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James Webb will be in a halo orbit, station keeping around the Sun-Earth L2 point. This means it needs to monitor its position with regard to L2, for periodic station keeping purposes.

But L2 isn't an object in space that it's orbiting. Its path is better described as a cyclical path round a point moving through space, that has no visible marker, and is identified by its property that it's a gravitational saddle point. But that feature doesn't have any specific prominent physical markings to identify it, and gravity probably doesn't change massively sharply at the saddle either.

From almost half a million miles away, I'm unclear the gravitational gradient at JWST is sufficient to identify with precision, where it is, relative to L2, enough for station keeping in its orbit. Perhaps it does just use very precise detectors of the local gravitational field, but how it obtains station keeping adjustment data from that alone still isn't clear, if so.

So how does JWST (or more accurately its ground control) identify station keeping corrections?

Update: to clarify, I'm mainly looking for answers with a list of "(item actually measured) to within (X amount/%) by (details of technique/method and how achieved)", and how those are then combined/used to produce an accurate enough location w.r.t. L2. Plus any interesting/relevant detail about it, or about the techniques used.

Update 2: clarifying "ground control", I meant JW overall, not assuming at all that its done onboard the observatory. That wasn't clear, so I've fixed it.

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    $\begingroup$ How do you detect the local gravitational field when you're in freefall? $\endgroup$
    – PM 2Ring
    Jan 27, 2022 at 6:08
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    $\begingroup$ Not a clue. If I had to guess, it can be done indirectly (precision measurements of motion w.r.t. known objects, or their distances? ), but honestly, not a clue what method JW uses, hence why I asked the question. $\endgroup$
    – Stilez
    Jan 27, 2022 at 6:14
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    $\begingroup$ @PM2Ring The devices we have are good enough to measure the tidal acceleration in LEO with great precision, and dedicated ones will even get results in solar orbit at 1AU. The thing about Lagrangian points though is gravitational gradient gets completely flat there. At Webb's halo orbit it's no longer the complete zilch of what it's at L2, but it's still insufficient to be useful. $\endgroup$
    – SF.
    Jan 27, 2022 at 9:09
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    $\begingroup$ @SF. Good point. :) And in fact my code in space.stackexchange.com/a/57679/38535 locates L1, L2, & L3 by finding the zeroes of the derivative of the effective potential in the rotating frame. $\endgroup$
    – PM 2Ring
    Jan 27, 2022 at 9:18
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    $\begingroup$ If it bumps into a teapot, it's probably gone too far. $\endgroup$
    – Steve Melnikoff
    Jan 27, 2022 at 16:03

2 Answers 2

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So how does JWST identify station keeping corrections?

It doesn't.

While the JWST does know where it is pointing, it does not know where it is in space. It doesn't need to. The JWST Flight Dynamics Team, operating out of the Goddard Space Flight Center in Maryland, maintain a regularly updated estimate of where the JWST is in space. This ephemeris is based on range and range rate readings provided by NASA's Deep Space Network (DSN) plus knowledge of delta Vs from prior momentum unloading and prior orbit correction maneuvers.

It is the JWST Flight Dynamics Team that calculates the occasional delta V maneuvers needed to keep the JWST in its pseudo-orbit about the Sun-Earth L2 point. The JWST itself simply executes those commands: Point in such and such a direction and fire until either some commanded amount of time has passed or until some commanded delta-V has been achieved.

The latter (achieving commanded delta V) requires accelerometers. I don't know if the JWST has accelerometers. For the last 20 years there apparently have been internal debates regarding whether the JWST needs accelerometers for this purpose. It does not need accelerometers for self-navigation because the JWST does not do that.

What the Flight Dynamics Team uses to estimate the translational state (position & velocity) of the JWST is a batch least squares orbit determination algorithm based on a history of range and range rate readings provided by NASA's Deep Space Network, a history of momentum unloading and orbital maneuver thruster firings, and estimates of solar radiation pressure.

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    $\begingroup$ This is a correct answer. Using radio telescopes on ground and a transponder on the JWST it is possible to measure range to the satellite with very high precision (down to meter precision) and range rate (speed compared to the earth) down to millimeter per second. If necessary (very seldom) radio telescopes can be used in pairs to get the direction with high precision. Most of the rest is simply Newton mechanical "laws" and a bit of math. The satellite itself has very little knowledge of where it is, but can point the teleccope using star trackers. $\endgroup$
    – ghellquist
    Jan 27, 2022 at 17:51
  • $\begingroup$ @ghellquist Isn't the JWST in the radio shadow of the moon from the Earth? (I honestly don't know, but L2 to my naive mental map looks like it should be?) $\endgroup$
    – Yakk
    Jan 27, 2022 at 21:54
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    $\begingroup$ @Yakk, it's orbiting around Earth-Sun L2, not Earth-Moon L2. The Moon's radio shadow only sweeps over the orbital path twice a month, and since JWST has a pseudo-orbital period of six months, it won't be in the shadow both times. (If the JWST team has timed things right, it'll never be in the Moon's radio shadow.) $\endgroup$
    – Mark
    Jan 27, 2022 at 22:05
  • $\begingroup$ @mark oh, now that was a long misunderstanding in my part. Earth sun L2 makes much more sense if you want to keep it cool. I am now surprised at how close it is! $\endgroup$
    – Yakk
    Jan 27, 2022 at 22:41
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    $\begingroup$ @Mark: the JWST halo orbit is so large that when looking from L2 towards earth the entire moon orbit is less than 1/2 the halo width. JWST will NEVER be in moon shadow. This was a primary requirement of the JWST halo orbit design. No coordination with lunar orbit required. $\endgroup$
    – BradV
    Jan 28, 2022 at 4:19
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Mathematics.

JWST will use the general techniques of locating itself in space - doppler shift, star trackers etc - same thing deep space probes use. Knowing the Sun position, Earth position and their masses, you know the position of L2. Knowing JWST position from its instrumentation, you can calculate what it is with relation to L2.

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  • $\begingroup$ But it seems that it needs to know its position a lot more accurately than most deep space probes, for fine decisions about station keeping and tiny delta-v corrections (and star trackers presumably rely on parallax and have limited fine accuracy? Doppler radio only gets you 1D not 3D?) , and is also much further away from earth and moon and other nearby useful reference objects that earth-local objects might use. $\endgroup$
    – Stilez
    Jan 27, 2022 at 7:06
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    $\begingroup$ @Stilez Doppler radio gets you 1D when going from 1 ground station, 3D if it goes from 3 around the globe. And it's exceptionally precise - using timing you can narrow down the precision to a couple meters, but from there, using phase shift measurement, you can get down to centimeters. It doesn't work so well for "3D" for probes in the Kuiper Belt, but at distances like L2, the parallax between one side of Earth and the other is more than enough to keep the probe in its orbit. Never mind the orbits aren't that sensitive, Webb drifting a couple kilometers off course is still not a problem. $\endgroup$
    – SF.
    Jan 27, 2022 at 9:19
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    $\begingroup$ @Stilez Regarding "But it seems that it needs to know its position a lot more accurately than most deep space probes ..." :That is not the case. The JWST does not know one <<expletive deleted>> thing regarding where it is in space. It's the JWST Flight Dynamics Team who work out of the Goddard Space Flight Center in Maryland (plus all the programs those people use) who know where the JWST is in space, plus or minus a few tens of kilometers. $\endgroup$
    – David Hammen
    Jan 27, 2022 at 9:29
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    $\begingroup$ @RichardHammen - when I say "it needs to know", I mean the ground crew, not the observatory itself. Sorry, figured that was implicit. But my question is, what exact measurement data, and what accuracy is achieved with the measurement data, that is used to accomplish "knowing where it is plus or minus a few tens of KM" with respect to L2. So I'm looking for answers with a list of "<item actually measured> to within <X amount/%> by <technique/method>" etc, and how those are then combined/used to produce an accurate enough location w.r.t. L2 $\endgroup$
    – Stilez
    Jan 27, 2022 at 9:49
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    $\begingroup$ @Stilez There is no such a thing as exact measurement data. Every measurement has errors and uncertainties. Range and range rate (doppler) give two measurements out of the six needed to specify the position & velocity state. However, dozens of readings, or dozens of dozens of readings (or more), coupled with mathematics result in an over-specified system. That's a good thing. It allows the estimator to zero in on a best fit solution. $\endgroup$
    – David Hammen
    Jan 28, 2022 at 0:57

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