It has been said that the PM2 does not record the Almanac file from the satellites, making it virtually impossible for a PM2 only user to do mission planning without information obtained from an outside source. Will Ashtech/Thales make available to PM2 users a way to download current almanacs or do online mission planning to circumvent this delimna? Perhaps this already exists and I do not know about it.

Shawn Billings
SIT Texas

ftp://ftp.ashtech.com/pub/almanacs/

will the PM2 be tweaked in the future to handle the almanac file or is that not possible due to hardware constraints?

Thanks

J.D.

That is one I will have to defer to California on, sorry.

Mike,

Gosh, you're quick on the answer.

The fact that the almanacs need to be downloaded from internet rather than directly by receiver probably isn't a concern to anyone. Actually shouldn't be to me either. Guess I'm just "old enough" to remember the days before we weren't so dependent in internet for information. Ah, for the days of transit, chain, pen and ink......Nahh.

Thanks

J.D.

J.D.,

Don't hold me to this, but I think a future firmware version will store almanacs.

Regards,
Brian
Modified By Brian D. Ewing, PLS on 10/5/2001 at 11:31 PM

That would be a nice, if really not absolutely necessary, addition. Hey, you know how spoiled us Locus users are. Hard to change I guess.

J.D.

along the same general subject lines.

I seem to have forgotten something I thought I knew, but was probably mistaken in the first place...whatever.

Do I remember that there was a specific need to run the Locus units for a short session in the user's locale before the first actual project use???? A firmware thing, possibly?

Or was I having a pre-senior moment?

Thanks,

J.D.



p.s. Jimbo, if your lurkin', the eye doc sentenced me to at least 2 more weeks of semi-blindness before another exam. Still digressing. 20-3300+/- now.

Brian D. Ewing wrote:

Don't hold me to this, but I think a future firmware version will store almanacs.

Is the PM2 firmware upgradeable by the user?

ME lurk....never !!!

And the firmware would have to be in an eeprom ( I think) to be user upgradeable. Similar to a Motherboard bios upgrade by "flashing" the BIOS.

Pretty advanced thinking there if they planned that !!

Jimbo

EDIT: JD, I'm not sure about the "locale" so much as the need to receive the almanac. Generally if you do a hardware reboot and clean the entire system memory it takes about 5 minutes + before you start recording satellites.
Modified By James Webb on 10/6/2001 at 12:41 AM

JD,

That's only an issue when moving the units a large distance (at a guess, >500 miles). In that case, the units need to acquire a new almanac. Shorter distances, no problem. Jimbo's right about waiting for an almanac after a full memory clear.

C'mon, Jimbo, we know you're a lurker at heart. ;>}


Regards,
Brian
Modified By Brian D. Ewing, PLS on 10/6/2001 at 12:54 AM

Brian,

Now I'm wondering.....and you know how I get when I start that...hehe.

If one almanac suffices for I-net downloads then why would a different almanac be needed for different locales when receiving the almanac by air ???

What exactly is in the almanac ??

Jimbo
Modified By James Webb on 10/6/2001 at 7:38 AM

The almanac is a subset of the ephemeris..All GPS units use the almanac (or ephemeris) to quickly locate SV's (space vehicles)..based upon the last known fix..This is why when you take the unit more than 100 or so miles it takes so long to lock on to anything...

Compare the almanac to a train schedule..It tells the GPS unit where the trains are (in relation to the user), the T (Where they should be at any given time) and S (how fast they are moving)..From this the unit , based upon it's last fix, will begin searching that part of the sky ,based on elevation and azimuth..

Many of the GPS manufactures have online planning software and almanacs and ephemerides...

One almanac is good for the entire world..They are not separate for regions, so any one is fine (as long as it's not to old)...

PS-It's also a good idea to subscribe to the NOAA warning bulletins..We used to call the Coast Guard every morning to check the SV's, but now just check the web...This alerts you to outages, satellite maintenance and other things that are probably more important with RTK than static...

And ALWAY's wear you AFP when in doubt..

Trimbo

For once I may have properly baited a question.

The question to me is in the unit storing the almanac file and why the difference in the Locus and PM2. I take from the responses that it's not the almanac file itself, but the stored "last known fix" of the unit that makes the difference. Makes sense. I may have been off the track in thinking the stored almanac would have anything to do with that first fix in a new geographic area (which was in the context of my question).

So, if the almanac is used "to quickly locate sv's", how can units (such as PM2) without stored almanacs get a fix so fast? I do understand the difference in the ephemeris data (real time "here's where I'm at" data)and almanac data (predicted "based on where I'm at now, I should be at xyz on mmmdddyyy.hhhmmmsssss" data). The E file (ephemeris) is a necessary component in the processing. The almanac on the other hand is used for mission planning AND initialization?

Just trying to understand the difference in the Locus and PM2 as related to almanacs.

Thanks for the education Y'all.

J.D.

Trimbo, we'll make you guys a trade. You take Jerry Jones and send us your son's high school athletic director.

Trimbo,

Maybe...it seems to me the explanation you gave is close but in your scenario wouldn't the antenna need to be highly directional ?

And the more I think about it since the antenna are not directional then what does the last fix have to do with anything ?? Possibly as a rough starting point to calculate the new position but then would this be related to the almanac ?

Seems like the PM2 says its not related to the almanac directly, or is the PM2 receiving and using the almanac and it is just not currently available for user download ??

Questions, questions, questions, always more questions.........

Jimbo

OK all,

T-man is correct, the almanac is basically an approximate ephemeris.

PM2 stores almanac info, but not in a downloadable, mission-planning compatible format (So it can use the info to achieve lock, but you can't download it to use for planning.)

Jimbo, the criteria is less direction than signal strength. The computed direction is used primarily to differentiate multipath from direct signal. Also, an approximate position aids in achieving lock. Questions R good.

Regards,
Brian

Brian,

Now that makes more sense...at least the parts about signal strength. I can also see how an approximate position aids in acheiving a quicker position.

But I have a feeling its way over my head as to how the computed direction helps differentiate multipath from direct signal.

Thanks for the info and explaination.

Jimbo
EDIT: I thought trimbo was right about the almanac being an approximate ephermis I just didn't see how, with what I think is essentially an omnidirectional antenna, it could determine elev and az. of a signal thru the antenna. Like the multipath distinction...above my paygrade.
Modified By James Webb on 10/7/2001 at 9:29 AM

Jimbo (et al):

The statement regarding direction of the SV from the ground station and multipath was a late-night brain-fart. Please disregard it. I meant S/N versus elevation. This helps multipath mitigation in that with two signals from the same SV, the weaker may be rejected as multipath. Also, the GPS signal has a right-hand circular polarization. When reflected (once), the polarity is reversed. Of course, in extremely adverse environments, the signal could be reflected multiple times, so this technique doesn't always work.

Sorry,
Brian
Modified By Brian D. Ewing, PLS on 10/7/2001 at 6:48 PM

The time and effort you give all of us is well above the "call of duty" and is greatly appreciated by me and I'm sure others !!

Jimbo

Brian,

Thanks,

You answered my question about the almanac and PM2....I think.

So, the almanac is a necessary component in locking on sv's? (I thought that would be the case) And the PM2's do store the required portion of the almanac for computations? (obviously they do whatever is necessary....Gosh, the PM2's "lock and load" extremely fast)

My question was actually pertaining to the "New Geographic Location". Such as when receiving brand new units from California to South Looweezeeanna, one needs to initialize the units in that locale for proper use. Correct????? I know this seemed to be the case with Locus units. Is it also the same concern with PM2's?

Just a question to add to the new user FAQ's.

Thanks

J.D., still bumping into things in E Tx

JD,

Ya bringing one to me ?!?!!Or are you saying I need to order one and get Brian to deliver it just about time that the next xxxx season starts ?!?!(You know what the xxxx season is!)

But seriously, are you gonna put on that seminar ?

Jimbo
;o}

JD,

Yeah, if you move a receiver from the PRC (People's Republic of California) to LA (South Looweezeeanna), you need to enter a new position, or wait until the receiver has a new almanac and has computed position in the new locale. The difference is PM2 will let you enter a new locale instead of waiting for a new almanac. Hope this answers your question.

Jimbo, I'll be there for xxxx season!

You name the weekend for the "seminar". My next 2 are full. Gotta run some gps stuff this saturday, and crank up the brewery for a session. Family reunion next week end. Found out we do have a family tree and not family vine.

I suppose you are referring to the trade out of me bringing the solar observations and you bringing the kinematic? Also gotta find out about the eye surgery schedule. They put me off at least another 2 weeks. I now understand why senior citizens are more subject to falls. Damn bifocals.

J.D.

Brian,

You answered it. Thanks

JD,

I was really thinking more about solar weather ! But actually I was thinking more like xxxx season, ya know, early spring for us. But it might still be winter way up North there in Stumpwater !!

We'll just have to crawfish on any earlier date ;-}

Jimbo

PS: careful with them bifocals...heck, my brothertoolk a year to get used to them being in his pocket!! When he put them on he was really dangerous !! To himself and everybody else...Couldn't see with'm !!!

Been trying to get used to bifocals since March.