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68 votes
Accepted

How can a Mars helicopter be autonomous if there isn't a Martian GPS?

Edit: The JPL Mars Helicopter Scout will use inertial navigation: The inconsistent Mars magnetic field precludes the use of a compass for navigation, so it will use a solar tracker camera ...
Hobbes's user avatar
  • 128k
62 votes

Why would GPS availability be reduced by high demand (or solar eclipse)?

GPS isn't affected by demand, as it is transmitting only from satellites, and the receivers only receive, they do no transmit to the satellite at all. At best, there is a slight degradation by having ...
PearsonArtPhoto's user avatar
  • 121k
46 votes

What is GPS' 19 year rollover and does it present a cybersecurity issue?

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 ...
forest's user avatar
  • 485
46 votes

Why does GPS need the fourth satellite?

Why then do we need the fourth satellite ? Because there is a fourth unknown: time. GPS works one-way, so the receiver's clock needs to be aligned with the senders' (the GPS satellites) clocks in ...
Ludo's user avatar
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42 votes
Accepted

Could a Falcon Heavy really put six GPS Block III satellites in orbit at the same time?

Could you fit all six into a FH fairing? If not, would a slightly larger one be enough? I can't find dimensions for GPS III sats, but this Lockheed Martin image shows how much of a Delta Medium 4-...
Russell Borogove's user avatar
39 votes

Why would GPS availability be reduced by high demand (or solar eclipse)?

As PearsonArtPhoto says, it's not the GPS protocol itself that causes the problem. Cell phones use Assisted GPS, where cellular data is used to speed up obtaining a GPS fix. This should be just a ...
Hobbes's user avatar
  • 128k
33 votes

What is GPS' 19 year rollover and does it present a cybersecurity issue?

@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 ...
Darren's user avatar
  • 431
31 votes

How can a Mars helicopter be autonomous if there isn't a Martian GPS?

GPS is one of several possible technologies available for assisted navigation. It's commonly used on commercial drones because the framework is in place and GPS signal is usually available on Earth. ...
OnoSendai's user avatar
  • 636
25 votes

Why does GPS need the fourth satellite?

Your GPS cannot directly determine the distance from any satellite, it has to go indirectly. It gets a signal from the first satellite, say "it was exactly 10:30:25.123456789 seconds according to ...
gnasher729's user avatar
23 votes

Why does GPS need the fourth satellite?

The receiver has no very precise time, the fourth satellite is needed to calculate a 3D position without knowing the precise time. After knowing the position, the receiver may calculate the GPS signal ...
Uwe's user avatar
  • 49k
18 votes

How can cosmic ray muons be used to replace GPS for positioning information in 3D on Earth and underwater?

From what that paper says, it's based on the flight time of highly-relativistic muons between a reference detector at the surface and the final detector under the water. There are three or more ...
Roger Wood's user avatar
  • 3,864
18 votes
Accepted

Is the full GPS constellation a Walker Delta constellation?

I only had a quick look but the answer seems to be: do all GPS satellites have the same inclination? The intention is yes, the reality is no (small variations) And do they exist in a Walker Delta ...
blobbymcblobby's user avatar
17 votes
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Is GPS time at least "really close" to TAI (International Atomic Time)?

TL;DR: For all practical purposes except milliarcsecond radio astronomy, TAI and GPS time are separated by exactly 19 seconds. TAI and GPS time both attempt to represent time as ticked by an ideal ...
David Hammen's user avatar
  • 74.8k
17 votes

Why does GPS need the fourth satellite?

GPS is much easier to understand in the one-dimensional case. You are somewhere on the road between Springfield and Newtown. The road is straight and the two towns are six kilometers apart. Around ...
Rainer P.'s user avatar
  • 1,172
16 votes

How can a Mars helicopter be autonomous if there isn't a Martian GPS?

I've reused some material from this answer here to show that the MARS 2020 rover will land on Mars using optical navigation in part. A helicopter can use similar environmental learning techniques ...
uhoh's user avatar
  • 149k
16 votes
Accepted

What is this gap in the GNSS satellite trajectories?

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)....
Dave's user avatar
  • 851
14 votes

What is required to make GPS signals available indoors?

There are some big differences between GPS and cell phone signals: GPS relies on the exact timing of a signal (1 nanosecond off = 30 cm of position inaccuracy). Cell phones are much more tolerant to ...
Hobbes's user avatar
  • 128k
13 votes
Accepted

How are GPS satellite clocks synchronized with each other in the earth's frame of reference?

The satellites' location is very accurately determined using ranging techniques, where a pulse is sent from a ground station to the satellite and the satellite responds very quickly, with a known time ...
PearsonArtPhoto's user avatar
  • 121k
13 votes
Accepted

How far from earth have atomic clocks (or ultra-stable oscillators) been placed and monitored?

There was Rb-based Ultra Stable Oscillator in outer space, delivered to Titan (VI moon of Saturn) inside the Huygens, brought there by Cassini. (DWE RUSO in http://nssdc.gsfc.nasa.gov/image/spacecraft/...
osgx's user avatar
  • 2,315
12 votes
Accepted

Does GPS work at ISS?

Does GPS work at ISS? There's certainly no reason for it not to! It's likely to be "yes" for at least some of the other GNSS systems as well. See Will Glonass, Galileo, or BeiDou-2 satellites provide ...
uhoh's user avatar
  • 149k
12 votes

Height of navigation satellites above Earth

For satellite navigation, you are most likely using the Global Positioning System, which uses six planes of satellites at an altitude of 20,180 km. However, many systems can also use the GLONASS ...
SE - stop firing the good guys's user avatar
11 votes

Why does GPS need the fourth satellite?

A GPS satellite sends out a constant stream of "I transmitted this at time x" messages. There are two ways of turning this into a location fix: The three spheres method you describe ...
Mark's user avatar
  • 15.3k
10 votes

How does GPS receiver synchronize time with GPS satellites?

The problem The GPS receiver's time has to be synchronized with atomic clocks located in GPS satellites. It is kinda chicken or the egg problem. The receiver needs precise time to calculate precise ...
Vlastimil Ovčáčík's user avatar
10 votes

How does GPS work exactly?

Does the receiver calculate the distance that separates it from each satellite, or does it just calculate the distance difference between each pair of satellites? A basic answer is a global ...
mins's user avatar
  • 5,033
9 votes
Accepted

Current situation with CoCom regulations and GPS receivers for balloons and cubesats

There is a standard clause included in all GPS receiver manuals regarding COCOM Limits. I cannot find the source of this clause, but since it is worded exactly the same in all the manuals I could find,...
called2voyage's user avatar
  • 23.7k
9 votes
Accepted

Do operating GPS satellites ever make orbital maneuvers for station-keeping?

Yes, GPS satellites do execute station keeping maneuvers. Primary purpose is to keep them within the desired repeating ground track, which leads to a maneuver every year or so for each satellite. I ...
Carlos N's user avatar
  • 4,444
8 votes

Why are the GPS constellation satellites in such a high orbit?

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 ...
Camilo Rada's user avatar
8 votes
Accepted

How does GPS module gets time even before a fix?

Each satellite broadcasts time. Only one is needed to determine the time with accuracy to distance to the satellite in light seconds - under 0.088s of error. But you need at least three, preferably ...
SF.'s user avatar
  • 55k
8 votes

What is the deepest position in space we can get a GPS signal?

GPS is since a few years used by satellites in geostationary orbit. Check out for instance this paper by Lockheed Martin about GOES-R, from the ESA GNC 2017 proceedings: https://ntrs.nasa.gov/archive/...
yzokras's user avatar
  • 1,004
8 votes

How are GPS satellite clocks synchronized with each other in the earth's frame of reference?

Receiver clock synchronisation: GPS or GNSS applications are only dependend on the travel time of the signal. To put it simply: You only need the travel time of four GNSS satellites to calculate the ...
ben's user avatar
  • 409

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