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Tips and questions relating to the GPS modules from SFE
By Hadaak
Is it possible to use 2 gps active antennas with one venus chip to have a better gps fix?
I found some gps combiners but I'm not sure the dual setup is doable or will help with gps signal optimization.
I'm using the venus 10HZ chip in the car.
i also thought of using a SMA joiner to connect the two antennas to the chip.
By jremington
For a hobbyist, no. Antenna design and interconnections are absolutely critical, so unless you are capable of designing and building a "phased antenna array", you will just degrade the performance of the system.
By Hadaak
strange enough I just connected the 2 antennas to the venus (just tearing the cables and and joining the 2 antennas) and I this morning I got a fix in less than 2 minutes. usually It took mor than 10 mn to get a fix. The venus chip is powered by a 3v coin battery. the chip is connected to my carpc via usb and a breakthrough board. The antennas are positionned near each other and have clear view of the sky. The antennas are ebay active antennas: ... 5892fd1656
By jremington
Ten minutes to get a fix is very poor performance, but two minutes isn't exciting either. The modules I use typically get a cold-start fix in 38 seconds, using the built-in antenna.

It is possible to connect two antennas such that they work together better than one, but a single experiment showing a faster fix shouldn't convince anyone that the antenna connection is responsible. Try different antenna spacings, etc.
By Hadaak
not sure what cold start is exactly. I bought the venus logger (10HZ) to use with my carpc and here is how it is behaving:
When I start the car in the morning I get a fix in 2 min after getting out of the underground parking.
If I stop the car and shut down the computer for 5 mn and then restart it again (outside) I get a fix in les than 10 seconds.
what I'm looking for is to get a quick fix in the first situation. I'm powering the venus chip with a 3v coin battery with enough power.
By fll-freak
GPS receivers need orbital data about each GPS satelite vehicle (SV). That data changes on a regular basis so it is transmitted by the SV. To get all the needed data takes a while. How long depends on many factors, but 2 minutes is in the right ball park. Now if the GPS receiver already has that data, then it does not need to wait to download it before it can start providing solutions. This is the difference between a warm and cold start. During a cold start, the GPS receiver has no cached data. A warm start, the receiver has cached data and and start after just a few seconds of synchronization. There is also a hot start where the receiver starts to produce results right away, but needs even more cached information.

The cached information is only valid for a few hours. Even if you maintain backup battery power to the receiver, by morning that data will be invalid and your GPS will start with a cold start.

There are two ways to get around your long duration (2 minutes) for that first fix. The first is to buy a GPS unit that optimizes the time to first fix. Top of the line units have TTFF of around 20-30 seconds with clear view of the sky. The second is to download from the web the information needed by your GPS (the almanac, ephemeris, and time) and upload that to your GPS prior to wanting your first fix. You will now have a warm start scenario that should provide a solution in just a few seconds (assuming satelite visibility!).