Industry Insights

Understanding Radar Part 1 - Fundamentals

2020-09-01  |  7 min read 

The recent explosion in Beirut, Lebanon, rocked the world and dominated our news feeds. The fifth largest artificial, non-nuclear explosion in recorded history devastated the Lebanese capital, killing more than 200, injuring over 7,000, and leaving over 300,000 homeless. Measuring devastation on this scale is no easy task, and the local authorities were too preoccupied with those directly affected by it to conduct larger surveys. NASA’s Advanced Rapid Imaging and Analysis (ARIA) team stepped up to the plate, collecting and analysing satellite-derived synthetic aperture radar (SAR) data to map the extent of the damage to the port city, as shown in the image below. This gave response teams a greater understanding of the extent of the damage from a macro perspective, enabling them to identify and focus on the worst hit areas quickly.

NASA Map of Beirut Explosion Damage
NASA Map of Beirut Explosion Damage

Source: NASA Earth Observatory, August 5 2020 https://earthobservatory.nasa.gov/images/147098/scientists-map-beirut-blast-damage

Reading about this in a recent newsletter sent out by NASA’s Jet Propulsion Laboratory (JPL) piqued my curiosity and got me thinking. What is SAR? What else is it used for? To understand SAR, we first have to understand the fundamentals of Radar.

Radar imaging is not a new technology, in principle it is actually quite simple: a transmitter emits a radio signal, which reflects off a surface, and then is detected by a receiver. Distance is measured by the time taken for the transmitted signal to be received, divided by the speed of light (C), and then divided again by 2. By pulsing the transmitted signal, it is possible to measure the speed of the detected object through measuring the doppler shift of the received signal. Depending on the velocity, the amount of doppler shift will vary too, in the same way that an ambulance’s siren changes tone as it nears and then passes you.

Radar’s perhaps best-known application is air traffic control, where large rotating radar dishes scan the skies for aircraft. But these are specifically tuned to search for objects the size of aircraft and operate within the S band at a frequency of 2.7 to 2.9 GHz. Depending on the intended application of the radar, different frequencies must be used. There are a number of different frequency bands used for radars, ranging from 0.3 GHz in the P band, to 40 GHz in the Ka band. The bands, and their uses are outlined in Table 1, below.

Band

Frequency

Wavelength

Typical Application

Mm

40 – 300 GHz

 < 0.8cm

Airport security (body scanners), Automotive short-range radar, short-range fire-control radar and CIWS (detection of incoming projectiles and missiles)

Ka

27 – 40 GHz

1.1 – 0.8 cm

Air traffic control, Traffic police radar

K

18 – 27 GHz

1.7 – 1.1 cm

H2O absorption

Ku

12 – 18 GHz

2.4 – 1.7 cm

Satellite altimetry

X

8 – 12 GHz

3.8 – 2.4 cm

Urban monitoring, ice and snow, vegetation cover

C

4 – 8 GHz

7.5 – 3.8 cm

Global mapping; change detection; monitoring of areas with low to moderate penetration; higher coherence; ice, ocean maritime navigation (Commonly used for SAR)

S

2 – 4 GHz

15 – 7.5 cm

Earth observation; agriculture monitoring, vegetation density (little but increasing use in SAR)

L

1 – 2 GHz

30 – 15 cm

Geophysical monitoring, biomass and vegetation mapping (medium resolution SAR)

P

0.3 – 1 GHz

100 – 30 cm

Biomass

Table 1: Radar frequency bands, and their respective wavelengths and typical applications.

Source: NASA Earthdata 2020: https://earthdata.nasa.gov/learn/what-is-sar

The advantages of each band are explained by the size of the wavelength, and how the pulse is reflected. A shorter wavelength is reflected off smaller objects, while larger wavelengths will not be affected by these same objects, effectively enabling us to see through them. An example of this might be an application to measure characteristics of a forest. An X-band signal will be able to see the density of leaf cover in the forest as its wavelength is about that of the size of a leaf, while a P-band signal will see through the leaves and can measure the density of the tree trunks and branches.

So now we know what we can see with a radar, and what wavelength we need to see it. The next step is to get the signal to the target (and back again). There are a few ways to do this:

  • The first radars were static, and the field of view (FOV) did not move. They relied on the target entering the FOV.
  • Next came the radars that we are perhaps most familiar with – those with a rotating dish that houses the transceiver. Here the sensor moves the FOV, scanning to locate a target.
  • Placing one of the first two kinds of radars onto a moving vehicle is another way of imaging a larger area. SAR and side-looking aerial radar (SLAR) are examples of this.
  • Finally, the most complex way to change the FOV is to have multiple antennae, which is typically done in a phased-array.

Keysight has long provided solutions for radar applications, building up a wealth of design and test experience in the field. To learn more about Keysight’s end-to-end solutions for radar applications, covering transceiver module and antenna test, radar emulation, target simulation, and component test, click here. To learn more about SAR, keep an eye out for Part 2.