# How to calculate path length using sentinal ! image?

i have calculated path length using this formula the result(path length: R = (H1000)./cosd(theta); theta is incidence angle in degrees*) looks not ok can anyone help me to solve this problem

• which Sentinel satellite are you referring to?
• Where is the formula from and what is H?
• What results did you get and why do you think it’s wrong?

Maybe you can give a few examples on this to help us understand.

Dear Sir

Thank you so much for your quick reply. I am working on the snow permittivity and snow density estimation using Sentinal 1 iW, slc mode.

for preprocessing I need to calculate the:

1- incidence angle

2- Amplitude

3- sigma

4- Backscotrring db

To calculate the (Fresnel transmission coefficients) is required so I need to apply the following formula:

a. Compute Power ratio: VV =10log10(Pr./Pt); Pr is square of the amplitude, Pt is transmitted power
b. Convert S0vv to dB = 10
log10(S0vv)
c. Compute path length: R = (H*1000)./cosd(theta); theta is incidence angle in degrees

H is satellite altitude = 693
for calculating a and b i got result whoever during the calculation of c) my results looks

Thank you for the additional information.

Did you try to compute c based on incidence angle in radians?

I have only computed in SNAP,.
could you please guide me how do it ?

radians - as a unit - instead of degrees SNAP uses this syntax in the Band Maths: `rad(incident_angle)`
(found under Functions)

thank you so much for you reply but by changing degree to radiance i cant get my final result. results are concerned avv is the attenuation constant. Can you imagine the radar backscatter to be greater than the incident energy, this is what my result is showing.
if there is some way please let me know i really need it to complete this work.

yes, please have a look here: S1 bit-depth

thank you so much for you reply but by changing degree to radiance i cant get my final result. results are concerned avv is the attenuation constant. Can you imagine the radar backscatter to be greater than the incident energy, this is what my result is showing.
if there is some way please let me know i really need it to complete this work.

I think the error comes from incidence angle the range for incidence angle is (36-42) degree, could u please give an idea and which method should i use to normalize the incidence angle in mountains area.

SNAP only allows to calculate the normalized radar cross section (Sigma0) and the terrain corrected Gamama0. The latter incorporates the local incidence angle, so maybe it is an alternative to your Fresnel transmission coefficients?