PSSRa Algorithm Specification |

### PSSRa

The Pigment Specific Simple Ratio (chlorophyll index) algorithm was developed by Blackburn (1998). It investigates the potential of a range of spectral approaches for quantifying pigments at the scale of the whole plant canopy
When applying existing narrow-band pigment indices, the PSSR algorithms have the strongest and most linear relationships with canopy concentration
per unit area of Chl a (Chlorophyll a), Chl b (Chlorophyll b) and Cars (carotenoids).

The PSSRa results from the following equation:

PSSRa = (IR_factor * near_IR) / (red_factor * red)

For Sentinel-2 the formula is:
B7 / B4, where (Central wavelength/Bandwidth): B7 = 783 nm (15 nm) and B4 = 665 nm (30 nm)

### PSSRa-Flags

Also the processor computes an additional flags band called 'pssra_flags' with the following bit coding:

Bit Position | Description
--- | ---
Bit 0 | The computed value for PSSRa is NAN or is Infinite
Bit 1 | The computed value for PSSRa is less than -1 (minus one)
Bit 2 | The computed value for PSSRa is greater than 1 (one)