Heliocentric TDRS?

"Henry Spencer" wrote in message
...
In article yXMIh.213$Eg4.29@trnddc03,
Rocky Top wrote:
I always thought we should launch a chaser relay vehicle for the
Pioneer/Voyager missions. Pretty soon they will get so far away that
communications will be impossible. But if we launched a chaser
communications relay vehicle that could detect and communicate
with the Pioneer/Voyager spacecraft and with earth...

Unfortunately, if the chaser is at half the probe's distance, the signal
it hears from the probe is only a factor of four stronger than what Earth
hears. And the Earth stations have unlimited weight and power available
for big antennas and elaborate receivers, plus they have human maintenance
when things break. Consequently, good Earth stations are much better
receivers than what you could reasonably put on the chaser -- much more
than a factor of four better. So the chaser doesn't make sense.

Relay satellites only make sense when they are quite close to the probes
they are relaying for. But that won't help you much with sheer distance;
its big advantage is letting the probes (e.g., landers) use smaller
transmitters and antennas.

Couldn't the ISS be used to assemble a very-much-larger antenna
than could be assembled in an automated deployable capsule?

In a similar thought, the ISS is most useful to assemble structures
unencumbered by gravity that would be difficult to assemble on
earth or so fragile as to be unlaunchable. A large (inflatable?)
shirt-sleeve inhabitable enviroment assembly bay should be one
of the modules. Then the ISS would really be useful.

/thinking out loud

-rt

In article 4PsKh.3458$I56.3074@trnddc06,
Rocky Top wrote:
...the Earth stations have unlimited weight and power available
for big antennas and elaborate receivers, plus they have human maintenance
when things break. Consequently, good Earth stations are much better
receivers than what you could reasonably put on the chaser -- much more
than a factor of four better. So the chaser doesn't make sense.

Couldn't the ISS be used to assemble a very-much-larger antenna
than could be assembled in an automated deployable capsule?

Yes, in theory (although impressive things have been done with deployable
antennas). ISS currently isn't well set up for it, but it's possible in
principle. But you've still got mass, power, and maintenance issues to
contend with, so it's probably still not competitive with spending the
same money improving the Earth stations.

...A large (inflatable?)
shirt-sleeve inhabitable enviroment assembly bay should be one
of the modules. Then the ISS would really be useful.

Had ISS been built as a space operations base, a pressurized hangar would
certainly have been useful. But it wasn't, and there are no plans to
pursue that application. In fact, the official US position now is that
ISS is a dead end and will get a minimum of further investment.

As to *why* ISS wasn't built as an operations base, that is unfortunately
fairly simple: because the customers weren't there. ISS was clearly
going to be very expensive, so justifying it as an operations base
required that there be a lot of commmitted, funded missions that would
benefit. And there weren't.

There's obviously a bit of chicken-and-egg problem here, since people
aren't going to commit to missions that need an assembly base if there's
no assembly base. But still, at the very least you'd need to have
ambitious plans that are likely to materialize soon... and while NASA had
no shortage of ambitious plans, ones that had a reasonable chance of being
funded were in short supply. In particular, after the SEI debacle, trying
to sell ISS on the grounds that it would be used to assemble a manned Mars
expedition would have been political suicide.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article . com,
TVDad Jim wrote:
I was thinking there may be some advantages in putting relay devices
in heliocentric orbit at, say, the orbit of Mars, to pick up telemetry
on those occasions when the Sun is between Earth and the exploring
spacecraft.

Trouble is, the relay spacecraft would face requirements for low mass, low
power consumption, and extremely high reliability without maintenance,
which would make their performance greatly inferior to that of Earth
ground stations. For the same sort of budget, you get more data back
sooner if you stick with the Earth stations and just accept a short
interruption in communications once a year.

Relay spacecraft are useful only if they can be placed very close to the
client spacecraft, so the clients can talk to them at high data rates with
low-power radios. For example, the MERs on Mars would be crippled if they
couldn't relay data via Mars orbiters.

Also, wouldn't transmissions be easier to intercept from listening
spacecraft far away from Earth's ionosphere?

Most of the frequencies used aren't significantly affected by Earth's
ionosphere. (Although as frequencies go ever higher, there is starting
to be some potential for interference from bad weather.)

The one form of space communications that *is* badly hurt by Earth's
atmosphere is laser communication, which currently is mostly experimental
but is edging toward practical use. The problem there is not the
ionosphere, but clouds. Even with multiple well-separated ground
stations, there's a significant chance of being clouded out, and putting
the optical "ground stations" in space (with radio links to the ground)
has real advantages. However, such relays wouldn't be farther out than
a high Earth orbit, for fast and easy links to Earth.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

On Wed, 7 Mar 2007, Henry Spencer wrote:

Relay spacecraft are useful only if they can be placed very close to the
client spacecraft, so the clients can talk to them at high data rates with
low-power radios. For example, the MERs on Mars would be crippled if they
couldn't relay data via Mars orbiters.

I was surprised to learn that the Mars Phoenix lander will have no
high-gain antenna, and will be entirely dependent on orbiting relays. If
Mars Reconnaissance Orbiter and Mars Odyssey both die or lose the ability to
relay, Mars Phoenix will be unable to talk to Earth.

(Seems to me that ESA's Mars Express has one of these relay gadgets aboard,
but it wasn't mentioned in the briefing I saw.)

--
Bill Higgins | "The victors write the histories,
Fermilab | and also the DNA sequences."
| --Barry Gehm

In article ,
Bill Higgins wrote:
I was surprised to learn that the Mars Phoenix lander will have no
high-gain antenna, and will be entirely dependent on orbiting relays. If
Mars Reconnaissance Orbiter and Mars Odyssey both die or lose the ability to
relay, Mars Phoenix will be unable to talk to Earth.

It's the logical extension of past trends, I guess. Older landers were
capable of going it alone. The MERs can talk direct but would be badly
hampered by having to operate that way. The next step is a lander that
needs relay for its basic mission.

(Seems to me that ESA's Mars Express has one of these relay gadgets aboard,
but it wasn't mentioned in the briefing I saw.)

Yes, it does, and the MERs have used it experimentally. But (a) NASA
doesn't like to mention foreign help on general principles, (b) I think it
would take special negotiations to use the ME relay package, and most
important, (c) ME is in a fairly high elliptical orbit, which gives many
fewer usable communications passes for a lander that can only communicate
at short range.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

(Seems to me that ESA's Mars Express has one of these relay gadgets aboard,
but it wasn't mentioned in the briefing I saw.)

Yes, it does, and the MERs have used it experimentally. But (a) NASA
doesn't like to mention foreign help on general principles, (b) I think it
would take special negotiations to use the ME relay package, and most
important, (c) ME is in a fairly high elliptical orbit, which gives many
fewer usable communications passes for a lander that can only communicate
at short range.

Would it make sense, then, to build a *lunar* TDRS network before the
Orion/LSAM missions? Since the computer horsepower on the LSAM is
going to greatly reduce the need for immediate ground telemetry, I
can't imagine a need for high-gain equipment for direct transmissions
to Earth from the lunar surface.

Secondly: are stable orbits possible for a lunar TDRS network, given
the gravitational problems with MASCONs?

In article .com,
TVDad Jim wrote:
Would it make sense, then, to build a *lunar* TDRS network before the
Orion/LSAM missions? Since the computer horsepower on the LSAM is
going to greatly reduce the need for immediate ground telemetry, I
can't imagine a need for high-gain equipment for direct transmissions
to Earth from the lunar surface.

Manned expeditions are going to have ample resources to talk directly;
where a relay system really pays off is if you're trying to talk to quite
small probes, e.g. small rovers. (Mind you, a manned expedition *would*
need relay if it was going to the farside.)

Secondly: are stable orbits possible for a lunar TDRS network, given
the gravitational problems with MASCONs?

People have tried to find stable lunar orbits, but the results are all a
little uncertain, because our gravity maps of the lunar farside are still
a bit iffy. More important, though, is that it shouldn't be too hard to
find orbits which are stable *enough* that relatively small corrections
will suffice to maintain them. It helps considerably that a comsat
network would want a somewhat higher altitude than a lot of the past
lunar orbiters.
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spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |