Hot answers tagged

46

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 ...


34

@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 ...


29

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 ...


23

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 ...


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 ...


7

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 ...


7

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 ...


6

The JPL DESCANO book series is the absolute reference in precise orbit determination. It's free, and divided into separate chapters. It includes all of the calculations needed to understand and solve for orbit determination. In short, OD achieves much better precision than GPS for the following reasons: The signal is at least a two-way signal (GPS is one-...


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 ...


5

Ryan C's answer provides the crux and a lot of useful background information. The crux is: Yes, the C/A code repeats every millisecond, but that's not the only modulation. The other information that is modulated on the signal can provide a reference that is not ambiguous. It is difficult to find details on how receivers solve these problems, probably ...


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.


4

The DDM term is mostly used in the GNSS Reflectometry (GNSS-R) scope, but is conceptually equivalent to the radar ambiguity function. In fact, a GNSS-R system can be understood as a bistatic radar, that is to say, a configuration in which the transmitter (the GPS/Galileo/GLONASS/Beidou satellite) and the receiver (which receives the reflected signal) are ...


4

Partial answer... Considering: https://www.sciencedaily.com/releases/2018/01/180111223914.htm NASA engineers has demonstrated fully autonomous X-ray navigation in space NICER/SEXTANT https://www.asterlabs.com/publications/2006/Sheikh_et_al,_AIAA_JGCD_Jan_Feb_2006.pdf Spacecraft Navigation Using X-Ray Pulsars China's XPNAV 1 Is NICER/SEXTANT the first ...


3

After insightful discussion about clock corrections of both GPS and GLONASS in Ephemeris time and clock corrections in RINEX navigation files, I understood better how to deal with the information provided in RINEX navigation files for both of them. In summary: It is indeed necessary to apply clock corrections if one aims to obtain a time in UTC as accurate ...


3

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. ...


3

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 ...


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 ...


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

Nothing in navigation signal processing is as simple as most descriptions make it sound. Signal detection theory is essentially probabilistic, so algorithms make their best guesses at the likeliest thing that might have happened, but there's always an error rate. Solving for the integer offsets can be done, if you are sufficiently careful and clever in ...


2

SaVi runs under Windows. It can be run under the Windows Linux Subsystem, under Cygwin, or under VirtualBox. And how to get it running is fully documented. See https://savi.sourceforge.io/install/ And SaVi includes GPS and GNSS simulations.


2

Since the offer for the answer to be updated wasn't taken up I'll post it as a new answer. As of August 6, 2021 it's still 70,135 km above Earth from 2015 Sources: NASA.gov's NASA’s MMS Breaks Guinness World Record Space.com's High-Flying NASA Mission Sets New GPS World Record Science Daily's NASA's MMS breaks Guinness World Record The Science Daily link ...


2

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 ...


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

First, on the practical side, it is most likely that GPS spoofing will come from a terrestrial spoofing source rather than a LEO, much less a rogue GEO. This allows several exploitable factors to detect spoofed signals. To answer your question(s), no, they are unlikely to come from space, and second, direction finding is one of the possibilities. Direction ...


2

tl;dr: Start with a terrestrial GNSS constellation at a medium lunar orbit, but add more satellites in polar orbits to make sure you can still get a good fix sitting at the bottom of a crater at one of the poles. The main spacecraft for all of Earth's GNSS systems are in medium Earth orbit (MEO) and this allows for enough members of a relatively small ...


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