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All,
This topic was created to discuss the communications and navigation infrastructure that we think needs to be in place either in orbit around Mars or on the surface of Mars to support the first crewed Mars missions in the coming decade. Although MRO was quite useful for increasing our understanding of Mars and sending back truckloads of data, that probe is rapidly aging and in need of replacement.
We can debate which communications technologies we think should go (Lasercom vs Ka-band vs X-band vs UHF, etc), which navigational aids we think should go (Ka-band radar vs LIDAR vs radio beacon vs IR beacon, etc), which science experiments to send (advanced versions of HiRISE, MARCI, SHARAD, CTX, etc), and which propulsion technologies (all-chemical, SEP, Combined Chemical SEP / CC-SEP). Keep in mind that Falcon 9 Heavy is now operational, but its TMI limit is 16t. Atlas V is also available, but it's TMI capability is considerably less. There's lots of stuff that can be done with either rocket using modern tech, but let's try to avoid unnecessary deployment complications such as on-orbit refueling or other highly experimental propulsion technologies.
Should we attempt to create a NASA Christmas ornament version of MRO as the replacement (1m telescope with multi-gigapixel imager, supercomputer with petabytes of memory, high power microwave radar, gigabit class Lasercom with Ka-band backup, high power SEP propulsion, etc), a slightly less advanced COTS Christmas ornament packed with all the next-gen COTS technology (ornaments with roughly 3 times the capability of MRO vs 10 times for the NASA ornaments packed with bleeding edge technology), or should we attempt to create constellations of much smaller and simpler single-purpose satellites that each perform specific functions (discrete telescopes for mapping, laser relay terminals for data relay, GPS, etc)?
The hardware price tags are in line with the sophistication of the tech and overall complexity of the design. The COTS stuff, while not as advanced as bleeding edge NASA stuff, would still be a significant improvement over what we have now and at a fraction of the price of the bleeding edge technology. The purpose-built satellites would be the most affordable option by far since they're the simplest to design, build, and deliver. The simple sats also carry distribute the risk of failure, whereas the loss of a NASA or COTS ornament would be devastating to the program.
A NASA ornament would be built by Lockheed-Martin, Boeing, Northrop-Grumman, or Orbital ATK with JPL and NASA input - these favored contractors have the standing armies of engineers and scientists to develop and integrate advanced tech
A COTS ornament could be built by JPL and NASA - JPL has a number of bright and talented engineers and scientists, but lacks some of the depth and breadth of resources that the major aerospace defense contractors have as a function of servicing military contracts
Simple sats could be built by academia and various mini and micro sat providers - universities and boutique / speciality providers can design, build, and integrate single-purpose satellites these days because the things are not loaded up with everything but the kitchen sink, as the NASA / COTS specials would be; this is another opportunity to help academia and small business flourish
Each F9H flight could potentially deliver a pair of the NASA or COTS ornament style MRO replacements or an entire constellation of the small single-purpose simple satellites. The ornaments have 1t to 2t dry mass, double or triple that with propellants. The simple satellites would likely have wet masses of 250kg or less. It should go without saying that whatever you dream up has to fit in a Falcon or Atlas deployment rack or payload fairing. There won't be any special vehicle design alterations.
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My main concern is that there should be live HD video feed from the surface of Mars direct to Earth.
On the previous thread I noted this from a discussion on stackexchange:
"However, the Mars Reconnaissance Orbiter also states that this link tops out at 4 megabits per second (worst case is 500 kbps), which leads us to the nail in the coffin:
Best-case data rates from Mars are on the order of 4 Mbps, whereas compressed-with-H.264 720p video needs 12 Mbps and 1080p needs closer to 22 Mbps. This rate from Mars is only achievable for a few months at closest approach. You would need to increase the link speed by a factor of at least 3 to get live, HD video even then."
https://space.stackexchange.com/questio … 20-mission
I asked: what is the problem with running say 4-6 MRO-style set ups in parallel to send back video data? After a period of delay (let's say 30 secs) back on Earth, the whole package could be re-synched to produce perfect video. That, to me, sounded well within our capabilities. The mass involved will also be well within the capability of a Space X 500 tonne cargo mission.
kbd I think it was confirmed that linking transceivers in that manner was feasible.
So that's the way I would get live HD video feed.
I mentioned the need for a large satellite dish than used in the lunar landings. But if we can run coms in series then we can use multiple dishes. The MRO dish is 3 metres wide. We're told it masses at 200 kgs or thereabouts.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Louis,
To simultaneously pump out multiple high data rate digital signals on different closely spaced channels or frequencies through the same satcom dish, something like Orthogonal Frequency Division Multiplexing (OFDM) is required. MRO had a diplexer onboard, which is a passive form of multiplexing, so Electra uses a form of FDM, but I don't think it's OFDM. I need to read more about the capabilities of the Electra radios are to understand exactly how it works. Although OFDM has existed since 1966 when the concept came out of Bell Labs, I'm not sure how many NASA mission have actually implemented it (maybe none since insufficient electrical and compute power existed to push the capabilities of the radios, until recently).
Electra radios are the first purpose-built SDR's created for deep space telecommunications and used extensively by JPL for Mars missions:
Picture of an Electra radio from Wikipedia (22cm in length and 13cm in height, weight approximately 4.9kg, according to NASA):
NASA / JPL has subsequently created miniaturized versions of the radio that weigh 2.1kg (Frontier Radio; S / X / Ka) and 0.4kg (FR Lite; UHF to C with option to add Ka band for missions that require it) for small sats.
Frontier Radio Lite: A Single-Board Software-Defined Radio for Demanding Small Satellite Missions
Edit #2: Frontier Radio was intended to handle 10mbps to 150mbps uplink speeds, according to the doc; so that means a single radio could theoretically handle a 1080p feed, but it can only transmit or receive over 2 channels. FR Lite only has 1 channel, but appears capable of 10mbps to 100mbps uplink and downlink (meaning same high data rate both ways) and uses very little power (a few watts). Some sort of output amp or signal relay may be required to boost the output wattage for Mars missions, or maybe the new equipment back at Earth is just so sensitive and selective that a few watts of power are enough. Either way, Electra was a cut above what came before it and Frontier Radio appears to be a cut above Electra.
Edit #3: Argh! I misread. The doc actually lists samples per second, not bits per second, so there's a conversion that must be done. If I have the data to convert it, I'll post the achievable bit rate.
Browse Wikipedia and look at the TDRS-M satellites that NASA uses to relay spacecraft telemetry data to ground stations from GEO to understand the packaging problem associated with stuffing multiple large dishes into the payload fairing. The pair of dishes that TDRS uses are larger than the satellite itself, although each dish provides S, Ku, and Ka band telemetry relay for satellites and spacecraft. Each TDRS-M weighs 3,454kg at launch. That's the wet mass I expect from a COTS ornament. A NASA ornament could be double that.
Irrespective of what solution you choose (an array of discrete satcom systems or SDR and OFDM with a single dish), eventually you'd run into thermal management issues that require an active cooling solution, which means even more power. A NASA ornament would have the excess electrical power to overcome such problems. A COTS ornament may have enough power for a half dozen Ka-band radios and amplifiers. The simple sats don't have sufficient space or power.
Let's use your example of a single 1080p transmission at the 500kbps data transfer rate attainable by MRO when Mars is farthest from the Earth.
500kbps = .5mbps
22mbps / .5mbps = 44
That means you need 44 separate channels to transmit at 22mbps. We're presuming use of SDR and OFDM here, as I think having 44 of the 3m satcom dishes would never fit in the launch shroud of a F9H or Atlas. Electra and Frontier Radio are both rad-hard low-power TRL-9 technology. I believe FR Lite is still awaiting flight qualification by flying it, but otherwise ready to go. OFDM would be a TRL-5 to TRL-6 technology that would have to be properly tested and demonstrated, but that could be done fairly quickly. The 4G/LTE service for cell phones is an application of OFDM, for example. I expect 2 years to take OFDM to TRL-7. The Gen-2 Lasercom stuff ranges in TRL from 4 to 8, but all of it based upon components taken directly from the telecom industry and TRL has progressed rapidly as a result.
Finally, I already gave the exact mass for the entire HGA and LGA subassemblies in your thread. The 3m Ka-band antenna (HGA) itself weighs 19.6kg. The mass for the gimbals and drive motors are 45kg. The waveguides and coax are 8.3kg. That's 75.9kg by my math and I left off some minor hardware that totals 3kg or less. The Ka-band amp requires 81W of input power and the gimbal drive motors require 14W.
I would agree about using multiple satcom dishes on Mars, but if they only have to communicate to an overhead satellite that's a few hundred (Edit: kilometers, not meters) away, then the dishes can be tiny. What I want to know is, how do you intend to get a 3m diameter antenna through the cargo hatch of BFS and to the surface without damage, and then wherever you intend to set it up at?
Last edited by kbd512 (2018-11-17 14:18:43)
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Communications technology is far from my area of expertise, but from what you said in your posts here and the other thread (starting roughly from this post) it seems to me that lasers are the obvious choice for any new interplanetary communications system.
Again, this is not something I know much about but it seems to me that laser comms are simply superior to any non-unidirectional system.
It seems to me that you'd want a constellation of three relay satellites offset by 120 degrees in an equatorial orbit. I imagine that the orbit will be as low as possible while still being high enough to get coverage to high and low latitudes.
One thing I want to point out: Electromagnetic spectrum is a limited resource because multiple sources will interfere with each other and render each other useless. NASA is a very small portion of the demand for spectrum on Earth because of commercial, military, and other government users. This is, in effect, why there are named portions of the spectrum that NASA can use.
In fact it is my understanding that the new X band opened up after TV transmission went from analog to digital. For whatever reason digital is a much more efficient way to transmit TV signals, so it opened up new wavelengths for use.
These limits don't apply on Mars. NASA can use the entire microwave-radio spectrum if it wants because there's nothing operating on or near Mars right now that it hasn't sent.
There's definitely value in using existing, off-the-shelf technology but there's off-the shelf technology for other bands too.
-Josh
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Josh,
My personal feeling about the use of lasers is that they work best when there's nothing in front of them, as is typically the case in space. If there's nothing to block the photons, and in the space between Mars and Earth there's not much matter to do that, then the in-space relays should probably be lasers. The data transfer rates for modern laser telecommunications equipment is measured in tens of gigabits per second and goes up from there. More importantly for a Mars applications, the transfers are largely completely error free. Error free transmissions quickly become critically important over the vast distances of interplanetary space. Error free terabit class data transfers appear to be on the near horizon for optical telecommunications here on Earth, thanks to some techniques lifted from radio telecommunications.
It's certainly possible to achieve much higher data transfer rates with radio transmissions using various TDM and FDM techniques, even gigabit class transfer speeds as cellular networks demonstrate, but typically at the expensive of additional power and an increased error rate. That's not terribly problematic when the distances involved are relatively short because the receiver can simply request a repeat of the missed data frames or packets. Unfortunately, one-way transmission times between Earth and Mars can exceed 30 minutes when Mars is farthest away from Earth. That's where high data transfer rate radio technology runs into practical limitations associated with increased error rates. What good is the data you received if you're frequently missing chunks of it and must wait another hour to get what you missed?
At the Earth end, only water vapor in the atmosphere has a substantial effect on laser transmissions, although other atmospheric effects can and do have deleterious effects. Even so, NASA tests of their Lasercom system have yielded some pretty spectacular results.
At the Mars end, microwave frequency transmitters are quite useful because the dust in the atmosphere doesn't substantially affect the transmission to an orbiting relay satellite in the same way that it would if lasers were used. There's certainly more of the frequency spectrum to play with on a planet with no FCC around, but the government is also a stickler for adherence to standards.
It’s not a question of laser over microwave when it comes to communications. Certain technologies are more appropriate for specific uses than others. If you need a 40mbps transfer rate over a few hundred kilometers or so, but your radio can only support 20mbps, then perhaps using a second radio or FDM is appropriate, even if that means greater mass and power consumption. That’s an example of a problem where throwing more horsepower at it is an acceptable solution to making a brick fly. If your application requires a 20gbps transfer rate, then you start running out of horses or places to put more horses.
I'm completely unopposed to using existing technologies if the determination is made that it will adequately service the requirement. I'm also of the opinion that the quantity of data being generated by increasingly sophisticated sensors is beginning to overwhelm traditional radio communications technology. Asking scientists how much data they want is a bit like asking fighter pilots how much speed they want. The question invariably produces a one-word response- MORE!
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It would be incredibly frustrating if the FCC felt the need to block people from using certain frequencies in certain ways, but that's life I suppose.
Anyway I don't know a ton about this stuff. If you were creating a network for Martian communications with Earth on a minimum budget, how would you do it?
-Josh
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Add in to that a need for over the horizon comms on the surface, so exploration expeditions, or resource gathering activities can communicate with the base(s) and each with one another. To my mind this means satellite relays, which might be the same ones as provide earth communications.
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Josh,
If I was forced to do this project using existing commercial technology, then I'd skip the science mission entirely and send a half dozen GEOStar-3 satellites equipped with X-band and Ka-band equipment. The satellites would use all-electric propulsion. It's already an option and not an economic penalty in this case since there's no revenue benefit from earlier on-station times achievable using heavier hybrid chemical and electric propulsion. Two unique features of Northrop-Grumman / Orbital ATK's GEOStar-3 is the on-orbit satellite servicing dock to transfer propellants and the ability to support tandem launch. Boeing's BSS-702HP bus can support up to 18kW of power compared to GEOStar-3's 8kW, but the 702HP is substantially heavier, doesn't support tandem launch, and doesn't support on-orbit servicing capability.
The ViaSat-2 program (702HP-based) provided 300gbps total throughput using four Ka-band transceivers. Those are 6,400kg satellites that cost $600M each. It should go without saying that the antennas used would have to be modified / optimized for communication over greater distances.
The ViaSat-3 (also 702HP-based and Ka-band) program will essentially provide greater total throughput than all other communication satellites combined, but not for long as Lasercom advancements continue to increase total throughput. Each Viasat-3 satellite is expected to achieve terabit class throughput using multiple Ka-band transceivers. No word on the cost or electrical power requirements, just that each satellite is a 6400kg class unit like the ViaSat-2's.
The NASA / USAF Lasercom program would use 2U micro sats, each only slightly larger than a shoebox, that would provide 200gbps each. If the point isn't clear, the 2U satellites wouldn't cost a couple million each. SpaceX's STARLink also uses Lasercom for satellite-to-satellite communications. The entire STARLink network is expected to have a 20tbps+ total throughput when the constellation is fully deployed.
Anyway, there's little doubt that the future of in-space communications will be optical. If this program could be deferred for about 5 years, then I think multi-terabit class transfer speeds using a combination of Ka-band (for the downlink) and lasers (for the link back to Earth) are achievable. There also appears to be a long term trend towards larger / heavier satellites with more electrical power and redundancy. If modular communications packages, solar power packages, and refillable propellants that can be replaced on-orbit become standardized, then there will be an incentive to use larger and more capable satellites with much greater useful service lives. At some point, it'll also make sense to incorporate some sort of composite armor that protects the chassis from radiation and space debris.
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Optical under dust storm condition is not going to work and that means you would still want radio up-links even if these satellites to send signals back to earth. So what we will need is the hybrid of both worlds for mars applications.
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Here is what Nasa is working in technology development missions
Laser Communications Relay Demonstration, or LCRD
https://www.nasa.gov/sites/default/file … ct2018.pdf
https://www.nasa.gov/feature/goddard/20 … nd-testing
Proven technology at this point as built for Lunar Laser Communications Demonstration (LLCD), which flew aboard a moon-orbiting spacecraft in 2013. Overall, compared to traditional communications systems on spacecraft today, LLCD used half the mass, 25 percent less power, and still transmitted six times as much data per second.
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I suppose there are questions to be asked about reliability and durability, but a 10-kg, 200 gbps, ~$2 million lasercom satellite seems like a slam dunk compared to a 6400 kg, $600 million satellite, even if its throughput is in the terabit range.
We seem to be converging on Ka/X-band for surface-to-orbit communications and laser systems for orbit-to-Earth communications. This seems like a solid design choice to me, with each being well-suited to its respective application.
This does create something of a mismatch, though: The Earth-to-Mars orbit leg might have a higher throughput than the Mars Orbit-to-Mars Surface leg.
-Josh
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That is due to orbiting speed of each planet and that they are locked into synchronous orbits for each.
That is why you now need to solve for how many are required and to make sure that a relay link from each can be made over several possible choice of satellites for if the next closest does fail to relay correctly..
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Josh,
I look at the initial problem as priority-based time-sharing of available bandwidth on the planetary side and ensuring that transmissions between Mars and Earth are error-free. Virtually all crewed and robotic missions have substantial data storage requirements so that data can be held in queue until an opportune time to transmit presents itself. If there are any errors on the orbit-to-orbit transfer side, then wide pipes are necessary to retransmit mangled content with current content as transmission priority and throughput capability permits. The latency between the surface of Mars and Mars orbit is less than a second. The latency between Mars and Earth can be more than half an hour. The relay satellite must store a bit over an hour of content, at whatever a realistic or maximum throughput rate is, to ensure that content not received by the Earth end can be retransmitted to Earth from the laser relay. If that high bandwidth orbit-to-orbit link is not available, then content quickly overflows the relay satellite's buffer cache or content may be irretrievably lost. This is about the best we can do with what we have or will have in the immediate future, so we'll have to learn to make do.
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I see that someone working for NASA already thought about the communications problem. An aerostat could rise above the dust storms to provide a clear-enough line-of-sight to an orbital laser relay satellite.
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Sounds like NASA - "let's complicate things" is their secret motto.
I see that someone working for NASA already thought about the communications problem. An aerostat could rise above the dust storms to provide a clear-enough line-of-sight to an orbital laser relay satellite.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Louis,
Seeing as how the company you've pinned your Mars colonization dreams to would never have done anything more than blow up a few sounding rockets without a lot of help and good faith from the good people who work for NASA, maybe you should give the agency a little credit for all the things they've done to make exploration of any place besides Earth a possibility. Last time I checked, the only people who have ever sent anyone anywhere outside of LEO worked for NASA. Snazzy promotional videos and power point presentations for products that don't exist count for nothing in the real world.
I give SpaceX credit where credit is due for all the good work they've done. Maybe you should learn to do the same when it comes to NASA. No other space agency or single corporation holds a candle to NASA when it comes to total contribution to space exploration technology, even though ROSCOSMOS has contributed mightily and also deserves a lot of credit.
It's Thanksgiving. Be thankful that NASA exists, because without them SpaceX never would.
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There was some discussion about the communications net and GPS requirements for Mars at the last chapter meeting in Boulder this past Monday night. Due to scheduling conflict, Bob Zubrin was unable to attend. So--rather than discussing the Moon Direct, we "played it by ear,' and we tossed around the idea of a Mars GPS system. It became apparent that this would be a major undertaking, but in long run, entirely necessary. According to one of the engineers present, Earth has about 80 GPS satellites in orbit at a given time. But that is for Earth-military applications where flying a missile through Bin Laden's window was considered necessary. IMHO, a constellation of 10 GPS satellites would be adequate for incoming and landing spacecraft, when combined with some ground-based transponders. We didn't get around to discussion of Mars-Earth commo.
I just noticed my typo only after noticing the comment in subsequent post of 8.
Last edited by Oldfart1939 (2018-11-22 21:06:45)
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I would suggest at a minimum a count of 8 relaying optical to earth and RF to surface GPS capable systems as that would give communication overlap for the surface. I do agree that 80 is not required and that the landing beacons would in addition be useful as you can only make minor landing hovering time only just so long at the end of the journey to mars.
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Oldfart1939,
Provisioning of adequate communications and positioning can't be done properly on-the-cheap. The laser relays could be fabricated and delivered to Mars in a relatively cost-effective manner (but they could also be incorporated into a Boeing 702 bus or Northrop-Grumman / Orbital ATK GEOStar bus), but Ka or Ku and X require lots of power and that means satellites that aren't small or cheap. I think that Boeing's 702HP line of products is overkill for this particular application (702MP or even 702SP may be more appropriate), but Northrop-Grumman / Orbital ATK's line of products are perfectly adequate and have features we'd really need to maintain the infrastructure. Some startup may come along with something even better, but those are the two players with the tech that fits the requirements. Lockheed-Martin's capabilities should be reserved for things that have never been done before, like supersonic vertical-takeoff stealth fighters or the most powerful planetary science / observation satellite in existence, like MRO (when it was created). I don't think that level of effort is required here to fulfill operational requirements. A half-dozen of the GEOStar-3 class satellites would be required, but 2 spares would be really nice to have. At the end of the day, this requires a lot of forward thinking.
A forward thinking communications infrastructure would employ Ka and X for present-day surface-to-orbit data transfers and lasers for orbit-to-orbit transfers, with the option to use laser for surface-to-orbit in conjunction with communications aerostats that can float above a lot of the dust in the lower atmosphere.
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kbd512-I would agree that the constellation of 6 satellites would probably be functional enough, but 8 would be better, and with 2 spares. This comes to a total of 10 GEOStar-3 class satellites. Any idea how much loot this might cost? Also, do you have the weights (mass) of each of these? Could a single Falcon Heavy send more than one or two per mission?
One way to see how many GPS satellites are needed would be to check your car GPS and click on Status-satellites; that will immediately let you know what you are using to drive down to supermarket.
As an afterthought added as edit: My Garmin 530W was very good onboard my airplane; always did satellite check before starting a cross country flight of any major distance.
Last edited by Oldfart1939 (2018-11-23 23:01:01)
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Oldfart1939,
Unfortunately, I don't have a good cost estimate on the Lasercom stuff. It should be on par with existing Ka-band stuff. The equipment itself is less costly / smaller / faster / lighter, but there's a major test and integration campaign underway at NASA to integrate the COTS technology and ensure that everything is up to NASA standards. Incidentally, GEOStar-3 has provisions for docking / releasing micro satellites, such as the 1U and 2U satellites that could provide the relays back to Earth. That means the high-throughput laser relays could be farther out or deployed to Phobos or Deimos, if mission planners deem that architecture to be beneficial to network architecture design goals. It could be beneficial to maintaining connectivity when the Sun is between Earth and Mars to have some relays in the main belt, for example, even if that means an even greater lag time to reach Earth.
Al Yah Satellite Communications Company's / Yahsat's (UAE) pure Ka-band "Al Yah 3" communications satellite, which is based upon GEOStar-3 bus, weighed 3,790kg at launch, uses 7.5kW of power, provides 53 Ka-band spot beams, and 4 gateway beams. It was delivered to orbit by an Ariane-5ECA rocket. I've no clue what the total cost was, but I suspect the satellite itself was north of $300M since it was the first of its kind and developed by Orbital ATK exclusively for Yahsat.
The "Hylas 4" satellite, also Ka-band and using the GEOStar-3 bus, is reputed to cost around $300M, inclusive of the Ariane-5ECA launch. The company reportedly raised $350M and then determined that they overshot the actual cost by about $50M, or so the story goes. That would mean the satellite itself only cost $100M, which seems quite reasonable for a satellite of this type. The list price for an Ariane-5ECA launch is $200M. The bird weighed 4,050kg at launch. I believe the propulsion systems for Al Yah 3 and Hylas 4 were identical, meaning hybrid chemical (for orbital transfer) and SEP (for station keeping). Hylas 4 was supposedly low-cost Ka-band, whatever that is. Anyway, I have two different sources, one a financial evaluation of Avanti (the company that owns Hylas 4), that pin launch and construction costs for Hylas 4 at $300M.
Backup:
Edison Investment Research - Avanti Communications
To get an idea of what NG / OATK has deployed thus far, see this link:
Northrop-Grumman: Communications Satellites
So, we're looking at least $1.2B for the satellites. Since we're ordering a dozen of them, we may get a discount on any special features required, such as added radiation shielding. GEOStar-3 supports tandem launch, but Falcon Heavy's payload shroud can only accommodate 2 of them. Falcon Heavy can TMI 16,800kg, so there's plenty of margin with a prospective 8,000kg payload. Launch costs are $90M reusable or $150M expendable, so the low end of the launch costs is $540M for 12 satellites. If the launches are spread across 3 launch opportunities, that works out to roughly $290M per year over the course of 6 years. We're taking the minimum energy transfer trajectory in order to save fuel aboard the satellites for orbital insertion. In this case, there are only disincentives for going there faster.
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Just thinking about the legs of the mars journey and if optical is a possible or are we still in RF mode for transmitting back to earth while in flight to and from mars.
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If the local comms and positioning satellites are small they won't have the power to effectively do the Earth communications. The functions could be split, however, between say six small satellites and two or three large ones. The point is that earth comms is not needed in all of them, just enough that this function is available virtually all of the time.
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Reposted
In another topic, I ** think ** it was kbd512 who pointed out the need for a communications pathway for transmissions between Mars and Earth when the Sun is between the two. I searched but could not find the reference, but if someone can find it I'll be happy to add it here.
There would appear to be a business opportunity for a corporation interested in passing information between various outposts in the Solar system, for a modest handling fee. As I recall, the post I am thinking of mentioned the possibility of using the asteroid belt as a location for a communications hub.
I'd like to build on that idea a bit, to suggest a communications hub in a facility in Solar Polar orbit. This would be comparable to the polar satellite orbits in heavy use today on Earth. The corporation that would undertake this project could serve customers throughout the Solar System for most of the orbit. The exception would be when the facility crosses through the plane of the elliptic.
SearchTerm:SolarCommunicationsHub
(th)
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I guess then if a polar orbit can have the more complete satellites for mars then the ones along the equator can be as elderflower suggested a mix of lesser units. Do we need 2 in the polar orbit to get the coverage or will we need 4 and 4 for mars?
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