<p>The high density of energetic charged particles causes problems for any instrument system transiting through the South Atlantic Anomaly. Here, for example, is a description from the <em>Hubble Space Telescope</em> <a href="https://www.stsci.edu/files/live/sites/www/files/home/hst/instrumentation/wfc3/documentation/instrument-science-reports-isrs/_documents/2009/WFC3-2009-40.pdf" rel="nofollow noreferrer">WFC3 Instrument Science Report 2009-40 (Barker et al. 2010, &quot;WFC3 IR SAA Passage Behavior&quot;)</a> of its effects on <em>HST</em>:</p>
<blockquote>
<p>The South Atlantic Anomaly (SAA) is the region where the Van Allen
belts are closest to the Earth and is distinguished by elevated levels
of high-energy particle radiation (protons and electrons) which
adversely affects the scientific instruments of low-earth orbit
satellites such as HST. Short and long term effects may include: high
cosmic ray (CR) hit rates, high dark and background levels, residual
glow and count rates, damage to the detector lattice, reduced charge
transfer efficiency, flipped memory bits, and Single Event Upsets that
can suspend the operations of HST instruments.</p>
</blockquote>
<p>Below is a figure from that report, showing three 159-second exposures taken by the infrared channel of WFC3 <em>with the shutter closed</em>. The left and right exposures were taken before and after transiting the SAA (the left image's brighter background is Earth light leaking into the camera). The middle image was taken deep inside the SAA. They note that the cosmic-ray incidence is about 50 times higher in the middle image.</p>
<p><a href="https://i.sstatic.net/bnqsK.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bnqsK.png" alt="enter image description here" /></a></p>
<p>It's standard to not do observations when passing through the South Atlantic Anomaly. From the <a href="https://hst-docs.stsci.edu/hsp/the-hubble-space-telescope-primer-for-cycle-29/hst-cycle-29-primer-orbital-constraints" rel="nofollow noreferrer"><em>HST</em> Cycle 29 Primer's &quot;Orbital Constraints&quot; section</a>:</p>
<blockquote>
<p>No astronomical or calibration observations are possible (except for
some specialized and infrequently executed WFC3 observations) during
spacecraft passage through the SAA because of high background induced
in the scientific instruments and FGSs.</p>
</blockquote>
<p>(While certain types of observations with the WFC3 instrument <em>may</em> be permitted, the document also notes that some instruments can't be used on an orbit that passes even through the edge of the SAA, so different instruments can evidently be affected more or less strongly.)</p>
<p>My guess (knowing very little about the details of how CHEOPS observes) is that &quot;no data were downlinked&quot; means either:</p>
<ol>
<li>The telescope kept taking data during the time it passed through the SAA, but the data were not downlinked because it was known they would be too noisy to use;</li>
<li>&quot;No data were downlinked&quot; is a compact (and maybe slightly awkward) way of saying, &quot;No data were downlinked because the telescope was not taking data during that time&quot;.</li>
</ol>
<p><strong>Edited to add:</strong> A quick look at the most recent <a href="https://www.cosmos.esa.int/documents/1416855/4841242/CHEOPS-UGE-PSO-MAN-001_i2.0-Observers+Manual.pdf/ae8c9725-51d6-97bd-6efe-0bb4c2d4f2c5?t=1604331831450" rel="nofollow noreferrer"><em>CHEOPS Observers Manual</em></a> reveals this discussion of the SAA</p>
<blockquote>
<p>... a target is typically observable whilst flying through the SAA,
however the data taken will show an increased number of glitches and
noise spikes due to the impact of high energy particles. Data taken
during IOC showed that images acquired during SAA passage are not
amenable to photometric reduction due to the huge number of cosmic ray
hits. Therefore, the images taken during SAA passage are not
downlinked to ground in order to save bandwidth.</p>
</blockquote>
<p>(So it's #1 of the two possibilities I suggested.)</p>

CC BY-SA 4.0
2021-01-27T19:29:05.337Z
It seems that we are lucky to have a cozy shell of LEO sufficiently above the atmosphere to not require constant propulsive boosts and at the same time sufficiently below the Van Allen belt that heavy shielding is not necessary for CCDs, and the SAA is there to remind us of this :-) Thanks for the great answer!
uhoh
CC BY-SA 4.0
2021-01-27T17:54:08.063Z
2021-01-27T18:04:56.117Z
2021-01-27T18:04:56.117Z
Peter Erwin
<p>Section 4.1.2. CHEOPS in <a href="https://arxiv.org/abs/2101.09260" rel="nofollow noreferrer">Six transiting planets and a chain of Laplace resonances in TOI-178</a> says</p>
<blockquote>
<p>Due to the low-Earth orbit of CHEOPS, the spacecraft-target line of sight was interrupted by Earth occultations and passages through the South Atlantic Anomaly (SAA), where no data were downlinked. This resulted in gaps in the photometry on CHEOPS orbit timescales (around 100 min). For our observations of TOI-178, this resulted in light curve efficiencies of 51%, 54%, 65%, and 86%. For all four visits, we used an exposure time of 60 s.</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/CHEOPS" rel="nofollow noreferrer">CHEOPS</a> is in a ~700 km dawn-dusk sun-synchronous 92.8° inclination orbit aligned with local sunset/sunrise (06:00, 18:00) and an image of the <a href="https://en.wikipedia.org/wiki/South_Atlantic_Anomaly" rel="nofollow noreferrer">South Atlantic Anomaly</a> at 560 km in the 1990's is shown below. It is also discussed in Space SE's</p>
<ul>
<li><a href="https://space.stackexchange.com/q/10/12102">What is the South Atlantic Anomaly, and what can I do to protect satellites from it?</a></li>
<li><a href="https://space.stackexchange.com/q/20421/12102">Why would Space Cube 1.0 have so many On-Orbit Upsets over South America?</a></li>
</ul>
<p>We can probably assume that the enhanced energetic charged particle flux due to Earth's lower magnetic field in that region of space has something to do with it, but what exactly?</p>
<p><strong>Questions:</strong></p>
<ol>
<li>Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry? Is this out of an abundance of caution at each investigator's discretion, or is it standard, or at least recommended to do so?</li>
<li>Is the problem enhanced noise in the focal plane due to charged particle tracks in the CCD itself, or the potential for digital disruptions like flipped bits in registers or solid state memories? In other words, is the problem mostly <em>analog</em> or <em>digital</em>?</li>
</ol>
<p><strong>note:</strong> CHEOPS observations are already constrained by needing to shield its cryogenic focal plane from radiative heating from both the Sun and the Earth in low Earth orbit:</p>
<ul>
<li><a href="https://space.stackexchange.com/q/40494/12102">For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS?</a></li>
<li><a href="https://space.stackexchange.com/q/40495/12102">Will CHEOPS use DSN or ESA ground stations, or both?</a></li>
<li><a href="https://space.stackexchange.com/q/40514/12102">How many flavors do dawn-dusk Sun-synchronous orbits come in?</a></li>
<li>Wikipedia's <a href="https://en.wikipedia.org/wiki/CHEOPS#CHEOPS_Instrument_System_(CIS)" rel="nofollow noreferrer">CHEOPS Instrument System (CIS)</a>:</li>
</ul>
<blockquote>
<p>he required photometric precision will be achieved using a single frame-transfer, back-illuminated CCD detector from Teledyne e2v with 1024 × 1024 pixels and a pixel pitch of 13 µm. The CCD is mounted in the focal plane of the telescope, and will be passively cooled to 233 K (−40 °C), with a thermal stability of 10 mK.</p>
</blockquote>
<p><a href="https://i.sstatic.net/DaPEq.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DaPEq.gif" alt="South Atlantic Anomaly at 560 km in the 1990's" /></a></p>
<p>South Atlantic Anomaly at 560 km in the 1990's from <a href="https://en.wikipedia.org/wiki/File:ROSAT_SAA.gif" rel="nofollow noreferrer">source</a></p>

CC BY-SA 4.0
2021-01-27T01:54:30.180Z
related: [Why does the exoplanetary system TOI-178 challenges current theories of planet formation?](https://astronomy.stackexchange.com/q/41049/7982)
uhoh
<observational-astronomy><space-telescope><radiation><charge-coupled-devices><cheops>
Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry?
uhoh
# Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry?

(Asked by: username_0 on 2021-01-26)

Section 4.1.2. CHEOPS in [Six transiting planets and a chain of Laplace resonances in TOI-178](https://arxiv.org/abs/2101.09260) says

> Due to the low-Earth orbit of CHEOPS, the spacecraft-target line of sight was interrupted by Earth occultations and passages through the South Atlantic Anomaly (SAA), where no data were downlinked. This resulted in gaps in the photometry on CHEOPS orbit timescales (around 100 min). For our observations of TOI-178, this resulted in light curve efficiencies of 51%, 54%, 65%, and 86%. For all four visits, we used an exposure time of 60 s.

[CHEOPS](https://en.wikipedia.org/wiki/CHEOPS) is in a ~700 km dawn-dusk sun-synchronous 92.8° inclination orbit aligned with local sunset/sunrise (06:00, 18:00) and an image of the [South Atlantic Anomaly](https://en.wikipedia.org/wiki/South_Atlantic_Anomaly) at 560 km in the 1990's is shown below. It is also discussed in Space SE's

* [What is the South Atlantic Anomaly, and what can I do to protect satellites from it?](https://space.stackexchange.com/q/10/12102)
* [Why would Space Cube 1.0 have so many On-Orbit Upsets over South America?](https://space.stackexchange.com/q/20421/12102)

We can probably assume that the enhanced energetic charged particle flux due to Earth's lower magnetic field in that region of space has something to do with it, but what exactly?

**Questions:**

1. Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry? Is this out of an abundance of caution at each investigator's discretion, or is it standard, or at least recommended to do so?
2. Is the problem enhanced noise in the focal plane due to charged particle tracks in the CCD itself, or the potential for digital disruptions like flipped bits in registers or solid state memories? In other words, is the problem mostly *analog* or *digital*?

**note:** CHEOPS observations are already constrained by needing to shield its cryogenic focal plane from radiative heating from both the Sun and the Earth in low Earth orbit:

* [For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS?](https://space.stackexchange.com/q/40494/12102)
* [Will CHEOPS use DSN or ESA ground stations, or both?](https://space.stackexchange.com/q/40495/12102)
* [How many flavors do dawn-dusk Sun-synchronous orbits come in?](https://space.stackexchange.com/q/40514/12102)
* Wikipedia's [CHEOPS Instrument System (CIS)](https://en.wikipedia.org/wiki/CHEOPS#CHEOPS_Instrument_System_(CIS)):

> he required photometric precision will be achieved using a single frame-transfer, back-illuminated CCD detector from Teledyne e2v with 1024 × 1024 pixels and a pixel pitch of 13 µm. The CCD is mounted in the focal plane of the telescope, and will be passively cooled to 233 K (−40 °C), with a thermal stability of 10 mK.

[![South Atlantic Anomaly at 560 km in the 1990's](https://i.sstatic.net/DaPEq.gif)](https://i.sstatic.net/DaPEq.gif)

South Atlantic Anomaly at 560 km in the 1990's from [source](https://en.wikipedia.org/wiki/File:ROSAT_SAA.gif)

**Tags:** observational-astronomy, space-telescope, radiation, charge-coupled-devices, cheops

> **Comments:**
> **username_0**: related: [Why does the exoplanetary system TOI-178 challenges current theories of planet formation?](https://astronomy.stackexchange.com/q/41049/7982)  

---

## 1 answers

### Answer by username_1 on 2021-01-27. Score: 4 | Accepted answer

The high density of energetic charged particles causes problems for any instrument system transiting through the South Atlantic Anomaly. Here, for example, is a description from the *Hubble Space Telescope* [WFC3 Instrument Science Report 2009-40 (Barker et al. 2010, "WFC3 IR SAA Passage Behavior")](https://www.stsci.edu/files/live/sites/www/files/home/hst/instrumentation/wfc3/documentation/instrument-science-reports-isrs/_documents/2009/WFC3-2009-40.pdf) of its effects on *HST*:

> The South Atlantic Anomaly (SAA) is the region where the Van Allen
> belts are closest to the Earth and is distinguished by elevated levels
> of high-energy particle radiation (protons and electrons) which
> adversely affects the scientific instruments of low-earth orbit
> satellites such as HST. Short and long term effects may include: high
> cosmic ray (CR) hit rates, high dark and background levels, residual
> glow and count rates, damage to the detector lattice, reduced charge
> transfer efficiency, flipped memory bits, and Single Event Upsets that
> can suspend the operations of HST instruments.

Below is a figure from that report, showing three 159-second exposures taken by the infrared channel of WFC3 *with the shutter closed*. The left and right exposures were taken before and after transiting the SAA (the left image's brighter background is Earth light leaking into the camera). The middle image was taken deep inside the SAA. They note that the cosmic-ray incidence is about 50 times higher in the middle image.

[![enter image description here](https://i.sstatic.net/bnqsK.png)](https://i.sstatic.net/bnqsK.png)

It's standard to not do observations when passing through the South Atlantic Anomaly. From the [*HST* Cycle 29 Primer's "Orbital Constraints" section](https://hst-docs.stsci.edu/hsp/the-hubble-space-telescope-primer-for-cycle-29/hst-cycle-29-primer-orbital-constraints):

> No astronomical or calibration observations are possible (except for
> some specialized and infrequently executed WFC3 observations) during
> spacecraft passage through the SAA because of high background induced
> in the scientific instruments and FGSs.

(While certain types of observations with the WFC3 instrument *may* be permitted, the document also notes that some instruments can't be used on an orbit that passes even through the edge of the SAA, so different instruments can evidently be affected more or less strongly.)

My guess (knowing very little about the details of how CHEOPS observes) is that "no data were downlinked" means either:

1. The telescope kept taking data during the time it passed through the SAA, but the data were not downlinked because it was known they would be too noisy to use;
2. "No data were downlinked" is a compact (and maybe slightly awkward) way of saying, "No data were downlinked because the telescope was not taking data during that time".

**Edited to add:** A quick look at the most recent [*CHEOPS Observers Manual*](https://www.cosmos.esa.int/documents/1416855/4841242/CHEOPS-UGE-PSO-MAN-001_i2.0-Observers+Manual.pdf/ae8c9725-51d6-97bd-6efe-0bb4c2d4f2c5?t=1604331831450) reveals this discussion of the SAA

> ... a target is typically observable whilst flying through the SAA,
> however the data taken will show an increased number of glitches and
> noise spikes due to the impact of high energy particles. Data taken
> during IOC showed that images acquired during SAA passage are not
> amenable to photometric reduction due to the huge number of cosmic ray
> hits. Therefore, the images taken during SAA passage are not
> downlinked to ground in order to save bandwidth.

(So it's #1 of the two possibilities I suggested.)

> **Comments:**
> **username_0**: It seems that we are lucky to have a cozy shell of LEO sufficiently above the atmosphere to not require constant propulsive boosts and at the same time sufficiently below the Van Allen belt that heavy shielding is not necessary for CCDs, and the SAA is there to remind us of this :-) Thanks for the great answer!  

---

