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5.2 Meter Radio Astronomy Project for 1420 MHz

 

ELEVATION POSITIONING SYSTEM

The elevation of the antenna is measured using a digital inclinometer. A digital inclinometer is a small device which contains a sensor and additional miniture electronics to convert its mechanical position into digital data which is sent to a serial port of the antenna tracking computer.

 

DIGITAL INCLINOMETER: The inclinometer I used is available from SmartTool Technologies of 1717 Grant St., Santa Clara, CA 95050 (408-653-1680) as their "ISU Circuit Board - Part No. 90104001". Its intended use is as a component of a digital carpenter's level, and the cost is about $100. It is built on a small circuit board measuring 1.3 x 2.6 inches and requires 5 volts at about 2 milliamps for power. In addition to the power lead, there are connections for receive data (RX), transmit data (TX), and a common ground. When the inclinometer receives the proper 5-byte digital command it responds with a 6-byte message containing the elevation information accurate to about 0.1 to 0.2 degrees.

 

SPECIFICATIONS: The following links will provide copies of the SmartTool inclinometer spec sheet:

 

MECHANICAL INSTALLATION:
The inclinometer needs to be mounted securely to the antenna in a weatherproof container large enough for the inclinometer as well as a TTL to RS-232 interface (see below). The actual sensor element is a disk about 1.25 inches in diameter and 0.25 inches thick. The plane of the sensor must be mounted perpendicular to the elevation axis of the antenna.

 

ELECTRICAL INTERFACE:
The digital inclinometer operates at 9600 baud (no parity, 8 data bits, 1 stop bit), TTL logic levels of 0 and +5 volts. A small additional circuit is required to convert the TTL logic levels to the RS-232 levels required by the computer serial port. You can build your own (a circuit diagram is included in the application notes provided with the inclinometer, see below) or you can use a commercially available equivalent as I did. The interface circuits typically require 9 to 12 volts D.C. I supplied 12 volts to the interface circuit and obtained the 5 volts required by the inclinometer with a 5-volt regulator fed from the same source. There are four wires required to operate the inclinometer/level converter: +12 volts, ground, RS-232 transmit data, and RS-232 receive data. The two RS-232 lines and ground connect to the serial port of the computer (COM1, COM2, etc.) and regulated 12 volts D.C. is supplied to the 12-volt line (and ground) from a separate source.

 

APPLICATION NOTES: The following links will provide copies of the SmartTool application notes (Note latest address & phone no. above):

 

DATA REQUIREMENTS:
The elevation data is requested from the inclinometer by sending the following five hexadecimal bytes to the inclinometer:
03 04 58 02 5E
 
The inclinometer responds with the following six hex bytes:
04 05 58 HH LL CC, where HH is the angle high-order byte, LL is the angle low-order byte, and CC is a checksum byte.
 
The actual angle is calculated as follows:
ANGLE = [(256 x HH) + LL] x 360/65536

 

SAMPLE PROGRAM: The following simple BASIC program will request data from the inclinometer, receive and convert it to an angle, and display it continuously:
REM Requests and receives data from SmartLevel modules.
REM Outputs 0 to 360 degrees.
CLS
OPEN "COM1:9600,N,8,1,ASC,RS,RB4096" FOR RANDOM AS #1
start:
PRINT #1, CHR$(3) + CHR$(4) + CHR$(88) + CHR$(2) + CHR$(94)
a$ = INPUT$(6, #1)

FOR n = 1 TO 6
b(n) = ASC(MID$(a$, n, 1))
NEXT n
angle = ((256 * b(4)) + b(5)) * 360 / 65536
el = 90 - angle
corr = 3
el = el + corr
LOCATE 1, 1: PRINT USING "####.##"; el
GOTO start
END

 

FACTORY SUPPLIED SOFTWARE:
Included with the inclinometer is a computer disk containing a utility program "ST.EXE." This program can be used to test the inclinometer and has a number of other features, including the ability to read and write to the inclinometer's read-only memory (ROM). During manufacture, the inclinometers are factory-calibrated and linearized. The calibration data is stored in an on-board ROM. It is possible to corrupt the contents of the ROM when experimenting with hardware and software for the device, thereby rendering it unusable. Fortunately, the program ST.EXE allows the user to save the initial contents of the ROM to disk and restore them if corruption should occur. The user must be sure to save the contents of the ROM immediately upon establishing serial communication with the device! The following links provide additional information about the supplied software:

 

COARSE CALIBRATION:
Calibration of the inclinometer is done by simply adding or subtracting a fixed amount from the reading provided by the above program. In the program above, a 90 degree adjustment is first made to the angle which compensates for the fact that my inclinometer happened to be mounted with its reference direction at roughly a right angle to the horizon. In fact, the mounting position is arbitrary as any position can be accomodated by adding or subtracting the correct amount. Additionally, a 3-degree rough correction factor was needed for my installation. This is equivalent to mechanically rotating the inclinometer with respect to the antenna.

 

FINE CALIBRATION:
More precise calibration can be obtained by using a signal source from the sky, such as sun noise or a signal from a satellite. For example, software can calculate the elevation of the sun at a given time. By peaking the noise in the receiver at that time and comparing the calculated elevation with the inclinometer readout the amount of error can be determined. A small addition or subtraction to the readout can then be made. This method has the additional value of not only compensating for the physical position of the antenna but also for any errors caused if the antenna pattern itself is skewed off center. After all, it is the center of the antenna pattern we need to position accurately. Sun noise is useful because it is broadband and is available at the frequency we are interested in. The use of a satellite signal will often require shifting to a different frequency for the measurement. If the frequency change is too great the pattern of the antenna may be different than at the operating frequency, introducing a position error.

 

ACCURACY:
The necessary position accuracy required depends on the beamwidth of the antenna. It is hardly necessary to point an antenna with 0.1-degree accuracy if the antenna pattern is 30 degrees wide. However, a 10-foot parabolic antenna operating at 4 GHz, for example, has a beamwidth of only about 1.8 degrees and it would be desirable to be able to position it to within 0.2-degrees or so.

 


 

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