Bushwalk Australia

Satellite GPS Differentials! Are they here at last?

Have a look at the picture below and note the little 'D' in the signal bars which denotes a differential signal from the satellites and it's the southern hemisphere.



Not sure why this is happening because I thought that differential sats were only for North America (WAAS). The satellite map image shows multiple sat positions at 45° from the horizon line so it's not just at the horizon either!

Could it be that us southerners are getting some WAAS sats after all?

The NMEA numbers tell the story!

As I suspected in my previous post here: viewtopic.php?f=21&t=20169#p269419, it is the Indian GAGAN GPS satellite network. Satellite NMEA 41 is India's Gagan GSAT-10 (PRN 128). It went live in May this year, I think their system is meant to be fully operational by 2016.

To explain the other sats in your screenshots, Garmin use the satellite NMEA 01-32 for GPS, and 65-88 for Glonass. If you see any other satellites in the 33-63 range, these will be Space Based Augmentation Systems.

Here is the pic I forgot to post to the previous thread showing the potential coverage of the Indian GAGAN system (from Africa to Australia):


The dots on the equator are the positions of the various geostationary satellites.

Boo for me though, I've not seen the SBAS birds show up on any of my Garmin GPS in Melbourne.

Interesting! How come all the Ds are in the GPS satellite range? What does it mean? In the meantime, I always thought that WAAS was a ground based system in N America. Ummm... I need to brush up.

What the guvmnt wants us to know about DGPS without hacking the systems...



https://www.amsa.gov.au/navigation/services/dgps/

Differential Global Positioning System

What is DGPS?

Differential Global Positioning System (DGPS) is a method of providing differential corrections to a Global Positioning System (GPS) receiver in order to improve the accuracy of the navigation solution. DGPS corrections originate from a reference station at a known location. The receivers in these reference stations can estimate errors in the GPS because, unlike the general population of GPS receivers, they have an accurate knowledge of their position. As a result of applying DGPS corrections, the horizontal accuracy of the system can be improved from 100m (95% of the time) to better than 10m (95% of the time).

More importantly, the reference stations provide integrity monitoring, warning users to disregard a satellite which is operating outside of specification. With DGPS, this warning happens within a few seconds of the satellite becoming 'unhealthy', compared to GPS warnings where some hours can elapse.

The most common form of DGPS used internationally for maritime navigation operates in the MF Radiobeacon band (285-325 kHz) and conforms to the RTCM Recommended Standards for Differential Navstar GPS Service for the transmission of data. This form of DGPS uses pseudorange corrections and range-rate corrections from a single reference station which has sufficient channels (typically 12) to track all satellites in view. Pseudoranges (distance measurements) are simultaneously measured to all satellites in view, and using the known (surveyed) position of the receiver's antenna and the positional (ephemeris) data from each satellite, the errors in the pseudoranges are calculated. These errors are converted to corrections and are broadcast to user receivers.

Differential Global Positioning System (DGPS) Fact Sheet [PDF Icon PDF: 124KB]
https://www.amsa.gov.au/forms-and-publi ... _Sheet.pdf

So, is it GPS system specific?

GPSGuided wrote: How come all the Ds are in the GPS satellite range? What does it mean? In the meantime, I always thought that WAAS was a ground based system in N America. Ummm... I need to brush up.

On my Oregon, when in Europe and using EGNOS, the D shows up in the bar when the SBAS data is located and shows the D above the satellites it is going to apply the correction to. When the SBAS data gets applied to the satellite data, the D changes colour/goes solid signifying that satellite's data is now is Differentially corrected.

The AMSA DGPS is a ground based augementation system (GBAS). There's a picture in Zone-5's link that shows the locations of these beacons (on the coastlines) and the coverage and it looks like this:


Of course, if you are on land, within the coverage area, you'd get the differential corrections if your GPS could receive the data.

GPSGuided wrote:So, is it GPS system specific?

Handhelds like the Garmin Oregon/Etrex currently only do differential corrections based on SBAS. I think some older Garmins could take differential from GBAS, so you need GPS receivers that can accept the DGPS data input, like for marine use, they use a separate antenna.

Check this out! Multiple 'D' symbols over all GPS satellite numbers...

I took a series of screen captures on a hike today between 10am and 3pm...



The one thing I did notice that in some cases while the 'D' symbols appeared the accuracy went up to 1m!

When the 'D' symbols disappeared the accuracy dropped down to 4-6m average.

Definitely something is going on here!

That's interesting. You're also picking up the Japanese MTSAT Satellite Augmentation System (MSAS) which is NMEA 42 (PRN 129). What latitude are you when you pick up the 42 satellite?

I haven't been able to see any of the Indian GAGAN or Japanese MSAS SBAS in Melbourne on my Garmins, but I'll take my kit out tonight and have a test.

I do have another GPS receiver which has GPS/Glosnas/Beidou capabilities, so I'll take that out as well and see if they can see any of the geostationary satellites.

keithy wrote:I do have another GPS receiver which has GPS/Glosnas/Beidou capabilities.

These units exist?

BTW, which Garmin unit are you guys getting those screen grabs? I don't get the same level of detail with my Oregon 6xx.

GPSGuided wrote:which Garmin unit are you guys getting those screen grabs?

keithy wrote:What latitude are you when you pick up the 42 satellite?

Sydney 'Blue Mountains' latitude on my Etrex 20 (v4.30/16GB).

GPSGuided wrote:...I don't get the same level of detail with my Oregon 6xx.

...this is what your garminoregon6xx 'Sat Page' should look like: same level of detail!

http://www.tramsoft.ch/gps/screenshots/ ... tes_en.jpg

Thanks Zone-5. Interesting the screen grab seemed to show more detail than just looking at my O650's screen and the eTrex 20 (smaller screen and lower pixel count) seemed to have more info on that satellite screen. Not seeing any 'D' symbols yet here in Sydney with WAAS activated. Ummm...

GPSGuided wrote:...the eTrex 20 (smaller screen and lower pixel count) seemed to have more info on that satellite screen...

The Etrex 20 screen captures are OS generated in the unit and dumped into the memory and not raster screen images as suggested.

GPSGuided wrote:These units exist?

I have a few newer tablets/phones that use Qualcomm's IZat Gen8C chip which does GPS/Glosnass and Beidou. They pickup satellites 201, 203, 205 and 207 in Melbourne at least. But they currently don't use them for geolocation. Ridiculously quick satellite lock from these things - without wifi or a sim card in either device. I'm not sure why the signal acquisition is so quick, whether it's to do with the antenna, or the internal Android clock that keeps a rough location from the last time the GPS locked.

BTW, which Garmin unit are you guys getting those screen grabs? I don't get the same level of detail with my Oregon 6xx.

On the Oregon 6xx go to Setup/Display/Screen Capture -> ON and you use the power button to take screen shots, saved as 240x400 96dpi bitmap files in the \Garmin\scrn directory. Unfortunately there is an annoying bug on the Oregons that when the screen capture is enabled, all User Button functions are disabled.

GPSGuided wrote:Not seeing any 'D' symbols yet here in Sydney with WAAS activated. Ummm...

Have a look for satellites in the 33 - 54 range. These are SBAS needed to get the differential data and the "D" to appear. I'm not seeing any of them either in Melbourne at least. On the Oregon, I can't remember the colours but the "D" in the satellite page appears when SBAS satellites are located, and the "D" changes colour when the Differentiation is applied to the GPS data.

Thanks guys and I'll play and learn some more of my little unit. They are so feature rich that it's almost impossible to know and remember all.

GPSGuided wrote:Thanks guys and I'll play and learn some more of my little unit. They are so feature rich that it's almost impossible to know and remember all.

It's the lack of any meaningful 'Reference Manual' ever produced from Garmin that make it almost impossible to remember features especially when most don't even know what features are actually available...

Boo to Garmin, again!

- - - - - - - - -

...this might help discover those unknown unknowns:

http://www.tramsoft.ch/gps/garmin_oregon600_en.html

Fortunately I find reading manuals a bore. Much faster to just rummage through the settings and see what's there. With a vibrant net community on these units, not too much is lost. I am not complaining. In any case, a highly detailed manual will be out of date with successive versions of its firmware. Can see why Garmin and other tech companies have opted for a basic version in print.

Heh... I'm a doofus. I realised why I wasn't seeing the SBAS satellites when I went outside last night to test my Oregon 600 and eTrex 30. I normally use a profile made up for Aus walking (GDA94, UTM, Glosnass off, and Waas/Egnos off) .

I forgot to turn it back on when testing. Only after I took the first load of screenshots, I realised that this was why I wasn't seeing any of the SBAS satellites.

Anyway, turning Waas/Egnos on and setting the GPS to both GPS/Glosnass, these are the screenshots in the exact same location (Oregon and eTrex side by side).



I am seeing the Japanese MSAS 42 & 50 satellites, and the Indian GAGAN 41 satellite popped in briefly on the eTrex as well, but disappeared quickly and never came back. The "D" on the Oregon blinks when it locates the Differential data, and goes solid when it applies the differential information to the satellites it is using for geolocation.

In this case, the Oregon was using the Japanese MSAS 50 for differential data while the eTrex didn't see the 50, and was using the 42 satellite. The eTrex took about 10 minutes longer to download the SBAS satellite data. This is shown by the satellite number blinking on the satellite page. And the "D" blinking as well. The Differential Data received from the two different sats though were applied to the same Satellites on both the Oregon and eTrex with the exception of satellite 5 missing the differential data on the eTrex.

With the differential data from the geostationary satellites, the EPE/Accuracy improves around 3-4 metres. From my screenshots above, my EPE is around 4-5m. With the Waas/Egnos turned off in the same location, the EPE increases to around 6-8m on both devices.

Ignore the elevation differences, as I had not calibrated the elevation data - I have both barometers set to manual calibration. I did have some three storey buildings close by, which is probably why the EPE/Accuracy with Waas on is still around 4-5m.

Still, it's good to know that we getting some SBAS differential data from the Japanese and Indian systems, and it might improve accuracy, so I'll have to change my default Aus settings now and see how it goes.

I don't agree with your sentiment... I am allowed to do that, yes?

...the base functions of all GPS's have remained the same with only 'new' features being added. Unless you have come to grips with the base functions which can only be understood by reading a good GPS manual or navigation training manual then the great fun you can have with these can only be discovered by accident.

Last time I posted the best tech manual available on the net here, I got poo-pooed for it!

my 2¢...

Glad you are seeing them now because I was convinced my unit had a software fault! Phew!!!

Ummm... Still not seeing any 'D's here in Sydney (WAAS/GLONASS both activated). On those SBAS differential data from locations well outside of Australia, would they still have the same level of accuracy when applied in Australia? Is it reasonable to assume that ideally the source of correction is near the user, or at least on the same continent.

GPSGuided wrote:Ummm... Still not seeing any 'D's here in Sydney (WAAS/GLONASS both activated).

Look out for those satellites in the 33-50 range. When the GPS sees them, it might take a while to download the data before the "D" even appears. The satellite number will blink while it is searching/downloading data. For my eTrex, it took about 10 minutes longer than for the Oregon to download the data from these geostationary satellites.

The ground reference stations are in totally different continents, so I had assumed that our distance from the reference stations meant that the SBAS data wasn't applicable. Much like GPS receivers in South American can see the WAAS satellites, but without ground reference stations in proximity, the data isn't useful. But interestingly, when I turned off WAAS/Egnos the EPE/Accuracy on both GPS devices increased.

I'll have to do more testing to see if it was a fluke, or whether the differential data actually makes a difference.

For Australia, I don't think there's been a review on the establishment of an SBAS system for a while. I remember reading when the Indian GAGAN system was announced that they would have capacity for ground stations in Australia given it's coverage.

I've only been getting them for the last 3 weeks only!
At first thought it was a glitch but even my Etrex 'H' (yellow) is getting them now.
If the Nuvi get's them then they are real, must test that ASAP!

I'm getting some real different numbers for satellites of late!
Are there any new ones that have just gone up?

Zone-5 wrote:Are there any new ones that have just gone up?

Not to my knowledge. What satellites are you seeing? The numbers will make it easier to identify which system it belongs to.

I know that the Chinese Beidou/Compass system is proposing launching something like a total of 20 satellites this year. A few of them I have seen on my Beidou capable receivers (These are in the 200-230 range).

India's third satellite GSAT-15 was scheduled between 2014 and 2016, but I don't think it has launched yet (was planned for later this year).

If you're curious, have a look at the FAA's website which has the real time satellite positions and WAAS Status http://www.nstb.tc.faa.gov/RT_WaasSatelliteStatus.htm

And I have this page bookmarked for satellite outages http://www.navcen.uscg.gov/?Do=constellationStatus or http://adn.agi.com/SatelliteOutageCalen ... endar.aspx The PRNs equate to the Garmin Satellite Numbers

The Russian Space Agency also has a page with the status of the Glonass Constellation http://www.glonass-iac.ru/en/GLONASS/

Funny how GLONASS and GLASNOS look so similar?

keithy wrote:

That last sat bar of yours Keith that indicates a '255'!
Where the heck is that one from...

...and should I expect to see this and others in that range as well on my E20 also?

Zone-5 wrote:That last sat bar of yours Keith that indicates a '255'! Where the heck is that one from...

I'm not sure, but the 255 seems to pop up only on my eTrex 30, not on my other GPS devices. And not all the time. I've noticed it populates the bottom row only when the Glonass satellites haven't aren't taking up the whole row. With the bottom row populated by Glonass satellites, the 255 doesn't appear.

The original guidelines set by IS-GPS-200F had the PRN assigned satellite numbers 120-158 reserved for SBAS, 64-119 and 159-210 for other Global Navigation Satellite systems. They've added the 200-235 range for Beidou satellites now I believe.

On the Garmin, Satellites 1-32 are GPS, 33-63 are SBAS, 65-88 are Glonass (I think this has increased to 96 for Glonass etensions). I haven't done much reading on the Beidou satellites, but I was sure that the top range was 235. So I'm not sure what the 255 satellite signal is.

I'm guessing here, but given that the 255 satellite only shows up when I have GPS+Glonass enabled, and WAAS/EGNOS on, it could be is one of the other dGPS signals that broadcast on the same L1 frequency, like the John Deere's Starfire system, or Fugro's Starfix, or Ominstar's system. With GPS only, or WAAS/EGNOS off, 255 does not show up.

What I'm not sure about is why it shows up on the eTrex 30 chip but not the Oregon 600 or my other GPS devices.

I've done a bit more testing on the Differentials, and noticed that during the day in Melbourne (I've tested around about midday to 5pm), I rarely seem to get the differential satellites.
The screenshots I took above were taken around about 10pm. Not sure why that would make a difference or whether it was an abberation.

I did some digging around, and thought it might be the Japanese QZSS. I haven't found out the NMEA ID of the QZSS, but they have this nifty tool: http://qz-vision.jaxa.jp/USE/en/qz_radar

It shows that the QZSS isn't viewable during the day due to it's geo synchronous orbit http://www.navipedia.net/index.php/File ... 5-0_09.jpg, but is visible on the horizon where I am at night.