What date ?
SPOT4 will observe each of its 42 sites every 5th day. The orbit change will be done January 29th and on one of the first days of February, perhaps even the first, the Take5 data acquisition will start. Several users have already asked about the date of acquisitions to schedule ground measurements simultaneously to the satellite overpass.To know on which day of the cycle your site will be observed, download this kmz file and open it with google-earth. Click on the footprint of the site you are interested in. You may read the value of the day number in the 5 days cycle, as well as the value of the mirror step.Knowing that the day 1 of the first Take5 cycle will be January the 31st, you can easily compute the overpass date :
- If you are interested in a site observed on the first day of the cycle, the observations will therefore take place in January, the 31st, in February, the 5, 10, 15, 20, 25, March, 2, 7 ...
- If the site is observed on the 3rd day of the cycle, it will be, in February, 2, 7, 12, 17, 22, 27, March 4, March 9 ...
What time ?
The overpass time is a little more complicated to calculate since the inclination of SPOT4'orbit is no longer maintained to save propellants and increase the satellite lifetime. This causes a drift of overpass times, towards earlier overpasses.
- In February, the satellite passes the equator at 9:25 UTC,
- In June, the satellite will pass the equator at 9:10 UTC.
- In both cases, at 45° North latitude, it will be 12 minutes earlier.
This overpass time is valid if your site is below the satellite track.
- If your site is viewed from the West (mirror step greater than 46), it will be observed a few minutes later. For instance, add 15 minutes if the site is observed at an angle of 27 degrees (mirror step close to 91).
- If your site is viewed from the East (mirror step lower than 46), it will be observed a few minutes earlier. For instance, subtract 15 minutes at an angle of 25 degrees (mirror step close to 1)
We have tried, whenever possible to program site acquisitions from the West, to have higher sun elevations.In fact, if you really need to know the exact overpass time, the easiest way is to ask the overpass time of the first images acquired on your site, and apply a linear drift of 15 minutes in 4 months.