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?ÿThe attached pfd article explains the meanings of DTM, GGA, and GNS NMEA sentence types.
Have you ever looked at your smartphone??s displayed ground elevation, often referred to as Alt MSL, and wondered if something wasn??t quite correct? Well, trying to get to the bottom of it will take a little effort, but thanks to the growing list of Android apps capable of capturing NMEA sentences, most of the work is in deciphering the terse NMEA messages.
There are 2 primary sentence types to look for, GGA and GNS as they contain the geoidal separation values(1) from which the orthometric (H) values are derived. To discover the correct geoidal separation (N) value for your location, search online for interactive calculation tools(2), and then compare it to the value in the NMEA sentence. The DTM message is also worth looking at as it can play a role in determination of heights(3).
Most apps will give you your ellipsoid height (h), often referred to as Alt, and some apps may only display this value.
There are many Android apps capable of logging NMEA messages. You will need only one for this request: 2 to 3 minutes worth of static data on any station under open view of the sky. If you're undecided, these 3 apps offer NMEA logging, and much more.
- GPSTest (barbeauDev) - can also log raw obs!
- GnssLogger (Google) - can also log raw obs!
- Ultra GPS Logger (FlashLight)
My Ask of You
Email your results with any screenshots, and please include the name of the manufacturer, make and model of your android device to kellybellis at gwi.net with the subject: NMEA data for you!, and thank you very much for your participation!
(1) Geoid models that are used by chipset manufacturers and OEMs may vary
(2) E.g., xGEOID20 Interactive Computation Page; Computation of GEOID18 Geoid Height; GeoidEval; Geoid Height Calculator (EGM96); any my personal free stand alone fav, VDatum.
(3) Local geodetic datums and datum offsets from the reference datum used by chipset manufacturers and OEMs may vary
Wow.
The GnssGoogle thing is awesome!!!
My Ask of You
It's not clear to me what information you want:?ÿ just a couple minutes worth of logged data in an open area plus device info and a screenshot, or does the data have to be taken at a known geodetic station, along with antenna height?
Hi Jim,
Thank you for asking and for considering my request.
The minimum ask is for the NMEA logged (static) data from your Android device's internal GNSS taken in an open area for 2 to 3 minutes, whatever screenshots you like to include along with the manufacturer, make and model of the Android device. If you want to setup over a known point (which would be absolutely wonderful if you want), please include your ARP Hi and published coordinates for the control point; e.g., link to an NGS datasheet, or OPUS report would be fine.
By the way, GPSTest (barbeauDev) has a share feature which crowd sources the GPSTest database, the purpose of which is to document the multi-constellation multi-frequency GNSS capabilities of Android devices.
Kind regards,
Kelly
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I have looked at my elevations on my Android. It is "off" by the Geoid Height. An interesting factoid, I do not know how to collect GPS data on my phone, good luck getting what you want.?ÿ
Thanks Moe. If you have an iOS device, I'm not sure what options are available, if any; however, Android devices are supported and app developers are starting to take advantage of this. But to be clear, I'm not asking for RINEX data, just the NMEA logs.
From the developers page:
Raw GNSS measurements support is mandatory on devices that run Android?ÿ10 (API level 29) or higher. On Android 9 (API level 28) and lower, raw GNSS measurements support is mandatory in all Android devices that contain hardware year 2016 or newer. Currently, 84% of existing Android phones have raw measurements.
The support for some of the raw GNSS measurement fields is optional and can vary based on GNSS chipset used. Examples of these fields include the following:
- Pseudorange and pseudorange rate.
- Navigation message.
- Automatic Gain Controller (AGC) value.
- Accumulated delta range (ADR) or carrier phase.
Kelly, I do this everyday, but with an app. I resurrected a old S7 that can not do it, but the app and an external antenna can. Are you looking for data collected this way?
Thank you for the question and for considering helping out. The main idea is getting NMEA sentences from the internal GNSS receiver's chipset, for example, on your Samsung Galaxy S7, insomuch as the Android device and its OS version supports it. In theory, NMEA sentences ought to be supported on your old smartphone; e.g., Android v7 and up.?ÿ There are many Android apps capable of logging NMEA messages and any that you want to use should be fine. DTM, GGA and GNS are the lines we're hoping to see. Likely, on an older *GPS only* device, they'll be seen beginning with GP, the code for USA Space Vehicles (SVs), but if GLONASS is also used on your device, the 2-character prefix may be GN, code for mixed GNSS.
$GPDTM, $GPGGA, $GPGNS, or $GNDTM, $GNGGA, $GNGNS,
Using an external antenna; not a so-called Smart-Antenna, should still allow the NMEA sentences to reflect what the Android device's internal GNSS receiver is reporting.
So-called Smart Antennas are really more than just a GNSS antenna and include the GNSS receiver all in one package.
Kind regards,
Kelly
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The reasons for my requesting the NMEA data is concerning datums because of what I have so far observed from a small set of devices. In short, I need more data. I've written to Google about this more fully here: https://issuetracker.google.com/issues/191375130
The larger issue I'm seeing in my tests is the whacky geoidal separation values as revealed in the NMEA GGA and GNS. Most apps reports the ellipsoidal heights and they're all within a reasonable range of values.
Based upon the small dataset provided as of this date, the errant geoidal separation (N) values appear to vary for a single given location based on the specific Android device being tested.
Using the horizontal position revealed through the NMEA sentences (GGA and GNS), point features were first created, followed by loading NGA's EGM2008 grid ( https://earth-info.nga.mil/ ) into the workspace, and then Apply Elevations to the Selected Features using NGA's EGM2008 surface model as the source 'terrain'. Lastly, the NGA (N) EGM2008 values were subtracted from the NMEA geoidal separation (N) values. These were their differences.
I could still use more NMEA data (DTM, GGA & GNS) from more devices, if anybody is willing.
Thanks!
Kelly
@kelly?ÿ
I can try to find some time this weekend with my OnePlus 8 pro, but we're very busy packing to move to our new home, so no guarantees.?ÿ