if and its a big if, someone has a modern GPS jammer that actually works on modern GPS units, then niether the iphone or a tracker would get a GPS fix. However on GSM you can get a rough location fix. Any GSM based tracker could, if programmed, use GSM positional data to back up a GPS failure.

Most of the easily available GPS jammers simply dont work on anything made in the last few years.

the civilian available frequencys are currently based on 2 seperate frequency groups with a third in development. the early civilian GPS used whats known as the L1 frequency band.

In simple terms the early civvy GPS network uses a message that contains 3 core bits of info, satellite time/date info, emphemeris data (wherethat satellite is in orbit NOW), and almanac data (rough positions of all satellites).On top of that is a coarse/aquisation code which in essence helps you calculate the "offset". Then the computer once its locked onto 3 satellites (2d fix) and 4 or more satellites (3d fix) it uses the data to calculate a position.

it is more complicated than that in reality as its all the signals are modulated onto a carrier wave so have to be demodulated and broken down, errors calculated etc etc etc before you even get to the point of calculating a position.... but google if you want to know more

now the good old US has upgraded the civvy GPS to use the L2 frequency which does not use a CA code but uses two data streams to do ranging and hence offsets. so works slightly different as well as being on a different frequency.

most new GPS recievers use a combination of both frequencys and methods to get better positional accuracy and faster fixs.

most newer GPS recievers have anti-jam capabilities and sufficient redundancy to get past most basic GPS jammers.

Almost all jammers are focused on the L1 frequency. Especially anything you and I (and more importantly criminal scum) can afford. Anything sub £200 (including the ebay jammers) arfe simple continous wave jammers operating on the L1 frequency.

most modern GPS units can filter out continous wave jamming on the L1 frequency unless the signal is very strong or directly in line of sight of satellites.

the better jammers that cost several hundreds use noise generation on the L1 frequency and hence are much more effective.

Even so their range is very limited due to the very low power of these types of devices. It doesnt help that most are based on cigarette power supplies so low down in car, and its 50/50 whether the aerial is high up or low down. It will be minimally effective on a high up GPS aerial. But they can be quite effective on low down GPS units such as cheap/older units or the magnetic undercar trackers (which is what they were designed for countering really).

all this wont effect L2 frequencies so the GPS will still get a fix in quick order due to stored C/A data and alamanac data. Such jammers would only slow down a fix on L2 frequency for a cold start on a GPS unit thats got no data whatsoever in it.

of course none of this effects the precise positioning systems or the p-code military systems.

GPS tracking and electronic warfare against both jammers and GPS is a very interesting subject... some of the latest kit uses either electronically or physically steerable phased array aerials that can remove all ground interference for advanced anti-jamming meaning youd need an aircraft with a jammer to stand a chance at all of jamming such modern devices. However thats a bit beyond the scope of car usage and generally reserved for top level civvy and mostly military usage.

oh and most people dont realise that one of the other uses of the GPS satellites is to detect nuclear missile launches and detonations as part of the US defence network. Thats a small part of how in the nuclear war films NORAD can see where all the missiles come from and detonate cool or what???