# GPS update rate

My thought process: I i'm am looking for a GPS that updates fast enough to actually read its position traveling 130 miles an hour ( I did some math and think 5 times a sec up date rate would get me 17 yard accuracy differing from the datasheet)

MY 1st QUESTION: How would I interpret the datasheet below. I fear I am miss understanding, On page 6 it says maximum update rate is 5HZ for the NEO-6G/Q/M/T but under what conditions? what can I expect from the 6M as far as the update rate be on average? I believe it should have he information listed...

NEO-6 datasheet

My 2nd question: What are the advantages of the 6M and 6T I was wondering if it was worth the extra cash and hoping someone understands GPS compilations I may be over looking...

$20 6M$170 6T

• I think you're misunderstanding the terms. Update rate is how often the GPS shouts its last known position up the serial link (baud rate permitting) and is entirely unrelated to the accuracy of the data. Reading the answer out more often does not mean the answer gets any better. – John U May 7 '14 at 8:22

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., Kalman filter) in such a way that the absolute position data from GPS corrects for the long-term drift components of the inertial measurements. The result is an output that has the accuracy of the GPS, but the high sample rate of the inertial measurements.

Note that the GPS output message includes a timestamp in addition to the position values. The position values give the absolute position of the receiver at the time given by the timestamp. Even if there is a lag in terms of computing that position or transferring that message over a serial link or whatever, the data within the message is always self-consistent, within the accuracy limits of the receiver.

A lot of projects (especially those done by hobbyists) ignore the timestamp and the transmission delay and simply take the position values directly as the "most current" position, but if you really care about high sample rates and low-latency data, this is an overly-simplistic approach. A properly-constructed hybrid GPS+inertial system can provide data that's "current" within a few milliseconds.

The common term for such a system is "AHRS", which stands for Attitude and Heading Reference System, and it's a common element in pretty much any autonomous vehicle control system.

• I guess if your missile or whatever it is is not changing velocity or attitude rapidly (relative to the GPS measurements) you could infer the location without the complexity of an IMU. The inertial measurements effectively give the high frequency response and the GPS the low frequency response. – Spehro Pefhany May 7 '14 at 12:59

For the uBlox modules when they mention the maximum update rate you can normally expect to receive data at that rate. The exception is when the combination of baud rate and GPS sentences selected exceeds what is possible to send at that rate. In that case the receiver will start skipping some sentences, but if you use 38,400bps you shouldn't have a problem in that regard.

The T suffix can be found on page 24 of the datasheet under ordering information and denotes "Precision Timing, TCXO". As well as higher timing accuracy you'll see on page 6 of the datasheet you also get some additional options for the timepulse output. They don't specify the T modules as offering more accurate position information so you probably don't need it unless you need some of the extra precision timing features.

• If i am not mistaken precision timing is used more for altitude calculations so I would say I don't need it in my case, anyways You really helped alot. – Rex Fender Baird May 7 '14 at 6:44