Author Topic: Mars Exploration Rovers Update  (Read 203013 times)

Offline Jim

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Re: Mars Exploration Rovers Update
« Reply #940 on: 07/14/2016 02:36 AM »

1.  During the landing operations phase on the martian surface, the Mars Science Laboratory is expecting to utilize the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to two spacecraft simultaneously.


2.  Unlike traditional MSPA in which the number of spacecraft that can be supported is limited by the number of available receivers, OMSPA makes use of a digital recorder at each station that is capable of capturing IF signals from every spacecraft in the antenna beam within the frequency bands of interest.

1.  Two is a stretch of the word multiple. More dual. But it is still limiting.

2.  So if the spacecraft aren't in the frequency bands of interest, it is useless.  Plus, if other things are going on, the recorded data may be useless since it isn't acted upon.


Offline Jim

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Re: Mars Exploration Rovers Update
« Reply #941 on: 07/14/2016 02:39 AM »

There are a few constraints. First, only a single uplink frequency can be transmitted.
Generally, this means that only one spacecraft at a time can operate in a two-way coherent
mode, while the remainder must be in a one-way (i.e., non-coherent) mode. Second, multiple independent
receivers are required at the Earth station. This sets a practical limit of two
spacecraft that can be served simultaneously. Third, ranging and two-way coherent Doppler
data can only be obtained from the single spacecraft operating in a two-way coherent mode.


And how often are those constraints not in effect.

Offline joek

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Re: Mars Exploration Rovers Update
« Reply #942 on: 07/14/2016 02:43 AM »
And how often are those constraints not in effect.

Often.  E.g., MSPA is used more often than not when receiving from Mars orbiters these days.

Offline Jim

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Re: Mars Exploration Rovers Update
« Reply #943 on: 07/14/2016 02:50 AM »
The DSN could support quite a few more rovers with fairly small changes, I think.  The DSN already has at least one dish pointed at Mars all the time.  The downlinks don't interfere with each other, and all of Mars is within the beam, so one dish can in theory receive all the data from all the rovers.

No, the MER rovers were purposely put on opposite sides of the planet (just like Viking) to avoid interference.  The dishes do not receive multiple downlinks.  They can only receive one spacecraft at a time

The basic premise is wrong.  DSN doesn't directly communicate with the rovers.  Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.
« Last Edit: 07/14/2016 02:54 AM by Jim »

Online LouScheffer

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Re: Mars Exploration Rovers Update
« Reply #944 on: 07/14/2016 02:53 AM »

1.  During the landing operations phase on the martian surface, the Mars Science Laboratory is expecting to utilize the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to two spacecraft simultaneously.

2.  Unlike traditional MSPA in which the number of spacecraft that can be supported is limited by the number of available receivers, OMSPA makes use of a digital recorder at each station that is capable of capturing IF signals from every spacecraft in the antenna beam within the frequency bands of interest.

1.  Two is a stretch of the word multiple. More dual. But it is still limiting.

2.  So if the spacecraft aren't in the frequency bands of interest, it is useless.  Plus, if other things are going on, the recorded data may be useless since it isn't acted upon.

Right now, they can do 2 downlinks in real time (limited by two receivers), and all downlinks within the beam via recording.  By adding more receivers they could do more missions real time, up to the number within the beam (that's why I said "could support more rovers with minor changes"). 

Bands of interest is not a limitation here.  X band encompasses all the rovers current or planned, and the dishes and low noise amplifiers support all of X band without tuning.

Offline Jim

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Re: Mars Exploration Rovers Update
« Reply #945 on: 07/14/2016 02:57 AM »

1.  Right now, they can do 2 downlinks in real time (limited by two receivers), and all downlinks within the beam via recording.  By adding more receivers they could do more missions real time, up to the number within the beam (that's why I said "could support more rovers with minor changes"). 

2.  Bands of interest is not a limitation here.  X band encompasses all the rovers current or planned, and the dishes and low noise amplifiers support all of X band without tuning.

1.  No, rovers need more than downlink, so the changes aren't minor.  And per your reference "But, sharing an antenna amongst many spacecraft by adding lots of receivers and associated telemetry processing chains could prove prohibitively expensive for the DSN."

2, OMSPA is for smallsat and not major missions because "OMSPA may require up to a couple of days for missions to access and recover their downlinked data,"  As I said, it is not acted upon when received.
« Last Edit: 07/14/2016 03:07 AM by Jim »

Online LouScheffer

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Re: Mars Exploration Rovers Update
« Reply #946 on: 07/14/2016 03:10 AM »
DSN doesn't directly communicate with the rovers.

Rovers talk both directly to Earth and through orbiters.  Uplink is normally straight from the DSN to the rover.  From Communications With Earth:
Quote
The data rate direct-to-Earth varies from about 12,000 bits per second to 3,500 bits per second (roughly a third as fast as a standard home modem). The data rate to the orbiters is a constant 128,000 bits per second (4 times faster than a home modem). An orbiter passes over the rover and is in the vicinity of the sky to communicate with the rovers for about eight minutes at a time, per sol. In that time, about 60 megabits of data (about 1/100 of a CD) can be transmitted to an orbiter. That same 60 megabits would take between 1.5 and 5 hours to transmit direct to Earth. The rovers can only transmit direct-to-Earth for at most three hours a day due to power and thermal limitations, even though Earth may be in view much longer.
Note that if using the satellite relays, even more rovers could be supported.   At 8 minutes per sol per rover, NASA could support dozens of rovers using only the relay satellites already in orbit around Mars.
Quote
Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.
See above, where the uplink was estimated to take about 1 hour/day/rover.  So a lot more rovers could be supported without any additional uplink capabilities.  And downlink from many more rovers could be supported now though the relays (without modification) or direct to Earth (with minor upgrades).  So there may be good arguments against many rovers, but communications is not one of them.

Offline joek

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Re: Mars Exploration Rovers Update
« Reply #947 on: 07/14/2016 03:57 AM »
The basic premise is wrong.  DSN doesn't directly communicate with the rovers.  Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.

There is much more down-link data (from Mars) than up-link (to Mars).  MSPA allows more down-link data from more surface or orbiting assets with fewer DSN assets.  Real-time or near-real-time interaction based on received data is the exception, not the norm.  That virtually all data is relayed through orbiters these days does not change matters.

The basic premise is correct and proven, even if it may not solve all perceived problems.

edit: For example, it is very typical to see one DSN dish receiving data from two Mars orbiters, while sending data to one of them.  Then they will switch transmission to the other orbiter.  They do that because: (a) all communications are generally scheduled well in advance; and (b) there is a lot more data coming down than there is going up.
« Last Edit: 07/14/2016 05:38 AM by joek »

Online LouScheffer

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Re: Mars Exploration Rovers Update
« Reply #948 on: 07/14/2016 12:38 PM »

There are a few constraints. First, only a single uplink frequency can be transmitted.
Generally, this means that only one spacecraft at a time can operate in a two-way coherent
mode, while the remainder must be in a one-way (i.e., non-coherent) mode. Second, multiple independent
receivers are required at the Earth station. This sets a practical limit of two
spacecraft that can be served simultaneously. Third, ranging and two-way coherent Doppler
data can only be obtained from the single spacecraft operating in a two-way coherent mode.


And how often are those constraints not in effect.
For rovers in particular, the constraints are almost never in effect.  For orbiters, coherent doppler and ranging are needed for orbit determination.  For stationary landers, doppler and ranging determine the solar system ephemeris and the geological properties of Mars. But since rovers move, there is almost no science value in coherent doppler or ranging.   So they can always work in incoherent mode, meaning they don't need the uplink.  And the practical limit of 2 is just that - it's how many receivers DSN has purchased.  There is no technical problem to acquiring more.  Cost is not a serious issue in this case, since the lower data rates from the rovers can use Software Receiver Processing for Deep Space Telemetry Applications and don't need the full (and much more expensive) hardware telemetry decoding.

Online LouScheffer

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Re: Mars Exploration Rovers Update
« Reply #949 on: 07/14/2016 12:55 PM »
The basic premise is wrong.  DSN doesn't directly communicate with the rovers.  Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.

There is much more down-link data (from Mars) than up-link (to Mars).  MSPA allows more down-link data from more surface or orbiting assets with fewer DSN assets. 
If more uplink is needed, both JPL (Noise Bursts and Intermodulation Products Caused by Multiple Carriers at X-Band ) and ESA (ESA’s Views on MSPA (Multiple Spacecraft Per Aperture) on the Uplink ) have studied the problem.  The main technical problem is trying to transmit multiple full-power carriers while the same dish is receiving.  (very high order IM products become important since the transmitter is about 10^20 times stronger than the received data).  You can back off the power somewhat, or (in the worst case) go to two dishes, with one doing all the transmitting, and the other all the receiving.  This will work for any combination of uplinks/downlinks.

Offline joek

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Re: Mars Exploration Rovers Update
« Reply #950 on: 07/14/2016 09:44 PM »
For rovers in particular, the constraints are almost never in effect.
...

True, but DSN rarely communicates directly with the rovers these days; relay through the orbiters is far more efficient for both DSN and the rovers.  IIRC in the last several months, there was only one such instance (29-Jun), which used DSS43 (70m Canberra) to talk directly to Curiosity; may have been related to a rover software update.*

...
You can back off the power somewhat, or (in the worst case) go to two dishes, with one doing all the transmitting, and the other all the receiving.  This will work for any combination of uplinks/downlinks.

That is how DSN often operates.  You will often see one antenna with two down and one up; or sometimes one antenna with two down and a different antenna with one up.


* But my memory is not what it use to be and my logs are a bit scattershot, so take with a grain of salt. In any case, in my experience, it is extremely rare to see DSN talking directly to a rover.
« Last Edit: 07/14/2016 09:51 PM by joek »

Offline Jim

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Re: Mars Exploration Rovers Update
« Reply #951 on: 07/14/2016 11:11 PM »
The basic premise is wrong.  DSN doesn't directly communicate with the rovers.  Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.

There is much more down-link data (from Mars) than up-link (to Mars).  MSPA allows more down-link data from more surface or orbiting assets with fewer DSN assets.  Real-time or near-real-time interaction based on received data is the exception, not the norm.  That virtually all data is relayed through orbiters these days does not change matters.

The basic premise is correct and proven, even if it may not solve all perceived problems.

edit: For example, it is very typical to see one DSN dish receiving data from two Mars orbiters, while sending data to one of them.  Then they will switch transmission to the other orbiter.  They do that because: (a) all communications are generally scheduled well in advance; and (b) there is a lot more data coming down than there is going up.

No, the premise is not correct.  If the DSN only communicates via orbiters, then more DSN capability doesn't help.  More orbiters are needed

Offline joek

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Re: Mars Exploration Rovers Update
« Reply #952 on: 07/14/2016 11:32 PM »
No, the premise is not correct.  If the DSN only communicates via orbiters, then more DSN capability doesn't help.  More orbiters are needed

Stop that.  This discussion started with whether DSN MSPA is useful.  It has proven useful.  It is used on a daily basis.  It maximizes Earth-Mars rover bandwidth and minimizes DSN resources.  The premise is correct; you are incorrect in your conclusion.  Or you appear to be constructing a straw-man in order to prove an irrelevant conclusion.

Certainly more orbiter Earth-Mars bandwidth would be desirable, but is the current orbiter capability a limitation to the number of rovers which could be supported?  You have presented no data to support such a claim.  So do the numbers and present, then we can have a substantive discussion.  Otherwise you are just hand-waving.
« Last Edit: 07/14/2016 11:39 PM by joek »

Online LouScheffer

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Re: Mars Exploration Rovers Update
« Reply #953 on: 07/15/2016 02:13 AM »
If the DSN only communicates via orbiters, then more DSN capability doesn't help.  More orbiters are needed
No, a single orbiter spends 8 minutes per day communicating with each rover, plus somewhat less time than that to send the data back to Earth.  So a single orbiter (and there are several in Mars orbit with relay capability) could handle any practical number of rovers.  The reason they put relays on each new orbiter is not for capacity, it's more that the old orbiters could die at any time.

So there are at least two possible ways to support lots of rovers via DSN.  You could use the existing orbiters, with capacity to spare, or you could make minor enhancements to DSN to enable direct-to-Earth, both uplink and downlink, for all of them.  The original argument was that DSN capacity was one argument (among many) for not wanting a fleet or rovers.  What we have shown here is that DSN capacity is not the limiting factor.   This does not affect the other arguments against many rovers, which are more persuasive.


Offline mtakala24

Re: Mars Exploration Rovers Update
« Reply #954 on: 10/17/2016 09:51 PM »
Anyone know if Opportunity is actually going to take pictures of the sky/horizon during ESA Schiaparelli landing?

Re: Mars Exploration Rovers Update
« Reply #955 on: 10/17/2016 10:03 PM »

Offline Star One

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Re: Mars Exploration Rovers Update
« Reply #956 on: 02/01/2017 05:58 PM »
OUTSTANDING OPPORTUNITY ROVER MAKING ‘AMAZING NEW DISCOVERIES’ 13 YEARS AFTER MARS TOUCHDOWN – SCIENTIST TELLS UT

http://www.universetoday.com/133012/outstanding-opportunity-rover-making-amazing-new-discoveries-13-years-after-mars-touchdown/

Offline Dalhousie

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Re: Mars Exploration Rovers Update
« Reply #957 on: 02/01/2017 09:58 PM »
Update on recurring slope lineae (RSL) based on yet to be released abstracts at the forth-coming LPSCs,  Includes  possible RSL imaged by the Opportunity team in Victoria crater http://www.leonarddavid.com/mars-mystery-new-research-on-recurring-slope-lineae/
"There is nobody who is a bigger fan of sending robots to Mars than me... But I believe firmly that the best, the most comprehensive, the most successful exploration will be done by humans" Steve Squyres

Offline FutureSpaceTourist

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Re: Mars Exploration Rovers Update
« Reply #958 on: 02/05/2017 10:30 PM »
Quote
"#Opportunitys latest #panorama from Mars- Click to zoom!" #space #cosmos #nasa #universe

https://twitter.com/step_holt/status/828315708429844480

Edit: Hmm, ok appears to be from last June? Higher resolution attached from http://marsmobile.jpl.nasa.gov/multimedia/images/?ImageID=7861
« Last Edit: 02/05/2017 10:37 PM by FutureSpaceTourist »

Offline FutureSpaceTourist

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Re: Mars Exploration Rovers Update
« Reply #959 on: 03/16/2017 06:12 AM »
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View from @NASAJPL's Opportunity rover on March 12, 2017, hand-colored to a Mars palette. (Yes, that's another planet!)

https://twitter.com/jpmajor/status/842046296458436612

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