What is the correct positioning of an external GPS Antenna?

Giulio Buccini shared this question 23 months ago

Hello everybody,

I don't know if the locus community is the right place for a question like this, but I think that gps-devices and gps-software are somehow linked themes.

Here the story: I just bought the well-know (and cheap) Globalsat BU-353S4 (i.e. the one with the sensitive SiRF Star IV chipset), something like the one in the attached photo. I plan to use it with my Samsung 7" tablet with android 4.4.

Doubt: "where I should mount the receiver on my motorcycle? In what position?"

By googling I found fragment of info fighting against the real life...

1. Your receiver should face-up the sky!

But I see people mounting it even on the lateral window of the car. In a vertical position!

So seems that the horizontal placement is not mandatory... or not?

I will not go to paradise when mounting the antenna in angled position???

2. Avoid position with objects obstructing the clear view of the sky!

But, again, people are mounting/placing the antenna just over the dashboard of the car, below the windscreen.The roof of the auto is not covering a huge portion of the sky???

3. Big objects close to the antenna can reflect the GPS-signal causing multi-path signals reaching the receiver!

But, again, what about the metal part close to the antenna inside a car? Lateral frame and the roof are not reflecting the GPS signal?

4. Keep the antenna away from radio interference!

Well, inside an auto there is a lot of electronic stuff, Bluetooth devices and... the smartphone of the driver. But seems that the receivers works the same... or not?

I'm quite confused... my plan is to mount the receiver behind the windshield made of poly-carbonate, but... is the plastic stuff an "obstructing" material?Why the glass of the car windscreen is not obstructing the GPS signals?

Replies (5)


To be honest I wouldn't worry about where you mount it on a motorcycle. A plastic windshield will probably be negligible regarding accuracy. I get 3m accuracy (standard max accuracy) with various devices (smartphones, not external antennas) from within the car under the dash so anywhere on the upper part of a motorcycle should be near perfect accuracy.


Thanks Jimmy, I'm still googling about the argument.

i have found an interesting answer on Stack Exchange: https://electronics.stackexchange.com/questions/131218/material-that-degrade-gps-signal-reception

Seems that degradation of the radio signal largely depends on the dielectric constant of the material (around 2 for ABS plastic and 4 for glass).Here what surprised me: degradation depends also on the angle of the incoming signal.

But that's not all. At increasing angles of incidence, the plate becomes "thicker" to the incident wave, the reflection becomes stronger and the transmission weaker. Also, the two polarisation components have different transmission coefficients, so the circular polarisation of the antenna will be degraded. A GPS antenna needs to operate with signals from as low as 10 degrees above the horizon, 80 degrees from normal incidence, and at this angle, even a thin sheet of plastic has a significant effect.

So, the inclination of the car windscreen can have a big effect even if the GPS-satellite is perpendicular to the car (and the dashboard inside). Most probably one cannot appreciate the difference because most navigation software automatically translate you on the center of the road.

I was in the Alps with my motorcycle and I have noticed some intermittent losses in precision even with open blue sky above me...

So, maybe, even the 3 mm polycarbonate material of my windshield can cause some signal degradation. (Polycarbonate dielectric constant = from 2.8 to 3.4)

Is still unclear to me if a "patch" antenna (i.e. a planar one - common on all GPS-antennas having the shape of a mouse) can be mounted with some angle, or it has to lay on the horizontally plane.


Hello Giulio,

this is a very interesting topic and I have absolutely no knowledge about it. Except ... satellites usually (almost always with modern device that supports more than just USA GPS system) cover the whole sky above you. So even if from one direction, you have an obstacle, from a different direction, the situation will be completely different. So from my unprofessional point of view, the position of the antenna is important in cases of more complex objects, like a train, truck etc, where is the possibility to have antenna simply covered too much. But in the case of a motorcycle, bike, etc. I believe it makes no significant difference because number of available satellites is too huge to be negatively affected by one or two sats with low data quality.



I've just got answered on Stack Exchange about the tilted mounting of commercial GPS-receiver (usually called "patch" antennas because the planar shape of the antenna itself):


I did not considered that there is a little metal plane below the antenna to shield it and get more gain/efficiency. When tilted, this plane is going to obstruct the view of satellites low on the horizon. Since there are many satellites in view (8? 10?), excluding a couple of them should not be a big issue in terms of precision.


Little update
I have received my external GPS antenna (namely a Globsat BU353S4 equipped with the SiRF Star IV chip), but now I'm challenging a new problem: my Samsung Tab 4 tablet does not support peripherals attached to his USB port (i.e. it does not support USB-OTG cables).

Now I have to solve this problem before going further with the mounting tests.

Any indication/help about cheap android devices working with an external USB antenna will be greatly appreciated!

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