History[ edit ] Radiometric scanning for Venus by Mariner 2 , for its December flyby of that planet First developments of microwave radiometer were dedicated to the measurement of radiation of extraterrestrial origin in the s and s. The most common form of microwave radiometer was introduced by Robert Dicke in in the Radiation Laboratory of Massachusetts Institute of Technology to better determine the temperature of the microwave background radiation. This first radiometer worked at a wavelength 1. Dicke also first discovered weak atmospheric absorption in the MW using three different radiometers at wavelengths of 1.
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A radio receiver used to measure the average power of the noise coming from a radio telescope in a well-defined frequency range is called a radiometer.
Noise voltage has zero mean and varies randomly on the very short time scales nanoseconds comparable with the inverse bandwidth of the radiometer. A square-law detector in the radiometer squares the input noise voltage to produce an output voltage proportional to the input noise power. Noise power is always greater than zero and usually steady when averaged over much longer times seconds to hours.
The ideal radiometer equation expresses this result in terms of the receiver bandwidth and the averaging time. Gain variations in practical radiometers, fluctuations in atmospheric emission, and confusion by unresolved radio sources may significantly degrade the actual sensitivity compared with the sensitivity predicted by the ideal radiometer equation.
Band-limited noise The voltage at the output of a radio telescope is the sum of noise voltages from many independent random contributions. The central limit theorem states that the amplitude distribution of such noise is nearly Gaussian. The sampling theorem Eq. This is what the band-limited noise output voltage of a radio telescope looks like.
It is convenient to describe noise power in units of temperature. The temperature equivalent to the total noise power from all sources referenced to the input of an ideal receiver connected to the output of a radio telescope is called the system noise temperature. Receiver noise is usually minimized by cooling the receiver to cryogenic temperatures. The simplest radiometer consists of four stages in series: 1 an ideal lossless bandpass filter that passes input noise only in the desired frequency range, 2 an ideal square-law detector whose output voltage is proportional to the square of its input voltage; that is, its output voltage is proportional to its input power, 3 a signal averager or integrator that smoothes out the rapidly fluctuating detector output, and 4 a voltmeter or other device to measure and record the smoothed voltage.
The simplest radiometer filters the broadband noise coming from the telescope, multiplies the filtered voltage by itself square-law detection , smoothes the detected voltage, and measures the smoothed voltage. The function of the detector is to convert the noise voltage, which has zero mean, to noise power, which is proportional to the square of voltage.
The filtered output is sent to a square-law detector, a device whose output voltage is proportional to the square of its input voltage, so the detector output voltage is proportional to its input power. It is always positive, so its mean DC component is positive and is proportional to the input power. The detector output also has frequency components near zero DC since the mean output voltage is greater than zero. The output voltage histogram of a square-law detector fed with Gaussian noise is peaked sharply near zero and has a long positive tail.
For a detailed derivation of the detector output distribution and its rms, click here. This integration might be done electronically by smoothing with an RC resistance plus capacitance filter or numerically by sampling and digitizing the detector output voltage and then computing its running mean. Integration greatly reduces the receiver output fluctuations. This important equation is called the ideal radiometer equation for a total-power receiver. This survey used total-power radiometers very similar to the radiometer described above, but with multistage RF amplifiers that simultaneously amplified and filtered the input signals.
By far the biggest time-dependent signal spanning a range of about 1 K is caused by ground radiation entering the prime-focus feed via leakage through the reflector mesh and spillover.
Fortunately, this unwanted ground signal varies smoothly with telescope elevation, so subtracting a short about 40 arcmin long running-median baseline takes out the spillover signal without removing compact radio sources.
Only now are the faint radio sources visible above the noise fluctuations. Data from all 14 receivers after subtraction of running-median baselines. Sources appear as spikes in both polarization channels R and L of one or two beams. Interference is usually visible in all 14 receivers simultaneously. In practice, systematic errors set a floor to the noise level that can be reached.
Receiver gain changes, erratic fluctuations in atmospheric emission, or "confusion" by the unresolved background of continuum radio sources usually limit the sensitivity of single-dish continuum observations. Fluctuating atmospheric emission Fluctuations in atmospheric emission also add to the noise in the output of a simple total-power receiver.
One way to minimize the effects of fluctuations in both receiver gain and atmospheric emission is to make a differential measurement by comparing signals from two adjacant feeds. The method of switching rapidly between beams or loads is called Dicke switching after Robert Dicke, its inventor. Block diagram of a beamswitching differential radiometer. The total-power receiver is switched between two feeds, one pointing at the source and one displaced by a few beamwidths to avoid the source but measure emission from nearly the same sample of atmosphere.
Fluctuations in atmospheric emission and in receiver gain are effectively suppressed for frequencies below the switching rate, which is typically in the range 10 to Hz. Nearly all discrete continuum sources are extragalactic and extremely distant, so they are distributed randomly and isotropically on the sky. The amplitude distribution of confusion is distinctly non-Gaussian, with a long positive-going tail.
The underlying gray-scale plot is a 1. Some of the faint sources seen by the foot telescope are "real" and some are blends of two or more fainter sources resolved by the VLA. Confusion by steady continuum sources has a much smaller effect on observations of spectral lines or rapidly varying sources such as pulsars. Superheterodyne Receivers Few actual radiometers are as simple as those described above. Block diagram of a simple superheterodyne receiver. Only the local oscillator is tuned to change the observing frequency range.
A radio receiver used to measure the average power of the noise coming from a radio telescope in a well-defined frequency range is called a radiometer. Noise voltage has zero mean and varies randomly on the very short time scales nanoseconds comparable with the inverse bandwidth of the radiometer. A square-law detector in the radiometer squares the input noise voltage to produce an output voltage proportional to the input noise power. Noise power is always greater than zero and usually steady when averaged over much longer times seconds to hours. The ideal radiometer equation expresses this result in terms of the receiver bandwidth and the averaging time. Gain variations in practical radiometers, fluctuations in atmospheric emission, and confusion by unresolved radio sources may significantly degrade the actual sensitivity compared with the sensitivity predicted by the ideal radiometer equation.