Episode #125 of the Stack Overflow podcast is here. We talk Tilde Club and mechanical keyboards. Listen now
28

No, 75 meters is not exceptionally close — several satellites show predicted passages that close or closer every single week of the year. But it is close enough to potentially appear in the top 10 list of satellite conjunctions for the week: http://celestrak.com/cgi-bin/searchSOCRATES.pl?IDENT=NAME&NAME_TEXT1=&NAME_TEXT2=&ORDER=MINRANGE&MAX=...


27

Exceptionally rare? No. Uncommon, well, yes. From what I can recall, these happened about once every 10 years per satellite for the constellation of satellites I worked with, at least, prior to doing a collision avoidance maneuver. That of course included debris/ satellite close approaches. Of course, I note that the two you listed are both live satellites,...


25

Nodal precession doesn't matter for a plane of satellites like this, they will rotate around in unison, so the coverage will remain the same. Okay, so why the unusual dual inclination constellation? The inclination of a satellite band tends to let you know what latitude it will work best at. A 0 degree satellite works best at the equator, a 90 at the poles. ...


21

The dimensions of the test satellites are indeed 400kg each, in a box measuring 1.1 m × 0.7 m × 0.7 m . Something I did not take into account in these calculations is the likely reduction in weight of the satellite during the mass production. For example, the Orbcomm OG2 fleet weighs in at 172 kg per satellite. This is a likely goal for SpaceX's satellites (...


19

88! PSLV-C37 launched 88 identical satellites into a sun-synchronous orbit. These satellites were for an imaging company called "Planet Labs". Technically the only ones that were identical were the CubeSats (Which were named Doves) but they were launching 4 other types of satellites from this launch too. The rocket launched Cartosat-2D, a few other ...


17

Uhoh touches one side of the problem: "Why" - the lower the orbit, the less of Earth is covered in a single pass, and the closer to equator the orbit, the less do the passes vary further narrowing the area. MEO equatorial satellites make sense. The lower the orbit though, the less useful they become. Still, there's a slew of tasks that wouldn't be hurt by ...


15

GPS satellite orbits go up to only 55 degrees inclination, so there are regions over the poles that they do not fly directly over (they are high up enough that they give coverage in the polar regions). If you were sitting up at the North Pole, you would never see a GPS satellite climb higher than 55 degrees from the horizon, whereas if you were on the ...


12

75 meters isn't all that rare - for the full public catalog there are 75 meter predictions about 2-3 times per day. What you have to consider is the uncertainty in that number. For TLE data, that's 75 meters +- several kilometers. The 75 meter miss distance is from the nominal position of each object at closest approach. But TLE data has a significant ...


11

Nothing physically prevents equatorial orbits at any altitude above the Kármán line. The question is really; what would be the point in flying over exactly the same equatorial band roughly every 90 minutes, when a higher-inclination orbit would let you cover much more of the planet (all of it, eventually, for polar orbits) or a higher orbit would let you ...


11

480,000,000!† †Depending on how you define satellite. I think Project West Ford is the current record holder, with 480,000,000 individual satellites. Well technically 480,000,000 copper dipole antennas, designed to act as an artificial ionosphere. Here is a photo of a few of the dipoles:


9

Does each satellite slightly raise or lower it's orbit to start spreading out compared to one reference satellite? Yes. So off the bat there's a deltaV penalty for distributing the constellation? The ∆v required to phase the orbit can be arbitrarily small if you're not in a hurry to reach the final configuration, as described in this answer. I ...


8

I don't think anyone can give you a detailed answer on how they will make money, because their business plans are their own, and proprietary. But the premise of the question seems essentially flawed - There are huge areas of the world that do not enjoy reasonable (or any) broadband access. There are also existing markets for mobile broadband (passenger ...


6

The space segment of the Cospas-Sarsat system has two components, SAR signal repeaters (SARR) and SAR signal processors (SARP). The Sarsat instruments [...], receive these messages. They are downlinked to the distress terminals — known as Local User Terminals (LUT) — distributed around the globe. The message are then processed and distress alerts ...


6

The answer is given within the question, the emergency signals are received by the the International Cospas-Sarsat Programme satellites. From wikipedia: Space segment The Cospas-Sarsat system space segment consists of SARR and/or SARP instruments aboard: Five satellites in polar low-altitude Earth orbit called LEOSARs Seven satellites in ...


6

Your arithmetic getting to 50 birds per plane about 7° apart seems reasonable. I would be surprised if 50 satellites could be put on a single launch, if for no other reason that I don't think they'd fit in the fairing. Regardless of how many go up on a single launch, I think we can safely assume it would be more than one, in which case the answer below is ...


6

Bart's correct. To elaborate, I was part the USAF team that tested the first DoD acquired GPS receivers. From what I remember, the GPS and common to all positioning systems, the orbital geometry was mostly limited by economic practicality. + and - 55 degrees, as stated in the first answer, covered the planet's highest population density. The higher ...


5

While there is a currently accepted answer explaining why there are "no LEO satellites in the earth's equatorial plane", I found two of them with inclinations currently less than 3 degrees! Both are science missions and detect gamma rays. I don't yet know why equatorial orbits were chosen, but I can forward two theories until such time that I find out. ...


5

We are living in the 21st century now. One phased array can support a large number of beams, each pointing in a different direction, each moving at a different rate. All the steering can be done electronically. So no, you don't need one antenna for every link. Just a few spacecraft-fixed, phased arrays is all that's necessary. Here is a drawing of an ...


4

Planet Labs' Flock 3p, a constellation of 88 Dove satellites, was recently launched aboard ISRO's PSLV-C37, the record-breaking launch of 104 satellites. Wikipedia says: They head to a morning crossing time, sun-synchronous orbit (SSO) at an approximate altitude of 500 kilometres (310 mi). According to the Wikipedia Planet Labs article, Flock 3p is the ...


4

One useful attribute of any LEO broadband system is agility, where you can deliver the service for the cost and shipping of a ground station. Which means anytime existing providers fail to keep up you have an opportunity to charge a premium until infra structure catches up with user demand. Failing to find useful numbers but the complaint level in Australia ...


3

Here's a table for gromain's answer, which assumes 400kg/satellite and 12k satellites. It includes prices from spacex's website as a rough estimate. ====reusable===== ===expendable==== F9 FH BFR F9 FH BFR payload to LEO 10t 27t? 150t ...


3

Neither. In the old times, geostationary satellites had their longitude windows, each with around 1°, and tried to keep themselves at the center os this window. This strategy was good enough for its time, when few satellites with poor attitude controls were located in this orbit. Nowadays, geostationary spacecraft are usually in a "collocation window", ...


3

About the second part: changing the declination of an orbit is always so costly, that no one does it. Consider a 180 ${}^\circ$ change, i.e. if we would reverse the orbit direction, it would require the $\Delta v$ of the double of the second cosmical speed, $\approx 15.5 \frac{km}{s}$. The typical $\Delta v$ reserve of the deployed satellites is measured ...


3

You can, but it will take time and fuel. The Right Ascension changes with time. The higher you are, the longer an orbit takes, and the slower your drift will be. Thus, you can start drifting your satellites by setting them at different altitude. Once you have reached your desired separation, you then have the satellites burn to be in the same plane. ...


3

The carrier aircraft you highlight are large so they can carry large enough rockets to lift large enough payloads to satisfy the market segments their companies are aiming for. Smaller aircraft could only loft smaller rockets, limiting the final payload size to much smaller payloads. Any launch vehicle, whether it's a traditional ground launched rocket or ...


3

Planet Labs opened a new factory in September that can produce 40 of their Dove satellites per week. Doves are 3U CubeSats - you can read more about the Doves themselves here. Outside of Planet, OneWeb/Airbus, and SpaceX in the future, I am not familiar with anyone who has concrete plans to mass-manufacture satellites, though I have read of a number of ...


2

Mathematically, "single frequency" implies an impossible transmitter that never turns on or off (Fourier-wise). but really there are all kinds of radar that uses a wide spectral range. Chirped radar, e.g. SHARAD at Mars and ICEYE at Earth is a standard radar technique. According to this ICEYE's radar chirp is spread to about 300 MHz around it's 9.65 GHz ...


2

Not just one spectral band, but a single frequency. Classical radars use a tuned microwave cavity to produce their radar emissions. These operate at a single frequency. The radar receiver operates in a narrow band around this frequency (to allow for Doppler shift). "C band" roughly indicates at which frequency the radar operates (C band is anything between ...


2

The launch provider is supposed to look at the licenses. I know with one of the launches that I have been involved with, the license was approved only a few days before launch, and the launch provider was getting pretty nervous and expecting that we would work on improving that.


2

I agree with the lift capacity of BFR at 304 Starlink satellites, by weight and probably dimensionally. But there is the problem of each orbital plane on the first Starlink Phase I only has 34 satellites in it. See the section on "How many launches...". But changing from one orbital plane to another costs a substantial amount of fuel, it is unlikely that ...


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