Wrong predictions for Molniya orbits

I’m trying to predict the position of a satellite with a Molniya orbit, Molniya 3-47:

1 23642U 95042A   18058.62866679  .00000210  00000-0 -75405-2 0  9996
2 23642  62.9606 257.1441 7218342 288.9618  10.0525  2.01983768165381

The prediction turns out to be completely incorrect. The minimal example below is taken from the readme almost line-by-line:

var tleLine1 = '1 23642U 95042A   18058.62866679  .00000210  00000-0 -75405-2 0  9996',
    tleLine2 = '2 23642  62.9606 257.1441 7218342 288.9618  10.0525  2.01983768165381';
var satrec = twoline2satrec(tleLine1, tleLine2);
var positionAndVelocity = propagate(satrec, new Date());
var positionEci = positionAndVelocity.position;
var gmst = gstime(new Date());
var positionGd = eciToGeodetic(positionEci, gmst);
console.log('Molniya', positionGd.latitude * (180 / Math.PI), positionGd.longitude * (180 / Math.PI));

It gives me 39.8491516985329 -157.00043723551485, while the correct coordinates are 38.64 72.56. At other times it’s even more off. Here’s the plotting of the orbit on Google Maps:

In reality it’s supposed to look like this (see n2yo):

Predictions for any other orbit (including highly elliptical orbits, like the Tundra orbit of QZSS satellites) seem to be correct.

[ezze]

@nhamer, nice catch! Thanks a lot for this.

Following Airbnb code style forced me to add many parentheses to satisfy no-mixed-operators rule. As we see now, it doesn’t give any benefits but adds headaches like this issue. Probably, it would be better to disable this rule in .eslintrc.json as it was done for function-paren-newline later. It was my mistake, sorry for this, guys.

A fix is made in https://github.com/shashwatak/satellite-js/commit/d398bd5fb880a48708c3ef6be712a595360c12b3. You can update the library to 2.0.2. @emedvedev, please check whether everything works fine for you. @dangodev, hope nothing is broken for you too.

All tests are passed but we don’t have too much tests to cover everything. If someone of you, guys, can prepare some test fixtures for Molniya 3-47 with C++ or Python version of the library, you are welcome. 😃

[nhamer]

Almost certain that the issue is https://github.com/shashwatak/satellite-js/blob/226d8d5520c0cf03a591a6f0bd4ddfd8a1de0965/src/propagation/dspace.js#L248

The equivalent C code (sgpunit.cpp@1114) is: mm = xl - 2.0 * nodem + 2.0 * theta;

(slightly different runs over a few seconds, so gmstime is not identical):

Old satellite.js (irez=0): Molniya 62.97302163853729 -152.31526401326246 Current satellite.js (irez=2): Molniya 30.900731334965187 -76.42315864995993 Correcting mm irez=2) gives: Molniya 62.864247526501345 -155.86674453746895