connect 2 gps antenna to one device

i want to connect 2 gps antenna to sim908,one internal and one external antenna so that if external antenna has been disconnected the internal would replace it
can u please help me with designing of the circiut

You can possibly use a GaAs RF switch. How do you want to detect disconnection of the external antenna? Is it an active antenna?

If your antennas are not active antennas, using additional switch may decrease your GPS receiver performance. As you may know, according to Friss equation, it should be very low loss.

as i explain when an external antenna has disconnect by user the internal antenna activaced .....
yes both internal and external are active antenna...
can u expalin how gaAs work?
it would be very good if you give me a circiut of this switch...tnx

You can use a signal combiner

http://www.gpssource.com/products/gps-splitter/145

Consider -4dBi of a linear antenna

Parallel requires impedance matching Pi then T filter C-R-C-R for 50 Ohms.

Separate GPS antenna requires careful location and ground pad size.

SIM908 datasheet.pdf

I would expect unwanted interference of both antennas and direction dependent signal fading when using a combiner.

Fading loss or signal cancellation will occur on the fringe of the stronger antenna where its sensitivity matches the lower sensitivity of the internal antenna and both amplitude is matched and phases cancel. Otherwise the stronger antenna swamps the weaker internal one in gain. DIversity or reflections off axis will also affect fading losses, otherwise known as Rician Fading which inherently occurs on Wifi in homes with multipath distortion.

Therefore directionality or diversity and gain of antenna must be carefully selected for the desired target.. Omni vs high gain directional.
If both are omni directional then location of the external antenna must get significantly higher signal to avoid Rician fading when in parallel. Otherwise it is best to use a pin diode dual antenna diversity switch to avoid Rician Fading or dead-bad drop outs from a static null position. The best wireless transceivers have diversity antenna control and chose the higher SNR from RSSI of the two sources when error rate is excessive.

Position errors in GPS are huge in downtown high rise situations due to the above multipath distortions in many GPS car receivers locating you on the wrong street. ref : my experience with Garmin windshield mounted in downtown Toronto. In this case the strongest signals from 7 satellites is not always the most accurate and algorithms to ignore rather than average must be used when errors are detected in positioning. Details are beyond scope here.