56
4 Measuring Principles and Techniques …
Fig. 4.1 Scheme of pulse range measurement method
can be constructed on analog principle (continuous time countdown from a radiation moment) or on digital principle (discrete time countdown) using fast-response
electronic devices. Two lower positions in Fig. 4.1 illustrate both principles of time
countdown. Each of meters starts at the moment of sounding pulse transmitting and
performs time countdown at the moment of receiving of reflected signal.
In analog measuring instrument the reading represents a recording of instantaneous value of saw-tooth voltage at receiving moment of target pulse (U = k 1 t D ),
and in digital instrument—the reading of a number of uniformly coming pulses, accumulated by arrival moment of target pulse (n = k 2 t D ). In both cases, timely uniform
processes (saw-tooth voltage or pulses counting) are associated with uniform time
flow, as it takes place in any watches. Definitely, that dwell time meters can be directly
graduated in range digits, i.e., permits to read (pick up) a target distance D. Regular,
with T period, repetition of range measuring process of the same target (Fig. 4.1)
gives a more stable measuring result and reduces a measuring error.
It should be noted that the ground radar directly measures not a horizontal range
(distance) to a target D g , but slant distance D, since exactly slant distance serves as
a trace of radio waves passing to a target and back. These distances are connected
with a relation D g cosβ =
√
D 2 − H 2 , where H—target height; β—target elevation
angle.
Besides, for ground and sea radars, the range depends on antenna installation
height H A . This limits a detection range of a target at assigned altitude H t , which in
assumption of standard refraction is defined by the following formula:
D(km) ≈ 4.12(
H t (m) +
H A (m).
4 Measuring Principles and Techniques …
Fig. 4.1 Scheme of pulse range measurement method
can be constructed on analog principle (continuous time countdown from a radiation moment) or on digital principle (discrete time countdown) using fast-response
electronic devices. Two lower positions in Fig. 4.1 illustrate both principles of time
countdown. Each of meters starts at the moment of sounding pulse transmitting and
performs time countdown at the moment of receiving of reflected signal.
In analog measuring instrument the reading represents a recording of instantaneous value of saw-tooth voltage at receiving moment of target pulse (U = k 1 t D ),
and in digital instrument—the reading of a number of uniformly coming pulses, accumulated by arrival moment of target pulse (n = k 2 t D ). In both cases, timely uniform
processes (saw-tooth voltage or pulses counting) are associated with uniform time
flow, as it takes place in any watches. Definitely, that dwell time meters can be directly
graduated in range digits, i.e., permits to read (pick up) a target distance D. Regular,
with T period, repetition of range measuring process of the same target (Fig. 4.1)
gives a more stable measuring result and reduces a measuring error.
It should be noted that the ground radar directly measures not a horizontal range
(distance) to a target D g , but slant distance D, since exactly slant distance serves as
a trace of radio waves passing to a target and back. These distances are connected
with a relation D g cosβ =
√
D 2 − H 2 , where H—target height; β—target elevation
angle.
Besides, for ground and sea radars, the range depends on antenna installation
height H A . This limits a detection range of a target at assigned altitude H t , which in
assumption of standard refraction is defined by the following formula:
D(km) ≈ 4.12(
H t (m) +
H A (m).