A couple of GPS questions

[Skeets]

Hi folks!

1) What would make the H lose GPS? Does the number of GPS satellites on the H increase the redundancy in case a satellite is lost or does it just increase the position accuracy of the H? Why would you lose total GPS and have to enter into manual flight if you begin with 15-18 satellites? Surely you do not lose all satellites at once....or do you? Or is losing GPS a flaw in these things? Not that I have lost GPS while flying (knock on wood) but have lost GPS while on the ground and it made me think of these questions.

2) Why does the number of satellites on the H not always match the number of satellites on the ST-16 since they are in the same area and should "see" the same number of satellites?

Thanks!

 

[Tuna]

Satellite signals are incredibly weak and need sensitive detectors to receive them. Your body next to the ST-16 will be blocking it's view of the sky, so it will more often than not see fewer satellites than the H does - the Typhoon's GPS receiver is usually out in the open. You'll often loose a couple of satellites when you stand over the Typhoon on the ground - again, your body is blocking its view of the sky.

You get a satellite lost message whenever the Typhoon looses two or more satellites - that doesn't mean it has no positional information, just that the information is less accurate. Basically, the more satellites you can see, the more accurately you can estimate your position (within the normal drift of GPS signal - it is deliberately inaccurate so that terrorists and so on can't rely on the signal for pin-point positioning).

Anything above 8 satellites will give you a pretty good fix. As they're orbiting the earth, they go in and out of view, so more is better to give consistent positioning and recover from places where the GPS receiver can see less sky. You will never lose all satellites at once, unless the GPS unit fails. That's pretty unlikely. The warning is a little misleading, as dropping from 20 to 18 satellites will have no effect on your positioning at all.

 

[nechsproject]

I always lose GPS sats when I lean over or go near h for adjustment. It always picks them back up when I move away. As for the ST-16, I never see them match as far as sat count, could be slight difference in location, orientation of the controller, sat antenna in the units. But as Tuna said if you have at least 8 on both you should be good. Happy flying!

 

[Photo]

Hi folks!

1) What would make the H lose GPS? Does the number of GPS satellites on the H increase the redundancy in case a satellite is lost or does it just increase the position accuracy of the H? Why would you lose total GPS and have to enter into manual flight if you begin with 15-18 satellites? Surely you do not lose all satellites at once....or do you? Or is losing GPS a flaw in these things? Not that I have lost GPS while flying (knock on wood) but have lost GPS while on the ground and it made me think of these questions.

2) Why does the number of satellites on the H not always match the number of satellites on the ST-16 since they are in the same area and should "see" the same number of satellites?

Thanks!

Hi, again I want to remind that sats lock is far more important than the exact number of sats seen on the H or ST16. Read this and you'll happily wait for 13-14 minutes to take off just to save your day, wallet and your H! http://www.measys.com/docs/TTFFstartup.pdf You can't see sats lock on the ST16 screen, its only time that saves you.People bother too much on nbr of sats and to less about sats lock.

 

[FlushVision]

I always lose GPS sats when I lean over or go near h for adjustment. It always picks them back up when I move away. As for the ST-16, I never see them match as far as sat count, could be slight difference in location, orientation of the controller, sat antenna in the units. But as Tuna said if you have at least 8 on both you should be good. Happy flying!

I respect Tuna. He is a very knowledgeable guy, but I can't agree that having 8 satellites makes you good to fly. I look for double that (and almost always get them if I wait a few minutes) before I have confidence to push the red button..

 

[nechsproject]

Ok, maybe not "good" to fly. But capable of flying. I agree that more the better but if it were horribly unsafe I imagine yuneec would have required more before allow motors to spin.

 

[PatR]

When flying Angle mode I often take off with inly 8 or 9 sats showing on the -16 as long as I have at least 11 or more on the H itself. However, I never rush a lift off and spend time on the ground adjusting video and photo settings, so everything "GPS soaks" before flying.

If I was planning on using Smart mode I would wait for more than 10 sats for the -16 and a good green/white light indication from the H before lifting off.

 

[Tuna]

Hi, again I want to remind that sats lock is far more important than the exact number of sats seen on the H or ST16. Read this and you'll happily wait for 13-14 minutes to take off just to save your day, wallet and your H! http://www.measys.com/docs/TTFFstartup.pdf You can't see sats lock on the ST16 screen, its only time that saves you.People bother too much on nbr of sats and to less about sats lock.

That document is for an entirely different GPS module. Even so, if you read carefully you'll see that TTFF from cold is still only 3-4 minutes (and happens when the number of sats reported reaches a minimum threshold) and ephemeris data is acquired in 45 seconds. You don't need the full almanac for accurate positioning, just for quicker start up times.

 

[FlushVision]

@Tuna and @ PatR
Typically, for me, it only takes 3-4 minutes for my aircraft to acquire 16 satellites and quite often I can get 18-20 satellites in that time. Also in that time my ST16 has usually acquired 2-3 satellites less (probably caused by my shadow mask). As far as I can recall I have never seen an instance where my H has not been able to acquire more than 15 satellites in that time frame.
My Phantom 2 Vision Plus, on the other hand, is rather more awkward. Since it will only see the American GPS constellation, I have never ever seen it acquire more than 13 satellites and it seeing 13 is a very rare event indeed. A typical flight using the Phantom, then, will often involve only having 8 or 9 satellites locked and sometimes I have difficulty getting the minimum 6 satellites locked. Below 6 sats on the P2V+ will put it into ATTI mode...you can take off but won't have the benefit of GPS assistance. Indeed, it was with the P2V+ that I gained most of my non-GPS flight experience

I agree with Tuna, then, that the H would have a fair 'fix' at 8 satellites (9 better) but I would advise a flight only if you are experienced in non-GPS piloting and don't rely on GPS being there to save your skin.

 

[Photo]

@Tuna and @ PatR
Typically, for me, it only takes 3-4 minutes for my aircraft to acquire 16 satellites and quite often I can get 18-20 satellites in that time. Also in that time my ST16 has usually acquired 2-3 satellites less (probably caused by my shadow mask). As far as I can recall I have never seen an instance where my H has not been able to acquire more than 15 satellites in that time frame.
My Phantom 2 Vision Plus, on the other hand, is rather more awkward. Since it will only see the American GPS constellation, I have never ever seen it acquire more than 13 satellites and it seeing 13 is a very rare event indeed. A typical flight using the Phantom, then, will often involve only having 8 or 9 satellites locked and sometimes I have difficulty getting the minimum 6 satellites locked. Below 6 sats on the P2V+ will put it into ATTI mode...you can take off but won't have the benefit of GPS assistance. Indeed, it was with the P2V+ that I gained most of my non-GPS flight experience

I agree with Tuna, then, that the H would have a fair 'fix' at 8 satellites (9 better) but I would advise a flight only if you are experienced in non-GPS piloting and don't use any GPS dependent flight modes.

Ok Tuna and Flash. so you are SURE that 4-5 minutes are enough to LOCK from COLD start?

 

[Tuna]

Ok Tuna and Flash. so you are SURE that 4-5 minutes are enough to LOCK from COLD start?

That document says:

This allows the GPS to discover the satellite vehicles which are in view and eventually establish a position. The TTFF for a cold start can be from 2 to 4 minutes, since discovery of the overhead SVs has a random element.

and

The receiver can predict which SVs are overhead but needs to download current Ephemeris data. TTFF for this start mode is typically 45 seconds.

- which suggests Ephemeris data will be downloaded in 45 seconds, less time than cold start TTFF. The only thing it says about almanac is that a full almanac is needed for fast starts, and it takes 15 minutes to acquire.

I'm not a GPS expert, but that's my reading of the literature. You can get a position from 8 or 9 sats, but that is not a stable number for flights as it's easy to lose enough as they go over the horizon that your position becomes unreliable. 12 or more seems to give a reasonable margin of error, but of course, the higher the better.

There's no reason to rush take-offs (and in the EU anyway, the slow connection time for the camera rather enforces a bit of a wait), but I can't see justification for waiting 15 minutes. The GPS module in the Typhoon is pretty up to date and should be able to retain the almanac for weeks (not days, like the module in that document). In all of the telemetry I've looked at, I've not seen problems once 12 or more sats are identified, and the Typhoon internally does not have the concept of 'locked' sats - there's just a count.

 

[ArnhemAnt]

Ok Tuna and Flash. so you are SURE that 4-5 minutes are enough to LOCK from COLD start?

Everyone will have a slightly different opinion on this and that is fine.
For me, if I haven't flown in a couple of weeks, or if I am taking off in a different location that is over 500klm away (for example), then I will always wait at least 12 minutes with the H powered up, prior to take off. All subsequent flights will have a shorter wait period obviously.

 

[Photo]

That document is for an entirely different GPS module. Even so, if you read carefully you'll see that TTFF from cold is still only 3-4 minutes (and happens when the number of sats reported reaches a minimum threshold) and ephemeris data is acquired in 45 seconds. You don't need the full almanac for accurate positioning, just for quicker start up times.

Tuna, I am definetly not an GPS expert but how can a modul keep/remember the almanack for weeks when the sats move? Or have I missunderstod..

 

[Tuna]

Tuna, I am definetly not an GPS expert but how can a modul keep/remember the almanack for weeks when the sats move? Or have I missunderstod..

There are (I think) currently 33 operational GPS satellites in orbit out of 72 that have been launched, and essentially the GPS module can tune into each one to detect it's position.

The almanac stores the orbits, which basically tells it when each sat should rise and set at the current location. Those orbits remain valid for quite a long time, so long as you don't move too far from your current location.

Without that information (a 'cold start') the GPS module has no idea which sats it should be able to see, so it just has to go through the full list, tuning into each sat until it finds enough. With that information (a 'warm start') it knows which sats should be in the sky, so rather than going through the full list, it can just tune into the specific ones that it knows are definitely available.

 

[FlushVision]

Everyone will have a slightly different opinion on this and that is fine.
For me, if I haven't flown in a couple of weeks, or if I am taking off in a different location that is over 500klm away (for example), then I will always wait at least 12 minutes with the H powered up, prior to take off. All subsequent flights will have a shorter wait period obviously.

As part of my take-off procedure I will always have myself a cigarette and I light up once I've switched the H on. I know, it's bad for me but there you go. So I stand there scratching myself having the cig while the H does it's thing and I only pick up the RC once I've finished the cig and coughed my guts up. By the time I've picked the RC up I've usually acquired at least 16 sats and very often 18 to 20 sats. Works for me.

I would advocate, though, the practice of the longer wait after a firmware update or if the aircraft hasn't been used for some time, irrespective of any technical reason why you should or should not need to wait...there's no harm in having that caution in your procedures. The bottom line is that it is always good practice not to hurry to the red button. In that respect, for me, a second cigarette is always welcome

 

[PatR]

Tuna, I am definitely not an GPS expert but how can a module keep/remember the almanac for weeks when the sats move? Or have I misunderstood..

I don't know the system can remember. The system code will look for reference information and find nothing current that resembles the previous data. It the greater scope of things, the number of satellites is important but more important is the amount of time each satellite has been in view of the system. Longer times generate higher accuracy. This is one of the things reviewed during UAV pre-flight checks. There was a requirement for a minimum number of satellites, and that number cannot be shared, but there was also a requirement to establish a minimum amount of time in view for that minimum number of satellites.

The debate over "how long to wait" for GPS acquisition in consumer grade multirotors will likely go on forever but the fact is, waiting for some period of time is better that launching immediately after acquiring "X" number of satellites. How long is up to you but be assured battery voltage isn't going to fall more than 0.1v over the course of 5-7 minutes or so. I've done too many lens focus processes, which require quite a bit of time, and know that unless the motors are turning the battery is not being heavily impacted. The ST-16 battery experiences greater impact but unless you plan on flying 8 or more batteries that day it's not much of a concern.

You can rush, or not rush. We'll read a lot more negative experience posts from those that rush than from those that don't. There's an old adage, which is pretty much true, that flying is not inherently dangerous but flying is not tolerant of those that make mistakes.

 

[Photo]

When I have to wait for a "cold" GPS start I use a "start H battery" for that purpose and when I know I will fly an assgnment that demands more than 2-3 batteries I always connect a powerbank to the ST16.. I never drain the H and ST16 batteries for power up and sats lock purposes
.

 

[Rubik]

This topic keeps coming up. When I presented the question to Yuneec Customer Service a while ago, I was assured that when the GPS indicator turns to a green READY, I was good to go. Since then I experienced a flight at a significantly different location than the last flight where I had the READY indication but immediately after takeoff, the TH drifted around a bit too much in hover. But after flying around for a few minutes and coming back to the same hover location, it was rock solid. That has put me in the "wait a while" camp (after a significant change in location or time lapse, of course).

I guess, if you don't mind a bit of drift, then you are good to go as soon as you get the green READY indication.

 

[PatR]

I've experienced the same as you when at new locations. I chose to start waiting a little longer when launching at a new spot and the drifting experienced after a fast launch never happened again. But that's just me.