Minimum parameters to get "rtk fixed"

Q&A forumCategory: QuestionsMinimum parameters to get "rtk fixed"
amorosik asked 4 months ago

I’m using a SimpleRTK2B card with original U-blox ANN-MB-00 antenna (with 10m cable extension)
Using U-Center configured to connect to external ntrip server, the receiver enters rtk float mode, and sometimes enters fixed mode
The satellite systems are enabled by default and therefore Glonass, Gps, Galileo, Beidou
In all, about 40 satellites are visible, at least 18-20 of which are with a level between 40 and 50 dB-Hz
The corrections provided are from a base about 10 km away
the question is: what are the minimum parameters to allow the gps receiver to permanently enter “rtk fixed” mode?

1 Answers

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.
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.
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.
For the static base , 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  and rover , 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?
Ardusimple 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 F9P , 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.
Internet connection for RTK system.
RTK links commonly used on ESPrtk or others are internet with 2 services, MQTT and NTRIP. Therefore, internet access needs to be fully supported and reliable at all times.
RTCM transmission speed depends on the internet speed it is using, so make sure your internet service has a minimum speed that is greater than the RTCM data rate.
Especially the application using for Base station, all connections need to be stable, connection is high speed but sometimes flickering and losing connection is also a serious problem.
Before using the internet for RTK systems, please make sure they are of good quality and are available 24 hours a day. For systems that operate continuously, it is necessary to take internet quality tests at many times during the day-week or even the month.
RTCM 3.3 data stream currently only requires less than 60Kbit / s (minimum of 10Kbit / s), however, you should not choose low-competitive networks. Please use internet service packages from 3G or higher.
Connect to the router:
For users using ethernet cables, the connection quality is reliable because the connections are transmitted directly by electrical wires, the maximum cable length should be within 30 meters.
With Wifi, should pay attention to the maximum distance of wifi. It depend type antenna and the maximum transmit power .
Power supply for the whole system:
Recommended power supply should be a DC battery or Power bank. Before the power input should add filters, they need to stabilize the voltage when  starts wifi, they should support anti-short circuit, self-recovery fuse, anti-interference ….
Electric wire:
The connections will fail if the wires do not work properly, making sure the wires are connected by welding method.
( ESPrtk Beginer document)