[Discuss-gnuradio] Mode-S/ADS-B and UAT

[Eric Cottrell]

Hello,

I was emailed by a few people off list that are interested in the Mode-S/ADS-B 
code.  I am still working on it.

I got the DPD Productions antenna just before Thanksgiving, but it was too 
windy to put up right away.  Sitting on the first floor in the middle of a wood 
frame house, the antenna was picking up aircraft from 40 to 140 miles away 
depending on direction.  I could not see low altitude aircraft to the south of 
the nearest busy airport, which is eight miles south.  44 feet above the ground 
on my roof is a different story and I can see aircraft from 150 to 210 miles 
depending on direction.  Although the DPD antenna is higher gain, I do not get 
the same level of interference as the colinear.  The DPD antenna is suppose to 
be a narrow bandwidth antenna.  I tried the wideband preamp with the DPD 
antenna and had the same problem with the noise floor rising and decreasing 
range.

I bought a SBS-1 last week.  It is a commercial Mode-S/ADS-B receiver that is 
smaller than the USRP.  The sensitivity seems slightly better.  It could handle 
the wideband preamp better with just a slight decrease in range.  I am 
currently working on providing a port 30003 compatible output for my receiver.  
Then I can use Planeplotter and can compare my code to the SBS-1.

My nearby major airport has a ASDE-X system for monitoring aircraft and 
vehicles on the airport surface.  It appears they are installing an ADS-B 
Multilateration system as I get a couple of the sensors.  They use DF18 
(Non-Transponder Extended Squitter) and just send an ID and Multilateration 
status.  Two of them ID as STEST123 with the category of Fixed Ground/Tethered 
Obstacle.  The third recently appeared with an ID of PTEST123.  The SBS-1 also 
picked up the IDs so it can decode at least some DF18 formats.

I want to do a UAT receiver and was planning a trip to Philly to look at the 
TIS-B/FIS-B ground segment transmissions.  I got a surprise when I found out 
the New England area has TIS-B/FIS-B services.  Someone did a demo of a receive 
only system.  The TIS-B service only transmits if there is a ADS-B aircraft 
using UAT in the area.  Being able to receive TIS-B would be good because it 
includes non-ADS-B aircraft.  There is also cross-linking of UAT and Mode-S so 
ADS-B aircraft on Mode S ADS-B should show up on UAT.
http://www.youtube.com/watch?v=NmYrUGxRC88

It is an iPhone application using a separate UAT receiver costing $1100.
http://www.skyscope.net/

I looked at the 978 MHz UAT frequency and think I see the signal but it does 
not completely match up with the specifications.  The system uses slightly over 
1 megabit per second FSK with a over 600K shift.  I see what looks like the 
mark frequency, but instead of the space frequency I get another signal at the 
center frequency (978.0 MHz).  If I use a simple PLL FM detector flow graph 
with a scope sink I do see what looks like a bitstream.  The current plan is to 
get my laptop updated and setup, then go mobile.  I suspect there is a 
transmitter at a nearby airport.

73 Eric