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i am writing as essay evaulating accuracy of smart phone gnsss for GIS data collection

for part of the essay i thougt it appropriate to mention DGNSS and Assisted GNSS. So far i understand Assited gnss is using cell towers to send over sateltie information and DGNSS is using basestations to do the same thing.

Is this accurate and are their benefits of one over the other

Tom Haywood
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Assisted GPS does not enhance accuracy but helps with availability. There is a tradeoff between reliability and availiability. Because users want to find the next car rental instead of landing a plane with zero vis, smartphones give you a position, even if it is very rough.

Differential GPS improves accuracy by comparing the receivers signal to one received by nearby receivers with fixed known position. If the signal carriers are used (for example 1575.42 MHz), an accuracy at centimeter level is possible. Carrier differential GPS either needs long observation time at fixed position (tripod and choke antenna) or permanent satellite reception while moving from reference station to the measured point (RTK). Carrier differential is available in handheld systems, but not in smartphones (to my knowledge).

Another variation of differential GPS is Augmentation Systems. These also compare their signal with nearby reference stations, but they do only for the modulation. This improves accuracy somewhat (8 meters DOP best case) by eliminating ionospheric effects common to both stations.

To cut a long story short, surveying with smartphones only works, if you average over a large number of measurements.

Andreas
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  • My answer is just a terse summary of readily available information, maybe mixed with a grain of opinion. I consider this a *lack of Research* type question and I would not mind, if it was closed. – Andreas Feb 20 '19 at 12:23
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I took a look and found this GPS vs A-GPS. It discusses some limitations that you can extrapolate into "benefits".

As GPS devices are communicating with satellite, it can get information only under clear sky conditions and when satellite is reachable without any interference. Due to this it is slower in responding the requested service. While A-GPS devices are communicating with network stations,it gets information even in cloudy atmosphere and worst network conditions,but it will have problem when network is not reachable. Under this conditions AGPS devices fall back to GPS feature if available to provide location information as per user request. A-GPS devices are faster as it need not have to go to satellite for information.

I also saw somewhere else that A-GPS is "faster start-up for acquisition". And, it may be worth mentioning that IMUs are used for those times when GPS signal is missing... and updated more accurately once GPS is reacquired. Found This article on IMU usage:

Current smartphone devices embody miniaturized Global Navigation Satellite System (GNSS), Inertial Measurement Unit (IMU) and other sensors capable of providing user position, velocity and attitude.

CapnJJ
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