Re: Longitude or Right ascension

Hi Gustavo,

The term Omega0 is the longitude of ascending node at the weekly epoch,
which means the angle between the orbital plane and the ECEF x-axis at the
beginning of the GPS week (GPS seconds = 0). It is expressed in ECEF, not
inertial, coordinates.

Maybe an easy way to look at this is the following:
- consider that Omega0 is the longitude of the ascending node at GPS Second
= 0

- then Omega' = Omega0 - OmegaDot_E * toe is the longitude of ascending node
at the time of ephemeris toe (simple rotation by toe seconds)

- then Omega'' = Omega' - OmegaDot_E * tk = Omega' - OmegaDot_E *
(currentGPSTime - toe) is the longitude of ascending node at the current GPS
time (here we just did an additional rotation by tk seconds)

- in an unperturbed orbit, the term OmegaDot (RAAN rate) would be zero, and
Omega'' would be equal to the Omegak given in the ICD. However, because the
RAAN rate is not quite zero, we have to further correct the longitude at
currentGPSTime: Omegak = Omega'' + RAANRate * (currentGPSTime - toe)

I hope this helps,

Cedric









______________________________________________
Dr. Cedric Seynat
Satellite Navigation Systems Engineer
GPSat System Australia Pty. Ltd.
Suite1, 22 Aberdeen Road
MacLeod, Victoria
Australia 3085
Ph.: +61(0)3 9455 0041
Fax: +61(0)3 9455 0042
www.gpsatsys.com.au

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Gustavo
Sent: Tuesday, 12 September 2006 5:32 AM
To: [email protected]
Subject: [OpenSource_GPS] Longitude or Right ascension

Dears,

I have a very basic but important question.

In GPS-ICD the definition for the term Omega0
is: Longitude of ascending node

Well, from that definition I understand that
Omega0 is then the angle in equator from X-ECEF
to ascending node at ref time t0 (definition of Longitude).

However, when wee look at the equation for the corrected longitude:

Omega = Omega0 + (Omegadot - Omegadot_E)*tk - Omegadot_E*t0

we see a term Omegadot_E*t0 which only makes sense if Omega0 is
a right ascension (angle in the equator between X-ECI and ascending
node) instead of longitude.

See that Long0 = RA0 - Omegadot_E*t0

Using proper names...
Long = RA0 + (Omegadot - Omegadot_E)*tk - Omegadot_E*t0
     = (RA0 - Omegadot_E*t0) + (Omegadot - Omegadot_E)*tk
     = Long0 + (Omegadot - Omegadot_E)*tk

See that if we got from GPS message is Long0 (instead of RA0), the
equation is quite different...
By the way, in Hofmann's book (GPS Theory and practice pg 38), he uses
the term Right ascension instead of Longitude...

Can anyone clarify that?

Msc. Eng. Gustavo Baldo Carvalho
ZARM - Uni-Bremen - Room 1210
Am Fallturm, 28359 Bremen
www.zarm.uni-bremen.de
Tel:(0049)-(0)421-218-4796
Fax:(0049)-(0)421-218-2521

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> Today's Topics:
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>    1. Building a GPS Receiver (Redefined Horizons)
>    2. Re: Building a GPS Receiver (Andrew Greenberg)
>    3. Re: Building a GPS Receiver (David Bengtson)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Wed, 3 May 2006 14:36:59 -0700
> From: "Redefined Horizons" <[email protected]>
> Subject: [OpenSource_GPS] Building a GPS Receiver
> To: [email protected]
> Message-ID:
> 	<[email protected]>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> I am a Land Surveyor and software developer that is very interested in
> building a GPS receiver that uses an off-the-shelf commercial GPS receiver
> and open source software.
> 
> I had a couple questions for this mailing list. I'm hoping to determine if
> this is a project that I can tackle.
> 
> [1] Is there any active development on a "homegrown" GPS receiver? If so
> which project? Would I be able to help with those development efforts?
> 
> [2] I'm not very familiar with low-level programming and digital
> electronics. I do know some basic prinicples of logic circuit design and
> dabble in the C programming language. I do have a lot of experience with
> high-level programming languages, and lots of time using survey-grade GPS.
I
> am a Linux user, but I'm no Unix pro. I am willing to learn new skills,
but
> am I getting in over my head if I try to build my own GPS receiver?
> 
> Thanks,
> 
> Scott Huey
> 
> P.S. - If I can get this thing to work I will be developing GPS data
> collection software for mobile devices. This would be released back to the
> community under an open source license. (Probably GPL.)
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> ------------------------------
> 
> Message: 2
> Date: Wed, 03 May 2006 15:34:01 -0700
> From: Andrew Greenberg <[email protected]>
> Subject: Re: [OpenSource_GPS] Building a GPS Receiver
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> Hi Scott,
> 
> 
>>I am a Land Surveyor and software developer that is very interested in
>>building a GPS receiver that uses an off-the-shelf commercial GPS
>>receiver and open source software.
> 
> 
> If you're talking about an OEM GPS receiver board, then it's called the
> GPL-GPS project, and you can read about it on our website:
> 
>  http://gps.psas.pdx.edu/
> 
> Feel free to join the GPL-GPS mailing list if this is what you're
> interested in.
> 
> 
>>[1] Is there any active development on a "homegrown" GPS receiver? If so
>>which project? Would I be able to help with those development efforts?
> 
> 
> There's lots of "homegrown" GPS projects out there. Most require a PC to
> operate. For example, see the OpenSource GPS project at:
> 
>  http://home.earthlink.net/~cwkelley/
> 
> And here's a list of most of the open source projects:
> 
>  http://gps.psas.pdx.edu/OpenGnssProjects
> 
> There's unfortunately no one working on a embedded open hardware GPS
> receiver... it's a project we'd like to do with GPL-GPS, but it's a lot
> of RF work and a pretty expensive endeavor as well.
> 
> 
>>[2] I'm not very familiar with low-level programming and digital
>>electronics. I do know some basic prinicples of logic circuit design and
>>dabble in the C programming language. I do have a lot of experience with
>>high-level programming languages, and lots of time using survey-grade
>>GPS. I am a Linux user, but I'm no Unix pro. I am willing to learn new
>>skills, but am I getting in over my head if I try to build my own GPS
>>receiver?
> 
> 
> Getting involved in GPS is by its very nature an "over your head" kind
> of thing because it involves everything from RF to DSP to software. But
> it's fun! And as long as you have the time, it's possible to do. There
> are tons of books and resources to help you out on your way. You can
> focus on the things you want to, and let other people handle the details
> you don't want to.
> 
> In terms of GPL-GPS, all you need is a Linux box with a few serial ports
> and you're on your way... everything is detailed on the website. We'll
> soon have instructions for Windows development as well.
> 
> I hope that helpes!
> 
> Andrew
> 
> 

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