# Tag Info

68

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 integrated to JPL's visual inertial navigation system. Some additional inputs might include gyros, visual odometry, tilt sensors, altimeter, and hazard detectors.[15] ...

62

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 antennas in really close proximity. A million man march, each with a GPS device, might cause some degradation as each device will absorb a bit of the energy ...

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 ...

41

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-meter fairing is taken up by a single one: Looking at the dimensions of the fairing in the Payload Guide, I think the satellite is about 5m long by 2.5m wide. ...

39

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 few kb per session though. Many mapping applications also download map data as you go along, again causing lots of network traffic. This may be an ...

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 ...

31

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. It requires a flotilla of satellites around our planet to work, though - something we don't have (yet) around other celestial bodies. In that case we need ...

26

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 ...

24

"...the time value received wouldn't be accurate insofar as I know GPS receivers aren't able to determine their position until they've got accurate time." The time value isn't used to tell the receiver what time it is (at least not directly, although that is helpful later). It's used so that the receiver can tell relatively what the distance is to each ...

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 ...

18

Not the way GPS works, no, since the Mars orbiters don't have atomic clocks. (Though future Mars orbiters may someday.) The relay radios on the Mars orbiters provide a 2-way Doppler data type that can be used to locate surface assets to, as I recall, within about 100 meters. Even a single orbiter with Doppler data collected over a few passes can get this ...

17

To give a big picture view of how the GPS solution is determined, consider the following equation: $\rho_i = \sqrt{(x_i-x_u)^2+(y_i-y_u)^2+(z_i-z_u)^2} +c\Delta t$ where $\rho$ is essentially a range from the user to the GPS satellite, $x,y,z$ are position coordinates, the subscript $i$ indicates the particular satellite, $c$ is the speed of light, and \$\...

16

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 developed for robots on Earth. This isn't a perfect example, but it gives the idea that the robot builds up a map over time. At each point you can identify the ...

15

GPS is regularly used in low earth orbit. It is possible to use the signals from the sidelobes of the GPS transmit antennas to receive service in a high earth orbit up to and potentially beyond geosynchronous altitude: (Image credit: GPS World) However, it is challenging because the signals are substantially weaker and coverage is intermittent. These ...

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 ...

14

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 clock at mean sea level on the surface of the Earth. Both differ from UT1, time as ticked by the Earth's rotation (this is the source of leap seconds), and ...

13

The only class of orbit in which a satellite always remains over the same point on the earth's surface is a geostationary orbit, which can only be directly above the equator. Putting the GPS satellites in such orbits would have several disadvantages: Poor satellite visibility for users at high latitudes - the satellites would always be low on the southern ...

13

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 variations in signal timing. Basically they don't care as long as the packets arrive quickly enough not to cause a gap in the audio. GPS transmitters are 18,...

12

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 between receive/transmit. This allows one to determine exactly how far away the satellite is. Combining this information with orbital tracks allows one to ...

12

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 satellite constellation to get more satellites visible, especially near the poles. They orbit in three planes at 19,130 km. The less complete systems BeiDou and ...

11

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 ...

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

Mark Adler already explained that the satellites aren't equipped anyway to provide a satellite navigation system. But even when they were technically capable of working as a GPS satellite, 5 satellites would not be enough. You need a direct line of sight to at least 4 satellites, and with only 5 satellites this will happen very rarely, because even ...

10

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/huygens_cutaway.jpg). Schemes and paper - RUBIDIUM ULTRA-STABLE OSCILLATORS AT TITAN: THE HUYGENS DOPPLER WIND EXPERIMENT, 1997. THE HUYGENS DOPPLER WIND ...

9

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 distance and precise distance to synchronize time (to calculate the time difference). The answer It is the fourth satellite that gives your receiver the ...

9

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 believe primary source of disturbances at that altitude are geodetic variations in gravity field. The maneuvers are needed less than once a year on average. Of ...

8

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 position. Three satellites for determination of the position (x,y,z) and one for the receiver clock error. This clock error is the synchronisation between the ...

8

Knowledge and maintenance of a spacecraft's state (its position, velocity, attitude, and attitude rate) is a collaboration between ground controllers and the spacecraft itself. Vehicle guidance, navigation, and control systems Some vehicles keep track of both translational state (where they are) and rotational state (where they are pointing). Others just ...

8

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 better cis-lunar navigation than GPS? Also see answers to How far up have satellites used a GNSS for positioning, and how does the precision degrade with ...

7

I think GPS receivers do not exclusively need at least 4 satellites, but rather 3 as a minimum for trilateration. A fourth satellite signal is necessary to synchronize the receiver clock with the satellite clocks. As for TOF/TOA vs. TDOA, I believe the difference relies whether on if the GPS receiver has an internal clock synchronized with the satellite ...

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