[time-nuts] Aircraft ping timing
brian at lloyd.com
Sun Mar 23 10:08:08 EDT 2014
On Sun, Mar 23, 2014 at 3:37 AM, nuts <nuts at lazygranch.com> wrote:
> Note that the ADS-B mentioned is just a fancy version of the
> transponder that was turned off.
I guess it depends on your concept of what a transponder does. Yes, ADS-B
does transmit in the same band as the standard transponder (1090MHz) but
the function and content of the transmissions are very different. With the
standard mode-A, mode-C, and mode-S system, the transponder only replies to
interrogation, and then only replies with the 12-bit ID code (mode-A) or
12-bit ID code plus 12-bit altitude code (mode-C). Mode-S turns it into a
two-way data link and adds a bit more more data (aircraft ID) to the reply.
The key here is that, if nothing is sending an interrogation, the
transponder is silent. The interrogating station must do the position
determination based on distance and azimuth of the reply.
In the case of ADS-B, the transponder is no longer just a transponder. It
is a full transceiver with an application, ADS-B, sending unsolicited data.
In this case the aircraft actively transmits its position and velocity data
along with its ID. No interrogation is required. Anyone within range and
with a receiver can pick up the signal and determine the location of the
transmitting aircraft. This means it is possible for a receiver to collect
this data passively.
The major problem with ADS-B is that they came up with two systems and,
rather than fight it out to determine which would win, they just adopted
both systems. So we have ADS-B on 978MHz using a bit more sensible
modulation and framing, and on 1090MHz using the older OOK pulse-code
modulation (1090ES). And to add insult to injury, they have mandated that
anything that flies at FL180 (18,000') or above (meaning all airliners)
must use 1090ES.
The transmission mode for 1090ES (extended squitter) is very inefficient.
The signal is broad as a barn door due to the use of pulse code modulation
using on-off keying. Each transmission requires 5 frames. The end result is
that it isn't going to take a lot of airplanes transmitting in the same
airspace before you saturate the channel.
So you have two separate-and-incompatible ADS-B systems flying around. The
FAA "solved" this problem by deploying a series of ground stations that
will repeat ADS-B data from one channel onto the other. If a ground station
detects transmissions on 978MHz, it will repeat them on 1090MHz and repeat
transmissions from 1090MHz onto 978MHz, otherwise it is silent. This means
that a receiver-only system is not guaranteed to see all traffic unless one
has receivers for both bands. If an aircraft is to be completely autonomous
for traffic detection, the aircraft must transmit and receive on both
bands. Alternatively it can transmit and receive on 978 and rely on the
ground stations to repeat the 1090ES traffic.
Also, because of the capacity limitations of 1090ES, the FAA does not data
link weather data on 1090ES. That is only available on 978MHz. So any
aircraft wishing to avail itself of the ground-based weather radar data and
ground weather reporting must have a receiver for 978MHz.
Bottom line, ultimately every aircraft is essentially going to need both
(I'm sorry but the level of stupidity that led to the ADS-B design as it
now stands just boggles my mind. You have to work at it in order to take
something so conceptually simple and make it this brain-damaged.)
Brian Lloyd, WB6RQN/J79BPL
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
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