function of the signal energy per bit for various degrees coding (Reference 7). of With the entry of the United States into space exploration, the need for a lightweight low-power telemetry system developed. Such a system (Reference 8), utilizing the noise-reducing properties of frequency modulation and the error-reducing properties of pulse-code modulation, was incorporated in the first Vanguard scientific space vehicle and ultimately achieved orbit in Vanguard I11*P.M. Rainey, U. S. patent 1,608,527, November 30, 1926, issued to Western Electric Co.,Inc.
2
This report is concerned with the heuristic development of the basic features of pulse-frequency modulation, an information encoding technique which has been used in a number of spacecraft. The primary advantages are its noise-immunity characteristics and
(1959~1). The telemetry encoder had a 48 channel capability, weighed 6 ounces unpotted, and required 12 milliwatts of power. Pulse-frequency modulation, as the system was called, was used again on the Ionosphere Direct Measurements Satellite, Explorer VI11 (1960 51). In April 1961 the space probe ExplorerX (1961~l), which measured the interplanetary magnetic field, was sent to an altitude of 240,000 km and again pulse-frequency modulation provided the encoding technique. Since that time, four more scientific satellites have been orbited with pulse-frequency-modulation telemeters. Explorer XII (1961 ul) provided continuous measurements of the energetic particles in the Van Allen radiation belts out to 80,000 hm; Ariel I (1962 01) was an ionospheric satellite (a joint effort between the United States and the United Kingdom); Explorer XIV (1962/3 yl) was a follow-on to Explorer XII; and Explorer XV (1962/3 X1) is providing a study of the artificial radiation belt. Two satellites to be launched in the near future, the Interplanetary Monitoring Probe and the successor to Ariel I, will use pulse-frequency modulation as the encoding technique.
General DescriptionConsider a time function f 1( t ) which is band-limited between zero and 1/2T0 cps. The function may be completely described by a series of impulses of separation T and area f l(nTo), with, n= - m, ..., -2, -1, 0, 1 2 - -,+ c A sequence of these time samples may be encoded for trans,, o . mission over a telemetry link in several ways. The method considered here is to encode the magnitude of the a r e a of each impulse as the frequency of a pulsed subcarrier, the duration of the pulse being some fraction of the sample time To.A s e t of k analog time functions f m( t ) may be multiplexed by sequentially sampling each function with spacings between samples of T,/k. In order that the spectrum may be utilized more efficiently, each pulse length should be equal to the sampling time, In practice the pulse length has been set at half this value to allow for response-time limitations in the crystal filters used in the detection process. Thus, a train of sequential pulses, the frequency of each being proportional to the amplitude of a sample, comprises the basic configuration for pulse-frequency modulation.
~,/k.
The signals f m( t ) may not necessarily be analog in nature. A sizable portion of the outputs of experiments aboard spa
cecraft occur as digital signals, and from a signal-to-noise-ratio viewpoint it is desirable to retain the digital character of the signal. The binary digits of the signal are combined and presented as a single digit of a higher order base. The present state of the art of PFM telemetry employs the encoding of three bits as one digit to the base eight; that is, three binary digits are encoded as one of eight frequencies. A special digital pulsed-subcarrier oscillator has been developed for this purpose; it is restricted to oscillation at only one of the eight possible frequencies, the frequency depending on the value of its three-bit input. In this manner the complexity of the switching circuitry in the encoder is materially reduced, since readout of the accumulators and s c a l e r s is accomplished three bits at a time instead of serially. Figure 1 illustrates the manner of commutating the digital oscillator to scan the stored data in the accumulator. Pulses from the experiment are counted and stored as a binary number in the accumulator. The digital oscillator is3
This report is concerned with the heuristic development of the basic features of pulse-frequency modulation, an information encoding technique which has been used in a number of spacecraft. The primary advantages are its noise-immunity characteristics and
EXPERIMENT INPUT
15
15 BITACCUMULATOR
100
-
1
DIGITAL:;::F F 1 OSCILLATOR
0100TIME (rnsec) Figure 1-Digital data readout system.
I
commutated three bits at a time in five steps through the stored fifteen bits in the accumulator. The output of the oscillator, for this case (octal number 47216), is represented, as in Figure 1, by five serial pulses of discrete frequencies. Since both analog and digital signals are encoded as frequencies of pulses, they may be intermixed in any order in multiplexing. Any one channel may be subcommutated to extend the number of signals f m( t ) which may be encoded, with a corresponding reduction in the signal bandwidth. The present format for PFM specifies that sixteen sequential channels comprise a telemetry frame. The first channel is devoted to synchronization, and the remaining fifteen are distributed between the analog and digital data to be telemetered. A group of sixteen sequential frames formsa telemetry sequence of 256 pulses (sixteen of these are devoted to synchronization). Figure 2 shows the frame and sequence formats. Synchronization is assured in two ways. First, the energy in the sync pulse is increased by increasing its length 50 percent. Second, a unique frequency is utilized outside the data band for the sync pulse. To provide a means for identifying the subcommutated data, the frequency of the sync pulse in every other telemetry frame is stepped in sequence through eight frequencies. These frequencies lie in the data band and are the same as the eight frequencies of the digital oscillator. When there is a poor signal-to-noise ratio, the energy at the sync frequency may be stored over a number of frames, and thus the sync signal-to-noise ratio is improved. This, of course, increases the acquisition time but it is a necessary compromise in regions of poor signal-to-noise ratios.