Date: 24 Aug 89 03:33:46 GMT
From: email@example.com (Phil Karn)
Subject: Voyager link calculations
Here's another link calculation for Voyager 2 at Neptune. I got some more
precise figures, and the result looks pretty reasonable. My references to
"Yuen" are to the book "Deep Space Telecommunications Systems Engineering",
edited by Joseph H. Yuen and published by Plenum (ISBN 0-306-41489-9).
8415 MHz transmitter power +11.04 dBW
(12.7 W - low power mode ref Yuen):
3.7 m spacecraft antenna gain
(ref Yuen p 4): +48.13 dB
EIRP: +59.17 dBW
Path loss, 4.416e9 km, 8415 MHz: 303.85 dB
= 20 log10(4*pi*d*f/c)
Receive signal flux: -244.68 dBW
70m receive antenna gain: +73.8 dB
= 10 log(4*pi*A/lambda^2) - N
lambda = C/8415e6 = 3.56cm
(assuming N = 2 dB illum loss)
Receive signal power: -170.88 dBW = 8.16e-18 W
= 8.16 attowatts
= 8.16 nano nano watts
Received energy per bit (Eb): -214.22 dBJ = 3.78e-22 J
at 21.6 kb/s (43.34 dB-b/s) = .000378 attojoules
= 0.378 piconanojoules
Receive noise density (N0): -218.60 dBW-Hz
= 10 log10(kT)
T = 10 Kelvin (ref USENET)
Eb/N0 ratio: 4.38 dB
This is quite consistent with the performance of the concatenated rate 1/2
Reed Solomon + rate 1/2 convolutional coder, which according to Yuen (p 255)
has a very steep "wall" or threshold just below 3 dB.
However, this figure does not take into account miscellaneous losses
(connectors, feedlines, etc) on board Voyager or at the receiving site, nor
does it allow for atmospheric absorption or transmitter degradation below
the nominal 12.7 watt figure.
According to an interview carried tonight on NASA select, the arraying of
the VLA to Goldstone effectively doubles the aperture of the receiver, so
this should give another 3 dB of margin to compensate for these factors.
As you can see, attention to detail is all-important here!
Date: 12 Oct 89 21:13:05 GMT
From: firstname.lastname@example.org (David Smith)
Subject: Re: Geyser-like plume discovered on Neptune's moon Triton (Forwarded)
In article <2500@ibmpa.UUCP> szabonj@ibmpa.UUCP (Nick Szabo) writes:
> I can't help but wonder about the proposed scientific merits
> of this [Voyager] extended mission. ...
>There are not enough resources to
>support both these new missions and the "extended missions" of the Voyagers,
>Pioneers, etc. Both Voyagers, two of the Pioneers, and ICE all require the
>use of our largest and rarest (70m) antennae. Building a new 70m dish
>would cost and estimated $2 billion: the price of a new space shuttle.
>If we support the extended missions, we could lose valuable data from the
>new planetary missions.
Hold on, the Voyagers and Pioneers do not require the use of our largest
and rarest antennas. They did in order to send back the large amount of
data from planetary encounters at high data rates, and for that, observatory
radio telescopes were linked in with the Deep Space Network. But for the
extended mission, they won't be tracked continuously, and they can be
serviced by fewer, smaller antennas. AW&ST says (Oct 9, p.117):
Voyager 1 and 2 will be tracked every day by NASA's Deep Space
Network (DSN), but the spacecraft will be reconfigured for a
baseline lower data rate of 160 bits/sec -- compared to 21,600
bits/sec at Neptune -- to enable the spacecraft to use the DSN's
smaller, 34-meter antenna systems.
David R. Smith, HP Labs