> >> > I am trying to find the field strength of a 1 Watt (30 dBm) signal with an
>> > isotropic antenna at a distance of 15 kilometers.
>> >
>> > I need to compare this to a GPS signal of -125 dBm.
>> >
>> > Anyone know how to find this?
>> Calculate the surface area of a sphere of 15 km radius
>> and divide your transmitting power onto it.
>>
>> To compare, the radiating power of a GPS satellite is about
>> 500 W, including antenna gain. The radius of its sphere is
>> about 20000 km.
>
> Where are you getting that 500W bullshit? This source says 45W.
>formatting link
says "It’s transmit power is 44.8 Watt at 1575.43 MHz and the antenna gain is 12 dBi."
Thus about 500W as an isotropic radiator as seen from inside the antenna beam.