Data acquisition - SBS-1 Multilateration trials

The SBS-1 Multilateration Trials Webpages

Introduction

Data acquisition

Data acquisition

Acquisition tool

Data are acquired through an offline tool that generates a standardized logfile over 2 minutes. Earlier trials have shown that an online tool is more comfortable, but the high data rate from some stations together with an increased requirement for configuration changes on the user side (routers, firewalls) deemed it more appropriate to use a tool that can operate completely on its own. The tool can be programmed in advance so to start data acquisition at a specific time, therefore an unmanned operation is possible.

Logfile formats

Logfiles are ASCII formatted with the PC timestamp added. This is for reference only and will not be used for the position calculation. Here are some examples:

20:00:00.412 - 07 - 00 2C 68 F4 - 5D A9 D1 E4 00 00 00 - 3FFD

20:00:00.413 - 01 - 00 6A 8A F4 - 8D A9 D1 E4 99 01 D2 0A 28 08 00 00 00 00 - 0424

20:00:00.415 - 05 - 00 C8 00 F8 - A0 00 16 91 FF F4 75 42 FF FC EE A2 BE BB - 7FDE

20:00:00.417 - 05 - 00 39 25 F8 - A0 00 13 1C 80 1D AD 31 60 0C E3 4C A0 BB - E9C6

20:00:00.418 - 01 - 00 33 96 F8 - 8D 40 06 8D 99 04 A5 21 80 5F 00 00 00 00 - 1047

Data are preformatted so that the clock, messagetype, counter value (LSB first), raw message and the checksum (in that order) appear separated by a hyphen. Each of these data blocks can be processed by other applications as is required.

Overview of Mode-S data types

During the various stages of data processing we will be required to identify and decode the raw data messages. The SBS-1 messagetype (01, 05, or 07) is not sufficient to properly identify the contents of a message. So we have a closer look onto the raw message itself and see how we can come to a quick check what the contents would be:

Raw data	Raw data	Download	SBS-1	Basestation TCP	Name of message and squitter rate	useful data	Aircraft ID
Byte 1	Byte 5	format	messagetype	socket messagetype			24-bit

20..27		04	07	MSG,5	Mode-S altitude reply	Altitude	end
28..2F		05	07	MSG,6	Mode-S id reply	Squawk	end
58..5F		11	07	MSG,8	All call reply/ACAS Squitter (every 1 s OPEN )	none	Byte 2-4
88..8F		17	01		Extended Squitter		Byte 2-4
	00..07	17	01			Altitude
	08..27	17	01	ID or MSG,1	Aircraft Identification Squitter (BDS 0,8) (every 4.8 to 5.2 s)	Callsign
	28..47	17	01	MSG,2	Surface Position Squitter (BDS 0,6)	Position, TTrack, GndSpd
	48..97	17	01	MSG,3	Airborne Position Squitter (BDS 0,5) (every 0.4 to 0.6 s)	Position, Altitude
	48..4F					Precision < 3m
	50..57					Precision < 10m
	58..5F					Precision < 0,05 NM
	60..67					Precision < 0,1 NM
	68..6F					Precision < 0,25 NM
	70..77					Precision < 0,5 NM
	78..7F					Precision < 1 NM
	80..87					Precision < 5 NM
	88..8F					Precision < 10 NM
	90..97					Precision > 10 NM
	99 (for subsonic)	17	01	MSG,4	Airborne Velocity Squitter (BDS 0,9) (every 0.4 to 0.6 s)	TTrack, GndSpd, VertRate, GNSS AltDiff
A0..A7		20	05	MSG,5	Comm-B altitude reply	Altitude	end
A8..AF		21	05	MSG,6	Comm-B id reply	Squawk	end

As it appears a look to the first and fifth byte of the raw message lets us quickly sort out, which data are interesting and which should be ignored. With the examples from above we can decode:

20:00:00.412 - 07 - 00 2C 68 F4 - 5D A9 D1 E4 00 00 00 - 3FFD

An all call reply or squitter (byte 1 = 5D) from aircraft A9D1E4 (bytes 2 to 4) without any further usable data. This is a message that just says, I am here. The message was received at SBS-1 counter value F4682C which translates to 16,017,452.

20:00:00.413 - 01 - 00 6A 8A F4 - 8D A9 D1 E4 99 01 D2 0A 28 08 00 00 00 00 - 0424

This is an ADS-B squitter message (byte 1 = 8D) from the same aircraft that provides velocity data (byte 5 = 99). The velocity is encoded in the sequence 01 D2 0A 28 08. The counter value is F48A6A, which is 16026218. This is 8766 counts later than the previous message which in turn is 8,766 * 0.05 µs = 438 µs.

20:00:00.417 - 05 - 00 39 25 F8 - A0 00 13 1C 80 1D AD 31 60 0C E3 4C A0 BB - E9C6

Here we have a no-pos message (byte 1 = A0) from aircraft 4CA0BB (the last three bytes). Messagetype 05 packets are only sent on specific request from a ground interrogator. This message was processed by SBS-1 at F82539 (16,262,457), which is 245,005 counts or 12.25 ms after the first message above.

What data can we use for what?

We will have to expect two stages of calculation that require known and unknown data.

The clock synchronization requires the following packets to decode a precisely known inertial position of a reference flight:

	ADS-B squitter high precision position and barometric altitude (Flight level) i.e. byte 1 = 88..8F and byte 5 = 48..77
	ADS-B squitter velocity data to correct the barometric altitude by the GNSS altitude difference, if available, i.e. byte 1 = 88..8F and byte 5 = 99

For flights with unknown position the data required can be derived from the other messages if it is secured that the message does no contain garbage. Therefore only those no-pos messages should be processed where the aircraft id had been confirmed by the reception of an all call reply/squitter message 58..5F. These messages, as are messages 88..8F, are CRC secured on the Mode-S downlink and the SBS-1 only lets them through if they have proven to be correct, i.e. the last three bytes are zero.