Dear fellow Sierra users,
today I have a question regarding the multiple antenna ports of the EM7455. The following is a snippet from the Product Technical Spec:
I.e.: three ports, Main is for 3GPP mobile data (WWAN), GNSS is for GPS (and friends), and, AUX is special: it can work as a second port either for the WWAN or for the GNSS.
In this topic, I’m asking specifically about the GNSS side of things.
And my main question is: how is this multiplex of GNSS input ports cotrolled?
The tech spec document actually provides more detail, there’s a block diagram:
Based on this picture, I can make my questions a little more detailed:
- What software component decides, which route gets taken?
- i.e. whether AUX gets to serve the GNSS or the “3GPP RX diversity” ?
- Also, speaking of GNSS input, how does the switch between “dedicated GNSS in” vs. AUX get flipped?
- Is there a way to learn the status of these switches?
- Is there a way to control these switches explicitly?
- If the AUX port gets to serve the WWAN function, does it mean that the GNSS route gets cut, or do the two work simultaneously?
Why am I asking: an end customer claims that on several pieces of newly deployed tablet PC’s, containing the EM7455, “out of the box” the GNSS fails to use an external antenna (likely on the AUX port - will check). With some degree of certainty (not very high) it seems that this gets rectified, if you bring the tablet PC outdoor and let it catch a fix using its build-in antenna (likely on the GNSS port). After that, the failover to an external antenna works like a charm.
I’ve noticed some forum posts mentioning AT!CUSTOM=“GPSLPM”,[0|1] as a possible influence… except that, in my case W_DISABLE doesn’t seem to be in the game (pending a response from the vendor), and the problem description doesn’t match the LPM explanation: in the “flawed” state, the NMEA does not shut off entirely, we do get NMEA sentences, just the number of SV’s reported “good” is low (3-4) and the signal quality is poor. That said, I have yet to make the customer check AT!GSTATUS? if the problem resurfaces with another factory-fresh piece.
We’re aware that it takes at least 12 minutes to download the complete almanach.
The receiver actually looks happy much faster - in a minute or two.
While trying to reproduce the problem, I’ve turned off the PC, removed its main battery (it’s a mobile platform with a hefty LiIon pack) and let it “chill out” overnight. In the morning, after start, the GPS engine of the EM7455 initially reported no satellites in view and “all zeroes” position, but achieved an initial fix in a minute or two, with about 6 satellites good already. I don’t think this meant that it had the full almanach already - I know that the cheap receivers are “casual” like that. After a couple more minutes of runtime, I have around 9 satellites good (if not more). I have a 40dB antenna on the roof, with about 10 m of H500 cable. The SNR levels correspond to that - typically over 40 dB.
I’ve politely scorned the customer enough for potential obvious PEBKAC - poor view of the sky, broken antenna cable or some such. He is not a rookie, he has multiple pieces of the hardware and antennas, he seems to know what he’s doing and reporting.
Anyway - I’m getting off topic. Any clues as to how/why the initial reception could be so poor, would be welcome.