4 ms·
I was curious to see how accurate the displayed ephemeris are (http://voyager.jpl.nasa.gov/Scripts/distance.js http://voyager.jpl.nasa.gov/Scripts/distance.js)
by mturmon 13y ago
I was curious to see how accurate the displayed ephemeris are (http://voyager.jpl.nasa.gov/Scripts/distance.js http://voyager.jpl.nasa.gov/Scripts/distance.js) given that they are known very precisely indeed.
For instance, ranges to Saturn are known to better than 1km, and Mars to better than 10m (http://tmo.jpl.nasa.gov/progress_report/42-178/178C.pdf http://tmo.jpl.nasa.gov/progress_report/42-178/178C.pdf, e.g., fig. 1 for Mars), and we don't even get to take direct measurements of those bodies as we can for the Voyagers.
Alas, the js seems to simply be interpolating between two known values, at a spacing of 1 year (86400s):
var epoch_0 = 1373990400;
var epoch_1 = 1374076800;
[...]
var dist_0_v1 = 17396281625.1305;
var dist_1_v1 = 17395807835.8111;
current_dist_km_v1 = ( ( ( current_time - epoch_0 ) / ( epoch_1 - epoch_0 ) ) * ( dist_1_v1 - dist_0_v1 ) ) + dist_0_v1;
I'm not sure how to square this linear progression over a 1-year span with the comment in the OP that "Because Earth moves around the sun faster than Voyager 1 is traveling from Earth, the distance between Earth and the spacecraft actually decreases at certain times of the year."
- D9u 13y agoMaybe take an average of the difference between the nearest point and the farthest then settle for the middle?
- marshray 13y agoMy understanding is that we have been able to take direct radio interferometery distance measurements of Mars since the Viking and other landers were there.
- mturmon 13y agoThat sounds right. But, to get the numbers in the www page, you can't just measure where it is. You have to about predict where it will be, which requires forward integration of a motion model for all the mass in the solar system. The paper I referenced cites Mars range measurements, and related measurements, as a check on this forward integration. Planetary radar is another option for a ground truth check even if you don't have a radio source (like Viking) on or near the planet in question.
- dschleef 13y ago86400s is one day, not one year.
- mturmon 13y agoThanks -- apparently I have not been getting enough sleep. That explains how they can get distances to increase, then decrease (as noted nearby in this thread).
- MrMeker 13y agoAs dschleef said, the epoch span is one day. You will notice that dist_1_v1 is less than dist_0_v1. This fixes the problem you noticed, leading to a reasonably correct, shrinking value for the distance between Voyager and the Earth.