Egnos - I made it!

[Werner]

After deleting the WAAS Almanac data once again a few days ago, I finally managed to get a differential lock. For the last few days, I never received the usually active Egnos birds (33, 37, 39), but received 44 reliably. Living in a hilly area, I never got 44 for longer uninterupted periods while driving and never got a differential lock until today, when I stopped on the way home. It took the iQue about 3 minutes to supply a lot of d's to the other birds.
:)

[jonasolof]

Werner,

Would you please repeat how you deleted the WAAS almanac?

Or rather, for those that have the Non Vol Util, eexactly which library should be deleted?

Only the PWDM WAAS almanac data?

[Werner]

Originally posted by jonasolof
Werner,

Would you please repeat how you deleted the WAAS almanac?

Or rather, for those that have the Non Vol Util, eexactly which library should be deleted?

Only the PWDM WAAS almanac data?
Yes, I Only deleted the (P)WDM WAAS Almanac Data. It is easy to find, as it is the last option of NonVolUtility.

[Bavarian]

Is it the same as ESC & Phonebook & Reset?

[landy110]

But ESC & Datebook & Reset also resets the ique language setting to english.:(
Only a hard reset does help to change the ique-language after WAAS-almanac reset.

[Werner]

On the first day after I deleted the almanac, I received 44 quite often. One day later, the iQue started to scan for all Egnos birds again once fired up in the morning, but only received 44 (without differential lock). On day three (yesterday), the iQue only scanned for 44 and no other bird and I finally got a d-fix after standing still for a short while. For all these days, I used the Gilsson antenna mounted between just behind the Miata's passenger compartment (pretty good look at the sky). Today I used the internal antenna from the dashboard of the Subaru, and I never saw a glimpse of 44, but the iQue never looked for any other Egnos birds. I guess the lack of reception is due to the fact that the roof of the car shielded the "look" towards 44, but it could also be off today. Guess I will see that on the way home when I head more eastward. So it seems my new WAAS almanac just says: Look for 44 and no other birds, and that information was received on day two when I did not get a d-fix.

FWIW (not much, but I find it interesting)

[apersson850]

Originally posted by Bavarian
Is it the same as ESC & Phonebook & Reset? No, it's not. What you describe resets all of the about 40 records that exist in the NonVol RAM. But with NonVolUtil, you can select which records to delete. In this particular case the {P}WDM WAAS Almanac Data. That record contains information about which WAAS/Egnos satellites that are receivable.

[snoopy-doopy]

I deleted the almanac file than I tried to get #44 but I got the same as before , only #39 with a white bar without any 'D'.
What did I wrong :(

[mbvt]

Hmm, interesting... I see we have some birds in common, so I can try this too... :)

[apersson850]

Hide the #39 from the iQue (put your hand over the antenna). Then remove the hand when you've passed #39 in the scanning sequence.

[jonasolof]

Ander's, how do I see that the scanning process is going on?

[apersson850]

Once the almanac has been deleted, the iQue will know nothing about any SBAS. It will only know that satellite numbers in the range from 33 to 51 are reserved for this purpose, and that two of the receiver channels on the iQue are to be used.

So the iQue will start scanning #33 and #34. After a while, if it wasn't successful, it will continue with #35 and #36, and so on, until it comes to #51. Then the scanning starts over again at #33 and #34.

Now, if you have a problem with that #39 transmits a satellite almanac, which doesn't include #44, but #44 is the one you want to use, then you can obscure the antenna while the iQue is scanning for #33-#43, and then remove your hand (or whatever you used) once it starts looking for #44. That makes it possible to receive #44, without listening to #39, for example.

The relation between these numbers and the PRN numbers normally used for the sats, is PRN-87=Garmin ID, in case someone is interested.

[snoopy-doopy]

When I force the 'Q' to get #44 (covering with my hand the antenna) must I have a signal from #44 ??

[apersson850]

If you don't get any signal from #44, the iQue will continue with #45-#51, and then start over from #33 again.
This repeats itself until you turn it off. So if you can't receive #44 from where you are, then you'll never succeed.
#44 is at the southeast, from most locations in Europe. Try tilting the unit somewhat in that direction, and boost reception by holding some fingers under the antenna (poor man's ground plane). Or even better, use an external amplified antenna on a car roof (or a cast iron frying pan).

[snoopy-doopy]

OK I'm gonna trying this , putting my Gilson on the roof of my car and driving on a hill direction southeast.....wish me luck:D

[apersson850]

Where in the world are you, roughly?

[snoopy-doopy]

+- N 49°30.560 E 005°53.011 ;)

[apersson850]

Now I know "roughly".
Well, from there it shouldn't be impossible to receive #44.

[snoopy-doopy]

??? from the country or from this place ???

[apersson850]

From your general area. But if you have a big hill in the most unfavourable direction, not necessarily exactly from your house (if that's what you gave the position for). But if you have a good view of the sky in the south-eastern quadrant, then it should be possible to receive #44. I'm further north (57º), which makes it more difficult for me, but also further east (14º), which makes it easier. So it evens out, to some extent.

I have no problem receiving #44, even if it's not always of any use.

[snoopy-doopy]

The coordinates of the place I gave is on a hill (+- 425m) with a clear sky-view around.

[apersson850]

In that case I assume that it should be possible to receive from there.

[Werner]

I received # 44 in the Alsace last weekend whenever Betty had a clear view of the sky to the east. IOW, not very often, as we mainly drove on very small twisty mountain roads heavily covered by branches. Never got a "Lost satellite reception" - thanks Gilsson!

[mbvt]

I'm not getting this EGNOS thing...

Even though I have some birds in common with Werner (26, 27, 29 for example) I never get a Differential lock, see no D at all.

Then I thought I'd delete the (P)WDM WAAS Almanac Data with NonVolUtility. But I don't have that option at all!

What gives? :confused:

(yes, I have WAAS enabled)

PS I do have a {P}HWM Temperature record in NonVolUtility. What's that? There's a thermometer inside my iQue?

[apersson850]

You can have all birds in common, that's not the point.
The point is that these supplementary sats, like #44, will transmit additional data for the other sats! So even if you receive signals from the same ordinary sats (#1-#32), you may not be able to get the correctional data for them. This may even be true in spite of actually successfully receiving #44, since they are still only testing the system. I received #44 for more than ten minutes two days ago, continiously, with a good signal. But no reaction at all, so obviously they weren't transmitting any useful signal.

If you can't find any {P}WDM WAAS Almanac Data when using NonVolUtil, then that does indeed indicate that you haven't had any successful reception of any Egnos sat yet. This record will be created automatically, but not until you do get some data from Egnos (or WAAS). Just enabling the function isn't enough.

Yes, there is a thermometer inside the iQue, just like there is in all Garmin GPS receivers. Probably in other manufacturers units as well.
Time is very essential to GPS operation. Since position is measured by timing radio signals, and these signals propagate at about the speed of light, then just a nanosecond is equivalent to 0.3 meters, or a foot, if you think like (with?) that.
Once GPS reception has been established, and the PVT equations solved, then timing is one of the cornerstones in the GPS system. But before that, the receiver has to lock on to the satellite signals. To make that easier, it keeps track of time even when there is no reception. This is done using a crystal oscillator, just like in an ordinary watch. But to give better precision, this crystal is temperature compensated. Each receiver has a personal compensation table, with entries for different temperatures, so that it knows how much to compensate at different temperature levels. So it has to know the temperature to know how to apply this correctional table.
Also, during operation, when a position has been established, precision timing of the electronics in the correlator is essential.

[mbvt]

Holy cow! Never knew it was that complicated! :rolleyes:

BTW, I always thought that differential signals were broadcast from transmitters stationed on the ground. Is that WAAS/EGNOS too?

[apersson850]

There are different differential (!) systems.
The earlier systems did transmit the corrections from ground stations (they still do, by the way). One drawback with these systems is that they need yet another antenna, since these signals can't use the same frequency as is used for GPS. That's not possible due to physical constraints.

But SBAS (Satellite Based Augmentation Systems) uses satellites to transmit the differential corrections as well. These corrections are computed in the same way as before, by finding the position of ground stations. Since their position is very well known from the beginning, it's possible to compute the difference between what the satellites say and the real position, thus finding a differential correction. Then this data is uploaded to the WAAS/Egnos sats, which re-transmit it back to the earth again.
There, you can receive it with any WAAS-capable receiver, since the frequency used is the same as for any other GPS satellite.

By the way, my description of what goes on when a GPS receiver finds the solution to the PVT equation was very brief...:eek:

[mbvt]

:cool: Tack så mycket!

[JMckie]

Some idle questions:

If you were to add more satellites at significantly higher orbits, would the elevation reading become more accurate?

How well would GPS work in low earth orbit, say on the Space Shuttle?

[apersson850]

Some idle answers:

The computation of elevation is inferior to the horizontal position, due to inferior satellite geometry. The extra satellites needed to get a better geometry are available allready. The problem is that they are on the other side of the earth, so the signals from them are significantly attenauted by that big globe we are walking on.
Adding more satellites on top of the constellation isn't too helpful, it's the bottom ones that would do the trick.
Maybe if Captain Zoom can transform the earth into transparent glass? :confused:

I don't know if there is any directional characteristic for the transmission from the GPS satellites, although it seems likely. Otherwise, there wouldn't be any theoretical problem with using the GPS system even above the satellites themselves. At least not as far as I can see.
The algoritms may have to be adjusted, so that they don't reject such high altitudes as being impossible.

[stephanpls]

Originally posted by JMckie

How well would GPS work in low earth orbit, say on the Space Shuttle?

GPS in LEO would not be that good. For good positioning you need a good satellite coverage (geometry), which could be provided with low satellites as well, if there are enough of them and if they would be spreaded well in the sky. However, the problem with LEO satellites is that their orbits are much more prone to gravity field irregularities, and therefore their positions are less precisely known. The position precision you get with GPS is not only a matter of geometry but also of the precision of the positions of the satellites.

[sPo-One]

Originally posted by apersson850
I received #44 for more than ten minutes two days ago, continiously, with a good signal. But no reaction at all, so obviously they weren't transmitting any useful signal.


I noticed the same behaviour. Good reception of #44, no "D's" in the status bars, no differential location, BUT my accuracy went down considerably from ca. 5m to ca. 2m.
Strange but true...

[apersson850]

Ah, I interpreted the question as if you could use the current GPS sats to determine your position, if you yourself are in LEO. Not the GPS satellites being in that orbit.

That's something else, of course.

Without knowing, I can assume that the current orbit for the GPS satellites was chosen as a compromise between distance (transmitter power), gravitational effects and a repeatable pattern every 12 hours.

[stephanpls]

Originally posted by apersson850
Without knowing, I can assume that the current orbit for the GPS satellites was chosen as a compromise between distance (transmitter power), gravitational effects and a repeatable pattern every 12 hours.

Correct. Also important is the number of satellites. The lower the orbit of a satellite, the smaller is the area where that satellite is visible and therefore the more sats you need for a good coverage. Also a weak point with LEO GPS would be the speed of those sats. Ever seen the Space Shuttle or ISS or Mir or something similar crossing the sky? The change rate of the coverage would be high.

I do not know all ins and outs about the Space Shuttle and the ISS, but I can imagine GPS is used for the attitude control of those vehicles. What I certainly know is that GPS is used in geodetic satellites that are used to compute the Earth's gravity field and so the geoid.
And once on a GPS symposium there was a paper about assessing the use of GPS in some probe that was to circle the Moon. I don't remember the conclusions though.... it's 10 years ago and since years I'm out of the GPS community.