Inverse Square Law: Explaining Reduced Data Rate w/ Distance

Click For Summary

Discussion Overview

The discussion revolves around the implications of the inverse-square law in radio communications, particularly in relation to the decreasing data rates of signals received from the Voyager spacecraft as they move farther from Earth. Participants explore the relationship between signal strength, distance, and data transmission rates, considering both theoretical and practical aspects of the phenomenon.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants note that as the distance from a signal source increases, the signal strength decreases, which necessitates longer listening times to accurately determine transmitted bits.
  • There is a suggestion that the modulation of the carrier frequency may play a role in how longer listening times affect the ability to discern between binary signals.
  • One participant introduces the concept of statistical sampling, explaining that a signal below the noise floor requires longer integration times to improve the signal-to-noise ratio, which is compounded by the inverse-square law.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between distance, signal strength, and data rates, but there are varying levels of understanding regarding the mechanisms involved, particularly in relation to modulation and statistical effects.

Contextual Notes

Some assumptions regarding the nature of signal modulation and the specifics of integration time in relation to the inverse-square law remain unresolved.

Drakkith
Mentor
Messages
23,205
Reaction score
7,687
Quoting from Wiki, bolded section mine:

Because of the inverse-square law in radio communications, the digital data rates used in the downlinks from the Voyagers has been continually decreasing the farther that they get from the Earth. For example, the data rate used from Jupiter was about 115,000 bits per second. That was halved at the distance of Saturn, and it has gone down continually since then. Some measures were taken on the ground along the way to reduce the effects of the inverse-square law. In between 1982 and 1985, the diameters of the three main parabolic dish antennas of the Deep Space Network was increased from 240 feet to 270 feet, dramatically increasing their areas for gathering weak microwave signals.

What exactly is going on here with the data rate? Is it just the strength of the signal that is falling off as distance increases? If so, how does that reduce the data rates that can be used? If not, what limits the data rates?
 
Physics news on Phys.org
The farther away a source is, the weaker its signal, and the longer one needs to listen to determine whether a 1 or 0 is sent, so the slower the data rate has to be.
 
Vanadium 50 said:
The farther away a source is, the weaker its signal, and the longer one needs to listen to determine whether a 1 or 0 is sent, so the slower the data rate has to be.

Can you elaborate on this?
 
What part is unclear?
 
Vanadium 50 said:
What part is unclear?

How listening longer helps you tell if it's a one or a zero. Something to do with how the carrier frequency is modulated?
 
This is a statistical sampling effect. The further the signal is below the noise floor, the longer you must integrate to raise the signal or lower the noise. Integration time is a square law. To change the Signal to noise ratio by a factor of two takes four times longer. This is compounded by the range inverse square law.
 
Last edited:
Ok, I see what you're getting at.
 

Similar threads

Undergrad Evidence for Inverse Square Law at Extremely Large Distance?
  • · Replies 5 ·
Replies
5
Views
2K
High School Does the inverse square law equalize gravity at different spots?
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Sound pressure and inverse distance law
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Inverse Square Law for Black Holes
  • · Replies 32 ·
2
Replies
32
Views
3K
Undergrad Questions about the inverse square law
  • · Replies 3 ·
Replies
3
Views
2K
Intensity of Stars and the Inverse Square Law Problem
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad How does the inverse square law apply to a focused detector?
  • · Replies 18 ·
Replies
18
Views
8K
Undergrad Are there upper and lower limits to the inverse square law?
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad Inverse square law and ensembles of photons
  • · Replies 25 ·
Replies
25
Views
4K
Undergrad Inverse Square Law, Temperature Change, and Heat Source Temp
  • · Replies 2 ·
Replies
2
Views
5K