# Current status of the use of GPS mutli-antenna time differentials for satellite attitude determination?

I am certain that I read a year or three ago about an unmanned satellite that was going to be testing a method of attitude determination that relied upon two (or more) separate antennas on one spacecraft, and used the difference in arrival times of GPS signals at the two antennas to calculate an approximate attitude.

I am not sure if it used a standard "relative GPS" implementation (see also this answer and this mathematical discussion), or if it was more advanced because all signals were available simultaneously to one correlator cluster, whereas relative GPS relies on the exchange of digital information using a link between two systems.

@DavidHammen points out in comments that this has been done before. This is not new. It seems examples include the Soyuz spacecraft, the ISS, and even much earlier experiments.

Question: However I'm looking for a recent unmanned satellite and if possible an understanding if it uses relative GPS or a more advanced technique running the output of all correlators from all antennas through a single fitting procedure.

• Highly related, if not a duplicate: Do any spacecraft use GNSS for attitude determination? The two answers to that question are incomplete; this idea was tried over 20 years ago. – David Hammen Oct 8 '18 at 23:09
• @DavidHammen Thanks for finding that. I see some of my up votes there which means I have read those in the past. But I don't believe that Soyuz is the instance I'm thinking of (I'm still trying to verify that it's actually GPS that gives it 0.5 degrees attitude as Wikipedia claims), and certainly the ISS isn't what I'm remembering. I'm not interested in when it was first used I'm interested in current status, and the example I remember is an unmanned satellite. – uhoh Oct 9 '18 at 0:37
• @DavidHammen I'll look more closely into this today to try to clear this up. In the mean time I've asked Does the Soyuz spacecraft really try to achieve attitude accuracy of 0.5° from GLONASS and GPS signals? and edited the question to make sure it's not a duplicate. – uhoh Oct 9 '18 at 0:55
• This is put to test repeatedly for the same reason laser propulsion is. Both are sexy, both appear to be high tech, both represent large sunken investments, and both have backers who are very good at selling the ideas to politicians. That both are losers is irrelevant. – David Hammen Oct 9 '18 at 2:20
• Another way to look at it: Would SpaceX use this? I doubt it. Star trackers aren't useful when a rocket is still in the atmosphere, but when a rocket is in the atmosphere the atmosphere tells the spacecraft part of its attitude. The unknown part is subject to gyro drift, but how much do gyros drift in the five minutes or so before a star tracker can see the stars? – David Hammen Oct 9 '18 at 2:28

To give a recent reference:

According to this paper quoted below, a set of 4 GPS receivers connected to indepenent antennas can be used to determine attitude and:

"To the authors’ knowledge, GAP also represents the first practical demonstration of dual-frequency-based attitude determination in space.[...] attitude solutions with 0.1–0.3° precision can be achieved in the GAP data processing. "

(By *"data processing" he means it's not done onboard)

With a few caveats tough, such as:

"precise orbit determination of CASSIOPE using GPS observations can achieve decimeter-level accuracy during continued operations but suffers from onboard and mission restrictions that limit the typical data availability to less than 50% of each day and induce regular long-duration gaps of 4–10 h."

Yikes. Admittedly, this was a mission to test a low cost GPS receivers.

The CASSIOPE minisatellite was launched on Sept. 29, 2013, so we are talking flight-proven performance (alledgedly)

To give a spec on the receivers:

"NovAtel OEM4-G2L receiver (Fig. 2) is a miniature GPS receiver offering L1 C/A and L2 P(Y) tracking of up to 12 satellites. It exhibits a small form factor (60 mm×100 mm) and power consumption (2.5 W) which are well below those of established space receivers [...] Measurements are nominally provided at a 1-Hz data rate by receivers GPS-0 to GPS-3 for orbit and attitude determination"

"The GAP-A experiment on board CASSIOPE is specifically designed to study GPS-based attitude determination using observations from three concurrently operated receivers. GAP-A was configured to optionally perform coarse real-time attitude determination on board the spacecraft (Kim and Langley 2007), but while this feature operated flawlessly in ground testing, GAP failed to respond to data requests on orbit. Nevertheless, raw GAP-A data could still be downloaded."

Furthermore:

"GPS attitude determination in space was frst demonstrated in the early 1990s as part of the RADCAL (Radar Calibration) mission and was later applied on a variety of other missions such as APEX, REX-II, UoSat-12, TopSat, and Flying Laptop (see, e.g., Georgi 2017; Hauschild et al. 2019, and references therein). On the International Space Station (ISS), a four-antenna GPS receiver system coupled with an inertial measurement unit is used to provide attitude information on a routine basis. Compared to the use of GPS receivers for position and timing, GPS attitude determination of satellites has, however, remained a niche application due to the higher system complexity and inherent limitations in the achievable performance, which cannot compete with well-established star sensors"

I've tried checking for GPS units built by "serious, big companies". I recalled that this 1997 paper by Thales claimed they had developed a GPS with the performance summarized in a table below, taken from a brochure from 2012. These kind of brochure is subject to change without previous notice, and I coulnd't find an updated version.

Apparently, a unit of the GPS Tensor by Thaes flew onboard the SAC-C satellite as an experiment, and this satellite also had star trackers.

In conclusion

Due to the complexity and limitations of star sensors, I was expecting GPS-based attitude sensors to become the go-to option for low accuracy attitude determination systems. It seems that even today, these sensors are not something I would recommend to anyone.

1 Montenbruck, O., Hauschild, A., Langley, R. B., & Siemes, C. (2019). CASSIOPE orbit and attitude determination using commercial off-the-shelf GPS receivers. GPS Solutions, 23(4). doi:10.1007/s10291-019-0907-2

[2] Marradi, L., & Fossati, D. (1997, February). The GPS Tensor™ Receiver Development. In Spacecraft Guidance, Navigation and Control Systems (Vol. 381, p. 311).

• Wow! Thank you for the very thorough and well-sourced r̶e̶v̶i̶e̶w̶ ̶a̶r̶t̶i̶c̶l̶e answer! – uhoh Oct 26 '19 at 5:40
• @uhoh: You can call it "article review". If I had written the article, I'd be very happy someone on any SO site posted an answer that is basically a review of it. Also, science and engineering are about standing on the shoulders of giants, I'd me a much happier person if I could find an article solving my problem every time. – Mefitico Oct 26 '19 at 17:18