I was hired for a job to take some drone shots of a church in Camberwell, London. While doing my preflight checks I noticed that I was getting very low Satellites linking up to my Drone (about 6, but if I moved around it would drop to 1 or 2 or even 0). I was standing quite close to the church and there were a few big trees around and large buildings. The Buildings were all lined up along the north side of the road with a park with trees in it but basically an open space. The wind was on the threshold of acceptable and weather was beginning to turn. I also noticed that there were flights to Heathrow that were directly above the church (but still way way up there). Needless to say I decided not to fly. - my question is what could have caused the low Sats numbers? just the buildings? The trees in the park? both?
Satellite lock for each Satellite is based on basically a radio wave with a timing and positioning code in it. If you are in the wrong location these signals can be bounced off anything from windows, gloss paint, wet leaves water (including water in the air) if these are bounced around then when the signal gets to the receiver if it doesnât match the check signal that is sent for enough milliseconds the receiver will report that Satellite as not locked . Thats a simplified version but gives you an idea
Years ago, I worked for a shop fitting company. My sat nav, used to lose satellites in places with high buildings, London, Birmingham, etc. I suspect this is a similar phenomenon. Was a bit disconcerting when approaching certain junctions and the sat nav had just told me â In one hundred yards turn,â ,then lose the satellites
Horizontal positional accuracy (longitude/latitude) is increased with satellites that are lower in the sky, and the more of them that are locked. Unfortunately these are also the ones that are lost first when the number decreases - leaving only those higher in the sky as locked.
Obviously there will be fewer chances of those higher in the sky / overhead being obscured, and hence why more is better and why a minimum number is recommended (depending on which of the satellite constellations the drone can receive).
The signals of satellites lower in the sky will be obscured by buildings and trees (particularly wet trees!) when positioned amongst them, and the more/higher/closer they are the more they will obscure those signals - and the less accurate the horizontal position will be.
The satellites are constantly moving (as they orbit the earth) so, even in ideal conditions, the potential number visible can vary. Add in the problem of buildings and trees, etc., and the remaining number visible can drop to a very low number.
Depending on the scenario, taking off with few and rising above buildings/trees will result in more satellites (whilst above those obstacles) and greater accuracy for the flight.
Being surrounded by tall buildings, trees, or the bottom of a steep valley reducing the amount of horizon-to-horizon sky impacts the number of satellites available which will of course reduce the positional accuracy, aquisition and retention of a âfixâ. The signals used by GNSS are astonishingly weak too - conditions in the ionsphere brought on by Space Weather impacts their strength and travel time to your recievers, which also affects accuracy. IMPC Website: Near Real-Time TEC Map, Europe gives a map of this and an indication of the sort of accuracy error you can expect where you are fying. When TEC (Total Electron Content) is very high - the âwarmerâ colours shown in the map - GNSS errors can be 10s of metres and position fix lost easily as the GNSS firmware discards satellite signals it decides are unreliable.
In addition to all of the above, the very weak signal from satellites hundreds of miles away can get swamped by the multitude of electromagnetic noise from other radio sources (phones and phone masts, police radio, speed radar, microwave ovens, wifi routers, bluetooth, etc), particularly within cities.
(And thatâs without even mentioning all the radio noise being generated by people flying radio-controlled camera drones.)
Certainly. Doubt they count any lower than 10 to 15° above the horizon. They may say on their site what the lower limit is.
I donât know because i donât use their (or anyoneâs satellite predictions.
Iâve never had insufficient if Iâm in an open space or above the trees/buildings ⌠and thatâs ok any old MP that doesnât benefit from the newer constellations.
So, itâs something I never bother with.
This was sort of true with many GPS receiving circuits used in drones around ten years or so ago. Those used in the original Phantom 1 and DJI Naza flight controllers were notorious for this. Turning on a Gopro and other similar action cameras on drones would cause the satellite count to decrease significantly. Nowadays SAW Filters (Surface Acoustic Wave Filters), which are excellent at rejecting out of band Interference, are commonly used in GPS Receivers. However they can be adversely affected by temperature extremes. In cold environments their center frequency goes high and in hot environments the CF goes low, effectively making the receiver less sensitive. More sophisticated GPS Receivers, such as those used in RTK systems, will have additional temperature compensating circuitry, known as TC-SAW filters.
Sat lock I would ney worry about. As long as youâre getting into double figures I wouldnât worry about how many. I find I can have difficulty sometimes with the 2S. Once I get double figures (around 12/13), I launch, let hover, Sat count goes up quicker and gain home point.
This is the thing, Saturday I was struggling to get a lock & when it did, it crept very slow it dropped back to orange occasionally & was less than 20, although I did say to Chris clinks I wonder if the weather / cloud cover played part,
However as said in the other post even today with clear skies where lock is usually perfect, when I booted up the drone again it was like the weekend struggling
I read ages ago I think on a DJI forum about this where someone said to calibrate the compass, which I never do unless prompted, anyways done that fired it back up & I was hitting 29-30 pretty much straight away, donât know the reason behind it but it worked so happy days.
