Hello everyone. Let me begin by saying that this is one of the most informative/beneficial surveying sites I've come across.
Anyway, I'm not sure if I'm building a mountain out of a mole hill, but in looking at the GPS almanac from the Trimble ftp website
( ftp://ftp.trimble.com/pub/eph/almanac.alm ) I can see that the GPS week is stated on the last line for each SV. In this case, it is 1798. This number (1798) corresponds with the NGS week listed on their page ( http://www.ngs.noaa.gov/CORS/Gpscal.shtml ). However, when I look at the Yuma almanac from the U.S. Coast Guard Nav. Centers webpage ( http://www.navcen.uscg.gov/?pageName=gpsAlmanacs ) I get a GPS week of 774.
Does anyone have any ideas as to what I'm forgetting or not thinking of when comparing these dates?
Any help/suggestions would be greatly appreciated.
Thanks guys and keep posting! The discussions are incredibly informative!
This is the GPS week rollover problem. See: http://www.npl.co.uk/science-technology/time-frequency/time/faqs/when-and-what-is-the-gps-week-rollover-problem-(faq-time)
Here is the description of the "YUMA Almanac:
DEFINITION OF A YUMA ALMANAC
ID: PRN of the SVN
Health: 000=usable
Eccentricity: This shows the amount of the orbit deviation from circular (orbit). It is the distance between the foci divided by the length of the semi-major axis (our orbits are very circular).
Time of Applicability: The number of seconds in the orbit when the almanac was generated. Kind of a time tag.
Orbital Inclination: The angle to which the SV orbit meets the equator (GPS is at approx. 55 degrees). Roughly, the SV's orbit will not rise above approx. 55 degrees latitude. The number is part of an equation: #= pi/180 = the true inclination.
Rate of Right Ascension: Rate of change in the measurement of the angle of right ascension as defined in the Right Ascension mnemonic.
SQRT(A) Square Root of Semi-Major Axis: This is defined as the measurement from the center of the orbit to either the point of apogee or the point of perigee.
Argument of Perigee: An angular measurement along the orbital path measured from the ascending node to the point of perigee, measured in the direction of the SV's motion.
Mean Anomaly: Angle (arc) traveled past the longitude of ascending node (value= 0-180 degrees or 0-negative 180 degrees). If the value exceeds 180 degrees, subtract 360 degrees to find the mean anomaly. When the SV has passed perigee and heading towards apogee, the mean anomaly is positive. After the point of apogee, the mean anomaly value will be negative to the point of perigee.
Af(0): SV clock bias in seconds
Af(1): SV clock Drift in seconds per seconds
week: GPS week (0000-1024), every 7 days since 22 Aug 1999
The "key is in the LAST LINE:
"week: GPS week (0000-1024), every 7 days since 22 Aug 1999"
1798 - 1024 = 774
GPS Week "rollover"
Loyal
Thanks guys! GeeOdd and Loyal both explanations and links clarified my confusion. So a follow-up question (and forgive me if it comes across as stupid) but should there be any concern when downloading the Almanacs from either Trimble or the USCG Nav. Center. Trimbel give the GPS week as if its still counting from 1980 whereas USCG Nav. Center provides the GPS week based on the rollover, which last occurred in 1999.
I see how the 1798 and 774 correspond but I don't want to make the assumption when uploading the almanac to my planning software. I don't see why it would make any difference, but just wanted to verify/validate or just simply educate myself on the these details
Again, thanks for the input!