[time-nuts] Need advice for multilateration setup

Bob Camp kb8tq at n1k.org
Sun Mar 29 09:15:19 EDT 2015


Hi

Calibrating the G sensitivity of the oscillator can be done much 
more easily by simply rotating it 360 degrees while carefully reading
out the frequency. If you want the full vector, you will need to rotate 
it through two circles, with the plane of one 90 degrees out relative to the other.

The net result is that you get a 2G change in acceleration in each axis. Measure
the frequency to 1x10^-10 every 10 degrees and you have what you need. You 
will need to keep the temperature / voltage / whatever stable enough that you 
don’t have more than 1x10^-10 drift through the process. That’s the main reason
for taking two readings at the the same angle, one at the start and one at the 
end of the process. 

Far easier to do in a static fixture on the ground than to extract it from telemetry
after the fact. The temperature outside your rocket is dropping at around 3C for 
every 1,000 feet you go up. At 10G’s your are going through 1,000 feet pretty quick.
Just the 3 C in the first 1,000 feet will move your frequency 3 ppm while you are trying 
to measure a 2x10^-8 shift. 

============

So, if you put a double oven in the rocket and put a thermal shield around it, (possibly
using the lead acid batteries you are powering it with) - you could get around the 
thermal shift to some degree. Of course the extra 20 or 30 pounds of weight *might*
impact your weight budget a bit :)

============

Bottom line is still the same, you don’t need to worry about the acceleration impact
on the static frequency. You do need to worry about it’s impact on phase noise and your
carefully worked out modulation scheme. This does not just apply to amateur rockets and
working out the RF systems on them. Some fairly *large* defense systems have run into this
issue pretty hard. 

Bob

> On Mar 28, 2015, at 10:34 PM, Bill Hawkins <bill at iaxs.net> wrote:
> 
> An idea occurred (always a surprise):
> 
> The rocket's acceleration increases from 1 g as the mass of fuel is
> ejected energetically, according to f=ma, with pretty constant force
> from the motor. At some point, the fuel and oxidizer tanks are empty
> (MECO), causing the acceleration to revert to 1 g or less, depending on
> altitude. The change from max acceleration to free flight offers an
> opportunity to calibrate the effect of max g on the oscillator. The
> velocity is almost unchanged at that point, so the change in Doppler
> shift comes only from the effect of acceleration on the oscillator. It
> should be possible to use linear interpolation for the effect of
> acceleration during powered flight, since f=ma is a first order
> equation.
> 
> Bill Hawkins
> 
> 
> -----Original Message-----
> From: Bob Camp
> Sent: Saturday, March 28, 2015 6:22 PM
> 
> The point being that, to even get acceleration into the picture, you
> need have impossibly high accelerations .
> 
> At 10 G, your oscillator needs to be temperature  stable to < 0.01C to
> even see the acceleration. If you are climbing 100K feet during the
> acceleration phase the oscillator will see a *lot* more than that.  
> 
> Bob
> 
>> On Mar 28, 2015, at 5:01 PM, Jim Lux <jimlux at earthlink.net> wrote:
>> 
>> On 3/28/15 10:27 AM, Bob Camp wrote:
>>> So If the rocket continuously accelerates  at 10,000 G's, you will 
>>> get a 20 ppm shift with typical sensitivity.  If you do this for very
> long, you will also get into time dilation issues.
>>> (you hit 0.1C in < 2 minutes).
>> 
>> 10,000G is more like an artillery shell.
>> 
>> A large amateur rocket might be more like 20-30G maximum.
> 
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