While watching the Mars Pathfinder, I got interested in the details of the downlink calculations. So I worked up a link budget, finding the numbers I needed on the JPL web pages and by email from the Pathfinder telecom chief engineer.
According to JPL, the high gain antenna is a flat plate array with about 24dbic gain. The low gain antenna is a choked circular waveguide that points straight up. It has 8 dBic gain.
Both antennas are easily seen in some of the pictures. For example, the high gain antenna is in the right side of this picture (made before the camera was raised) and the low gain antenna shows up at the very edges (left and right) of this 360 degree pan (made after the camera was raised).
Back to Phil Karn's Home Page
|Transmit Power||Pt||+10||dBW||(10W) email from JPL|
high gain antenna: 24 dBic on beam
low gain antenna: 8dBic at zenith
|Operating Frequency||f||8420||MHz||X-band deep space allocation|
An article on rec.radio.amateur.space gives the actual frequencies as 8427.22222 and 8429.938271 MHz
|Wavelength||lambda||3.56||cm||lambda = c/f = 3e8 m/s / 8420MHz|
|Mars/Earth distance on July 4 1997||d||191e6||km||http://mars.sgi.com/mpf/mpftechtraj.html#SUMTAB|
|Path Loss on July 4||Pl||276.6||dB||path loss in dB = 22 + 20log10(d/lambda)|
|Antenna gain, 34m DSN antenna||Gr||68||dBi||http://deepspace1.jpl.nasa.gov/dsndocs/810-5/tc31/tc31.html#24Gain|
|Estimated Rx signal power, 34m DSN antenna, July 4||Pr||-144.6||dBm||Pt + Gt - Pl + Gr|
optimistic - no allowance for misc losses
|Estimated Rx signal power, 34m DSN antenna, July 4||Pr||-146||dBm||
JPL pre-landing estimate
|Actual Rx signal power, 34m DSN antenna DSS-15, July 4||Pr||-145.4||dBm||email from JPL|
|Rx system noise temp, 34m antenna||T||28||K||
(30 deg elevation)
|Rx noise power density||No||-214.13||dBJ||No = kT, k=1.38e-23 dBJ/K|
|Rx power/noise density||P/N0||38.73||dB-Hz||Pr - No|
|Required bit SNR||Eb/No||1.2||dB||
http://edms-www.jpl.nasa.gov/tda/progress_report/42-129/129I.pdf, page 1.|
MPF uses a rate 1/6 constraint length 15 Viterbi-decoded convolutional inner code concatenated with a (252,230) Reed-Solomon outer code, with 5-way interleaving between the two.
Modulation is binary phase shift keying (BPSK) with residual carrier; low data rates use a greater carrier power fraction, resulting in a higher Eb/No requirement (about 4 dB higher at 5bps).
|Data rate||r||4740||bps||ibid, page 4.|
|Required power density at 4740 bps||Pr/N0||38.1||dB-Hz||ibid, page 4. Note slight inconsistency with (r - Eb/No), which gives 37.96 dB-Hz|