[time-nuts] Need advice for multilateration setup

[Bob Camp]

Hi

Depending on construction of the resonator, an oscillator can have maximum
sensitivities anywhere from 5x10^-8 / g to 5x10^-11 per G. Typical numbers
for “good but not great” parts are in the 5x10^-10 to 2x10^-9 per G.

Since the sensitivity is *not* the same in every axis, a device with 2x10^-9 in (say)
the X-Y axis might have a 1x10^-10 sensitivity in (say) the Z axis. In something 
like a rocket, your acceleration is likely to have a dominant axis. With characterization
data on the individual oscillator, you might be able to reduce the impact by 10:1.

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).

If the oscillator has a 1 ppm / C temperature coefficient, a 20C change will give you
the same (static) frequency shift. If you change temperature quickly (as you would in this 
case, you hit outer space in a few seconds) figure a 5 to 10X increase in that shift. 

Simply put - temperature will get you before acceleration does in terms of static shift. There
are other things that will be a problem before either of these get in your way. 

Most tracking  *assumes* good phase noise on the signal. Oddly enough rockets are not
very quiet devices while accelerating. The same sensitivities that give you the issues from
static acceleration give you phase noise under vibration. It is not at all unusual to see 
phase noise degradation of >60 db on physical small platforms doing high levels of acceleration.

Bob

> On Mar 28, 2015, at 8:25 AM, Peter Reilley <peter at reilley.com> wrote:
> 
> Some crystal oscillators specify their sensitivity to G forces.
> Here is one:
> http://www.abracon.com/Precisiontiming/AOCJYR-24.576MHz-M6069LF.pdf
> 
> Available here:
> http://www.digikey.com/product-detail/en/AOCJYR-24.576MHZ-M6069LF/535-12627-
> 1-ND/4989033
> 
> Others specify shock and vibration limits but say nothing about 
> frequency stability.
> 
> Pete.
> 
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Chris
> Albertson
> Sent: Friday, March 27, 2015 9:55 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Need advice for multilateration setup
> 
> On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris <cfharris at erols.com> wrote:
> 
>> The biggest problem I see is the crystal oscillator in the rocket is 
>> going to notice the G forces during acceleration in a pretty big way.
> 
> 
> But all of the ground stations will see the same frequency shift on the
> rocket's transmitter.   I think this can be backed out in processing.
> 
> Someone needs to write the equations and post them here.
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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