Homework Help: Signal to Noise Ratio Between Spacecraft Transmitter and Receiver Antenna

1. Jan 28, 2012

grrgh

1. The problem statement, all variables and given/known data
Determine the SNR for the spacecraft that uses a transmitter power of
16 W at a frequency of 2.4 GHz. The transmitter and receiver antenna
gain are 28 dBi and 60 dBi, respectively. The distance from the space-
craft to ground is 2 ϫ 1010 m, the effective noise temperature of antenna
plus receiver is 14 degrees Kelvin, and a bit rate of 120 kbps. Assume
the bandwidth of the system to be half of the bit rate, 60 kHz.

2. Relevant equations
The equation below calculates received power in free space (no loss due to non free space).
Pr = Pt * (λ/(4πd))^2 * (Gt * Gr)

Pt is transmitted power.
λ is wavelength.
d is distance.
Gt is transmitter gain.

Also, SNR = Pr/Pn [Pn = noise power]

3. The attempt at a solution
I calculated Pr plugging in the following...
Pt = 16 W
λ = 0.125 m
d = 2E10 m
Gt = 28 dBi = 630.96
Gr = 60 dBi = 1,000,000

giving me Pr = 2.497 * 10^(-15) W = -146.03 dBW

The main problem is I don't know how to calculate Pn, or how/why bandwidth, bit rate, and temp come into play here. Textbook not very helpful and struggled deciphering meaning on wikipedia.