[Ground-station] Gateway update + Gateway orbit modeling

Mon Mar 23 10:52:38 PDT 2020

 Hi Michelle
I wouldn't rule out a dish/feed that puts out an Oval pattern much likethe FAA RADARS.   Now pointing at the moon would be enough.
Tips of the oval to match the satellite's max distance from the moon.

Kent WA5VJB

    On Monday, March 23, 2020, 12:46:03 PM CDT, Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute> wrote:  

 Thank you Zach, super helpful. I am really looking forward to seeing what your students can achieve. Is anyone at the point where they would be interested in a teleconference or recording a ~15 minute video presentation?

It's clear from simply drawing the orbit in my notebook based on Frank Bauer's presentations that simply tracking the moon will result in too much loss over the week it takes for Gateway to zip around the moon (at microwave). We assumed we'd need to track, but getting real numbers means the link budgets and antenna assumptions get more real. 

GMAT is up and running and I'm working through tutorials. They're well done! GMAT has a MATLAB and python interface. 

I learned about Poliastro at the Open Source Cubesat Workshop. See more about it here: https://docs.poliastro.space/en/stable/

-Michelle W5NYV

On Mon, Mar 23, 2020 at 9:42 AM Leffke, Zachary via Ground-Station <ground-station at lists.openresearch.institute> wrote:

Hi Michelle and Everyone!

We need FOSS tools for this sort of thing for both mission analysis and design as well as for tracking programs for the Amateur Radio community.  SGP4 and TLEs from celestrak aren’t going to cut it for this kind of mission.  Pointing ‘at the moon’ is probably not going to be sufficient for Gateway, especially with tighter microwave beams and the NRHO of the Gateway.  I am unfamiliar with GMAT, and encourage as many folks as possible to look into it as Michelle mentioned.  I just wanted to let folks know that I am pulling a couple parallel threads related to the Gateway and modelling its orbit.

Thread 1 (mission modelling, nearer term).  AGI’s STK (not FOSS) and a few VT AOE students.

I mentioned in a previous thread that we have a senior AOE student working on building models in STK for Lunar Gateway (with help from two grad students).  That is progressing fairly well (despite all the remote work / COVID-19 isolation) and we expect the student to be finished by end of the semester with a basic model that we can build upon.  For those that aren’t aware, System’s Tool Kit (STK) is a powerful orbital/trajectory modelling tool from Analytical Graphics Incorporated (AGI) and includes tools like a communications toolbox for modelling antenna patterns, transmitters, receivers, performing link analysis, etc. etc.  It also has the ‘astrogator’ tool which is a required package for modelling the orbits/trajectories for things that are a bit more complicated than SGP4 and LEO/MEO/GEO.  The downside of STK is that it is hyper-expensive for individual users.  We are fortunate at VT to be part of the AGI academic partnership and have access to the Pro version (with the comms toolbox), and the Lunar Gateway problem is a good learning experience for students.  GMAT is widely considered to be the open source alternative to STK, but for the moment I’m sticking with what I know and have access to.  Perhaps results can be compared from one to the other in order to double check ourselves once things are a bit more ‘ripe.’

Thread 2.  Open Source Tools – Python, NAIF (SPK files), and JPL HORIZONS

I am currently playing around with a python package called Skyfield (https://rhodesmill.org/skyfield/) that is a follow on to the popular pyephem module.  For modelling major bodies (planets, moons, etc.) Skyfield uses ephemerides from NASA/JPL in the form ‘.bsp’ files.  More specifically it uses a subset of SPICE files from NASA’s NAIF group (https://naif.jpl.nasa.gov/naif/), which is a defined format for modelling a LOT of things related observation geometry in the solar system.  The ‘bsp’ files used in skyfield are actually Type 2 and/or Type 3 SPK files in the SPICE system (out of 21 possible Types), that it accesses through a separate module called ‘jplephem’.  To over-simplify it a bit, think of SPKs as containing the coefficients for various regressions of trajectory/orbit data that is derived from spacecraft tracking data (like ranging from DSN).  The different ‘Types’ of SPKs are related to what kind of regression and form of the equation is used (Chebyshev polynomials, Lagrange interpolation, Hermite Interpolation, etc.).  Skyfield reads in the coefficients, sets up the equations, you feed it a time, it pumps it through the equation returns a position, and then computes the ‘additional info’ most users are interested in (like az, el, range, range rate).  Related, but slightly different from SPICE SPK files is the JPL HORIZONS database (https://ssd.jpl.nasa.gov/horizons.cgi), which contains a number of ephemerides for comets, asteroids, planets, and a large number of spacecraft in the Solar System.  HORIZONS can be queried through a web interface and/or a telnet interface, and there is a python package called Astroquery that provides a python API for access (via the telnet interface).  HORIZONS can provide observational data (think az, el, range, range rate), state vector information (think time series of position and velocity), and/or orbital elements.  I am currently trying to figure out how to merge the two and get HORIZONS to export an SPK of various Spacecraft (currently using LRO as the ‘standin’ for Gateway).  HORIZONS definitely exports SPKs, but at the moment it looks like only for asteroids and comets and in a type that is not currently supported by Skyfield.  The ultimate goal is to be able to import a non-Earth Orbit spacecraft into Skyfield as a ‘body’ so that the rest of the code would flow like normal.  As an alternative…(though a hack in my current opinion, might change) skyfield can be ‘bypassed’ and HORIZONS can be directly queried for observer data.

I would encourage folks to take a look at the skyfield github page (https://github.com/skyfielders/python-skyfield/) and look for some of the issues related to this.  Contributing to Skyfield supporting various SPK types might go a good ways towards FOSS tools for tracking non-earth orbiting vehicles (like gateway…..or for use in an ‘Amateur DSN’ context).  Also, generating SPKs for spacecraft from the HORIZONS database data ultimately for use with Skyfield (and other tools) is also a good way to help…there is another python module called SpiceyPy (https://pypi.org/project/spiceypy/) that can be used to create SPKs.  Assuming Gateway data shows up in HORIZONS (or on the NAIF site), this could be the alternative to Celestrak/Space-Track for where Hams go to get data for tracking spacecraft not in Earth Orbit.  Eventually there may need to be an alternative site set up (maybe ORI, maybe AMSAT) where just the SPKs can be downloaded, maybe produced from HORIZONS data, to alleviate strain on the HORIZONS telnet interface (not sure what the limits are here).  Finally, I should also mention that SPICE files have support in multiple languages, not just Python (see the NAIF link above for more details), but again, I’m sticking with the tool I know for now.  SPKS can also be used in GMAT.  At the end of the day….SPK files seem to be the way the ‘pros’ do it, maybe we should consider it ourselves.

All good stuff! GMAT should definitely be explored for mission modelling and analysis purposes.  At VT we’re trying to get a handle on STK modelling for the same purpose.  Personally, I’m looking into Skyfield+Python+NAIF+HORIZONS for ultimate application in ‘real time tracking’ programs (to feed antenna controllers, Doppler tuning, etc.).  Can ‘gpredict’ (and popular windows equivalents) be modified to include this type of tracking?  I encourage as many folks as possible to start thinking about this and these kinds of questions and either jump in on what’s already going on or come up with other ways to do it.

-Zach, KJ4QLP

--

Research Associate

Aerospace & Ocean Systems Lab

Ted & Karyn Hume Center for National Security & Technology

Virginia Polytechnic Institute & State University

Work Phone: 540-231-4174

Cell Phone: 540-808-6305

From: Ground-Station <ground-station-bounces at lists.openresearch.institute>On Behalf Of Michelle Thompson via Ground-Station
Sent: Sunday, March 22, 2020 1:52 PM
To: Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute>
Subject: [Ground-station] Gateway update + Gateway orbit modeling

Things are moving on Gateway (lunar station)!

I’m trying to find an orbit model so we can make progress on a ground station design. JAMSAT asked for help at AMSAT-NA Symposium and I said yes. We’ve been talking ever since and are at the stage of writing formal commitments to AREx, the project from ARISS that is working to put amateur radio on the Gateway. 

Yes, I know, everything could be canceled or delayed. But, the work we’re doing is entirely re-usable and is exactly in line with our payload project.

I have not yet found a published orbital model for Gateway, but from the meetings and presentations and documents, I know it’s a near rectilinear halo orbit with a 7 day period. It’s highly elliptical, going from 3,000 (wikipedia) or 7,000 (Frank Bauer) out to 70,000 km. I am trying to get clarification on the perilune (closest distance to moon).

I have GMAT from NASA successfully installed and over 1000 pages of documentation to go along with it.

I don’t know anything about modeling orbits other than what I learned in AP Physics in high school, but I’m willing to try to get something nailed down and reviewed enough to help us with 1) tracking 2) link budgets and 3) antenna repercussions.

Plus, surely an animation of what things look like would be super fun and help everyone spread the word about this really neat amateur radio opportunity. 

So if you know anything about GMAT and can answer some newbie questions (as soon as we have some) please speak up. If you're interested in installing it and trying to help with this, then go here and get it stood up on your machine: 

https://sourceforge.net/projects/gmat/

-Michelle W5NYV

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