# THE SPOT SATELLITES

# SPOT on its orbit

Previous Page

## The SPOT orbit is circular, sun-synchronous and phased.

### A circular orbit

- altitude: 822 km
- inclination: 98 deg. (i.e near-polar orbit)
- revolutions per day: 14 + 5/26
- period: 101 minutes
- westward drift between successive ground tracks: 2823 km
- cycle duration: 26 days
- orbital revolutions per cycle: 369

If images of different locations are to be suitable for comparison, they must be acquired from the same altitude. Thus, the orbit must be circular, or have a constant altitude relative to the Earth's surface.

### A sun-synchronous orbit

Because the valid comparison of images of a given location acquired on different dates depends on the similarity of the conditions of illumination, the orbital plane must also form a constant angle relative to the Sun direction. This is achieved by ensuring that the satellite overflies any given point at the same local time, which in turn requires that the orbit be sun-synchronous (descending node at 10:30 a.m).

HRV imaging instruments |

### A phased orbit

It is essential to achieve an observation cycle giving repeat access to any particular point at regular intervals. This requirement is met by choosing a near-polar orbit. Each SPOT satellite passes over any given point on the Earth's surface every 26 days. In 26 days, it will make a whole number of revolutions following one complete track cycle. The same pattern is then repeated over and over.

To ensure that the satellite covers every point on the earth's surface during the cycle, HRV (High Resolution Visible) imaging instruments offer a combined field-of-view that is wider than the greatest distance between two adjacent tracks.

This is achieved using the 'twin-vertical' viewing configuration in which the two instruments acquire a combined swath centred on the ground track.

Ground tracks over a 24-hour period