Dear W. Halstead,
Two months ago you asked questions on this email list about the
planetary ephemeris used by program WSJT-X for astronomical
calculations. I set your query aside to be answered later -- and
somehow "later" has not arrived until now. Sorry for the delay.
Some details about the astronomical calculations in WSJT-X are described
in a 2016 QEX paper:
https://physics.princeton.edu//pulsar/k1jt/LunarEchoes_QEX.pdf
The paper includes results of experimental EME echo measurements made at
144 MHz (at the K1JT statiion) and 432 MHz (at W2PU). The measuremnents
included Doppler shifts accurate to about 0.1 Hz and linear polarization
angles accurate to a few degrees.
The measured Doppler shifts agree with WSJT-X predictions to within the
measurement uncertainties. The paper lists and describes the most
important requirements for predictions with the stated accuracy.
On your two specific questions:
1. Differences between the JPL DE405 and DE440 ephemerides have some
importance for interplanetary spacecraft navigation, but are completely
negligible for amateur EME work.
2. Yes, we should include the latest Leap Second (January 1, 2017) in
subroutine lib/slasubs.f -- and we have now done so. But again, the
errors resulting from a 1 s offset in Ephemeris Time are negligible for
all but the most precise amateur EME work. Computed Doppler shifts
might be off by around 1 Hz at 10 GHz.
-- 73, Joe, K1JT
On 8/18/2021 3:27 AM, halsteaw--- via wsjt-devel wrote:
(Not an astronomer)
I noticed in the contrib/Ephemeris folder that the JPLEPH file is
DE405 which was created back in 1997.
(Ref: https://ssd.jpl.nasa.gov/?planet_eph_export )
Is there a reason that such old data is being used?
For the best EME and other calculations, shouldn't DE440 which was
created June 2020 be used?
-ALSO-
lib/slasubs.f is missing the data for the Leap Second for Jan 1 2017 @ line 161
(Ref: https://www.ietf.org/timezones/data/leap-seconds.list )
Respectfully,
~W. Halstead
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