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119

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 ...


28

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

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 ...


23

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 ...


22

The first thing that comes to my mind is that a first fix starts with unknown position and with unknown time. The device has to find as much satellites and then calculate the position. All the subsequent fixes are easier to find, because you already have an approximate positon and you already have the correct time, then a GPS tracker would spend less time to ...


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

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 ...


14

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

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

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). ...


11

Question How come my GPS module takes so long time to fix, after TTFF (Time To First Fix)? Answer Update - I now realise my answer below will apply to many similar GPS signal acquisition problems but does not answer the OP's question in this specific case. The OP asks why first fix is reasonably long, but later fixes are disappointingly not that much ...


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

According to the datasheet: VBAT This is the battery backup power for the SRAM and RTC when main power is off. Without the external backup battery, ET-662 will always execute a cold star[t] after turning on. To achieve the faster start-up offered by a hot or warm start, a backup battery must be connected. The battery voltage should be between 2.0V and 3.5V ...


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.


9

GPS can be spoofed without decrypting or creating signals. Therefore, the system cannot be made secure by cryptographic signatures. The conceptually simplest way to spoof is to erect a number of highly directional antennae and point each of those at a GPS satellite, such that it receives exclusively signals from that satellite. Then feed those signals ...


8

GPS has two (maybe more - newer ones) startup modes. Cold start is when GPS doesn't know any satellites and tries to acquire the satellites. Once the number of required satellites are acquired, GPS gets current coordinates and time information from them. After it receives that information, it updates its local caches and RTC. The other mode is warm start ...


8

For 1.5GHz the exact type of capacitor is important as well the the capacitance value. Many types of capacitor will self resonate well below 1.5GHz. The capacitor does not act like a perfect capacitor, instead it acts like a capacitor with an inductor in series. If your capacitor was perfect and your antenna impedance at 1.5GHz is entirely resistive then ...


8

When you need position (and/or attitude) data at a high sample rate, the usual technique is to combine inertial measurement (e.g., MEMS accelerometers and rate gyros), which give you relative movement data at high sample rates, with GPS information that comes at a relatively slow rate (e.g., 1 - 10 Hz). This information is combined in a system model (e.g., ...


8

The receiver maintains its own internal timebase, and some of the unknowns that it needs to solve for are the frequency and phase offsets between that local timebase and "GPS system time" as inferred from the received signals. Once the receiver has those values, the 1PPS output is generated from that timebase. There is no "direct connection" to either the ...


8

I would not advise to use an integrated GPS solution (containing MCU and closed source firmware) for a satellite application. There are several reasons why this might fail to work: The frontend frequency plan might be optimized for a limited doppler range. Typically, the RF frontend will mix down the signal to an IF lower than 10MHz (higher IF will require ...


8

Part of the answer here is that cell phone GPS isn't just GPS. Cell also use other information for geolocation, such as triangulation of cell phone towers and the visibility of wifi networks. For example, the non-cellular version of the iPad Air doesn't have actual GPS but still knows where you are in built-up areas using these techniques.


7

You want a compass. Like the normal analog ones, it senses the orientation of the earth's magnetic field.


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