Month: August 2017

Physical fundamentals of the radar principle

The basic principle of operation of primary radar is simple to understand. However, the theory can be quite complex. An understanding of the theory is essential in order to be able to specify and operate primary radar systems correctly. The implementation and operation of primary radars systems involve a wide range of disciplines such as building works, heavy mechanical and electrical engineering, high power microwave engineering, and advanced high speed signal and data processing techniques. Some laws of nature have a greater importance here.
Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy.
Reflection of electromagnetic waves
The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.

Electromagnetic energy travels through air at a constant speed, at approximately the speed of light,
300,000 kilometers per second or
186,000 statute miles per second or
162,000 nautical miles per second.
This constant speed allows the determination of the distance between the reflecting objects (airplanes, ships or cars) and the radar site by measuring the running time of the transmitted pulses.

This energy normally travels through space in a straight line, and will vary only slightly because of atmospheric and weather conditions. By using of special radar antennas this energy can be focused into a desired direction. Thus the direction (in azimuth and elevation) of the reflecting objects can be measured.

These principles can basically be implemented in a radar system, and allow the determination of the distance, the direction and the height of the reflecting object.
(The effects atmosphere and weather have on the transmitted energy will be discussed later; however, for this discussion on determining range and direction, these effects will be temporarily ignored.)
Why Radar?

Radar has many advantages compared to an attempt of visual observation:
Radar is able to operate day or night, in lightness or darkness over a long range;
Radar is able to operate in all weathers, in fog and rain, it can even penetrate walls or layers of snow;
Radar has very broad coverage; it is possible to observe the whole hemisphere;
Radar detects and tracks moving objects, a high resolution imaging is possible, that results in an object recognition;
Radar can operate unmanned, 24 hours a day, 7 days a week.

Radar Historical Overview

 Neither a single nation nor a single person can say that the discovery and development of radar technology was his (or its) own invention. One must see the knowledge about “Radar” than an accumulation of many developments and improvements, in which any scientists from several nations took part in parallel. In the past, there are nevertheless some milestones, with the discovery of important basic knowledge and important inventions:

1865 The Scottish physicist James Clerk Maxwell presents his “Theory of the Electromagnetic Field” (description of the electromagnetic waves and their propagation) He demonstrated that electric and magnetic fields travel through space in the form of waves, and at the constant speed of light.

1886 The German physicist Heinrich Rudolf Hertz discovered electromagnetic waves, thus demonstrating the Maxwell theory.

1897 The Italian inventor Guglielmo Marconi achieved the first long distance transmission of electromagnetic waves. In his first experiments he used a wire to a wooden pole. In Italian a tent pole is known as l’antenna centrale, and the pole with a wire alongside it used as an aerial was simply called l’antenna. Today Marconi is known as pioneer of radio communication.

1900 Nicola Tesla suggested that the reflection of electromagnetic waves could be used for detecting of moving metallic objects.

1904 The German engineer Christian Hülsmeyer invents the “telemobiloscope” for a traffic monitoring on the water in poor visibility. This is the first practical radar test. Hülsmeyer apply his invention for a patent in Germany, France and the United Kingdom.

1921 The invention of the Magnetron as an efficient transmitting tube by the US-american physicist Albert Wallace Hull.

1922 The American electrical engineers Albert H. Taylor and Leo C. Young of the Naval Research Laboratory (USA) locate a wooden ship for the first time.

1930 Lawrence A. Hyland (also of the Naval Research Laboratory), locates an aircraft for the first time.

1931 In Britain the first known proposal for a radar system came from William A. S. Butement and P. E. Pollard in January 1931. They equipped a ship with radar. As antennae were used parabolic dishes with horn radiator. Although their equipment produced short-range results the work was abandoned for lack of government support.

1933 On the basis of the in 1931 from himself invented sonar, Rudolph Kühnhold presented a so called “Funkmessgerät”. It worked on a wavelength of 48 cm and the transmitter had a power of about 40 Watts. From these tests, the Freya-radar was developed, which was produced in series beginning in 1938.

1935 Robert Watson-Watt (later: Sir Robert) suggested that radio waves might be used to detect aircraft at a distance and outlined a means of doing so. Intensive research began and by 1939 Britain possessed a defensive chain of highly secret Radio Direction Finding (RDF) stations.

1936 The development of the Klystron by the technicians George F. Metcalf and William C. Hahn, both General Electric. This will be an important component in radar units as an amplifier or an oscillator tube.

1939 Two engineers from the university in Birmingham, John Turton Randall und Henry Albert Howard Boot built a small but powerful radar using a Multicavity-Magnetron. The B–17 airplanes were fitted with this radar. Now they could find and thus combat the German submarines in the night and in fog.

1940 Different radar equipments are developed in the USA, Russia, Germany, France and Japan.

Driven by general war events and the development of the Air Force to major branch of service, the radar technology undergoes a strong development boost during the World War II, and radar sets were used during the “Cold War” in large numbers along the inner German border.

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:


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.

The first frame:
[TC-] [-Altitude-] T F [—-Latitude—] [—Longitutde–]
01011 000 000011111111 0 0 10110011110111111 01001101110100110

The second frame:
[TC-] [-Altitude-] T F [—-Latitude—] [—Longitutde–]
01011 000 000011111111 0 1 10101100101000001 11110101101111010

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 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

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)
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)
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.

4. 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

5. 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)
M = -39

7. Compute the global longitude, Lon
Lon = dlon(T) * (modulo(M, ni) + Lon(T) / 131072)
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 …