Cross-Constellation Solutions

I wonder if is extra cost on my Topcon gear? The latest release of Magnet 2.7.1 is cable of using many constellations. But even I turn them all on, I only see GPS and GLN.

Not really enough SV's

From Topcon website:
Topcon receivers with Vanguard technology are already capable of supporting all planned signals for GPS, GLONASS, Galileo, BeiDou, and QZSS. This support will be incorporated in Topcon products when these constellations have matured and are ready for commercial use.

Current GNSS available.

GPS
SV's: 32
OK: 30.

GLN
SV's: 24
OK: 24.

BDS
SV's: 13
OK: 13.

GAL
SV's: 4
OK: 3.

QZSS
SV's: 1
OK: 0.

You have to see two satellites from a given constellation simultaneously to use them in your positioning solution.

I have not seen a price from Topcon to enable Galileo tracking. And I don't know about Beidou. With 11 or so, perhaps it could be useful.

after you get to enough, I wonder how much better the solution would be with more satellites...

Instead of simply more satellites, I was thinking (in some cases) better satellites.

The GPS and GLONASS constellations are both pretty old. I was wondering if a piecemeal approach could be applied to get 5-6 more modern signals.

A long time ago I made a list of which active SVs were "modern" across all constellations, you could see over five for a bit every day. The problem is I'm not sure which ones that are capable of broadcasting modern signals actually are. For instance, a handful of the new GPS SVs have been up there for a while now, but they only recently began sending a navigation message on the modern signals a few months ago.

I Would Not Dewpend On Two Satellites

You really need 4 minimum of a constellation to get a viable solution. However a precise solution may require 6 or more satellites from a single constellation. In my personal opinion I would wait a few years before investing in other than GPS and GLONASS.

Paul in PA

I Would Not Dewpend On Two Satellites

With respect to age of the satellites in service, the GLONASS constellation is relatively young. Especially compared to the time in service for the GPS constellation.

You can see a status report on their constellation here: GLONASS Status page

I Would Not Dewpend On Two Satellites

But what precludes the use of:

2 GPS
+
2 GLONASS
+
2 BeiDou
+
and so on

???

I Would Not Dewpend On Two Satellites

According to my genius son Daniel, the only receivers that will fix with less than four GPS satellites are Ashtech units.

Everyone else requires a minimum of four GPS satellites to fix a solution.

And as for Topcon, which is the only brand I know much about, you must see at least two of any constellation beyond that to be able to use the data from those birds in your positioning solution.

Ten years ago when the GLONASS constellation was in shambles, that was a real problem. But now day, with their constellation fully deployed, with spares in orbit, the GLONASS constellation always contributes at least four or five birds to your constellation in view.

I Would Not Dewpend On Two Satellites

Can you get a fix solution if the receiver only sees 3 GPS and 3 Glonass?

I Would Not Dewpend On Two Satellites

> According to my genius son Daniel, the only receivers that will fix with less than four GPS satellites are Ashtech units.
>
> Everyone else requires a minimum of four GPS satellites to fix a solution.

Hi Jerry,
These are screen captures from a Triumph LS from Javad. It runs with six parallel RTK engines which each use slightly varied algorithms for their solutions.

The number in the upper right corner of each block is the GLONASS count used in the current solution. In the upper left is the GPS count used in the current solution. The GPS satellites were intentionally disabled to see if the LS could fix with GLONASS only. I wouldn't want to depend on this as the time to fix was slower and it isn't often that I see seven GLONASS satellites in view at a time. But it was an accurate solution and shows that it can be done.

I have tested an Altus-APS3 (version 2) receiver that was able to fix with only 2 GPS satellites, but only Javad seems to be able to fix GLONASS only (that I am aware of). I'm curious if Trimble's R8-3 or R10 is capable of GLONASS only.

By the way, I still think Daniel is a genius.

I Would Not Dewpend On Two Satellites

Yes, part of Ashtech's Blade is a GLONASS only fix. It is outlined here:

http://www.ashtech.com/products_technology.html

I Would Not Dewpend On Two Satellites

my screen captures weren't very legible.

I Would Not Dewpend On Two Satellites

ah. cool. thanks.

I Would Not Dewpend On Two Satellites

I think this discussion has deviated from my original question. If you told the Triumph LS unit to only use two GPS satellites, two GLONASS satellites, two Beidou satellites, and two Galileo satellites would it still produce a fix?

Given that the two of each you pick are in view/viable.

I never meant two satellites period. I never even said two, was wondering if a lone Galileo satellite would add to the solution?

I Would Not Dewpend On Two Satellites

I'll see what I can find out.

I Would Not Dewpend On Two Satellites

Cool, thanks.

I Would Not Dewpend On Two Satellites

Not quite, but close I believe. The SP80 is a good example of the 'old Ashtech' but now 'Spectra Precision' Z-Blade technology.

The SP80 (and the PM800) will fix with only Beidou, only GLONASS, only GPS SV's (I would assume that it would work GAL too, if there were enough GAL SV's.) I have some pictures of GLONASS only fixes [ here] on a PM800.

But the really neat trick is you can fix with any mix of signals. So 2-GLO + 1-GAL + 2-GPS is good enough for a fix (depending on SV locations of course.) There is a neat picture and link to a white-paper [ here]

You pretty much need matched base and rover and need to use RTCM 3.2 MSM or one of the Atom correction formats. While Blade will back-compute corrections for SV's that are not tracked on the base, it seems iffy to me and I think you need to try to track everything at the base that is possible.

While all of these sparse constellation possibilities are intriguing, I think the reality is most of the time when I venture out now, I have so many SV's that it makes my head spin.

M

I Would Not Dewpend On Two Satellites

>
> But the really neat trick is you can fix with any mix of signals. So 2-GLO + 1-GAL + 2-GPS is good enough for a fix (depending on SV locations of course.) There is a neat picture and link to a white-paper [ here]

Thanks Mark, this is exactly what I was wondering about with my original post. Now I know at least one technology is capable. I was mostly wondering if someone made a receiver that could take advantage of all of the "modern" signals available, across multiple constellations to see if it would perform better under canopy (where better signals are more important than more satellites).

Repeat, I Would Not Depend On Two Satellites Per System

Getting a solution as Mark Silver indicates is good, but as surveyors we require more than just a possible solution.

Understand the whole geometry of GPS surveying. We are determining points on the ground to within 0.01' from satellites bouncing around in the sky 11-16,000 miles away. We have a very good idea where the GPS satellites are because we have a worldwide system to observe them precisely. We have hundreds of full time CORS stations to use for comparative analysis. The US recently refused the Russians the opportunity to place GLONASS monitoring stations in the USA and it is doubtful the Chinese will get an OK either. GPS orbit data includes corrections over various parts of the globe.

I am less able to discuss the timing issues from constellation to constellation, but it to is a problem.

I would not accept a multi constellation position as correct unless it can be verified by other means. A separate GPS position may be checked by a separate GLONASS position which may then be checked by an independent Galileo/Bedoui solution, but I would not let a few other constellation observations skew my known good data. More data is not always better data.

At times it may be necessary to accept whatever position you can get out in the field and the only reasonable verification is a full blown traverse through all GNSS observed points.

Mark, does recent software weigh the different precisions in broadcast orbit data?

Paul in PA

I Would Not Dewpend On Two Satellites

My initial thought is 'it depends'. If you are talking Static I see room for improved solutions. If you mean RTK I suspect you will simply get more varieties of bad data...