By Steve Hutt, FASVIG Programme Coordinator

LPAT Screen

Here’s what the LPAT screen looks like.

It’s been a bit quiet on the NATS/AOPA Project EVA LPAT front so I thought a bit of news would be welcome. On Saturday 26th September 2015 I had the opportunity to take part in a NATS/AOPA Project EVA LPAT flight trial.

Back in February 2015 when AOPA put out the call I volunteered to take part in the flight trials for the NATS portable Low Power ADS-B Transceiver (LPAT). Last Thursday 24th August I received a call from Bob Darby of AOPA asking if myself and my GHOCK Flying Group colleague, John Simper, were available for Saturday 26th August? The weather forecast was good;  John and I were available and so was our group PA28, G-HOCK, so we jumped at the chance. Bob forwarded a bunch of instructions and the proposed flying exercise. Bob would be flying down from Fairoaks with a pair of LPATs in his PA28, G-BPIU, to meet us at Goodwood. We would be setting them up in both planes and carrying out the detailed flight test protocol that has been developed for the LPAT flight trials. These involve two aircraft flying a pre-agreed route maintaining at least 500ft vertical separation, one flying direct at 80kts while the other flies a zigzag route above or below at 120kt, all the while observing and recording the LPAT’s effectiveness and influence on pilot traffic awareness.

The first flight of the day proved to be a damp squib. We overlooked the attenuator that is required to be connected to the LPAT’s coax antenna cable so our LPAT did not work. After landing back at Goodwood, we swapped over the LPAT’s between the planes and that was when we diagnosed the cause of the problem. Ordinarily we might have been able to spot the fault before take-off based on detecting airliner over flights, but it was our poor luck that there were no airliners flying in the vicinity when we needed them! The second flight of the day, with the corrected setup, was far more successful. Both aircraft were able to regularly identify each other as nearby traffic symbols on the LPAT screen.


LPAT with antenna and ground plane in G-HOCK

The LPAT units are prototype demonstrators so do not necessarily represent how any final production devices may look or function. As they operate on the standard 1090MHz frequency they use a conventional transponder rod antenna which is connected via a short length of coax cable. And as with a transponder, LPAT needs a metal ground plane for the antenna too so there was a 13x13cm aluminium plate with a rod antenna sticking through it, which needed to be accommodated on the glare shield.

For successful operational use of LPAT it is key to understand the implications of the location of the antenna and how the aircraft airframe may block both transmitted and received ADS-B messages. During our flight trial we encountered instances where we:

a) made visual contact with the other a/c first then acquired the target later on LPAT,
b) the other a/c was acquired as a target on LPAT first and we later made visual contact,
c) made visual contact but never acquired an LPAT contact, and
d) acquired target on LPAT but never made visual contact.

There were definitely times where LPAT alerted us to the other aircraft’s presence when we would never have seen it if relying just on mark one eyeball. In one instance LPAT reported the other aircraft and the only way I managed to see it was by dipping the left wing.

I found the LPAT screen a little on the small side and due to it being a very bright sunny day it did suffer from reflections. With the current setup the audible traffic proximity alarms are not fed into the intercom system so I found them hard to hear. At one point we also discovered that putting ones hand up to shield the screen from sunlight had the effect of blanking out the nearby antenna such that we disappeared from the other aircraft’s LPAT display.

All in all it was a very useful day’s flying where we learned a lot and NATS reported being very grateful for our feedback in return. Project EVA is not so much about ADS-B technology but how GA pilots can benefit from it. Project EVA cannot progress without user feedback and our experience along with many others is essential. I had a very useful and long telephone conversation with the NATS Project Leader afterwards. There is more work to do but the basic technology does work provided the user has a good understanding of the environment within which LPAT is operating and how LPAT will respond.