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Unstable RTK fix and low accuracy

Hi,

I’m using the Drotek GPS RTK kit (XXL base and XL rover).
I have used the boards is completely open areas, with no clouds and i’m having trouble getting a RTK fix. It happens but generally for a few seconds and than oscillates between float and fix.
The accuracy is also quite low in the end. If I take off and land on the exact same position the horizontal and vertical position might be different with distances over a meter.

This is almost the same as what I get without a standard Ublox module.

What can I do to increase accuracy ?

Regards,

A well-placed antenna is crucial to achieve good RTK positioning results

You can place your smartphone near the window in your house and it will obtain GPS coordinates after some time, but for RTK such environment won’t be sufficient. For RTK to work there are special requirements for antenna placement that are much more strict that for a standalone GPS.

No obstacles blocking the skyview

Antenna needs to have a clear skyview 30 degrees above the horizon. There should be no obstacles that could block the view like: buildings, trees, cars, humans, laptops etc.
antenna-mount
Examples of bad environments for antenna placement: indoors, urban areas, forest, close to the ground.

Examples of good environments for antenna placement: field, top of the hill, rooftop of a building.

No electronics nearby

Electronic devices may produce RF noise that could affect reception of the GPS signal. Keep all electronics (including the Reach itself) as far as possible from the antenna.

Ground plane

Antenna has to be placed on a ground plane which reduces multipath, provides shielding and improves signal reception. Ground plane is a conductive plate, it may be a piece of metal, a roof of a car, a metal roof of a building, etc.

Tallysman TW4721 requires ground plane no less than 70x70mm.
antenna-ground-plane

15004-GPS_Antenna_Ground_Plate-04
If you’re indoors you must run a SMA extension cable long enough to locate the antenna where it has a clear view of the sky. That means no trees, buildings, walls, vehicles, or concrete metally things between the antenna and the sky. Be sure to mount the antenna on a 4”/10cm metal ground plate to increase reception.


For RTK application, the active antenna must be able to provide signal over 38dB under open sky. Antennas vary in price, from under $10 to over $300; they mainly differ in interference rejection capability, multipath rejection capability, antenna phase center stability, and weight. Usually more expensive antenna claims to have superior interference and multipath rejection capability. Phase center of expensive survey grade antenna remains constant as the azimuth and elevation angle of the satellites change; signal reception is unaffected by the rotation of the antenna or satellite elevation. Helix antennas are expensive mostly due to light weight feature for UAV applications.
Under open sky no interference no multipath environments, performance difference between the wide price range of antennas may not be much. Depending on budget and application environment signal conditions, user can choose appropriate antenna for the required application.

For the static base (receiver A), I suggest to use geodetic antenna.
In my experience, I have tried to use Novatel GPS-701-GG and Harxon HX-CSX601A
for F9P-ZED, which perform well.

For precisely kinematic base (receiver B) and rover (receiver C), the best choice is
also geodetic antenna if the price is affordable.
If you plan to use patch antenna, I have tried Tallysman TW2710.

These antennas are just for your reference, you can find some similar antennas to
meet your demands. Just pay attention to the frequency bands, which need to cover
GPS L1/L2 and GLONASS L1/L2 + BEIDOU L1.
Why I cannot receive RTK fix for a long time?
TinyRTK (M8P) or F9P-ZED require at least 7 satellites, signal level above 38dB, elevation angle above 15 degrees, and good geometry to have position fix; more satellites the better. Currently there is only 3 satellites with 38dB or higher signal, thus cannot have RTK fix.
Keep antenna at least 10cm away from M8P, farther the better, to avoid picking up its radiated digital noise, signal level will be higher if placed at distance. 7cm x 7cm or larger metallic sheet (or tin foil) placed underneath the antenna could also boost signal couple dB more.