On 6/18/17 9:29 PM, Hal Murray wrote:
jim...@earthlink.net said:
Well, at JPL we regularly lock two crystal oscillators together that are
over a billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz. It's how we measure the
distance and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)
It's just how we do radio science/ranging - you transmit a spectrally pure
signal from earth (typically oscillator locked to a maser), at the
spacecraft you have a very narrow band PLL (traditionally a VCXO) that
locks to the received signal, and you generate the downlink signal from
that same oscillator, transmit it back to earth, and compare.
A "spectrally pure signal" gets you a frequency offset for velocity, but it
doesn't get any timing info. How do you get range?
Well, you start by just measuring the round trip light time - we know
(having measured it on the ground) the time delay through the
spacecraft, and we know, very precisely, the time delay from the
transmitter on the ground to the radiating aperture, and, equally
precisely, where that aperture is. (the latter is, in its own right, an
impressive feat - if you want to measure the distance to something at
Jupiter or Saturn (1E9 km) to the nearest mm, which is what 1E-15 ADEV
implies)
Once you know that (I'm not sure what kind of uncertainty that is, maybe
a microsecond?) you do the precise measurement in two different ways:
1) sequential tone ranging: by putting a "ranging tone" at, say, 1 MHz,
on the carrier, which gets turned around in the transponder. Remember
that there is typically an "approximate" estimate of the range, so if
you look at the phase difference of the ranging tone, you can figure out
where you are within the "cycle". Use a sequence of tones to resolve
ambiguities.
2) PN ranging - put a PN code on the transmitted signal which is turned
around by the transponder and measure the difference in code phase -
some do regeneration, some don't
And finally, you measure carrier phase.
In general, PN ranging is considered better, but we still use tone
ranging for most JPL missions - heritage and availability of spacecraft
radios that support it is a powerful force.
https://cwe.ccsds.org/css/docs/CSS-SM/Meeting%20Materials/2008/March-2008/DS%20Ranging%20type%20comparison%202001-11-03.pdf
Tone ranging takes power away from the telemetry (limiting the data
return), while PN ranging doesn't (after despreading, you've got your
whole signal power there). Tone ranging also requires that you have a
good a-priori estimate to pick a suitable set of tones.
A historical note.. before Apollo, tone ranging what what people did
(back in the 900 MHz Microlock days). Apollo introduced PN ranging as
part of the "unified s-band", but the acquisition time for the PN codes
was apparently very, very long, even with the composite codes in
use(called, oddly, JPL Ranging codes), and apparently everyone hated it
(so I am told, I was a little kid and didn't know PN from tones) - so
they went back to tone ranging for the most part. ESA makes a lot of
use of regenerative PN ranging, NASA does not - the SDST (Small Deep
Space Transponder) doesn't do regen PN ranging, the BepiColombo
transponder does.
https://deepspace.jpl.nasa.gov/dsndocs/810-005/203/203C.pdf - tone ranging
https://deepspace.jpl.nasa.gov/dsndocs/810-005/214/214A.pdf - PN ranging
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