measuring 10 km with 1m accuracy

The total effect from vacuum to sea level air is 300 ppm you say. So how much is it going to vary at sea level. I think the OP required

100 ppm accuracy, no? I wouldn't expect the 300 ppm effect to vary more than 10, 20, maybe 30 ppm. Do you disagree?

Rick

Can you do the calculations without the almanac? Why would the older systems require the downloading of the almanac if it isn't required?

Rick

S

You can start the calculation without the almanac, but you need it for fine tuning of the position. Newer devices might estimate the almanac before downloading it from the satellite.

IPS

ro

Isn't the almanac the data on where the satellites are? How could you even start an estimate before you know where the satellites are? Is there another way to get their position?

Rick

The almanac contains coarse keplerians for all satellites, sufficiently accurate for determining if a particular satellite is high enough above the horizon, so it is worth trying to lock into it.

Once you know which satellites look promising, you also know which their PRN sequence and it is easier to search for those sequences only.

In old single channel receivers, you had to try out all the 1023 "phase shifted" versions of a specific PRN sequence before lock, which may take some time. With the almanac, you can narrow the test to those interesting (above horizon) PRN sequences (satellites), which will speed up the startup on old receivers.

The message from an individual message will contain the exact keplerians for that particular satellite, so the orbit and hence position of that satellite can be determined exactly for navigation purposes.

A modern multichannel receiver will search for all possible PRN sequences (satellites) in parallel and once four satellite positions (with sufficiently geometry between each other and sufficiently high above horizon), the receiver position can be determined exactly.

The satellite almanac is intended to help old single channel receivers (dominant when the system was designed) performing a reasonable fast cold start.

On average, the drop in refractive index is about 40 ppm/km, but when you are seeing mirages, the drop is more than 157 ppm/km.

Thus, on average, staying below 2.5 km and the error budget will be acceptable, however, even on ground level, the error margin of 100 ppm could be exceeded in special climatological conditions.

With no initial information, the GPS set also has to receive enough almanac data to get information on the visible satellites. IIRC, it can take up to 20 minutes for the full almanac transmission.

Mark Borgerson

My experience with modern GPS receivers is that they often take 10 minutes to get a lock after a full reset which wipes all the satellite information. This is a unit I bought new about 3 years ago with some

16 or 20 channels. I think it may be based on the SiRF III chip. So it appears that searching for *all* possible PRN sequences is still not common. Am I wrong? Or do you have a different definition of "modern"?

Rick

The full almanac data cycle is 12,5 minutes, which will be the worst case lock-in time, regardless of the time taken for PRN sequence synchronization.

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Tauno Voipio
tauno voipio (at) iki fi

I wonder what it could be like to walk under a bridge unselectively...

Unless you compensate by using data collected from an accelerometer.

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IIUC, the latter is also called orthodromic distance.

Taking into account terrain features, you mean?