Accuracy vs Precision. So they can pin your position to within 30 centimeters, but how small of changes can be detected? Can they detect relative movement on the order of 1cm or less? that would be really useful for some things. For example if you wanted to measure the distance between 2 points on the ground, you don't care about offsets but do care about precision.
You mean absolute vs. relative measurement. Accuracy vs. precision is a different concept.
It is possible that a setup with high-precision absolute measurements can produce high-accuracy relative measurements, but the former does not imply the latter (imagine, for instance, any source of error in absolute measurements that's locally non-linear).
>Can they detect relative movement on the order of 1cm or less?
The short answer is no, largely due to localised ionospheric conditions. The longer answer is yes, GNSS systems can be enhanced in accuracy using RTK [0][1]. This allows for precision on the order of 1cm, but requires a (rather expensive) fixed position base station to provide corrections. Surveyors, precision ag, and other large machine control (think dozers, graders, etc) have used this technique for precision positioning for the past decade or so.
I do recall reading that upcoming GPS satellites add a second civilian frequency, which I believe will enable these sort of ionospheric corrections to be done more easily and inexpensively.
I don't know quite enough about the technical details to be sure, but RTK can defintiely go over IP networks with NTRIP and other protocols. The biggest limitation with RTK is that the corrections are limited to 10-15 miles of the base/reference station (more is possible, generally with less accuracy), since ionospheric conditions vary enough to make the corrections highly location dependant.
The antenna systems on RTK rovers (the devices receiving corrections, which also receive the GPS satellite signals) aren't huge, but I'm not sure if they could be reduced in size enough to fit in a smartphone.
The technology in good receivers like the Piksi is slowly drifting into consumer-level hardware. You need to correct for such errors as:
1. Jitter in the code phase vs. phase-locked-loop. As the article describes, most existing codes are long and slow, with bandwidths on the order of 300 m (specifically, 1.023 MHz). The receiver can measure the exact time at which this code changes to get your existing resolution, but the L5/E5 bands are at 20 MHz.
2. Integer offsets when interpreting the carrier frequency. The above slow codes are transmitted on a carrier at 1-2 GHz, giving much higher resolution, but there's no information in the signal as to which particular node of the carrier frequency you're observing. Comparing to a known location, and improving your guess over time, can let you make use of this information.
3. Unknown, slowly drifting ionospheric delay. Typically fixed in industrial or agricultural applications by using a base station and radio link to tell your remote link that the base station (which is bolted to a big chunk of concrete) is now reporting that it's 20 cm from where it was an hour ago, and the remote unit should probably just adjust any measurements by that much.
I only have enough knowledge of the system to be dangerous, but I've wondered whether it would be possible to correct for #3 at a consumer level with a phone app. If you had thousands of phones cooperating in a city, at any given time many would be stationary, even charging or on wifi, and you could theoretically trade off roles as reference base stations and remote receivers. I think it would require a lot more low-level access to the GPS chip than generic Apple/Android phones give you, but it's an idea - feel free to take it and run if you like it.
To answer the basic question - common NMEA protocol returns GPS data with latitude formatted as DDMM.MMMMM (Degrees, minutes, and decimal minutes) and longitude in DDDMM.MMMMM format. Four digits of precision get you a precision of about 1.6 meters, depending on where you are on the Earth, but that doesn't mean you have that level of accuracy.
Indeed, carrier phase and multi-frequency+constellation measurements are drifting into consumer hardware, but I can say that it's probably going to be a Good Long While before we full realize its benefits. The use of cell phones (and cheap cell phone antennas) is an active area of research, particularly for organizations aiming to make high-accuracy GNSS positioning widely accessible outside of specialty markets (i.e., agriculture and surveying). It's an interesting area to be in for sure:
The base station network density question @ac29 points out later is a compelling reason for some of this.
(Shill: We're hiring for firmware engineers right now, focused on a variety of different areas: embedded Linux, DSP basebands (C), and navigation algorithms (C++) design and implementation. See https://jobs.lever.co/swift-nav for more details or email jobs@swiftnav.com.)
Iono delay is typically corrected by using a dual-frequency receiver so that it can be calculated directly via difference in arrival times of the time pulse on each frequency. L2C can help here.
It's quite normal for words which might seem to have the same meaning in the common vernacular to have nuanced meanings within a domain. This doesn't just apply to science and engineering - consider law, medicine, religion, food service...really just about anything you can think of. And it's not just an English language phenomenon, either: two words that mean the same (exact, identical, similar) thing really implies there's a difference between the words.
And yeah, as a test engineer, metrology can be confusing, but that's because it's necessarily complex, not because of silly choices.
