0. Hardware

1. What is ADS-B?

1.1. What

In 2002 the Federal Aviation Administration (FAA) announced a dual link decision using the 1090 MHz extended squitter (1090 ES) link for air carrier and private or commercial operators of high-performance aircraft, and universal access transceiver link for the typical general aviation user.[31] In November 2012, the European Aviation Safety Agency confirmed that the European Union would also use 1090 ES for interoperability.[32] The format of extended squitter messages has been codified by the ICAO.[28]

With 1090 ES, the existing Mode S transponder (TSO C-112 or a standalone 1090 MHz transmitter) supports a message type known as the extended squitter message. It is a periodic message that provides position, velocity, time, and, in the future, intent. The basic ES does not offer intent since current flight management systems do not provide such data (called trajectory change points). To enable an aircraft to send an extended squitter message, the transponder is modified (TSO C-166A[33]) and aircraft position and other status information is routed to the transponder. ATC ground stations and aircraft equipped with traffic collision avoidance system (TCAS) already have the necessary 1090 MHz (Mode S) receivers to receive these signals, and would only require enhancements to accept and process the additional extended squitter information. As per the FAA ADS-B link decision, and the technical link standards, 1090 ES does not support FIS-B service.[31]

Relationship to ADS-A/ADS-C

There are two commonly recognized types of ADS for aircraft applications:

ADS-addressed (ADS-A), also known as ADS-Contract (ADS-C)
ADS-broadcast (ADS-B)

ADS-A is based on a negotiated one-to-one peer relationship between an aircraft providing ADS information and a ground facility requiring receipt of ADS messages. For example, ADS-A reports are employed in the Future Air Navigation System (FANS) using the Aircraft Communications Addressing and Reporting System (ACARS) as the communication protocol. During flight over areas without radar coverage, e.g., oceanic and polar, reports are periodically sent by an aircraft to the controlling air traffic region.[21]

The transmission delay caused by protocol or satellites is significant enough that significant aircraft separations are required. The cost of using the satellite channel leads to less frequent updates. Another drawback is that no other aircraft can benefit from the transmitted information as ACARS information is not re-broadcast from ground facilities to other aircraft.[citation needed]

Traffic information service – broadcast (TIS–B)

Traffic information service – broadcast (TIS–B) supplements ADS-B’s air-to-air services to provide complete situational awareness in the cockpit of all traffic known to the ATC system. TIS–B is an important service for an ADS-B link in airspace where not all aircraft are transmitting ADS-B information. The ground TIS–B station transmits surveillance target information on the ADS-B data link for unequipped targets or targets transmitting only on another ADS-B link.[citation needed]

TIS–B uplinks are derived from the best available ground surveillance sources:

ground radars for primary and secondary targets[citation needed]
multilateration systems for targets on the airport surface[34]
ADS-B systems for targets equipped with a different ADS-B link[15]

Flight information services-broadcast (FIS-B)

FIS-B provides weather text, weather graphics, NOTAMs, ATIS, and similar information. FIS-B is inherently different from ADS-B in that it requires sources of data external to the aircraft or broadcasting unit, and has different performance requirements such as periodicity of broadcast.[21]

In the United States, FIS-B services will be provided over the UAT link in areas that have a ground surveillance infrastructure.[15]

Another potential aircraft-based broadcast capability is to transmit aircraft measurements of meteorological data.[citation needed]

Documents

Source code

–> https://github.com/MalcolmRobb/dump1090

–> https://github.com/mutability/dump1090

–> https://github.com/flightaware/dump1090

How to decode

the position of the aircraft from an Odd and an Even CPR ADS-B Frame?

Say you receive the following two frames:

8D75804B580FF2CF7E9BA6F701D0 8D75804B580FF6B283EB7A157117

Bytes 2 to 4 give ICAO address 75804B which is CEB [5J] Cebu Pacific Air Registration RP-C3191 Airbus A319

The first 5 bits contain the Downlink Format (DF). First byte 8D is 10001-101 so DF=17 and CA=5 DF 17 means we have an extended 112 bit squitter. Not all extended squitters have the position. We need to check the Type Code (TC).

Byte 5 is the first byte of the extended squitter as such, which is an extra 56 bits compared to a short squitter. This makes up to the last 3 bytes not included. These last 3 bytes are an error check. The Type Code is contained in the first 5 bits which is in Byte 5 of the whole frame: 58hex is 01011-000bin Both TC are 01011bin which is 11dec. This TC is Airborne Position with Barometric Altitude as follows: Airborne position with Horizontal protection limit: (HPL) 25 m ≤ HPL < 185.2 m (0.1 NM) 95% Containment radius, μ and v, on horizontal and vertical position error: 10 m ≤ μ < 92.6 m (0.05 NM) Navigational uncertainty category: 7

This is still not sufficient, as to start decoding positions we need an ODD and an EVEN frame. These frames contain the position in CPR (Compact Position Reporting) format. Whether a frame is odd or even is indicated in bit 22 of the extended squitter. As we need more binary data lets change the extended squitters into binary.

Examples

In the first byte: First 5 bits are the TC which is 11. A TC 11 decodes to the following fields:

Bits 6 and 7 are the Surveillance Status. Bit 8 indicates the antennas used. Bits 9 (MSB) to 20 (LSB) contain the altitude. Bit 21 contains the T (Time) bit. T in this case is 0 which means we are not synchronized to UTC. Bit 22 contains the F flag which indicates which CPR format is used (odd or even). Bits 23 (MSB) to 39 (LSB) contain the encoded latitude. Bits 40 (MSB) to 56 (LSB) contain the encoded longitude.

So our first frame has F flag = 0 so is even and the second frame has F flag = 1 so odd. So we finaly know that we can use our information to find the position of this aircraft.

CPR

CPR uses several functions which are good to know before we start:

Nb is the number of bits for encoding. Airborne positions use Nb = 17 as we can confirm from above. Note Nb = 19 for surface positions.

CPR decodes positions to Zones Nz. The number of possible zones for airborne positions is Nz = 15 giving an unambiguous airborne range for decoding of 360 NM.

The floor notation floor(x) denotes which is the greatest integer k such that k<x. In VB6 this is done by the function INT.

Modulus MOD(x,y) is always positive. In VB6 I have written it as follows: Function modulo(val, modval As Double) As Double modulo = val Mod modval If val < 0 Then modulo = modulo + modval End Function

The NL(x) is a big one but only returns a number between 1 and 59. In my VB6 progam I use a lookup table as described in the PDF document 1090-WP-9-14.

So our starting point is:

Lat(0) = 10110011110111111 or 92095 dec Lat(1) = 10101100101000001 or 88385 dec Lon(0) = 01001101110100110 or 39846 dec Lon(1) = 11110101101111010 or 125818 dec

  1. Compute the latitude index j: Under VB6 that is done as follows: j = Int(((59 * Lat(0) - 60 * Lat(1)) / 131072) + 0.5) gives j = 1

  2. Compute the values of Rlat(0) and Rlat(1): rlat(0) = AirDlat0 * (modulo(j, 60) + Lat(0) / 131072) rlat(1) = AirDlat1 * (modulo(j, 59) + Lat(1) / 131072) where Const AirDlat0 As Double = 6 Const AirDlat1 As Double = 360 / 59 This gives rlat(0) = 10.2157745361328 rlat(1) = 10.2162144547802 Note: Southern hemisphere values are 270° to 360°. Subtract 360°.

3.NL for Rlat(0) and Rlat(1) , NL(0) and NL(1) are both equal to 59. Do not proceed from here if NL(0) not equal to NL(1). NL(0) = 59 NL(1) = 59 Both NL are equal so rlat(0) and rlat(1) are our latitudes.

  1. i being the frame to decode, the last frame is odd, i = 1, compute n(i) which is the greater of 1 and NL(i) - i ni = 58

  2. Next Dlon(i) = 360 / n(i) dlon(1) = 6.20689655172414

6.Find M, the longitude index. You need to know that in this case T = 1 (odd). M = Int((((Lon(0) * (nl(T) - 1)) - (Lon(1) * nl(T))) / 131072) + 0.5) gives M = -39

  1. Compute the global longitude, Lon Lon = dlon(T) * (modulo(M, ni) + Lon(T) / 131072) gives Lon = 123.889128586342

So there you have it!

On the second frame our aircraft was at

Lon = 123.889128586342 Lat = 10.2162144547802

Also on 2nd frame for example: Altitude is 2175 feet but that is a different story …