Not enough satellites for RTK - best option?

[Niels Laukens]

Hi list,

I've tried to get an RTK lock last week, but I wasn't able to get 5 satellites. I've tried for around an hour, and got 3 or 4 satellites locked. The locks themselves were stable, but with C/N0 of 7~10, so not super strong either. The location wasn't ideal (grass field surrounded by some trees), but is the intended location for our prototype to work in.

I know for sure that there were other satellites in view; I had a "reference" GPS receiver that reported other satellites with lock in its GPGSV sentence.

Is there a way to figure out what the problem is, and hence, what solutions would apply? I can get access to a spectrum analyser, but I'm not sure if that is of any help since GPS is at -13dB below noise... Also, I don't know if/where I can pick up the integrated antenna's signal.

* I know that a 20ms integration firmware is on its way for v1.0, so that will definitely help

* I can get an external antenna with 4dBi antenna gain + 27dB LNA gain - 6dB cable loss = 25dB gain. But this will only help if the problem actually is the weak signal, and not the SNR, since it will happily amplify all noise an well.

* I can get a metal plate to mount the Piksi (or the external antenna, for that matter) on, which should reduce the noise coming from the ground.

What would be my best option to try first?

thanks in advance,

Niels

[Goh Guo Dong]

Hi Niels,

I have tried both methods. I would say both methods work well and it is best if you mount an external antenna on a metal plate. The metal plate increases the S/N ratio by 30-75%, the effect is much obvious on weaker signals.

Regards,

Goh Guo Dong

[Simon Harst]

Hi Niels,

I'm struggling with the same problem and I've tried both possibilities. The antenna I'm using is the following:
http://www.newark.com/tallysman-wireless/32-3070-00/antenna-gps-1-585ghz/dp/64T4786

This antenna has not really increased the SNR in comparison to a 15-bucks-off-the-shelf antenna I was using before.
The metal plate however does help a lot against the noise created from your rover's electrical currents - so I'd try that first.
If you're buying an antenna after all, I'd be interested in the model and your experiences! Currently we've actually suspended development of the piksi until firmware v1.0 is out because we didn't arrive at any good results.

- Simon

[Niels Laukens]

On Mon, Jul 7, 2014 at 11:43 AM, Simon Harst <sih...@googlemail.com> wrote:

This antenna has not really increased the SNR in comparison to a 15-bucks-off-the-shelf antenna I was using before.
The metal plate however does help a lot against the noise created from your rover's electrical currents - so I'd try that first.

At the moment, I don't have a "rover", it's just the Piksi with a USB cable attached. In the future, I will add additional electronics (and hence, noise).

This is also the reason why I asked if/how it's possible to determine the core issue: Do I need to combat noise, or is simple amplification enough?

 

If you're buying an antenna after all, I'd be interested in the model and your experiences!

Currently, I'm considering this one https://www.sparkfun.com/products/464 (although I'm buying it from a local vendor).

Niels

[Fergus Noble]

Hi Niels,

The v1.0 firmware will help a lot here but in the mean time I would suggest getting an external active antenna, this will make a noticeable improvement in SNR. The extra gain does help a lot as the signal is very weak. A reasonable quality external antenna will usually have better selectivity and NF and is physically further away from the processor and other sourced of EMI.

A ground plane either on piksi or on the external antenna will also help a lot.

I don't think a spectrum analyser will help you much - like you say, its quite hard to see the GPS signal using one! And it doesn't sound like you have got a big source of interference nearby or you wouldn't lock any satellites.

Thanks,

Fergus

--
Fergus Noble

CTO, Swift Navigation Inc.
1155 Indiana St.
San Francisco, CA 94107

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[Fergus Noble]

Hi all,

You have probably already seen this in the user guide but we highly recommend the following antenna:

https://www.linxtechnologies.com/en/products/antennas/sh

http://www.digikey.com/product-detail/en/ANT-GPS-SH-SMA/ANT-GPS-SH-SMA-ND/1793609

It has very good performance for the price point, some of the other low cost mag mount patch antennas are of much lower quality.

Fergus

--
Fergus Noble

CTO, Swift Navigation Inc.
1155 Indiana St.
San Francisco, CA 94107

[Henry Hallam]

You can also remove the magnets easily if you're worried about
interfering with a magnetometer.

[LamorindaMarketing]

I think the antenna that Niels Laukens  is considering is like one of mine and removing the magnet was more difficult than the one the Swift Nav folks recommend. One advantage the Sparkfun model has is the nice long 5 meter cable versus 3 meter.

RoboBill