
Galileo satellites experiencing multiple clock failures - oherrala
http://www.bbc.com/news/science-environment-38664225
======
VLM
What are the legacy GPS system stats? Its kinda important to compare before
deciding the Russians hacked them.

Its surprisingly difficult to find real, detailed data about the GPS system.
The block II first production run of GPS sats that were launched around 1990
and decommissioned in at most 17 years had a design life of a pitiful 7 years
and used dual Rb clock and Cs clocks. I'm not sure the modern block details
are declassified. Anyway the failure rate for Galileo seems consistent with a
"less than a decade design life" I'm not sure block IIIa is even finished
designing yet.

People have this peculiar idea that "the gps satellite" was launched one time
in the 80s and its been running ever since... not so, gosh there must have
been a hundred or so launched over the last couple decades.

I have some limited experience with Rb-standards and on the ground in telco
land you'll get an amazing song and dance from the mfgr about how in theory
they should run for eternity but being a vacuum tube (actually conceptually
closer to a fluorescent light) they do wear out and end up on ebay after a
decade or so for ham radio microwave and EME operators to do peculiar things
with them cheaply. Not good enough to run a data center is still stable enough
to help bounce signals off the moon.

I have no experience with hydrogen standards they don't even appear on ebay in
my price range. Again perhaps its normal for them to wear out, etc.

So Rb standards "should" die off on a regular basis. Possibly the biggest
classified secret of the entire GPS project MIGHT be you have to massively
overspec your clocks or you get an economic kill shot of spending a billion
bucks (rubles, euros) on something that only runs six months LOL shoudda just
licensed our technology that learned the lessons the hard way in developmental
block I satellites in the mid 80s.

~~~
analog31
_Not good enough to run a data center is still stable enough to help bounce
signals off the moon._

Out of curiosity, why does a data center need a Rb clock?

~~~
electrum
Very accurate time is important for some applications:
[https://www.wired.com/2012/09/google-
spanner/](https://www.wired.com/2012/09/google-spanner/)

~~~
ajross
Not at this level. The Rb resonance in question is ~6.8 GHz, which means that
counting it (which is functionally all atomic clock is really doing) will give
you accuracy higher by definition than the cycle time of the CPU running your
timing-sensitive application.

GPS needs that because at the speed of light in vacuum that corresponds to
~2cm of wavelength, which is only 2 orders of magnitude from the design
accuracy of the whole system.

~~~
cnvogel
Cycle-to-cycle accuracy of an atomic clock (then called a timing and frequency
standard) is _never_ dominated by the resonance frequency, and you never
extract the timing information from the radio-frequency that interacts with
the physics directly.

The reference frequency used externally for timing is the reference oscillator
which typically was 5 or 10 MHz, but nowadays tends to be 100 MHz as HF
electronics became better.

For an older _Caesium_ clock, a block diagram is explained on this page:
[http://www.leapsecond.com/museum/hp5062c/theory.htm](http://www.leapsecond.com/museum/hp5062c/theory.htm)

{a Rb clock is very similar electronically, only you are not looking at count-
rates of Cs atoms flying through your spin filter but you are observing the
optical attenuation of a rubidium gas, both vary with radio-frequency waves
being injected into the package. Also Cs is a primary standard and Rb is very
stable - but adjustable and susceptible to extarnal magnetic fields.}

From the reference frequency, a series of synthesizers / multipliers generate
the radio frequency that is relevant to the physics process. And this
frequency is then modulated slightly (137 Hz label in Figure 5) so that
correlation in the physics process from being too high, or too low in
resonance frequency, can be observed (Figure 9, you vary between f2 and f1 137
times per second).

A good GPS steered quartz oscillator will use around 100s of loop time-
constant, the time constant for the control loop in a caesium clock will be
much higher.

~~~
ajross
I'm not following your ire. I don't understand why you think adding all the
detail above is inconsistent with "functionally, all an atomic clock is really
doing is counting cycles (and, OK, tracking phase) of a known transition of
some isotope or another. You're just describing how.

------
woliveirajr
> Esa is also in contact with the Indian space agency which is using the same
> clocks in its sat-nav system. So far, the Indians have not experienced the
> same failures.

Since the same technology and the same manufacturer is being used, it's a
weird problem to have after the satellites are already in space.

~~~
digi_owl
Would it be paranoid to contemplate sabotage?

After all, a global system has clear military value. In contrast the Indian
system is limited in coverage to India and nearby nations.

~~~
celticninja
my thoughts too, Stuxnet but for GPS satellites. Too much of a threat to the
US GPS system? Maybe a target for Russia but unlikely because they have no
competing system. The only thing that leads away from it being US orchestrated
is that they found Stuxnet, so if they found this you could be pretty sure
that US/EU relations would be very shaky. Of course perhaps then Russia make
it look like a US attack to divide a wedge between allies.

~~~
Gupie
>> Russia ... have no competing system

Russia has GLONASS, China has BeiDOU, India IRNSS, Japan QZ

~~~
celticninja
apologies, I thought GLONASS was military use, I then checked and got stuck in
wikipedia, meaning to come back and edit my comment, too late now though.

~~~
caf
If you have a recent smartphone, you might be a GLONASS user and not even know
it!

~~~
flyinghamster
If it's an Android smartphone, recent versions of MobiWIA - Eclipsim's GPS
Status & Toolbox app divide the satellite signal bar display into four
sections (GPS, GLONASS, Beidou, and Galileo). Since my phone only supports GPS
and GLONASS, the latter two don't show any bars.

------
jblok
With functioning clocks, Galileo is accurate to within a metre, according to
this article. That's crazy - I had no idea it was that accurate.

~~~
athenot
With India, China and Russia launching their own positionning systems, I
wonder if it would be possible to interpolate an even higher precision by
listening to all 5 signals at the same time.

(I don't know enough about signal processing to know if that's a possibility
or not.)

~~~
jessriedel
Not only is it possible, it's already done in many (most?) smartphones made
over the past few years, at least for the Russian GLONASS system. As China's
BeiDou and Galileo become operational, those will presumably be incorporated
too.

However, I'd guess that most of the improvement in user experience comes from
the higher reliability than higher accuracy. Even if the errors from all
systems were completely uncorrelated (which they are not), averaging the
location reported by four systems gives you at most a factor of 2 in improved
accuracy. On the other hand, if your chance of being able to lock onto enough
satellites (within a given number of seconds) for one system is 80%, the
chance that you can lock on to at least one of the four systems becomes 99.8%.

~~~
folli
I'm having trouble to grasp how you get a factor 2 improved accuracy if you
have data from four uncorrelated systems all having the same accuracy.

Where can I find more information about this?

~~~
jessriedel
If your N samples are independent and distributed with the same error sigma,
then the error of the averaged result is sigma/sqrt(N). (And of course,
sqrt(4) = 2.)

You can calculate this directly by computing the variance of a sum of
independent Gaussian random variables. Can't find a great link for further
reading, but someone else might be able to help you out.

~~~
RileyKyeden
I'm glad there are people who know what all this means so I can tap a button,
say "navigate to the grocery store" to my phone, and not spend brain cycles
learning and maintaining knowledge of the wide range of
math/programming/engineering specialties I'd otherwise have to master.

~~~
jessriedel
Everyone should know the central limit theorem though, simply for its own
sake! It's worth the brain cycles :)

[https://en.wikipedia.org/wiki/Central_limit_theorem](https://en.wikipedia.org/wiki/Central_limit_theorem)

~~~
RileyKyeden
I had an intuitive sense of this. People saying "half the population has a
below average IQ!" always elicited a head tilt from me. I just didn't know
there was enough to say on the subject to fill a 8k word wikipedia article.

I'll file this next to the formula I whip up when I want to figure out how
much a trip costs based on mileage and fuel prices.

~~~
robocat
The intuition should be that "Half the population has below average X" is
_untrue_ for the vast majority of situations, because most real things have a
skewed distribution.

AFAIK IQ is a peculiar case because of how IQ is defined.

~~~
halomru
A lot of natural processes produce results that are normally distributed. So
given a large sample, a continuous value range, no bounds (or far away bounds)
and a lack of obvious skew the intuition "half the population is below
average" has a good chance to be (approximately) true.

Of course IQ is a made up thing that we just defined to behave that way.

------
paglia_s
Is there a way to fix such problems with a software update?

~~~
Cyph0n
It sounds like a manufacturing issue to me, so I don't think so. The batch
they ordered could have been faulty, but I'm assuming that ESA uses samples
taken from multiple runs to minimize failures, so that's fairly unlikely.

~~~
planteen
It could be a design issue too. Like are they accidentally over-voltaging the
clock with a transient or some sort of power sequencing issue? Or does some
component have tin somewhere? Really hard to say at this point.

~~~
Cyph0n
I really hope it's not a design issue, but that could definitely be the case.
Why tin specifically?

~~~
kmm
Tin forms whiskers, small hairlike growths on the metal that can cause short
circuits. We've already lost a satellite to this phenomenon, although I would
hope it's not a problem anymore with current metallurgical techniques

[https://en.wikipedia.org/wiki/Whisker_(metallurgy)](https://en.wikipedia.org/wiki/Whisker_\(metallurgy\))

~~~
wrigby
If I remember right, tin whiskers were determined to be the cause in the crazy
Toyota runaway accelerator pedals a few years back. I think it's mainly an
issue with lead-free solders, which RoHS has forced into widespread usage over
the last decade.

~~~
Symbiote
ESA is not part of the EU -- it's an international organization, and is exempt
from national law in the same way as an embassy. Even if RoHS applies to
spacecraft, it probably doesn't apply to ESA's spacecraft.

That's not to say acquiring non-RoHS components isn't made more difficult by
RoHS.

~~~
planteen
RoHS does not apply to spacecraft. In fact, companies specialize in things
like replacing RoHS solder balls on BGA parts with lead solder ones.

------
truftruf
/r/conspiracy: The CIA probably fed them bad components or software for the
clocks. Higher levels of positioning accuracy is potentially dangerous and can
be misused by third parties, so the argument goes.

/r/conspiracy: #2 The CIA is downvoting this post.

