How much POWER do the GPS Satellites output on the 1575mhz L1 frequency?

In the frequency allocation filing the L1 C/A power is listed as 25.6 Watts.  The Antenna gain is listed at 13 dBi.  Thus, based on the frequency allocation  filing, the power would be about 500 Watts (27 dBW).

Now, the free space path loss from 21000 km is about 182 dB.  Take the 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) and you get  -155 dBW. The end of life spec is -160 dBW, which leaves a 5 dB margin.

And if you really get into it, you'll discover ALL of the following represent the same approximate signal strength for GPS on the face of the earth (m stands for milliwatts and m2 stands for meters squared):

-160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163 dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz

Once you figure out why they're all the same, you're well on your way to understanding power, power density, and power flux density as it relates to GPS. For those that wish to quibble, I am assuming an even distribution of
power density over a 2 MHz C/A bandwidth.

Other Interesting information about the latest (Block IIR)  GPS satellites:

Earth Weight (module in orbit): 2370 pounds
Orbit altitude: 10,988 nautical miles
Power source: solar panels generating 1136 watts
Launch vehicle: Delta II
Dimensions: 5 feet wide, 6.33 feet in diameter, 6.25 feet high
(38.025 feet wide including wing span)
Design life: 10 years