Mars Pathfinder X-band Downlink Budget

July 21, 1997

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).

Phil Karn

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ItemsymbolvalueUnitsSource
Transmit PowerPt+10dBW (10W) email from JPL
Antenna gainGt24dBicemail from JPL:
high gain antenna: 24 dBic on beam
low gain antenna: 8dBic at zenith
Transmit EIRP34dBWPt+Gt
Operating Frequencyf8420MHzX-band deep space allocation
An article on rec.radio.amateur.space gives the actual frequencies as 8427.22222 and 8429.938271 MHz
Wavelengthlambda3.56cmlambda = c/f = 3e8 m/s / 8420MHz
Mars/Earth distance on July 4 1997d191e6km http://mars.sgi.com/mpf/mpftechtraj.html#SUMTAB
Path Loss on July 4Pl276.6dBpath loss in dB = 22 + 20log10(d/lambda)
Antenna gain, 34m DSN antennaGr68dBi http://deepspace1.jpl.nasa.gov/dsndocs/810-5/tc31/tc31.html#24Gain
Estimated Rx signal power, 34m DSN antenna, July 4Pr-144.6dBmPt + Gt - Pl + Gr
optimistic - no allowance for misc losses
Estimated Rx signal power, 34m DSN antenna, July 4Pr-146dBm http://mpfwww.jpl.nasa.gov/mpf/faqs_surfaceops.html#strength
JPL pre-landing estimate
Actual Rx signal power, 34m DSN antenna DSS-15, July 4Pr-145.4dBm email from JPL
Rx system noise temp, 34m antennaT28K http://deepspace1.jpl.nasa.gov/dsndocs/810-5/tc31/tc31.html#24Noise
(30 deg elevation)
Rx noise power densityNo-214.13 dBJNo = kT, k=1.38e-23 dBJ/K
Rx power/noise densityP/N038.73dB-HzPr - No
Required bit SNREb/No1.2dB 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 rater4740bps ibid, page 4.
Required power density at 4740 bpsPr/N038.1dB-Hzibid, page 4. Note slight inconsistency with (r - Eb/No), which gives 37.96 dB-Hz