ISS position and AMS 02 pointing accuracy study



On March 27th, 2011 Grange Obs. contacted NASA JPL since an indipendent technology research on AMS 02 orientation was in progress.
At the time, it was noticed the JPL tool HORIZONS (on-line ephemerides calculator) provided ISS orbital prediction for a short period of time (the service was canceled on January 2011).
The AMS 02 payload has been mounted on May 19th, 2011 on ISS during the STS-134 Shuttle mission, but 2 months earlier it was clear that the Space Station orbit accuracy needed to be studied.

Dr. Jon Giorgini (JPL SSD Group senior analyst) replied the day after (cc'd Dr. Donald K. Yeomans, senior research scientist at JPL - CALTECH) that the service was temporarily provided since the ISS Crew wanted to observe an asteroid's close approach, so station-centered predictions were needed for a limited amount of time.
The AMS 02 problem was deemed fairly similar; in fact, the payload has the possibility to select the studied uncharged particles (gamma rays) incoming direction, so in principle it performs like an optical telescope observing the celestial sphere, and needing to be precisely pointed.
AMS 02 has a built-in star trackers (having a pointing accuracy of several arcsec), but ISS cannot orientate itself like the Space Telescope, in fact the station manouvers are mainly for the orbit circularization purpose, and are not intended for pointing a fixed celestial object.
Dr. Giorgini also offered the JPL support for re-activating the ISS service on Grange Obs. demand, while justified and clearly referenced.
A reply was then sent for investigating the ISS position accuracy and the involved algorithms to solve its orbit decay needed corrections on orbital elements.
Here is the Dr. Giorgini reply:

"As to uncertainties of the result, there is no general
answer, being a function of the target's orbit and
measurement dataset. Accuracy data is not distributed.

The models and solutions are relatively low accuracy
however, and you should expect km-level error even for
the "latest" set of elements, at least for low-Earth
orbit cases."

Why ISS orbit (thus position) seems so uncertain, considering celestial bodies position is known with high accuracy?
First of all, an astronomer would notice the sky rotation rate as seen from ISS is about 16 times faster than that observed on Earth, since our planet spins once a day; instead ISS Crew withnesses 16 sunrises and sunsets during 24 hours, speeding considerably faster.
Consequently, on ISS stars rise and set at high speed, and space station footages show for example how fast the Moon is setting or how short is the dusk duration.
That is due to the ISS orbital speed (about 28000 km/h) so in one second the position of a payload like AMS 02 changes by 8 km, and its pointing precision shall vary accordingly. Telescopes on ground have mounts which counterbalance Earth rotation, but AMS 02 is firmly fixed on station Truss.

Secondarily, in astrometry the higher is the target speed (like during a fast asteroid Earth fly-by), the highest is the rate of the measures obtained for having a good approximation on position and velocity.
Using optical methods, from target single positions (at least three, 2 components on celestial sphere times 3 equal six numbers, see over) the orbital elements can be calculated; fast targets simply need more measures to reach a given accuracy.
On the contrary, having the orbital elements of a fast object close to Earth it is important to predict the effect of its velocity decay (due to our planet difference in mass distribution and the presence of the atmosphere remnant, actually acting as a motion resistence, or drag).
That prediction is called orbit propagation.

The orbital elements are six conic geometry, length and angular parameters (plus a seventh one, i.e. the time or epoch), and can be provided also in a form called Two-Line Elements (TLE), frequently used by state-of-the-art orbital tools like AGI Satellite Tool Kit (STK), which propagate orbits using the SGP 4 and SGP 8 (Earth satellites) or SDP 8 (deep space probes) algorithms.
The SGP algorithm limit is the air drag effect is linearly applied, but solar effect on atmosphere dimensions and density is actually unpredictable.
Have a look of the ISS orbit decay statistics from the CalSky website based on distributed TLEs; the station period is mainly depending on the atmospheric drag and the decay appears linear, in reality being highly non-linear like solar activity (so the station position errors could be very high from time to time):



The AMS 02 local position and resolution is described in this paper to be purchased, where ISS tracking is mentioned.
The ISS orbital positional accuracy is discussed in this other paper, see page 1 and page 2.
A discussion about satellites orbit determination methods and the involved timing errors can be found in Bill Gray's website; the late MIR station (as well as the current ISS) precision would be about 700 meters.

It is concluded that since numbers on NASA JSC space station navigation consoles appear changing once a second or less, the first impression in the audience is that involves a great accuracy; in reality, they simply propagate the USSPACECOM distributed ISS TLE and State Vectors (i.e. 3 components for the target position in space and 3 for its velocity at a given epoch, again six numbers in total), updated every 36 hours nominally, by the aid of the SGP above mentioned algorithms, and selected co-variance methods.
The point remains the poor knowledge of the atmospheric drag effect, as well as time formats limitation.

Accuracy is a well know concept for the observatories certified in astrometry, having instruments and tools capable to estimate objects position on celestial sphere better than half arcsecond (at the ISS typical 350-400 km minimum distance from Earth, the station position approximation shall be theoretically equal to 1 meter in the best conditions).

Dr. Roberto Battiston, Deputy Spokeperson of AMS 02 consortium involving 16 Counries and leaded by Samuel Ting, the Boston MIT professor winner of the Nobel Prize, replied to a Grange Obs. offer to link the present page into the project website; he pointed out the payload, along with star trackers can use modern GPS receivers aboard ISS which could potentially lead to a precision of 5 meters, see this paper.

Apparently, reading info on the web and contacting experts in the aerospace sector, the AMS 02 GPS receivers are only used for timing purposes.
And if the exercise is having the AMS 02 position at the given microsecond a gamma ray count was received, the interpolation (of which type?) between 1 Hz ISS state vectors (from JSC console) basically means having a fish every second, but using Station's accurate TLEs is like to be thaught fishing.
Who knows if those notes helped the AMS 02 Team also for ISS tracking purposes; if so, it would be honest to point that out.

Grange Obs. with its telescopes and astrometry methodology application can indipendently derive and validate the ISS TLEs using optical methods at every visible passage, and can calculate accurate ephemerides of station mounted payloads like AMS 02 once the GPS receivers should have failed or be unused for localization purpose.
ISS shall be kept operative since 2020, and plans to extend its duration throught 2027 exist; it is a fact the Station electrical and electronic devices programmed lifetime has been noted to suddenly reduce while exposed to ISS mechanical vibro-acoustic and radiation environment.

To conclude the technology research, Grange obs. has developed a numerical method to translate the NASA JSC State Vectors J2000 into the ISS local flight system (GTOD with updated terrestrial longitude system), which shall permit to know the NASA referenced ISS position and the real AMS 02 orientation in space.

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