Electronic Distance Measurement - EDM

The velocity of light – History Review

• The accuracy of EDM instrument depends ultimately on the accuracy of the estimated velocity of light (velocity of the electromagnetic wave) through atmosphere.
  
• The first estimates of the velocity of electromagnetic wave were derived from astronomical observation – very large distances were necessary to produce effects measurable.
  
• In 1676 - By studying the time of the eclipses of the satellites of Jupiter, Romer first calculated the value to be about 187 000 miles per second.
  
• In 1727 – Bradley improved – by using observations made on the aberration of the light received from star – he obtained of 308 000km/s.    
  
  • Bradley method : because the motion of the earth round the sun, star in a direction at right angles to the motion will appear to have a small circular orbit and by measuring the diameter of     this circle, Bradley was able to calculate a value for the velocity light.    
    
• In more recent time – this Bradley method has been used to obtain a value of 299 714 – close agreement with values derived from other method.
  
• In 1849 – Fizeau making first direct terrestrial measurement. Fizeau method is in essence the one still employed in the modern EDM instrument – WHELL MODULATION.
  
• Fizeau estimated the velocity to be 313 000km/s
  
• Fizeau answer on WHEEL MODULATION was of no great accuracy – in experiment the timing of the eclipses was estimated by eye and any error in the light path length would contribute     directly to the error.
  
• With modern terminology – using visible light as a carrier wave, modulated to give a approximately square wave by a mechanical method – the phase of returning signal was estimated by     eye using a null-point method and a variable modulation frequency.
  
• In 1906 – Rusa and Dorsey – using ration of electromagnetic to electrostatic units – 299 784km/s ±10.
  
• In 1929 – Michelson, Pearson and Pease – using rotation mirror device– obtained 299 796km/s ±15
  
• In 1933 - Michelson, Pearson and Pease – mile long tube reduced to . of low vacuum in order to minimise errors due to atmospheric effect – 299 774km/s ±4
  
• During the 1939 – 1945 war, radar devices using microwaves were introduced into rangefinding techniques.
  
• In 1940 - Anderson – using Kerr Cells device – 299 776km/s ±6.
  
• In 1941 – Birge – using weighted mean from past result and obtained 299 776km/s ±4.
  
• In 1947 – Essen and Gordon-Smith – using cavity resonator – 299 792km/s ±4.
  
• In 1949 – 1951     i) Aslakson – radar measurement – 299 792km/ ±1.4 (1949) and 299 794.2km/s ±1.9 (1951).
  

          ii) Bergstand – Kerr Cell – Geodimeter device – 299 793km/s ±2 (1949) and 299 793.1km/s ±0.2 (1951).
  

• In 1950 – Essen – Cavity resonator – 299 792.5km/s ±1.
  
• In 1954 – Froome – microwave interferences – 299 792.7km/s ±0.3.
  
• In 1955 – Scholdstrom – Kerr Cell device - 299 792.4km/s ± 0.4.
  
• In 1956 – Ordnance Survey – Geodimeter 4 - 299 792.4km/s ±0.5.
  
• In 1957 – Froome – Microwave interferometer – 299 792.5km/s ±0.10.
  
• In 1957, the XIIth General Assembly of the International Scientific Radio Union recommended that the best available value for the velocity of electromagnetic waves in vacuum was : 299 792.5km/s ±0.4km/s.
  
• This value (299 792.5±0.4km/s) was also accepted by the International Union for Geodesy and Geophysics.
  
• Standard of velocity is itself dependent on two others
  

1. Standard of length and that of time – is based upon an atomic standard – the wavelength of the orange line of the krypton atom, known to accuracy of about on part in one hundred million (1 x 10^-8).
   
2. Atomic clock provide a standard of time accurate to about one part in a hundred thousand million (1 x 10^-11).