Monday, July 20, 2015

GPS on several fronts...

This week end I flew an open source Piksi GPS on an HPR to see if it could be made to keep
lock at high acceleration without any imu aiding. I modified the tracking constants of the piksi software .(actually used # defined for wide bandwidth tracking already in the latest software)

It lost lock at boost, got lock again at apogee (strong winds at FAR apogee horizontal V was 204knots!)  This pulled the electronics bay out of the vehicle when the drogue deployed at that speed.  Fortunately its a sturdy rocket and it survived a no main chute deploy with minor damage.

There were some issues I think I set the lock detection too too optimistic and it tracked some noise...
I will fly a Piksi again recording the Raw RF some time in early Aug.
(I'm also going to change out the TCXO for one with lower G sensitivity see discussion below)

On a 2nd GPS front PSAS flew a software GPS receiver on their rocket on Sunday.
They gathered raw data, but they don't have the full GPS solution working yet.
They posted the following graph

It shows doppler shift in the tracked sats.
I actually question that a little bit...

Doppler is what makes a trains whistle change tone as if goes past.
If you change the velocity with respect to a radio transmitter the frequency will shift.

There is another effect that can also effect this data... the RF system on a GPS uses a very precise clock (usually a TXCO)  in the receiver.  That clock may have acceleration sensitivity.
IE the frequency of this oscillator may vary with applied acceleration...

I'm somewhat suspicious that the graph above shows this more than Doppler shift.
I have not looked up the orbits of the gps sats at the time of flight or the position of PSAS flight, but I'll make some assumptions...

The GPS sats are scattered randomly about, ie some are almost straight up and some are at the horizon.  If the rocket flies straight up one would expect a bigger doppler shift for the sat straight over head and much less shift for the sats at the horizon as their relative velocity changes less.
Also if the rocket turns into the wind and has some horizontal velocity one might even expect a doppler shift in the other direction for the sat you are flying toward/away from.

However if the frequency shift is due to g effects on the reference clock one would expect all the doppler shifts to move the same direction....

So it looks like they got a shift of about 1400 hz  at 1.5Ghz that would be a velocity of  140 m/sec or so... at 1.5 Ghz. So the magnitude is withing the realm of possibility...

(The graph also shows an overshoot at the end of the rocket burn this is the acceleration changing direction from the perspective of the GPS receiver.)

There are vendors that specifically sell clock oscillators with low G sensitivity...

Given their data the PSAS should be able to determine this.
With a full GPS solution derived from the data (they have not done this yet)
They will get position velocity and time....
They also have sample of data precisely clocked out via their reference oscillator...
The difference between the time in the solution and the time via their local clock can
be measured and will give an idea of clock drift.

They can also calculate the expected doppler between them and the individual sats and see how that compares with the   measured doppler any difference is clock error...

All of these data extractions require they get a full solution running....

There is a quick and dirty test they can do with just what they have...

Run their gps sampler system on the ground with a fixed  stationary antenna...

Now take the GPS board and orient it it differently in the 1g gravity field.
6 combinations IE board component side up, component side down, tipped on its end, pointing up, down right left....

 and generate the doppler graph above...  if the doppler shifts with orientation of the sampler board then they probably need a more stable clock.

  At  bare minimum this test will show them what axis has the lowest clock sensitivity....

In any case I really applaud what the PSAS guys are doing!


Andy said...

How out was it at FAR?


Andy said...

"hot" not "out"

Paul Breed said...

Not too bad, I left by 11 am.

Iftekhar Ahmed said...

I must appreciate the way you have express your feelings through your blog!
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heroineworshipper said...

Interesting blog revival after 2 years. In those 2 years, cubesats have become the big thing. SpaceX & Orbital made headlines, then got snuffed out by accidents. Armadillo was dissolved & John Carmack moved to Facebook. won a government contract for a proof of concept of an engine & NASA lost its funding for yet another shuttle replacement.

boxing day 2015 said...

There is another effect that can also effect this data... the RF system on a GPS uses a very precise clock (usually a TXCO) in the receiver. That clock may have acceleration sensitivity.
IE the frequency of this oscillator may vary with applied acceleration...
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Anonymous said...

That's the point of the PSAS solution. They don't lose lock under boost because they don't *have* a lock. They record raw RF data, and they can post process it later. If they lose lock while processing, they can go back in time through the data, adjust for G's and reprocess. Not too useful for GPS guidance, but anything with the ability to go to orbit won't leave the pad at 10+ g's, and thus won't lose lock. Say nothing of inertial aiding, which they can develop *off-line* now that they have data. Exciting.

Regarding G tolerance, you are correct that the data contains both G effects and doppler, but note that the frequencies look like a rocket's velocity curve, and not the acceleration curve.

I think the doppler is pretty close to correct.

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