I generated an ephemeris for July 1, 2007 using both the current SEM and YUMA almanacs for the time.
A SEM formatted almanac has almost the same parameters as a YUMA formatted almanac, except for the inclination parameter.
To see this, I'll use the SEM ephemeris (S the YUMA ephemeris (Y) and the truth ephemeris (T and create a second difference to see the effects, so: T-S - (T-Y) Y-S or, the difference between how well the Yuma or SEM compare to truth.
Users can therefore download a single file each day or hour, which. The daily GPS broadcast ephemeris file is a merge of the individual site.
In particular, I'd be interested in hearing further results regarding which almanac is closer to the download truth using the second difference approach outlined above.
All in all - we're still getting what's promised, it's just not as enticing as before.
These differences ( Y-S ) are now no longer all positive as before - meaning YUMA almanacs aren't always less accurate than the SEM almanacs.
One reason for this is the accuracy of the parameters in the file, including the inclination vs.
The SEM minus YUMA difference plot now looks markedly different.
I'm plotting here for April 21, 2008: The SEM and YUMA almanacs no longer closely match each other.
A GPS receiver listens for the signals and uses the differences in times that it's hearing from the different satellites - due to speed-of-light delays and relativistic effects (!) - and knowledge of the orbits of the satellites (also broadcast by the satellites to triangulate its position on the earth.
So the new AEP system is no longer producing almanac parameters in the same manner as the previous system.
SEM almanacs use an inclination offset from the nominal 0.3 semi-circles (54 degrees) whereas the YUMA almanacs provide the actual inclination angle.
There are some other questions as well - Is the SEM almanac still slightly more accurate than the YUMA?
The almanacs are created to allow receivers to quickly assess the visibility of satellites - whether they will be above or below the horizon based on the receiver's current position.
The one major difference between July of last year and April of this year, is that the entire Master Control Station (MCS) computing systems were replaced.
Known as AEP (Architectural Evolution Plan this upgrade replaced decades old computing machinery and software with updated systems.
Did something get messed up with the upgrade?
I called a colleague of mine who's close to the GPS Operations Center to see if he'd seen these results.
Before I get to the issue, let's look at how well SEM and YUMA almanac generated ephemerides agree.
There is a constellation of satellites in orbit, with atomic clocks on board, that essentially just shout out their identifications and the time, for anybody who will listen.
Time to first fix (TTFF) is a measure of the time required for a GPS navigation device to acquire satellite signals and navigation data, and calculate a position solution (called a fix).
An animation depicting the orbits of GPS satellites in medium earth orbit.
Scenarios[edit]
The TTFF is commonly broken down into three more specific scenarios, as defined in the GPS equipment guide:
- Cold or factory: The receiver is missing, or has inaccurate estimates of, its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. Manufacturers typically claim the factory TTFF to be 15 minutes.
- Warm or normal: The receiver has estimates of the current time within 20 seconds, the current position within 100 kilometers, and its velocity within 25 m/s, and it has valid almanac data. It must acquire each satellite signal and obtain that satellite's detailed orbital information, called ephemeris data. Each satellite broadcasts its ephemeris data every 30 seconds, and is valid for up to four hours.
- Hot or standby: The receiver has valid time, position, almanac, and ephemeris data, enabling a rapid acquisition of satellite signals. The time required of a receiver in this state to calculate a position fix may also be termed Time to Subsequent fix (TTSF)
Many receivers can use as many as twelve channels simultaneously, allowing quicker fixes.[1] Many cell phones reduce the time to first fix by using assisted GPS (A-GPS): they acquire almanac and ephemeris data over a fast network connection from the cell phone operator rather than over the slow radio connection from the satellites.
See also[edit]
- Global Positioning System (GPS)
References[edit]
External links[edit]
- US Coast Guard, Navigation Center's NAVSTAR GPS User Equipment Introduction.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Time_to_first_fix&oldid=908060563'