altitude. Our initial choice of GPS was the Emlid RTK reach GPS however this seems to be a bit over kill.We have a target altitude of 45000 ft. and would like a GPS module that can output positional data to our main control board (and our redundant recovery board). Can anyone give me some advice as to what sort of board might fit our purpose any additional advice regarding recovery systems would be much appreciated!Thanks!Ali

author: SparkFly
author_rank: Regular Contributor
timestamp: January 19, 2018, 09:35:23 pm
content: Hi,I've had excellent success with NEO6MV2 modules, using a ublox NEO-6M on high altitude balloons to nearly 40km altitude. As your rocket isn't flying over 18km this should in theory not shut down the gps module as per COCOM limits. I can't say how well it would cope with the vibrations/accelerations/etc of a rocket flight though.Which student organisation are you part of? I'd be really interested to follow your progress.

author: james_s
author_rank: Super Contributor
timestamp: January 19, 2018, 09:37:13 pm
content: Is there a good reason to use a GPS rather than a more conventional atmospheric pressure sensor? GPS can be a bit fiddly, especially in fast moving vehicles.

author: eerocketman
author_rank: Contributor
timestamp: February 01, 2018, 06:27:25 pm
content: Sorry for the late response we are Students for the Exploration and Development of Space at UCSD!!!

author: texaspyro
author_rank: Super Contributor
timestamp: February 01, 2018, 06:36:28 pm
content: Quote from: SparkFly on January 19, 2018, 09:35:23 pmThe COCOM rules state that the GPS should not work if it exceeds a speed AND altitude limit. Some GPS modules interpret this as speed OR altiude.

author: james_s
author_rank: Super Contributor
timestamp: February 01, 2018, 07:17:27 pm
content: I wonder if this applies to all the various low cost Chinese GPS modules? I could see them not bothering to implement the shutdown rules just as some of the cheap DVD players ignored region coding.

author: retrolefty
author_rank: Super Contributor
timestamp: February 01, 2018, 07:53:52 pm
content: Quote from: james_s on January 19, 2018, 09:37:13 pmThisPressure sensor and calculations is lighter more reliable and accurate enough.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 01, 2018, 08:25:45 pm
content: Our university club was donated a popular telemetry board by its manufacturer... I accidently got involved in evaluating it.As it was only taking telemetry at 10 second intervals, on a Mach 1.4 rocket that gets pretty disappointing during boost. Not to mention GPS and 440 Mhz downlink antenna placement causing asymmetrical drag, plus roll rate issues. This resulting in my assisting in a serious evaluation of how GPS is attenuated by carbon fiber cases. Answer, it really depended on the fiber material chosen, weave, and curing bake...The mechies on the team model everything , drag, weight and balance, g-shock on internals, vibration etc., Thus an external antenna with a good field of view was ruled out.Moral of the story, GPS is not always a great choice, and requires careful testing.Microwave Doppler Shift velocimetery is reasonably easy, you need a good beacon on the bird with low phase noise. Choose not a popular DDS synthesizer, but use a crystal oscillator and multiplier chain. Add a photodiode gating the signal and you have roll etc.,.. When you reach apogee, the Doppler slows, then reverses and you see almost no velocity, then a negative Doppler, then the chute pops. A simple mixer on the ground, a preamp, and the reference oscillator makes it easy to record the data. So with a little work and the proper audio recording means, you have acceleration, velocity, flight timing, apogee, and tracking until it gets close to the ground.I'd fill in the details on how to build it using off the shelf parts for around 200$, but I do not wish to be stalked and pummeled by the university mascot.Your required to have a isolated barometric peak recording altimeter installed for the competition, and adding a few more such modules is cheap and easy.GPS sucks unless you have the right GPS..Our team (I'm a staff member asked to consult) has a limited number of Engines per year, so stuff has to work, and work well. Don't discount the temperatures reached on the pad and in storage at a certain desert completion site. Least of all, evaluate anything transmitting or powered for "HERO" aka Hazards of Electromagnetic Radiation to Ordnance. the Words Hey, switch on the telem would ya?, followed by a "Whoosh!, is not a good idea...You need solid ground handling and safety procedures.Have fun..Steve

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2018, 01:22:55 am
content: What use is a telemetry unit with a 10 second interval in any kind of moving vehicle? Even in something relatively slow like a model airplane 10 seconds is a long time. I could fly all the way across the field or gain or lose several hundred feet of altitude in that time.

author: texaspyro
author_rank: Super Contributor
timestamp: February 02, 2018, 03:13:16 am
content: Quote from: LaserSteve on February 01, 2018, 08:25:45 pmA little background... I have been doing high power rocketry since... well... forever. I have been level 3 certified since the certification program first started. I have held several world altitude records. I have also been doing GPS since the days of single channel sequencing receivers and have written firmware for some commercial GPS receivers.A GPS system with 10 second update rates would only be useful for recovery location tracking. I have rocket telemetry systems that do 20 Hz updates. There are now a LOT of very nice rocketry telemetry/tracking systems out there for reasonable prices.Generally you do not want to use external antennas, but external antennas can be successfully used. Also internal antennas are not used in a carbon fiber/metal body. GPS/radio based telemetry systems work best mounted in a fiberglass nose cone. I've put plastic nose cones to well over Mach 2 (ok, they were lined with a half inch of kevlar pulp reinforced epoxy).GPS (and accelerometer) based recovery deployment can be rather problematic. If your GPS loses lock or takes a long time to recover from a COCOM shutdown, you are screwed. Accelerometers are subject to integration/path errors, etc. Newer 10 axis IMU system can help, but beware of acceleration and gyro rate limits. Note that when you are deploying a recovery system at apogee, your rocket velocity is near 0... unless your rocket went horizontal...Barometric usually works well, but has issues at high altitudes (not much air pressure to work with at 100,000 feet) and can have problems at high velocities... Barometric tends to get iffy past 50,000 feet... with modern sensors maybe 100,000 feet. Mach transition can be a bitch (I've done 0 .. Mach 2.9 in one second flat) and can produce false apogee detections. Your firmware needs to correlate velocity calculations with altitude calculations.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 02, 2018, 04:44:41 am
content: We had no problems overcoming around 1.5 dB of added GPS link budget loss through this particular carbon shell. Measured loss at 446 Mhz for the beacon was negligible.Other carbon tube fiber tests showed far higher adsorption.Nasa very early on perfected Doppler interferometry and DOVAP. There are many government publications available for free on the subject.Fifty to 100 mW of RF as the CW beacon is easily obtainable with a 7$ MMIC these days.in fact, the receiver preamp, oscillator, mixer, 250 mW MMIC, and bandpass filters are all readily available as tiny demo PCBs on EBAY, that can be strung together using SMA jumpers.Only the audiodirect conversion amplifier after the mixer board is tricky, because it needs to terminate the Mixer's 50 Ohm RF signal yet pass and amplify the LF and Audio signal.Decoding/recording can be done with a sound card, provided you choose your carrier frequency carefully.Hats off to Texas Pyro for chiming in with the big picture.And yes, the donated board was intended for recovery, but has a slot for a usb stick for all GPS data. Only the RF downlink at a fixed 10 seconds was the issue.steve

author: texaspyro
author_rank: Super Contributor
timestamp: February 02, 2018, 05:11:39 pm
content: Quote from: LaserSteve on February 02, 2018, 04:44:41 amI did some tests with a 219 MHz pulsed tracking beacon. First I put it in a 98mm motor casing with solid closures on each end. Faraday cage, right... so no signal? Nope, no detectable signal loss.Then I tried it in some phenolic and fiberglass tubes that I laminated with carbon fiber cloth. Signal almost totally disappeared with most of the carbon fiber materials that I used. With the tracker in a rocket on the pad at 1000 feet it was hard to get a signal. When the rocket deployed, the trackers can be detected well over 20 miles away.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 02, 2018, 07:18:19 pm
content: Until the design team chose to go yet another way with their materials, designing a downlink antenna with the G10 based Fin was going good from both radiation pattern and reflected power, but made for one heck of a thick PCB substrate. They they wanted to go to "printed" carbon fins. A local company has a carbon fiber 3D weaver/printer and weaving in the copper tape log periodic near the surface looked good, although a bit lossy. Then they went yet another way and I have not heard from the telemetry chair in a long time... My bet is after his internship he has learned enough RF to do it on his own. So much for my getting to watch the launch...I'm not so sure I like Mr. Faraday so much anymore, either.Anyways, for the OP, I recommend the open source version of the US Navy Electronic Warfare and Radar handbook, and a good look at path loss thru the one way or two way Friis Equation. Then read up on noise figure on the various microwave amateur radio sites. This gives you tools for evaluating your GPS antenna and/or preamp.Steve