Author Topic: OGN-R uplink typical range  (Read 3984 times)

buzz53

OGN-R uplink typical range
« on: July 30, 2020, 09:08:53 pm »
Short question: up to what range should I expect to receive OGN-R uplink data?

Long version: Since I am based in rural East Anglia it can be rare to see enough PAW airborne traffic to make useful assessments, so in the past I have tried to use ground stations as a more repeatable test. I think this is probably more important anyway as the uplink is becoming the unique advantage of PAW.

Having had disappointing results last summer with internal antennas in my RV6, I decided it was time to drill a hole in my bottom and fit a proper one. My results today were a bit contradictory. In the hangar, I had intermittent contact out to 27km on a PAW target which was encouraging and gave confidence in my home-made antenna.

However, on the test flight, I didn’t seem to pick up the UKWRM OGN-R station beyond about 2 nm, which is pretty much what I found last year with the internal antenna, and clearly not a lot of use. This is based on: the appearance of the OGN-R label on the RADAR screen; the appearance of the UKWRM ground station icon on the RADAR screen; and on the reporting of MLAT traffic on the radar and in the track file (I think).

Is this a valid test? How are those indicators controlled? I believe the ground stations transmit a beacon every 15 seconds or so regardless of the detection of any nearby PAW aircraft. Is that correct and if so, is that used to drive the OGN-R flag, station ICON etc?

What else would give rise to this contradictory performance? Is it possible UKWRM is just broken? Last year I had similar results from UKTIB so I suspect not.

BTW looking at the datasheet for the PAW radio module, my calculations suggest free space range should be about 150km which seems an awful lot more than is seen in practice even air to air. Am I missing something?

Alan

PS no voltage or throttling issues!

exfirepro

Re: OGN-R uplink typical range
« Reply #1 on: July 30, 2020, 11:26:40 pm »
Hi Alan,

Short question, but unfortunately no short answer as there are so many variables involved.

Transmission and reception at these frequencies is theoretically ‘line of sight’, which would imply ‘to the horizon or beyond - especially at altitude’.

In practice, however, transmission/reception range is generally restricted - predominantly by topographical features (though not many hills in East Anglia) or more significantly by physical obscuration by aircraft metal or carbon fibre bodywork, engine(s), human bodies, fuel in tanks etc., between aircraft or between aircraft and Ground Station antennas. The type and position of antennas on each aircraft, whether they are fitted internally or externally and if appropriate on a suitable ground plane, plus the length and type of coax between the antennas and receivers or transceivers can also have a significant effect on transmission/reception range.

Rebroadcast is of course subject not only to the vagaries of installation in individual aircraft, but is also dependent on your aircraft and the target aircraft both being in range of at least one Ground Station at the same time, which in an area like East Anglia, (or in my case Scotland) where Ground Stations are widely spaced and not particularly numerous, is not always possible. Despite optimum siting and use of relatively high gain antennas (where permissible), geographical (and in particular topographical) location of Ground Stations can have a significant effect on rebroadcast range, particularly in certain directions.

Having said all that, if you give me your home airfield and aircraft registration / Hex ID, I can take a look on the Network Database to see from which Ground Stations and at what ranges it has been reported.

Looking forward to hearing from you.

Best Regards

Peter
« Last Edit: July 30, 2020, 11:28:23 pm by exfirepro »

steveu

Re: OGN-R uplink typical range
« Reply #2 on: July 31, 2020, 06:44:28 am »

What else would give rise to this contradictory performance? Is it possible UKWRM is just broken? Last year I had similar results from UKTIB so I suspect not.


You can go onto https://www.gliderradar.com/ and see the status of a ground station, and see coverage...

However, this is just "belt and braces" as it is not a map of received P3i in aircraft, it's a map of aircraft received...

Some coverage maps below, depends on who made the antennas, I'm guessing, and therefore gain...

Have you any way of testing antenna performance? Are you 100% happy with cable terminations?


steveu

Re: OGN-R uplink typical range
« Reply #3 on: July 31, 2020, 06:45:57 am »
Just to add a high performing ATOM-GRID station on the North Downs... topography allows excellent coverage to the South, but not to the North...

exfirepro

Re: OGN-R uplink typical range
« Reply #4 on: July 31, 2020, 09:20:03 am »
Hi Steve,

Thanks for the screenshots. I agree GliderRadar’s coverage maps can be a useful tool to show general coverage area, but as you say they display coverage for ‘aircraft’ and have limited scope for configuring them to obtain information on specific transmission types.

I find the OGNRange tools more useful in this respect, as you can tune them to display coverage range for a specific OGN or PAW receiver for a specific time period e.g. ‘Today’, ‘Yesterday’, ‘Last 7 Days’ etc. Looking at https://ognrange.glidernet.org for both UKWRM (FLARM) and PWUKWRM (P3i) for ‘yesterday’ and ‘last 7 days’ for example, clearly shows that the station was operating on both protocols over that period (though wouldn’t of course show any short term outages).

Unfortunately I don’t have direct access to PWUKWRM (Colchester) via the PilotAware Network at present, but if Alan gives me his Hex ID or Reg, I can do a search for him from one of my own sites using the ‘My Aircraft’ Tool, which will show all PAW ATOM Ground Stations on which his aircraft has  appeared, with maximum ranges for each Mode - ADSB, PAW and FLARM. Just one of the benefits of an efficient professionally created secure network.

p.s. Alan, if you don’t want to ‘publicise’ your details, send your Hex/Reg to me by PM and I’ll reply similarly and only report the basic outcome on here.

Best Regards

Peter

buzz53

Re: OGN-R uplink typical range
« Reply #5 on: July 31, 2020, 09:54:14 am »
Thank you Peter and Steve.

The HexID is F4406A, flight yesterday from Elmsett EGST about 16:50.

I appreciate there are many variables but I think it’s safe to say it should be better than 2nm? I imagine my setup is fairly ideal, being an external monopole on the bottom of a metal aircraft. Currently for convenience using 2m of RG174 so would expect a small reduction in performance but not the drastic result I am seeing.

I’m quite confident about the antenna as I have a VNA for tuning and which I use for other 868MHz activities. Although on a slight tangent, I did measure both the official PAW and ADSB antennas on another RV6 for comparison and found them both surprisingly over-long. The ADSB was 71.5mm compared to a standard transponder antenna at 56mm (measured from where they emerge from the metal base). I would have expected them to be the same. The PAW was 86mm which is the ideal freespace length but I would have expected less in an actual antenna.

Thank you for the pointer to the gliderradar site. I mentioned UKWRM in error yesterday, it should be PWUKWRM of course. It’s clear from the coverage maps that that PWUKWRM is a relatively poor performer compared to PWUKTIB. It’s also poor compared to UKWRM which seems odd given the much higher airborne power of PAW but perhaps this is due to the SDR receiver for UKTIB performing better than the PAW hardware receiver.

Back to the main subject, these maps are as you say air to ground and I wondered to what extent they reflect the performance in practice in the other direction? I assume the PAW base station power output is adjusted compared to a normal aircraft PAW to give the same max legal 500mW ERP allowing for the base station antenna gain? In which case, as the aircraft doesn’t have the benefit of a gain antenna, I assume the uplink performance is inevitably going to be significantly worse?

I will try to borrow another PAW for my next flight but with 30 degrees forecast today I think that will need to wait!

Alan

PS just seen Peter's later mention of OGNRange, I thought that had packed up but will take another look.

Admin

Re: OGN-R uplink typical range
« Reply #6 on: July 31, 2020, 11:23:53 am »
RG174 will give 2db of loss at that frequency
That is a lot of loss

Thx
Lee

exfirepro

Re: OGN-R uplink typical range
« Reply #7 on: July 31, 2020, 11:30:38 am »
Hi Alan,

There are several OGNRange sites, some of which seem to have come and gone recently. I find the one I posted the link to above,  the most reliable these days.


OK - I have done a search and only 2 records show up for F4406A * both from 30th July:

PWHempstd reporting only 4 ‘pings’ at between 20 and 30Km (with a max of 27Km) - so a bit of a ‘fluke’, and

PWUKWRM (Colchester) reporting 248 ‘pings’ at <10Km and 12 ‘pings’ at between 10 and 20Km (with a max of 12Km).

By comparison, my own most recent flight from 24th July - with a PAW 1/4 wave on a metal ground plane underneath my aircraft - shows a PAW range to 3 local Ground Stations over a fairly long flight over and around Edinburgh at ‘not above 2000ft’ of 29, 39 and 51Km.

From the number of ‘pings’ received, your PAW was obviously transmitting, though I might have expected a much higher ping report at close range from Colchester (mine reported 2372 pings from my ‘Home Airfield’ at <10Km during the period of approach and landing), but that of course depends how long you were in the vicinity. How long was your flight and how long were you within 10Km of Colchester?

BTW, I should have said - PAW Ground Stations only transmit in response to a signal from a PAW-equipped aircraft - though not necessarily your own.

As the OGNRange site clearly shows that PWUKWRM was receiving PAW traffic from up to 30Km yesterday, it seems obvious that, despite your external antenna, your installation isn’t operating to the expected level.

This is probably (at least in part) to do with losses in the coax, but that may not be the whole story. You say you checked your antenna setup with your VNA - I assume testing the antenna in situ underneath the aircraft on the 2m length of RG174. What sort of results did you get?

Other areas to check are that there is no short circuit between the ground and the inner of the coax at the SMA connector, that the PAW Bridge is properly seated in place (are we talking Rosetta or Classic?) and that you had a solid reliable GPS fix throughout the flight - as otherwise your PAW won’t transmit. If you were running SkyDemon, you should be able to see any GPS disconnects from your SD track log. BTW, do you run a transponder? If so, how far apart are the antennas?

If all else fails, we can ask Lee to analyse your track log (when he comes back) to see if that shows any other issues.

* F4406A BTW is the default Hex ID - allocated based on the MAC address of your PilotAware. You should replace this in PAW/Configure with the allocated ICAO Hex ID for your aircraft.

Best Regards

Peter
« Last Edit: July 31, 2020, 11:52:17 am by exfirepro »

exfirepro

Re: OGN-R uplink typical range
« Reply #8 on: July 31, 2020, 12:06:13 pm »
Supplement to above,

Lee is of course correct - at least 2dB loss at 869.5MHz in your RG174 cable + 1dB for each additional connector. That means at least half the signal (in each direction) is disappearing in the coax.

Also just checked the length of a PAW ‘Rohan’ 869.5MHz 1/4 wave - the ‘whip’ is 70mm from where it comes out of the body to the tip - and it matches at about 1.3:1 on 2m of good quality (LM240 or CLF200) coax with SMA and BNC connectors.

With reference to your comments on Uplink vs Downlink, the PAW maximum legal transmit power is the same in both cases i.e. 500mW ERP (Effective Radiated Power), so transmit and receive should be pretty much the same either way. We can only use fairly low gain antennas on the PAW side of the ground station to counteract cable and connector losses, whereas on the OGN and 1090 side (which are receive only) no such restriction applies.

Regards

Peter
« Last Edit: July 31, 2020, 12:53:38 pm by exfirepro »

buzz53

Re: OGN-R uplink typical range
« Reply #9 on: July 31, 2020, 02:40:21 pm »
Thank you all again.

Peter, your 70mm measured length for the PAW antenna sounds good. Mine, with a different construction, worked out at 66mm. I will have to quiz my chum on the provenance of his long ones, but I’m sure he said they came from PAW. I’ll recheck my own antenna tuning and post a couple of VNA plots later for interest.

I was working on a loss of a little under 2dB for my (perhaps temporary) RG174. But even CFL200 would be 0.6 dB so I’m probably less than 1.5 dB worse. You’d expect this to cause only a 15% loss of range so surely this is not really a major factor here? Of course the RG174 is not so well screened against interference but I don’t think this is the issue either. For test purposes I can probably reduce mine to only 30cm so will give that a go anyway to rule it out. My earlier tests using the directly mounted sleeve dipole were also similarly poor.

I was probably within 10km of PWUKWRM for 15-20 minutes. There were no GPS failures, looking at Skydemon and PAW logs. I have a mode C transponder, antenna is probably 1 m away. I’m fairly sure it isn’t affecting things since I did at one stage pinch its antenna to feed PAW for a quick test!

I see the groundstation antenna gain is +7dBi, I thought it would be more. So the uplink will be at most 3-6 dB worse off than the downlink i.e. uplink range should be at least half the downlink. Certainly not what I'm getting! I agree something is well off in my system.

Alan

exfirepro

Re: OGN-R uplink typical range
« Reply #10 on: July 31, 2020, 03:29:43 pm »
Alan,

All OK on your figures. I would certainly expect to see a bit better coverage from a 15 x20 minute stint. That’s not to say of course that PWUKWRM is optimal in all directions - it could well suffer from some local screening - hangars, trees, etc. I have never been there so wouldn’t know.

You didn’t say if your PAW is a Classic or Rosetta?

Peter
« Last Edit: July 31, 2020, 03:33:03 pm by exfirepro »

buzz53

Re: OGN-R uplink typical range
« Reply #11 on: July 31, 2020, 03:43:31 pm »
Sorry, it's a Classic. And it's secondhand so before you say anything that obviously that gives rise to a line of thought! But the fact that it has received an aircraft at 30km but can't receive a ground station at > 3km or so makes me think it's not a basket case. Also, not wishing to add further confusion, but I did dabble with the SoftRF DIY project last year with very similar results. It's this conflicting evidence that is making diagnosis difficult.

Alan

Deker

Re: OGN-R uplink typical range
« Reply #12 on: July 31, 2020, 05:39:04 pm »
I have also found that there is a significant difference between a PAW air to Air contact and Air to Ground station.
Initially I thought that it is that 'downward angle' and blanking by the aircraft structure, but at say 3,000ft and 15nm, the down slant is only 1.9 degrees,
so the antenna is getting a good view of stations in at least a 180 degree forward arc.
The antenna is the 'official PAW dipole with about 1.2m coax suckered to the windscreen.

I doubt if a 2db coax loss is the reason for only getting a 2nm range that the Op has reported.

Deker.

exfirepro

Re: OGN-R uplink typical range
« Reply #13 on: July 31, 2020, 05:58:25 pm »
Hi Deker,

OK on the down slant angle. As I reported earlier, the database shows that while Alan was ‘seeing’ the Ground Station reporting at 2nm, the database reports that the Ground Station was receiving him at up to 12Km for at least part of his flight. It still remains a fact that the majority of issues regarding transmission/reception range as you know come down in the end to antenna choice and placement, especially with the relatively low power we are using. Unfortunately, during recent tests, I have found considerable variation between some of the early PAW antennas, which certainly won’t help, though Alan is now using a ‘Homebrew’ 869.5 antenna mounted underneath his aircraft, albeit on 2 metres of RG174.

Alan,

FYI, just so you know, Deker makes the PAW Rohan P3i antennas for us, so also knows what he is talking about.

OK on dabbling with SoftRF - that doesn’t make you a bad person - I’m always ‘tinkering’ with something or other. The reason I asked is because older Classics sometimes display particular faults that we just don’t see on Rosettas. If you choose to run a second hand Classic, that’s perfectly fine with me, as long as it’s licence key is up to date. If it isn’t of course it certainly won’t work!

The ‘obvious’ suspect for a P3i fault would appear to be a Bridge issue, and I certainly wouldn’t rule it out - especially with an older unit, but the PAW Bridges are generally very reliable and Bridge faults are extremely rare. The more likely issue with the Classic would (as I alluded to earlier) be loss of GPS, which precludes P3i transmit and can be caused by a defective GPS dongle or a seriously overheating SDR (the black Realtek ones used on the Classic occasionally run very hot - especially as they get older, - or a faulty WiFi dongle - either of which can in a ‘perfect storm’ situation block the GPS signal, but you don’t appear to be experiencing either of these problems. My intuition (and experience) still directs me to address the antenna / coax issue first, before we start to suspect anything more complicated.

Best Regards

Peter

rogerabc

Re: OGN-R uplink typical range
« Reply #14 on: August 12, 2020, 08:55:43 pm »
I experience the same effect & when checking the log files on aircrew.co.uk the ground station beacons only light up at very close range.

Are there any PAW classic users picking up ground stations at long range?

It feels like I am not benefiting from the atom grid in flight.

Log file here

https://www.icloud.com/iclouddrive/0_XLgLjbxcx_48am7f7ghxExw#2020-08-08_09-30