4
$\begingroup$

As far as I know, the recently launched James Webb Space Telescope will be positioned at 1,500,000 km of the Earth, orbiting Sun-Earth L2 Lagrange point at roughly 4x the distance between the Earth and the Moon.

Usual positioning satellites constellations (GPS, GALILEO, GLONASS, BEIDOU,...) are located at roughly 20,000 km, which is ~ 0.013 of the JWST final position.

On the basis of this observation, I'm then wondering; could the position of the JWST be known accurately enough using GNSS satellites at such a great distance? I'm intuitively convinced that positional signals may be too collinear (if they still exist with a good enough S/N) to be precise enough at such distance. But I may be wrong.

And if not, how is the position of the telescope known at any given moment? Or how is the instrument positioned (i.e. relative to which reference frame,...? Maybe it's not an Earth-centric frame as we are used to down here)?

$\endgroup$
4

1 Answer 1

5
$\begingroup$

Partial answer...

Considering:

https://www.sciencedaily.com/releases/2018/01/180111223914.htm

NASA engineers has demonstrated fully autonomous X-ray navigation in space

NICER/SEXTANT

https://www.asterlabs.com/publications/2006/Sheikh_et_al,_AIAA_JGCD_Jan_Feb_2006.pdf

Spacecraft Navigation Using X-Ray Pulsars

China's XPNAV 1

Is NICER/SEXTANT the first civilian "spacecraft" to determine it's own position in space without GPS or uplinked data?

How can spacecraft navigate without contact from Earth?

Satellite Position

https://ntrs.nasa.gov/citations/20190000639

Predicted Performance of an X-Ray Navigation System for Future Deep Space and Lunar Missions

https://physicsworld.com/a/x-ray-pulsars-plot-the-way-for-deep-space-gps/

X-ray pulsars plot the way for deep-space GPS

https://www.wired.com/story/nasa-just-proved-it-can-navigate-space-using-pulsars-where-to-now/

NASA Just Proved It Can Navigate Space Using Pulsars. Where to Now?

https://arxiv.org/ftp/arxiv/papers/1704/1704.03316.pdf

Pulsar Positioning System

I had erroneously assumed that JWST would make use of XNAV. But no.

Maybe, probably, due to the time when the project was first started and the length of time it took to get to launch, XNAV probably wasn't matured enough to get on board.

So instead there is this:

Navigation Concepts for the James Webb Space Telescope

https://ntrs.nasa.gov/citations/20040082113

The Mission Engineering and Systems Analysis Division at the Goddard Space Flight Center (GSFC) is supporting the JWST project by developing navigation concepts that meet nominal orbit determination accuracy requirements on the order of 50 km in position and 20 millimeters per second (mm/s) in velocity (3-sigma).

These requirements are challenging because of the unusually large solar radiation pressure (SRP) forces that will be experienced by the spacecraft and the frequent attitude reorientations and unbalanced momentum unloads that are planned for this mission.

This paper evaluates ... approaches for meeting the navigation requirements:

  • Ground navigation using standard range and/or Doppler measurements from the Deep Space Network (DSN), which is the current mission baseline
  • Onboard navigation using the communications hardware and Sun sensor baselined for this mission
  • Onboard navigation using optical celestial navigation sensors that can measure the angle between the Earth and the Moon or a star and the Moon.

The ground navigation approach processes standard range and Doppler measurements from the Deep Space Network.

The onboard navigation approach processes celestial object measurements and/or ground-to-spacecraft Doppler measurements to autonomously estimate the spacecraft’s position and velocity and Doppler reference frequency.

The ground navigation approach provides stable navigation solutions using a tracking schedule of one 30-minute contact per day.

The onboard navigation approach that uses only optical quality celestial object measurements provides stable autonomous navigation solutions.

So, the answer seems to be a combined ground and onboard navigation approach making use of the DSN.

..and not GNSS

So, whilst celestial GPS via Pulsars seems to be the way forward, they do not seem to be on JWST..

Further:

Also see this answer regarding using GPS beyond Earth:

https://space.stackexchange.com/a/567/40489

Furthest distance that GPS was utilised from Earth is around 70k km.

and this answer for self navigation in deep space:

https://space.stackexchange.com/a/12138/40489

Probes don't locate themselves. We do.

location of a probe is measured by ground control, using data from the probe.

https://ntrs.nasa.gov/api/citations/20080030196/downloads/20080030196.pdf

Ranging is required for JWST, using alternate ground stations in the southern and northern hemisphere.

The NASA Spectrum office objected to provide more than the 10 MHz band in X-band range and suggested using Ka-band. JWST project decided to move to K-band and have one (1) 4-hour contact per day for communication and ranging.

https://www.esa.int/Enabling_Support/Operations/ESA_Ground_Stations/ESA_to_pick_up_as_Webb_phones_home

ESA to pick up as Webb phones home

https://www.esa.int/ESA_Multimedia/Images/2019/02/Ultra-precise_navigation

Ultra-precise navigation

"How not to lose a spacecraft"

enter image description here

https://www.esa.int/Enabling_Support/Preparing_for_the_Future/Discovery_and_Preparation/Deep_space_communication_and_navigation

https://journals.sagepub.com/doi/pdf/10.1177/002029400804100302

For near-earth space explorers, a variety of autonomous navigation methods have been proposed and explored, including a magnetometer-based navigation method, Global Position System (GPS), inter-satellite link, and celestial navigation method.

However, for deep space explorers, celestial navigation is the only feasible way.

$\endgroup$
4
  • 2
    $\begingroup$ voluminous answer with lots of goodies and gems! I have a bounty on Exact speed/distance of JWST (& others in L1/L2 halo) from Earth; is it measured any differently than for more distant deep-space missions? but the bounty message is pretty clear about what I'm hoping to see. Of course the OP's expectations may not be exactly the same. $\endgroup$
    – uhoh
    Jan 23 at 2:38
  • $\begingroup$ Based on what I've read, with JWST being the first L2 mission to be defined as a high data rate mission (270gb daily), a lot of ground services seemed to have been upgraded to cope with this and leads me to think yes, while it is not measured any differently it is perhaps measured more accurately due to the upgrades. OTTOMH though $\endgroup$ Jan 23 at 2:54
  • $\begingroup$ "270gb daily" Yikes! Let me know when you're ready for a question about a first high data rate Lagrange point mission. $\endgroup$
    – uhoh
    Jan 23 at 3:04
  • 1
    $\begingroup$ Apologies, forgot to capitalise, 270 Gb :P $\endgroup$ Jan 23 at 3:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.