Future long-range manned NASA missions will require very large reserves of consumables, such as food, and fuel. The cost of launching these consumables into orbit using conventional rocket technology is prohibitive and so alternative methods must be found. A proposed solution to the problem is to launch consumables into space with non-conventional means, such as a rail gun. Once the problem of getting these packages to orbit has been solved a new problem of locating and identifying them arises. These consumables will initially be located in a variety of orbits and it would be necessary to locate them and then aggregate them into a "warehouse in the sky". This task would involve finding the consumables, identifying them, and then guiding them to a central warehouse facility. MQW shutters are particularly suited to these applications because the technology enables fast data rates, requires very low drive powers, is lightweight, robust, and is not polarization-sensitive.
We report the results of a recent demonstration in which a MQW retromodulator array was used as a low power, lightweight means to provide optical tagging of a remotely located object consistent with these challenges. A laser diode integrated on a tracker/pointing system scanned without cueing for a modulated retro-reflected beam. The retro-reflected energy was received and the embedded code demodulated for tagging identification. Ranges were on the order of 40 meters using an array of 1/2 cm MQW devices. Data were transferred at a rate of one mega chip per second over the link. Device power requirements were on the order of several milliWatts. The entire transmit/receive assembly is shown in Figure 1.
At the heart of the feasibility test were the control algorithms used to identify the signals, correlate gain, control the tracker, and decode the received signals into information. A description of the process has two parts: (1) The tracking algorithm; and (2) the signal received. The acquisition software initiates a series of ever decreasing rectangular searches based on pre-set threshold levels, tn. A given swath is painted by the 3 mRad beam and stepped through a given pattern. The gimbal controller receives a relative signal level from the receiver controller and compares the received signal level to the thresholds to command the gimbal through four modes of target acquisition and signal optimization.
As the beam begins to paint the modulator array, the return signal level increases and the next threshold level is reached, initiating the smaller pattern. This continues until a maximum signal level is obtained and the interrogator locks onto the maximum signal. If the signal is lost for some reason, or if the dwell time is too short, after a period, the search pattern "restarts" from the beginning. A hypothetical search pattern is shown in Figure 2.
When a signal is received by the photodetector, the receive algorithm processes the bit stream in accordance with the following relationships for the matched filter and envelope detector respectively: where: s(filter) is the filtered signal value, c(seq) is the chip sequence which defines a symbol, and s(rec) is the signal received from the APD. N is the product of the number of samples per chip and the number of chips per symbol. A signal level, S(out), is sent to the tracking algorithm when the correlation gain between the received sequence and the matched sequence in the receiver software is maximized for a given tracking position.
When two sequences were alternatively sent and demodulated, the process demonstrated that a tag could be identified out of a crowded environment. The method also demonstrated that additional information, such as geolocation, photonics, or container content, could be sent with alternating data streams without affecting acquisition efficacy. More information is available in reference 2 on the publications page.