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 ...
- 131k
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 ...
- 122k
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 ...
- 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 ...
- 14.8k
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-...
- 172k
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 ...
- 131k
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 ...
- 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.
...
- 636
26
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 ...
- 360
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 ...
- 49.5k
18
votes
Accepted
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 ...
- 77.4k
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 ...
- 3,934
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 ...
- 12.6k
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 ...
- 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 ...
- 151k
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)....
- 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 ...
- 131k
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 ...
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 ...
- 151k
12
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 ...
- 15.9k
11
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 ...
- 5,083
9
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 ...
- 221
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 ...
- 4,504
8
votes
How can the CYGNSS spacecrafts (actually) measure ocean roughness?
Addressing multipath has long been a challenge with regard to GPS. Carrying a GPS-enabled smartphone toward the heart of a large city results in GPS-estimated positions and altitudes that bounce ...
- 77.4k
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/...
- 994
8
votes
Accepted
Why multiple GPS satellites are never launched on the same rocket?
Originally, it was because the GPS satellites are heavy. An Atlas E/F with SGS-1 upper stage could just barely put a GPS Block I satellite into a 12-hour orbit, while the Delta II was designed around ...
- 15.9k
8
votes
Is the full GPS constellation a Walker Delta constellation?
Partial answer only to address the apparent inclinations in the image
(refer to @blobbymcblobby's thorough answer and others if posted for the rest)
Through the extremely rigorous method1 of looking ...
- 151k
7
votes
Does GPS work at ISS?
Yes. In fact the ISS uses GPS as the primary source of information in its attitude control system.
The US segment of ISS has been using GPS as its primary source of
information for position, ...
- 189k
7
votes
Does GPS work at ISS?
As an addendum to the other answers, although GPS will work on the ISS, conventional, consumer receivers will not. GPS modules sold to civilians in phones, cars, or other devices have restrictions ...
- 21.7k
7
votes
Accepted
How many satellites would be required for Solar System GPS?
How many satellites would be required for Solar System GPS?
Let's try six, along the lines of @szulat's comment but a few more.
The question says we can ignore planetary shadows for the purposes of ...
- 151k
Only top scored, non community-wiki answers of a minimum length are eligible