高手请进，为什么我的迭代算法出了问题？

[rmn]

Public Sub XYZ2BLH(ByVal x As Double, ByVal y As Double, ByVal z As Double, b As Double, l As Double, h As Double)
Dim A As Double, E2 As Double, N As Double
Const conZero = 0.000000001
Dim B0 As Double, preB0 As Double
A = conA80: E2 = conE2_80
l = Atn(y / x)
l = l * 180# / conPi + 180 'rad->d 若不加180为与真值互补的负值
'----------迭代算法求b，h
B0 = 0.1
'MsgBox B0

Open App.Path + "\debug.txt" For Append As #1
Write #1, B0
Do
preB0 = B0
N = A / Sqr(1 - E2 * Sin(B0) * Sin(B0))
h = z / Sin(B0) - N * (1 - E2)
B0 = Atn(z * (N + h) / Sqr((x * x + y * y) * (N * (1 - E2) + h)))
'Print B0
Write #1, B0

Loop While (Abs(B0 - preB0) > conZero)
Close #1
b = B0 '----
h = z / Sin(b) - N * (1 - E2)
b = b * 180 / conPi
'-----------
'MsgBox "BL:" + Str(b) + "|" + Str(l)
End Sub

问题：求得b为一固值大概为1.57弧度，即89.*度。
x，x，z变量可赋值例x＝-2390290.3314，y＝5389867.5787，z＝2424507.8342

万分感谢！！