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[on:]

Hello all. I seem to have scrapped my spot-meter project cause I am getting a Digital SLR (yes, some of us actually still shoot film   ), that and I have a work related opportunity to develop something custom. Plus I'd rather be shooting pictures than building something helping me to shoot pictures - thanks Graynomad  

I have always wondered ever since I got my Garmin Garmap 60 what affects the accuracy of the unit, and what affects the speed of satellite acquisition. Comparing the Garmap 60 to the Garmap 60CSx, the CSx acquaries the satellites in a fraction of the time it takes my standard 60 to acquire satellites. Now I have read somewhere there is something about a "cold acquisition" time...almost like cold starts with auto engines.

Hence the questions, what affects the speed of satellite acquisition for a GPS module - and here I also presume it depends on the type or power of GPS antenna?

Also what affects the accuracy of said GPS antenna?

I want to build a GPS breadcrumb logging unit, that allows for a reference number to be entered where a coordinate "mark" is placed. Much like most GPS units only it wont really have a mapping screen, only a LCD or OLED screen of reference number entering purposes.

Thanks a million.

[Reply #1 on:]

I recall getting a GPS many years ago & reading that the first initialization could take a long time - cold acquisition. It certainly did, of the order of twenty minutes. I assumed at the time, that the issue was that the unit had last been used at its place of manufacture (Taiwan) and was really really confused by how it now found itself in the UK. The reality though may simply be that the GPS needs to know the time very accurately to use satellite data to compute position. If it knows neither good time nor approximate location, it's complex and time consuming to figure out those things, presumably by successive approximations, Modern units seem much better at this, likely because of s/w improvements.

[Reply #2 on:]

Regardless of the GPS module used it will typically take about 12 minutes to get a signal lock "out of the box".  The reason (to the best of my knowledge) is that initially the GPS module must download the ephemeris and almanac data so it knows where the satellites are orbiting so it can compute the data.  The important thing to consider when integrating a GPS module is that it has some kind of memory backup supply.  Some GPS modules will use supercaps and others will use a battery.  When you issue a cold start you are telling the GPS to effectively erase the current ephemeris and almanac data and download everything again.

[Reply #3 on:]

What constitutes a "cold start"? And following that is how to prevent a cold start...not that a cold start is a big problem come to think of it, only an annoyance.

Thanks for the help

[Reply #4 on:]

A cold start can be initiated by commands or if the unit does not have valid ephemeris data (ie no backup battery). I believe the data is also only valid for a certain amount of time but I don't remember what that is off the top of my head

[Reply #5 on:]

I thank all the wise members for there help.

[Reply #6 on:]

Acquisition speed mostly depends on the chipset used in the device, the antenna size and the surroundings (open space vs. inside a building). In any case, a cold start of several minutes sounds like *a lot*. Modern devices are able to start in less than a minute: http://wiki.openstreetmap.org/wiki/GPS_Chipset#Vendors

What do you intend to use the GPS for? Photo geotagging?

[Reply #7 on:]

Acquisition speed mostly depends on the chipset used in the device, the antenna size and the surroundings (open space vs. inside a building). In any case, a cold start of several minutes sounds like *a lot*. Modern devices are able to start in less than a minute: http://wiki.openstreetmap.org/wiki/GPS_Chipset#Vendors

What do you intend to use the GPS for? Photo geotagging?

I want to build a little bread crumb and position marking unit for geomarketing, something that looks less like a consumer item - our company had instances where people where badly assaulted and the Juno units stolen. If the unit just looks like simple box (nothing you will see or have seen in shops) the probability of it being stolen is less. For data quality we also need to check that the fieldworker actually covered a certain geographical area to obtain all the data required and didn't just stand in one spot for an hour and collected some data and thumbsucked the rest.

Nah the spotmeter thing I left, and as for photo tagging I'm not really that interested, I use my Garmin to mark points of interest for photography at a later stage.

Thanks for the info Dimko.