Difference Hot/Warm/Cold Fix

[myrrh]

In a review I saw TTFF shown as "factory, cold 1, cold 2, cold 3, warm, hot." I read an explanation of the difference between those, but it didn't sink in. Could someone please explain this to a technology-challenged person?

[Darren]

GPS receivers rely on two important pieces of information to calculate your location.

1. Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

2 . The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

Because the GPS satellite constellation is constantly moving above you, almanac data becomes 'stale' very quickly. In short, a HOT and or WARM start is one where the receiver has been off for only a short while and the Ephemeris and Almanac Data it holds is still current for your location. Because it has up to data Almanac data the receiver knows which satellites it should expect to see and where in the sky allowing a faster satellite signal aquisition.


A COLD start is one where the receiver has been off for a longer period and the receiver needs to download updated tables to allow it to acquire a lock. This download increases the time before a fix is acquired.
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Darren Griffin

[myrrh]

Okay, Darren, I get that much, thank you.

1)How do I determine what the length of time (for the receiver to be off) is to be considered a cold start and the length of time to be considered a hot or warm start?

2)What is the difference between hot and warm?

[Darren]

I think I'm correct in saying that HOT and WARM starts are one and the same. I'm certainly not aware of any differentiators except between Hot/Warm and Cold starts.

As for the length of time a receiver needs to be off before a COLD start is required, there is no hard and fast rule as so many factors affect this timing. Were you located on a high flat plateau with no obstructions then the ephemeris and almanac data may be valid for a little while longer but essentially any OFF period in excess of a few minutes renders the data stale and so switching ON after three or four minutes could be described as a COLD start.

p.s. Can you add your location to your profile, it helps us when answering software specific and location specific queries
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Darren Griffin

[myrrh]

Thank you, Darren - And here is my location!

[Dave]

Basically the difference between hot and warm starts is how much of the 30 second download of the ephemeris table you currently have which is accurate. Usually hot starts are anything up to around 20 mins, warm starts from 20 mins to a couple of hours and cold starts are anything above this.

A Cold start basically needs to download a full ephemeris table (min 30 seconds), so you'll find a cold start anything from a few hours to easily a week or more. The Almanac is current for around a couple of months, so if you have the GPS powered off for more than this, you will need to re-download the Almanac and Ephemeris data which will take around 12.5 mins (minimum).

[bigfatjohn]

I find that changing location slows down acquisition greatly too. For instance if I put my bike on a ferry in Portsmouth one night and then get off the next morning in St. Malo.

From the abovementioned explanation can I assume that this is, at least in part, because my receiver is not where it thinks it should be according to the almanac data it holds?

[Darren]

That's exactly the reason and the further you travel with the receiver off the greater its initial confusion!
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Darren Griffin

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