what is gravitational field of the earth?
Newtonian Gravity, objects of mass M and n:
F = force of attraction on each
F = G m M/r^2
if M is earth mass, call it Me
and r is earth radius, call it Re
then
G Me/Re^2 = 9.81 m/s^2 approximately
so
F = 9.81 m
F/m we call acceleration of gravity on earth, g = about 9.81
If you go further from earth then you have to do the real thing
F/m = G Me/r^2
where G = 6.67 *10^-11
and Me = 5.97*10^24 kg
or simply use the value at the surface and correct for bigger radius
F/m = 9.81 (Re^2/r^2)