as shown in the drawing for purposes of illustration , the present invention is concerned with laser communication systems , particularly laser communication systems used in air - to - air or air - to - ground applications . atmospheric turbulence causes scintillations in received beam power , which may fall below a threshold level , below which error - free communication is impossible . in accordance with the present invention , each transceiver terminal in a laser communication system detects and monitors the received beam power and adjusts the transmission rate to avoid loss of data . the received beam power fairly accurately reflects the turbulent state of the transmission path that is encountered by a transmit beam sent from the terminal that is monitoring the received beam power . therefore , monitoring the received beam power allows each terminal to adapt its transmission rate or , as further discussed below , its mode of transmission , to minimize transmission errors that might be caused by atmospheric effects . adapting the transmission rate includes ceasing transmission temporarily when the received beam power is very low ( below a selected threshold ), and increasing the data transmission rate above the average rate when the received beam power is relatively high ( above another selected threshold ). the data transmission rate may also be changed more often , either using other selected thresholds or on a continuously variable basis . fig1 shows two identical transceiver terminals , designated terminal a and terminal b . each terminal includes a transmit laser 10 , a data modulator 12 , a high power amplifier 14 and a telescope and optics 16 . data for transmission are first input , in digital form and at a uniform data rate , over line 18 to a data buffer 20 . the buffer 20 is a large first - in - first - out buffer of sufficient capacity to avoid data loss when the transmit function is slowed down or suspended for short periods . data bits are output from the data buffer 20 over line 22 to the data modulator 12 , under control of an adaptive data rate controller 24 , which controls both the rate of data output from the data buffer 20 and the speed of operation of the data modulator 12 , as indicated by lines 26 and 28 . in normal operation with no atmospheric turbulence , the data buffer 20 delays the input data but does not function to change the data transmission rate , which proceeds at the input data rate . the telescope and optics 16 include an optical component providing a separate receive signal path , such as a dichroic mirror 30 . a convenient way to recover the received signal is to employ different wavelengths for the transmit lasers 10 . then the dichroic mirror 30 can be transparent for the transmit signal 32 , but functions as a mirror to the receive signal 34 . the receive signal 34 is reflected onto a detector receiver 36 , which is an opto - electronic device that produces an electrical signal on line 38 proportional to the receive beam power . the detector / receiver / demodulator 36 also recovers a stream of data bits from the receive beam , as indicated by line 40 . this data stream may , of course , have a varying data rate as a result of adaptive control at the terminal that transmitted the data . the potentially varying data rate data stream is input to another data buffer 42 , having a similar capacity to that of data buffer 30 . an output data stream is withdrawn from the buffer 42 at a uniform data rate , as indicated by line 44 . the receive beam power provided by the detector / receiver / demodulator 36 on line 38 is monitored in a receive beam power monitor 46 , which generates appropriate control signals to the adaptive rate controller 24 , over line 48 . the receive signal beam power output on line 38 varies in accordance with the atmospheric conditions . when there is little or no turbulence , a graph depicting this power variation will be a relatively flat and unvarying curve . during turbulent conditions , the graph resembles the one shown by way of example at 50 in the drawing . the graph is characterized by sharp troughs with durations of 1 - 10 milliseconds . the troughs are separated by power peaks that look less abrupt relative to the troughs when the power level is plotted in decibels ( db ). it has been observed that the average received power in turbulent conditions is approximately the same as the received power in the absence of turbulence . the principle of operation of the invention relies on the assumption that the transmit signal 32 and the receive signal 34 will be subject to approximately the same turbulence conditions for a time interval long enough to allow transmission data rate changes to track the earlier detected changes in the receive beam power . since the turbulence conditions affect the beam power at a relatively slow rate , usually measured in milliseconds for a swing from maximum to minimum power , the assumption is valid as a practical matter . the assumption also seems to hold to a good approximation even when considering simultaneous motion of the sending and receiving terminals , through the atmosphere and possibly relative to each other . the received power monitor 46 and the adaptive data rate controller 24 can control the transmit data rate in accordance with one of several alternative techniques , examples of which are described below : the received power monitor 46 senses an excursion of receive beam power below a preselected threshold that represents the lowest power at which data can be transmitted at a nominal data rate , which may be the same as the input data rate on line 18 , without uncorrectable data errors . as is well known , some communication channel errors can be corrected by adding redundancy to the data stream and employing digital error correction techniques . even if such techniques are used , there will still be a power threshold below which uncorrectable errors will occur . in this example , when the received power crosses below this threshold , the adaptive rate controller 24 sends a signal over lines 26 and 28 to stop retrieving data from the data buffer 20 and to suspend operation of the data modulator 12 . when the receive beam power crosses above the threshold again , transmission is restarted . if the data buffer 20 is not to “ fall behind ” by overflowing , the data transmission rate on line 22 , when activated , will have to be greater than nominal or input data rate on line 18 . in addition to stopping and starting transmission based on a low threshold , the received power monitor 46 also detects when the receive power crosses above a higher selected threshold . above this higher threshold , it is possible to transmit data at a rate higher than the nominal or input rate . therefore , when this higher threshold is exceeded , the data buffer 20 and the data modulator 12 are controlled to transmit data at a higher rate , selected to compensate for the data delayed when transmission was suspended . instead of the higher threshold of example 2 , a sequence of thresholds is established . crossing above any of the thresholds results in an increased data rate . crossing below any of the thresholds results in a reduced data rate . selection of the thresholds and associated data rates is made with a view to maximizing the overall transmission rate that is achievable . a low threshold , below which data transmission is suspended , may not be necessary so long as some low - level data transmission rate may be maintained all the way down to the troughs of the power variation curve . as the selected thresholds are moved closer together , adaptation of the data rate becomes almost continuously variable , subject only to the resolution of power detection in the detector / receiver 36 , and to the fineness of rate control provided by the data buffer 20 and the data modulator 12 . in an important alternative implementation of the invention , the data modulator 12 does not vary the actual data transmission rate of the transmit signal 32 , but adaptively varies the mode of data modulation to change the user &# 39 ; s effective data rate . for example , the actual transmit data rate on line 32 can be maintained at a constant high data rate or “ chip rate .” a coding scheme is then used in the data modulator 12 to vary the rate at which data is retrieved from the data buffer 20 , without affecting the chip rate on the transmit line 32 . one possible data coding scheme uses a simple repeat algorithm wherein each data bit is transmitted only once when the received power is very high , but is transmitted multiple times when the received power is lower . other , more complex data coding schemes may be used to vary the effective data rate without changing the actual data rate on the transmit line 32 . obviously , every change in the effective data rate must be transmitted in some manner , such as in a message header , to the receiving terminal . this alternative form of the invention has the advantage that much of the transmit and receive hardware can be designed to operate at the single high speed chip rate . it will be apparent from the foregoing description that the invention provides a solution to what has been seen as a significant drawback to laser communication through the atmosphere . specifically , the invention provides for transmission data rate control to compensate for the effects of atmospheric turbulence , as measured by received beam power in a duplex communication system . adaptation of the transmit data rate may be based on sensing the power level in relation to one or more preselected thresholds , or may employ stepped rate control that may be made almost continuous within the resolution limits of the power detection and rate control components . it will be appreciated that , although a specific embodiment of the invention has been illustrated and described in detail , various modifications may be made without departing from the spirit and scope of the invention . accordingly , the invention should not be limited except as by the appended claims .