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I am a systems engineer working with various instrumentation devices for ground platforms. These platforms can vary in speed from 0-60+mph. Current instrumentation provides TSPI at 1Hz and is based on GPS. We are looking at a new system that claims to provide TSPI up to 20Hz. I have no additional details at this time on what additional sensors are in the loop.

Since GPS uses 1PPS, how do you get 20Hz TSPI? IMUs?

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    \$\begingroup\$ You are mixing up two different things. Most GPS modules provide a PPS signal for timekeeping purposes. But there is nothing to stop a GPS module from calculating fixes at a faster rate. And as you hint or suggest, if you have an IMU, you can also potentially use dead reckoning to estimate position in between fixes (regardless of fix rate). Some high end GPS units are export controlled in the USA. \$\endgroup\$ – mkeith Oct 5 '15 at 22:02
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    \$\begingroup\$ Be aware that even if you get a 10 Hz GPS, the position output is the result of a filtering process. It could re-compute position as often as it likes, but for the output to be sensible, it is smoothed over a long time, perhaps a second or more. For example the maximum acceleration might be limited to 2 g, which could make its step response to a sudden stop slower than 1 second anyway. \$\endgroup\$ – tomnexus Oct 6 '15 at 3:54
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All modern GPS receivers will provide you with a position update rate of 5Hz or 10Hz (e.g.MTK3339). However, the speed signal the GPS provides is derived from the satellite signals and not from the position update rate. The speed can alternatively be calculated from one position to the next one (difference of position divided by passed time). If that method is used a higher position update rate is desired because a sudden change of speed will need too long to be visible.

As someone else mentioned the 1PPS has nothing to do with that, it is only a clock reference independent from speed or update rate.

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  • \$\begingroup\$ All, Thank you for the information. I assumed that the signal from the satellites was at 1Hz and that was the restriction. What is the rate coming from the various satellites? \$\endgroup\$ – DJGlover7 Oct 7 '15 at 16:09
  • \$\begingroup\$ The limiting factor is the processing speed of the GPS receiver as well as the artificial noise on the consumer GPS frequencies. I would guess that the military GPS offer much higher update rates and also much better accuracy. But it does not sound like you need to control fast moving objects like rockets in your application. \$\endgroup\$ – optronik Oct 7 '15 at 19:46
  • \$\begingroup\$ It's difficult to pin a position update rate on the satellites as it's all in the receiver. The satellites simply transmit orbital ephemeris data and the time of day at 512 bits per second and a CDMA chip rate of 1.023 MHz, all precisely phase locked to an atomic frequency standard. The GPS receiver maintains a lock on the CDMA spreading code and uses that to determine the time of arrival differences between the satellites. Getting a lock in the first place takes a while, but after that the position can be updated at a rather high frequency. I'm not sure what the upper limit on that is. \$\endgroup\$ – alex.forencich Oct 9 '15 at 6:06
  • \$\begingroup\$ I should also note that even at 1.023 MHz, the chips are still very 'rough', each one corresponding to almost 300 meters. So the GPS receiver has to lock on to the chip edges very precisely so it can get a fix accurate to a couple of meters. \$\endgroup\$ – alex.forencich Oct 9 '15 at 6:12
  • \$\begingroup\$ And the ephemeris data is transmitted at 50 bps, not 512 bps, I remembered incorrectly. \$\endgroup\$ – alex.forencich Oct 9 '15 at 6:14
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It's certainly possible for a GPS to update faster than 1 Hz, it's just that the majority of GPS receivers report the updated position on 1 Hz intervals. I have personally used some GPS receivers that output position updates at 10 Hz, but they use a non-standard binary output format that's much more compact than the more common text-based formats.

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As other answers have alluded to, there are a number of challenges and errors which will or may be encountered when trying to get fast GPS positioning / logging with 'real' accuracy @ >1Hz. You CAN get 10Hz modules, but you may or rather will have more noise. There are also limits placed upon consumer GPS modules in terms of speed and altitude but it does not seem you would / will be approaching these.

Personally, I would combine an IMU with your GPS data, using the GPS updates for absolute position and IMU to generate the intermediate points / positioning data. You can achieve a very acceptable level of accuracy for most applications this way, along with additional data. This is commonly how a number of applications achieve fast update - and military applications as alluded to will not rely solely on the GPS alone, since GPS can be jammed quite easily, in which case where other tracking systems are unavailable the 'vehicle' (whatever it is), will rely upon calculations from last known fix, using tech like or same as the IMU.

EDIT: - Just realized this Q is a year old - well, hope this helps someone :)

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