*3.1. SPOT 6*/*7 Constellation*

The SPOT constellation is one of the few satellite sensor systems with the stereoscopic and tri-stereoscopic capabilities to generate DSMs with very high pixel resolution and coverage over large areas. Specifically, SPOT-6 and SPOT-7 are two twin satellites orbiting at an altitude of 694 km that were launched September 2012 and February 2014, respectively, and have a swath coverage of 60 km × 60 km at nadir. SPOT-6 satellite and SPOT-7 represent the service continuity of SPOT- 4 and SPOT-5 satellites (launched 1998 and 2002), and they provide a revisit on ground each 26 days with a total coverage of 6 million km2 per day.

As mention before, the SPOT data were acquired from the Station of Telemetry Reception in Mexico (Estación de Recepción México—ERMEX) through an inter-institutional agreement for delivering SPOT6 and 7 panchromatic (resolution 1.5 m) and multispectral (resolution 10 m) modes: blue: 0.450–0.520 μm, green: 0.530–0.590 μm, red: 0.625–0.695 μm and near infrared: 0.760–0.890 μm. Table 1 provides details of the images used here for stereoscopic processes and their acquisition dates.


**Table 1.** SPOT6 data used to generate the digital surface models (DSMs).

\* This image was used for the eruptive temporal sequence (see Section 3.2).

The SPOT stereoscopy capability consists of illuminating the same location from different points of view to create a stereoscopic view. The images are captured along the same orbit with customizable positions and orientations of the target (Figure 3).

**Figure 3.** (**a**) Stereoscopic and tri-stereo SPOT configuration capabilities. (**b**) Google Earth preview of the SPOT satellite configurations in stereo and tri-stereo modes. FW, forward acquisition; BW backward acquisition.

Here, we used the Level 1A data. All images were acquired with a cloud cover until 25% (according with the technical acquisition procedure), and these clouds were located in some areas over the volcano. Therefore, after the stereoscopy process, a mask was applied to remove the cloud cover because, although

the software attempts to triangulate distal data to compensate for areas without height information, the final height measurements could have been wrong.

SPOT6/7 works over a simple stereoscopy configuration, termed "agile view", which implies a customized position and orientation of the observed target. The sensor begins with forward acquisition (FW), switches to backward acquisition (BW) after 90 s and repeats this process once again and ends the stereopair acquisition after 180 s. Therefore, each line of the image is acquired twice with a 90 s delay. To acquire tri-stereo full coverage, an additional nadir acquisition is performed with 30◦ as the maximum incidence angle [32] (Figure 3).
