Radarbox is "blind" for the planes above my head

[vic_atc]

Hi,

I'm encountering something very unusual. I have no problem with the reception of my radarbox. My RB detects planes at aprox 230 nmi away. However, I've noticed something very weird: every plane that enters a circle around my radarbox (a cricle with aprox 10 km radius), it simply cannot be seen. Immediately after these planes exits this imaginary circle, the radarbox starts seeing them again. I have checked that there is nothing wrong configured on my Radarbox Software (like not showing the aircraft within a specific range). It simply doesn't receive the Mode S signal anymore for those planes being inside this "blind zone".
Have you seen this strange behavior too? Is there something wrong with my RB?
I can expect that this behavior could be due to:
1. Very strong signal. It saturates the LNA of the Radarbox.
2. Omnidirectional characteristic of the aircraft's transponder antenna. Thus, when the plane is above my head the signal would be very low.
3. Maybe the Doppler effect. Firstly, the plane is heading toward my radarbox so the signal's frequency is bigger: f_central + f_doppler. When it goes away, the frequency will rapidly decreases to f_central - f_doppler. Maybe the radarbox cannot adapt to this fast frequency changing.

If there is anyone who can explain me what is happening, I would be very grateful.

Regards!

[RodBearden]

Hi Vic

I reckon your point 2 is the answer - combine that with the fact that your aerial will be pointing straight at the transmitter, with no "length" showing.

It's probably not being overpowered - I quite often have a police helicopter hovering a few hundred metres from my location but not above it, and the reception (admittedly only of Mode-S) is fine.

Rod

[Jeremy]

Are the a/c your are seeing outside your blind zone on the network?
J.

[Fenris]

What antenna are you using?

Generally a colinear antenna like the supplied antenna will have a null along the axis of the antenna, as will the blade antennas on the aircraft.

When these nulls are aligned it can lead to a big increase in path loss and hence not enough signal received to decode.

Also note that the transponder antennas on the top of the aircraft are partially shielded by the aircraft structure when seen from below, so a similar effect applies even if there is no null in the antenna radiation pattern.

Do you see any aircraft at low altitudes over your location? In theory these should be received better than if they are at high altitude.

[vic_atc]

Hi,

To Rod: many thanks for the answer. I should expect that everyone has observed this behavior, am I right? If not... I agree that the signals for the plane right above my head are weaker, but I suspect that RB should still receive them. So, I'm not very confident that my RB is working properly. If everybody tells me that this is the normal behavior observed on all the RB units (using the provided antenna), I'll be ok.

Are the a/c your are seeing outside your blind zone on the network?
J.

Hi Jeremy,

No. I have disabled the network feature.

To Fenris:

Actually I'm very close to an airport so I'm not very sure that the pilot enables the ADS-B feature of the Mode S right after the plane takes off. Today, two airplanes have flown right over my head (in Climbing state) but I couldn't see them on the Radarbox Map. Probably they were listed in the left panel but I've missed to check that out. I repeat, I guess they didn't had the ADS-B enabled.
Otherwise, the most of the planes for which I'm reporting this problem were flying at cruising altitude, right over my head. That's way you and Rod are probably right.

[Fenris]

Do you know what aircraft they were that passed low overhead? Do you see aircraft on the ground? How close is the airport?

Most aircraft that transmit ADS-B do so all the time, on the ground too.

[vic_atc]

Do you know what aircraft they were that passed low overhead? Do you see aircraft on the ground? How close is the airport?

Most aircraft that transmit ADS-B do so all the time, on the ground too.

They were both B738s. Of course they have Mode -S with ADS-B. I was able to see them on the map 1-2 minutes after they've flown over my RB's antenna.

Regarding the airport: 2 weeks ago I went with my car near the airport (200-300 m). I wasn't able to see any 'ground' plane on the Radarbox software. Only after 10 minutes after taking off. Or 10 minutes before landing.

[Fenris]

Hmm, are you sure you don't have some sort of filter applied? An altitude one perhaps?

[MikeC]

I don't suffer from this problem using a GP-1090 external antenna:

[palmar]

Also note that the transponder antennas on the top of the aircraft are partially shielded by the aircraft structure when seen from below, so a similar effect applies even if there is no null in the antenna radiation pattern.

SSR, Mode-S and TCAS use two antennas, top and bottom of fuselage.

[palmar]

3. Maybe the Doppler effect. Firstly, the plane is heading toward my radarbox so the signal's frequency is bigger: f_central + f_doppler. When it goes away, the frequency will rapidly decreases to f_central - f_doppler. Maybe the radarbox cannot adapt to this fast frequency changing.

No issue. The input stage is broadband (more than necessary so it picks up DME signals) and signals will not be lost by Doppler shift.

[palmar]

Regarding the airport: 2 weeks ago I went with my car near the airport (200-300 m). I wasn't able to see any 'ground' plane on the Radarbox software. Only after 10 minutes after taking off. Or 10 minutes before landing.

If you have seen the same aircraft while en-route but not while landing or becoming airborne, something is definitely wrong. ADS-B is not switched on by pilots, it remains on on the ground. EXCEPTION: if you are in Singapore.

[vic_atc]

Hi,

I've just come back from the airport :) For the low level flights (app / dep) this is what really happens:

- The tracking stops 10 min before landing. But messages are still received (at 'Changed' i can see that it changes every second). I also could see planes with 'Grounded' status. But no position report.
- Something like that happens for departing planes. Only after 10 minutes they show up on the map. Until then, only basic Mode-S info.

I have triple checked that no filter was applied. No Range/Altitude filter.

There will be a long night because right now I'm going under a pretty busy route to check again what happens when I'm right under the plane. I'll send updates, I'm taking my wireless modem with me.

Regards.

[Fenris]

Also note that the transponder antennas on the top of the aircraft are partially shielded by the aircraft structure when seen from below, so a similar effect applies even if there is no null in the antenna radiation pattern.

SSR, Mode-S and TCAS use two antennas, top and bottom of fuselage.

Yes, the top antenna behaves differently from the bottom antenna, it has even less coverage below the aircraft.

[palmar]

Hi,

I've just come back from the airport :) For the low level flights (app / dep) this is what really happens:

- The tracking stops 10 min before landing. But messages are still received (at 'Changed' i can see that it changes every second). I also could see planes with 'Grounded' status. But no position report.
- Something like that happens for departing planes. Only after 10 minutes they show up on the map. Until then, only basic Mode-S info.

I have triple checked that no filter was applied. No Range/Altitude filter.

There will be a long night because right now I'm going under a pretty busy route to check again what happens when I'm right under the plane. I'll send updates, I'm taking my wireless modem with me.

Regards.

If you are not in Singapore from where the same behaivour was reported before:
Your box may not be able to detect 112 bit Mode-S signals when a certain reception power is exceeded. It seems that 56 bit signals still come through. There is a floating reference detector which may not work properly for 112 bit. I would think it is best to return the box.