4
$\begingroup$

I'm looking at a brochure for the Iris V2 Deep Space Transponder by JPL. At the end of the brochure there is some data on suggested antennas. For each antenna the bps at approximately 1 AU is shown for opposition, arrive, and conjunction.

What do opposition, arrive, and conjunction mean with respect to telecommunications?

$\endgroup$
3
$\begingroup$

Conjunction and opposition are terms used in positional astronomy, see the Wikipedia article Opposition (planets) for an illustration.

I think what we've got in the brochure you refer to is an example of poor presentation design. I believe the expression Mars ~1 AU (bps) in the 3-column heading of the tables you are referring to is supposed to be associated with the "arrive" column, i.e., the data rate is for "arrival at Mars when the distance to Earth is approximately 1 AU." This would be consistent with the other two columns where the data rates are higher at opposition (Mars less than 1 AU distant) and significantly lower at opposition where Mars is about 2.5 AU away. (The distances vary, as illustrated at How Far is Mars from Earth?)

I will have to write to Courtney Duncan to ensure my interpretation is correct. Once I get a response, I will update this answer.


UPDATE


Date: Wed, 02 Aug 2017 03:53:03 -0400
From: Fred Koschara
To: Courtney Duncan
Subject: Puzzling specs in Iris V2 CubeSat Deep Space Transponder data sheet

Hello Courtney --

I happened to look at the Iris V2 CubeSat Deep Space Transponder data sheet (https://deepspace.jpl.nasa.gov/files/dsn/Brochure_IrisV2_201507.pdf) and found the data rate tables on page 4 rather puzzling. After pondering the matter for a while, I came to the conclusion that the expression "Mars ~1 AU (bps)" in the 3-column heading of the tables on that page is supposed to be associated with the "Arrive" column, i.e., the data rate shown is for "arrival at Mars when the distance to Earth is approximately 1 AU." This would be consistent with the other two columns where the data rates are higher at opposition (Mars less than 1 AU distant) and significantly lower at opposition where Mars is about 2.5 AU away.

Is this interpretation correct?

Thank you for your time and information.

-- Fred Koschara, President
The L5 Development Group


Date: Thu, 3 Aug 2017 02:49:00 +0000
From: "Duncan, Courtney B (337G)"
To: Fred Koschara
Subject: Re: Puzzling specs in Iris V2 CubeSat Deep Space Transponder data sheet

Hi Fred,

Yes, that's correct. 1 A.U. is a nominal distance for a Mars arrival (or departure) from (to) earth which is a likely time for things to be happening with SmallSat payloads.

And at opposition, it's about half an AU (in round numbers, average is 0.524), and at conjunction 2.5 (2.524).

It just so happens that Mars is just leaving conjunction right now. We usually stand down missions for a couple of weeks in order to avoid communications issues that can occur due to lots of sun noise in this period.

That table is further confused by the fact that all data rates were rounded down to the next DSN supported data rate and so are, on average, root two pessimistic. DSN data rates go up and down from 1000 bits per second in factors of two.

Courtney

Courtney Duncan
Supervisor, Reprogrammable Signal Processing Group 337G
Flight Communications Systems Section
M/S 161-260
Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove
Pasadena, CA 91109

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.