Hot answers tagged

45

The field in the protocol that specifies the week number is a 10-bit value. In most computers, when an (unsigned) integer exceeds its maximum value, it wraps around to zero. This is roughly similar to Y2K, though is more like the upcoming year 2038 problem (but with weeks instead of seconds). This 10-bit value will wrap around, and the GPS system will hold ...


33

@forest’s answer is correct. But what makes the rollover slightly more problematic is that many GPS receiver manufacturers have accounted for it by pre-programming an internal “pivot date” in the firmware. That is, if a receiver was manufactured/programmed in, say, 2015, then there is internal logic that says “if the date appears to be prior to 2015, it’s ...


25

The main reason they are in such a high orbit is to allow for more of the Earth to be visible at any one time. In order to have a reasonable amount of the Earth visible, you have to be high up. A lower altitude could in theory work as well, but the chosen altitude seems to be a far enough distance to be useful, but not so far as to have communication link ...


19

GPS / GNSS satellites are orbiting at an altitude where their orbital period is half the Earth's mean sidereal day (23 hours, 56 minutes, 4.0916 seconds) so their nodal precession rate is both small (roughly 4 minutes, or ±222 km East-West drift along the Earth's equator per day) and fairly constant, or perhaps better said stable, over longer periods of ...


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 ...


10

The ISS does it. Source: http://spacestationlive.jsc.nasa.gov/handbooks/adcoHandbook.pdf Attitude Determination How am I currently oriented? The ISS also uses GPS to determine how the ISS is oriented, or facing, as it orbits the Earth. This orientation, or attitude, can be determined by measuring the difference in time that the GPS signals ...


10

The furthest satellite that I have seen use GPS is the AO-40, an amateur built satellite. In fact, they did a research paper on the subject. The paper states that it was able to achieve a navigation solution at 60,000 km, which is about 1/6th of the way to the moon, and well beyond the belt of Geosynchronous satellites. I'm sure there are other satellites ...


8

Exact radiation patterns and gain (you're probably not asking about signal strength since that depends on distance which isn't constant) will vary across all the different GNSS contellations, even individual generations / blocks of same systems, but for a fairly detailed analysis of GPS blocks, you can refer to e.g. GPS Space Service Volume: Ensuring ...


6

It seems that latitude and longitude can (or must?) be provided to PyEphem as a string, using the degrees:minutes format i.e. Longitude: 116:17.75718, Latitude: 40:3.00174 See rhodesmill.org/pyephem/quick.html#observers PyEphem also needs Universal Time (and not the time in the local Timezone) See rhodesmill.org/pyephem/quick.html#dates Given that 40 N, ...


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

TL;DR: It depends. The actual answer depends which generation of the BeiDou system one is referring to. The excellent Springer Handbook "Global Navigation Satellite Systems", edited by Teunissen and Montenbruck, has a section describing the BeiDou system. In fact, there are several generations of the system: BDS-1, BDS-2 and BDS-3: BDS-1: There were two ...


5

It appears BeiDou 1 location service was bi-directional, involving a central ground station calculating the position and sending it back "the system capacity is 540,000 users per hour, and serve up to 150 users simultaneously" The above source goes on to indicate BeiDou2 uses the more conventional GNSS method. The Asia times story doesn't give sources for ...


5

The short answers is to ensure ground track repeatability. And the period is not 12 hours but half a sidereal day (that is about 4 minutes shorter), so that when the earth have done one rotation, the satellites have done two and the geometry of the whole constellation relative to earth is the same than one sidereal day before. Repeatability is important for ...


4

I will make an attempt at an answer, gathered entirely from some papers and websites I've found. The first, from UMass Lowell's Center for Atmospheric Research, gives an explanation of the magnetic equatorial anomaly: Characterized as the occurrence of a trough in the ionization concentration at the equator and crests at about 17 degrees in magnetic ...


4

Maybe Systems Tool Kit could be helpful: https://www.agi.com/products/engineering-tools It's a powerful tool, and there is also the possibility to have a free license, although it does not include all capabilities. I have used it on Windows 10, but I cannot tell about Windows 8.


3

If his statements are credible, I think he's implying that Galileo is in some form dependent upon British scientific knowledge, and that replicating that knowledge on the Continent will require significant investments of money and time. That would fit in with his claim that the UK had much of the capability needed.


3

The new Soyuz MS does too. See https://en.wikipedia.org/wiki/Soyuz_MS Instead of relying on ground stations for orbital determination and correction, the now included Satellite Navigation System ASN-K (Russian: (АСН-К, Аппаратура Спутниковой Навигации) relying on GLONASS and GPS signals for navigation.[2][11] It uses four fixed antennas to ...


3

I wouldn't have suspected GPS satellites to be corrected less often than other GNSS. But I would guess they don't need to be as close to a circular orbit so long as the orbital parameters are reasonably accurate in the ephemeris the satellites send to GPS receivers. So long as the user can determine the satellite's position, it shouldn't matter too much if ...


2

Spoofing from a satellite would need a substantial amount of power , especially if you want to use a GEO satelitte. Doing it from LEO would potentially be lower power than the GPS sats themselves but pose difficulties denying an area long enough to be useful, and make locating the cause easier by matching complaints against orbital elements. It would ...


2

tl;dr: Celestrak offers those short lists for convenience only. If you want to know the TLE of a group of satellites, look them up explicitly using their IDs. It looks like once a GLONASS satellite gets on the list by being commissioned for GNSS, it stays on the list even if GLONASS stops using it. The two satellites that are not yet on the Celestrak ...


2

Other answers explain the WNRO problem as an overflow/high order truncation/rollover problem. This is correct. It is a security issue because services may be impacted. Not all security issues are about data loss. Business continuity is strangely important to people as well. This type of thing has been going on in the digital world for a while now. ...


1

The DDM measures both the delay that a reflected signal takes to get back to you and the doppler (frequency) shift of that signal. In your left most figure, what you are looking at is a picture of how a GPS signal reflected off of the ocean. The bright red spot is the point of "maximum specular reflection", that is, the point directly underneath the ...


1

Transmitters on GPS satellites are very weak (~60W) so by the time the signal reaches the Earth's surface, it is below the noise floor. That is; the signal is weaker than the normal level of background electromagnetic noise, making it hard to detect. This is why GPS signals can't usually be seen indoors without an external antenna, for example. It is ...


1

The GPS constellation wanted to be high enough where many of them could be seen, but not in Geostationary orbit, because it wasn't needed. Somewhat arbitrarily it was decided to set them up such that they orbit twice per sidereal day, as it is a good middle ground, not a lot of satellites in that area, and it is high enough to be useful. When other nations ...


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