A multiwavelength distance-measuring (MWDM) instrument that utilizes the dispersive nature of the atmosphere has been developed and tested. With this instrument there is no need for the usual meteorological observations and corrections. This MWDM instrument simultaneously measures the optical path length at three wavek/ngths, two in the optical region of the spectrum and one in the microwave region. The instrument calculates correction terms from the optical path length differences and then computes the corrected base line distance, the first-order effects of temperature, pressure, and water vapor fluctuations along the line thus being eliminated. Field testing demonstrated that the instrument was capable of making consecutive distance determinations with a standard deviation from the mean as small as 4 parts in l0 s. The long-term stability of the instrument was investigated from June 1974 to November 1974. The results showed a standard deviation of 1.3 parts in 107 when the data were fit to a sine wave of annual periodicity. The amplitude of this sine wave was 3 parts in 107, which was in good agreement with the predicted results of a two-dimensional t, hermoelastic model. o d c• i I ß ß ß ß ß ß ß 'l ß ß ß ß ß l' ß ß I. ß ß ß e e ß ß ß '.l ß ß ß ß ß .I t (•u •u) 30NV.L$10 SLATER AND HUGGETT: A MWDM INSTRUMENT

Dynamic mode locking of a He–Ne laser was accomplished by detecting the rf beats between axial modes, and by using this signal as external regenerative feedback to the laser, making the system a self-oscillator at the axial mode difference frequency of 75 MHz. The feedback signal was applied to the laser by means of a KD*P longitudinal modulator placed in the laser cavity to provide phase modulation. The average output power of the laser was 50 mW; the peak pulse power was about 1.25 W.

The most precise measurements of strain on long base lines (1–11 km) are currently being made in central California near Hollister. Base lines radiating from Hollister have been measured daily, except when measurements are limited by poor visibility, since September 1975. Strain episodes have been observed that are attributed to slip at depth on the major faults in the area. There is a suggestion that this slip begins several kilometers down and propagates upward to the surface. Many of these strain episodes have coincided with magnitude 2.5–3.5 earthquakes in the region. Another strain episode indicates that the eastern end of the Sargent Fault may continue past the Calaveras Fault for approximately 2 km.

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Thermally induced errors, dominant in a water‐tube tiltmeter, are eliminated by using two adjacent tubes containing fluids with densities having different temperature coefficients. Simultaneous measurements of the apparent difference in height between two stations indicated by the two fluids are used to provide the required correction for the density changes. The application of this method could allow precise measurements over long baselines (a resolution of about 10−8 to 10−9 radians in a 1‐kilometer instrument) without a level interconnecting tube or deep burial.