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.
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 coherentmode, while the remainder must be in a one-way (i.e., non-coherent) mode. Second, multiple independentreceivers are required at the Earth station. This sets a practical limit of twospacecraft that can be served simultaneously. Third, ranging and two-way coherent Dopplerdata can only be obtained from the single spacecraft operating in a two-way coherent mode.
And how often are those constraints not in effect.
Quote from: Jim on 07/14/2016 01:48 amQuote from: LouScheffer on 07/14/2016 01:31 amThe 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
Quote from: LouScheffer on 07/14/2016 01:31 amThe 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 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.
Quote from: LouScheffer on 07/14/2016 02:17 am1. 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.
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.
DSN doesn't directly communicate with the rovers.
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.
Rovers require more interaction than orbiters and so the upgrades like MSPA don't increase the capability to do more rovers.
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.
Quote from: hop on 07/14/2016 02:10 amThere 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 coherentmode, while the remainder must be in a one-way (i.e., non-coherent) mode. Second, multiple independentreceivers are required at the Earth station. This sets a practical limit of twospacecraft that can be served simultaneously. Third, ranging and two-way coherent Dopplerdata can only be obtained from the single spacecraft operating in a two-way coherent mode.And how often are those constraints not in effect.
Quote from: Jim on 07/14/2016 02:50 amThe 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.
For rovers in particular, the constraints are almost never in effect....
...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.
Quote from: Jim on 07/14/2016 02:50 amThe 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
If the DSN only communicates via orbiters, then more DSN capability doesn't help. More orbiters are needed
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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!)