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Congratulations to NASA JPL and the Perseverance Team and everyone else who participated and contributed in some way!

Watching the live feed of the EDL seven minutes of terror I heard there was a short "plasma blackout" of the radio link to the MRO in Mars orbit which was relaying data to Earth via DSN. (I also thought I heard a deceleration rate of 10 gees!)

The plasma is sufficiently dense and conductive that it attenuates the high frequency used between the two spacecraft.

There was a second link from Perseverance direct to Earth, referred to as a "heartbeat" or "tones" which was primarily a carrier whose Doppler shift could be accurately tracked to deceleration profile, along with some minimal health and condition data.

This VHF tone is around 400 MHz and was tracked by the Green Bank Observatory earlier. I don't know if it was Green Bank who received it during EDL also or not, but Mars was certainly high in the sky at the time (elev. > 65°), so it seems likely.

Questions:

  1. Roughly what frequency was used in the link(s) between Perseverance and MRO?
  2. Roughly how long was the plasma blackout for both the MRO link and for the VHF tones-to-earth signal? Did the blackout at the lower frequency last longer1?

1since plasma frequency varies with electron density and temperature.

As an aside, the authoritative-sounding and accepted answer to Was the time of Schiaparelli's landing chosen specifically so the Giant Meter Wave Radiotelescope could listen? is "No, because TGO was there to receive and relay it", and in fact this data did turn out to be extremely valuable due to the unfortunate events.

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  • $\begingroup$ related: When NASA spacecraft arrive at Mars, is Mars always always above the horizon in the US? and a related but somewhat moot Mars plasma blackout question: Will Red Dragon experience a “Communications Blackout” during SRP? $\endgroup$
    – uhoh
    Commented Feb 18, 2021 at 22:12
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    $\begingroup$ It wasn't too long; less than a minute, I think. And yes, I also heard the 10g callout. $\endgroup$
    – Phiteros
    Commented Feb 18, 2021 at 23:12
  • $\begingroup$ @Phiteros ya I think it was the LRO link that we heard mentioned, but I'm also interested in the VHF to Greenbank signal which I think could have been somewhat longer for reasons discussed in the question. $\endgroup$
    – uhoh
    Commented Feb 18, 2021 at 23:26
  • $\begingroup$ I suppose the down vote is because the question is not easy to answer? $\endgroup$
    – uhoh
    Commented Feb 21, 2021 at 0:51
  • $\begingroup$ I thought 'tones' were X-band (8 400–8 450 MHz)? $\endgroup$ Commented Apr 19, 2021 at 17:17

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There are some misunderstandings here (similar to this question) that make the question(s) a little malformed.

Roughly what frequency was used in the link(s) between Perseverance and MRO?

From Wagner et al. "Mars Reconnaissance Orbiter maneuver plan for Mars 2020 entry, descent, and landing support and beyond" (2019) (emphasis added):

Relay telecommunication support in the UHF frequency range is provided by the Electra Proximity Link Payload.

The document for Electra (especially in the MRO context) is Edwards et al. "The Electra Proximity Link Payload for Mars relay telecommunications and navigation" (2003). It gives a frequency range of 390-450 MHz in 56 kHz steps or "channels". Mars 2020 makes use of an Electra derivative; Electra-Lite, which "maintains the core functionality, performance and reprogrammability of the standard Electra" (it's lite-r lighter on power, mass).

There was a second link from Perseverance direct to Earth, referred to as a "heartbeat" or "tones" [...] This VHF tone is around [...]

Presumably this is a simple mistake as ~400 MHz is UHF in both ITU & IEEE definitions. More importantly; however, is that the direct-to-Earth (DTE) tones/heartbeats/semaphores/etc. are not UHF, they are X-band.

Roughly how long was the plasma blackout for both the MRO link and for the VHF tones-to-earth signal? Did the blackout at the lower frequency last longer?

The UHF link (MRO, MAVEN) is subject to plasma blackout, the X-band link (DTE "tones") is not. From DESCANSO Design and Performance Summary Series Article 14 Mars Science Laboratory Telecommunications System Design, Makovsky et al. (2009) (by extension applicable to Mars 2020 & Perseverance, emphasis added):

The period of maximum heating is expected to create a plasma envelope sufficient to produce a link dropout at UHF relay frequencies (but not at X-band), making DTE via a plasma-penetrating frequency highly desirable during this period.

Additionally, the DESCANSO article gives this table of expected link dropouts:

MSL Descanso Drop outs

So the blackout period was expected to be about 25-100 seconds (for MSL, it could have been less for M2020 due to it's lower entry speed). Looking at the webcast the blackout appears to last ~50 seconds based on operator callouts.

The lead communicators of the webcast also occasionally mix up the two links (X-band & UHF), though you can note how callouts are still made during the UHF blackout (because of data from the X-band DTE "tones").

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