After learning some about making altimeters reliable from my first altimeter project (the MARS altimeter) I started a second project over the winter to make a more advanced unit.

The main feature of this unit is that it has 4 output pyro channels and an LCD screen for easy programming on the board.

Here is a pic.

The unit is neccessarily larger than most altimeters but it will fit within 2.6" airframes but it is recommended for 3" and larger. 

Geneseo is our home flying field.  If you been there you know there are alot of priceless warbirds on the field.  Safe recovery deployment is something we take seriously. As such, reliability has been the primary goal of this altimeter.  Four outputs allow redundant charges to be in place for the apogee and main events.  The output can also be configured for other uses such as timers for motor air starts or to turn on other devices.

There is also an abundance of diagnostic and reliability routines built into the code of this altimeter.

User Interface

Most advanced altimeters require you to use a computer to change to configure the altimeter. Some tout this as a feature!  I think that this a big negative unless you enjoy needing a laptop, lugging it to every launch with you, not forgetting to bring the connection cable, making sure the laptop is charged, operating it in bright sunlight, blowing sand or light drizzle, etc.  This is like requiring you to use your computer to change the volume or song selection on your IPOD.

The MARS4 altimeter's programming ability is totally self-contained. There is a 4-position joystick just below the LCD screen to navigate the menus to configure the altimeter.  The menu layout and options are easy to set and inituitive.

Altimeter Design and Features

This altimeter has several innovative event, reliability and diagnostic features.

Event Programability

All four output channels are independently programmable for any of the following events.

• Apogee.  The method for detecting apogee is selectable. The options are barometric pressure, integrated acceleration, barometric OR acceleration or barometric AND acceleration.  Also you can set a delay from when apogee is detected to when the channel fires.  This is useful if the altimeter is configured for redundant apogee deployment and you do not want both channels to fire at the same time.
  
  
• Altitude. The channel will fire at a predetermined altitude after apogee. The altitude can be set in 100' increments anywhere from 100 to 65000 feet.
  
  
• Timer. The channel will fire after a predetermined delay after launch detect or motor burnout. Also you can configure the timer event to fire at a predetermine altitude on ascent.

Each output channel has a configurable backup timer.  This can be set to fire the channel after a selected delay after launch detect.  For instance if you have a simulation that says apogee will happen in 12 seconds, you can set the backup timer for 16 seconds in case of a sensor or other problem that prevents the proper apogee detection.

The on-time of each channel is also settable.  This useful if you are using an output to operate another device that needs to be on for an extended time like a camera or transmitter.

Output Current: Each output channel will source up to 10A for the first 200ms and 7A continuously.  A single 9v battery will source 4A and is all that is needed to operate the altimeter. There is no need for dual batteries as the altimeter uses sophisticated brown-out protection implemented in both hardware and software.  Although the unit can successfuly fire igniters it is recommended to use only ematches for the ultimate reliability.

Diagnostic Features

The most important function of an altimeter is to safely recover your rocket by making sure the deployment events happen as designed. There are several things that can go wrong. If those faults can be detected then unsafe flights can be avoided. The design of the MARS4A altimeter has fault detection and diagnostic modes that do not exist in other altimeters.

Pyro circuit continuity testing

Current altimeter designs test continuity by placing a high resistance in series with the ematch and looks for a tiny current flow. However just because you have continuity doesn't mean your ematch will fire. For instance a short in the pyro circuit will pass continuity but the ematch will not fire. Or a very high contanct resistance will conduct low current just fine but will fail when the device attempts to draw high current.

The MARS4A continuity check measures the ACTUAL resistance of the connected device (ematch) under FULL firing current.  If the measured resistance of the ematch is significantly off from where it should be then you can investigate. A short or abnormally high contact resistance will be obvious.  In addition the continuity check will determine how much current will actually be flowing through the connected device when turned on.

The above pictures show the pyro continuity screens with a Quest Q2D2 igniter attached on channel 1 and a J-Tek ematch on channel 2. Top screen is the resistance measurement and bottom screen is the current conducting through the attached devices.

Sensor Check

A faulty barometric pressure sensor or accelerometer will risk that the deployment events will not happen as planned.  The MARS4A altimeter has a mode where you can read the live sensor outputs on the LCD screen. You can see if the accelerometer is responding and if the barometric pressure sensor output makes sense for your location.

Other Features

• The altimeter knows which way is "up".  You cannot install the altimeter upside down.
• Automatic mach detection. There is no need to set mach delays for barometric based events.
• Peak altitude, velocity and acceleration of last flight recallable on LCD screen.
• 2 memory banks for storage of flight data.
• Download data is ASCII text and open source so that unit can be used with any computer or operating system. Windows, MAC, Linux.
  

(More to come)

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