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117

There are several things which affect the time to first fix (TTFX). Getting the almanac and ephemeris. These two things are technically a little different from each other, but for our purposes we'll treat them as the same. They are the locations of the satellites, and you need a to know where they are in order to work out your own position. Each ...


47

Clock errors are not corrected, they are compensated in two steps. 1. Error determination The GPS control segment uses reference receivers in well known locations to determine the actual orbital elements and the clock error of space vehicles. The reference for position is the WGS84 reference frame, for time it is the international atomic time. Even the ...


42

If someone stands on a hilltop over a large town and screams "the Mongols are coming!" then everybody knows what's up and they get out of town. The lookout doesn't have to say "Hey Timmy: The Mongols are coming! Hey John: The Mongols are coming! Hey Sarah..." GPS is just a bunch of satellites in orbit screaming "I'm over here!" in radio frequency. A GPS ...


35

GPS satellite is only a transmitter (as far as GPS signal is concerned) and the other end is only a receiver. There is no bi-directional communication and therefore there is no need for satellite to care about the number of devices. Basically satellite transmits its own location at certain time and the receiver calculates its position by using that ...


27

The official documentation for GPS is available online at: https://www.gps.gov/technical/ The portions you are probably most interested in are the Interface Control Documents, especially: IS-GPS-200, "NAVSTAR GPS Space Segment / Navigation User Segment Interfaces" IS-GPS-800, "NAVSTAR GPS Space Segment / User Segment L1C Interface" These documents ...


26

The exact position is the phase center of the antenna independent of the length of the cable and location of the chip. The time delay has to be calibrated by measuring the delay of the cable for the band. (L1 band). Many GPS receivers provide option to key in the delay parameter.


23

Long term the 1 Hz signal is probably the most accurate time, and so also frequency, reference that you will ever encounter. You are effectively obtaining something like a cesium clock time reference for the cost of a GPS module. A bargain. You can buy commercial "disciplined oscillator" units and designs for DIY ones are available. A DO is not frequency ...


23

No, this won't work in theory or practice because you do not have sensors to capture rotational motion. When you rotate an accelerometer, it is unable to detect that its coordinate system has rotated with respect to the desired coordinate system. What you are trying to do is called inertial navigation. In principle, to do inertial navigation, you need a ...


22

The cell phone operating system downloads the GPS almanac data (satellite ephemeris and status information) over the internet via the cell network and loads it into the GPS module much faster than the it would take to download that from the GPS satellites directly at 50 bps (yes, that's 50 bits per second, GPS is rather old tech optimized for operation at ...


20

The pull-up resistor to 3.3V and the diode means that even if you feed the circuit with 5V logic (most Arduino boards use 5V logic) the GPS chipset will see a maximum of 3.3v. The MT3339 device may be damaged or operate incorrectly if any of its pins go above its 3.3V supply rail. If the signal RX_5V goes to a logic low the diode will conduct and pull the ...


15

I work in an industry affected by the LightSquared system and may be able to provide some insight. The issue at hand does fall into the area of being with the GPS receiver. The bands that LightSquared wants to use are near the GPS L1 wavelength. These bands are currently employed by systems that send command and control packets to satellite systems. However ...


15

There is no free lunch here. You need a solid test plan. This is an area well worth investing in, because once you develop a reputation for shipping shoddy products, it's pretty much impossible to shake it. To a large extent, you need to be able to trust your component suppliers, and focus on testing only for the types of errors that occur during the ...


15

A GPS receiver creates a local replica of something called "GPS system time", which is a virtual timebase created from all of the clocks on the satellites and ground stations. This replica is integral to the process of coming up with a navigation solution, which is based on measuring the signal delay from each satellite to an accuracy on the order of ...


13

@DavidKessner's answer is in line with what I'm about to say, but I wanted to elaborate, and this is a bit more than a comment. This output could be used to, say, wake the MCU (from a deep sleep mode) once per second (to within a couple nano-seconds) in an application where you cared about the MCU doing something on a particular second, within great ...


13

If a GPS receiver doesn't see satellites, then what you see in the software is not the GPS time. Probably, your GPS unit has some backup timekeeping (quartz crystal). Some GPS units have a low power crystal with a backup battery. It provides an approximate time, which allows the receiver to reduce the search space, which reduces the time to first fix.


13

Cable delay adds an equal offset to the pseudoranges for all satellites. Since GPS uses the difference in the pseudoranges to each satellite to calculate the position, positioning isn't affected by cable delay. The position calculated will be at the antenna, not at the receiver, which you can see by realizing that moving the antenna has a different effect ...


12

Sattellites "in view" is different from full communication with that satellite. There are a few steps in eventually receiving the GPS data so that position can be found. Due to the modulation scheme, just being able to detect the carrier is not enough. In fact "carrier" is a bit misleading for GPS since the signal power is something like 20 or more dB ...


12

there are several things to keep in mind, when combining data from different systems. Combining different Satellite Navigation systems Typically it is possible to combine gps+glonass or any other navigation system. That is called GNSS (Global Navigation Satellite System). For a single GPS case you need 4 Satellites for the components [X,Y,Z, dt (the ...


12

50 mOhm resistance, 47nH inductance? Its probably an inductor. Murata make some thick film inductors in a green package with a black stripe. Its probably one of these: http://psearch.en.murata.com/inductor/product/LQG15HS10NJ02%23.pdf Assuming the pin is the antenna then having an inductor feed into another inductor down to ground is a common circuit ...


11

You should consider flipping the system around. There is no need for the robot itself to determine the location. It only needs to know what to do. This can be communicated to it from a fixed PC via a WiFi link. With such a link it doesn't matter whether the robot figures out the location or whether that is done in the fixed installation and then the ...


11

Note: This answer was originally written before we knew anything about the receiver, like its 1.8V power supply. Telaclavo's answer is good. As a more general answer the PNP/MOSFET solution remains; you don't want to power parts of your circuit from a microcontroller's I/O pin. You don't want to do that! You'd configure the pin as output and make it ...


11

The other answers have already explained the "how" and "why", so all that's left for me is the "what": it's called A-GPS (assisted GPS, sometimes also called accelerated or augmented GPS). In other words: the reason why a phone's GPS works faster than a "GPS's GPS" is that the phone isn't using "GPS", it's using aGPS.


11

None of the smartphones today have dedicated GPS chips in them, they only have a GPS capability that stems from byproduct features of the modem chips. That's not true. GPS functionality in phones is done either with a dedicated chip, or with a dedicated GPS receiver within a System-On-Chip (which is effectively the same as having a dedicated chip). ...


10

The constraining factor is the lowpass-filtering after despreading. If we assume -204dBW/Hz noise power density (~ 17°C noise temp), we can only allow around 25kHz of noise bandwidth before it reaches the L1 power of -160dBW. Our integration time must be at least 1/25.000s to detect the signal from the noise background (assuming omnidirectional antenna). ...


10

That is, no contact. I've currently come across 2 cold soldering and 3 no contact issues. Looks to me you have got a hole in your final testing. (I assume you have some sort of test-jig) I can imagine that waiting for a GPS lock requires too much test time but, especially with the detected errors you should add a resistance/contact/SWR test on the antenna.


9

There is no reason why this shouldn't be possible. Each satellite sends a clock signal with very high precision. The GPS or GLONASS module knows the location of these satellites, again with very high precision. Each signal received is one item of information giving a four-dimensional sphere on which the receiver is located. With four satellites, four such ...


9

There are few methods to accelerate GPS module assembly testing: Use AGPS to acquire a faster fix. TTFF should take few seconds, assuming you can load the AGPS data to the unit. Read the GPS raw data, specifically, C/n. Use a GPS repeater, or better yet, a GPS constellation simulator to get a known signal.


8

Satellites don't respond. They transmit signal and GPS clients receive it. I mean your GPS device doesn't need to send anything to satellite, it just receives from satellite and that's enough. Your GPS device needs to receive signals from more satellites and then it does some math to compute its own position. So the short answer is: It's massively ...


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