Backpack portable equipment was developed to measure vector baseline from approximately 1 km to 100 km in length with subcentimeter to few centimeter accuracy. The equipment design features as well as the instrumentation specifications are discussed. It is shown that the unit has the following advantages: it is simple in concept; it is reliable in unattended operation; and it is inexpensive (less than $15,000 per unit)

Connselman, C. C., III

Cox, D. B., Jr.

Greenspan, R. L.

Shapiro, I. I.

English

NASA Technical Reports Server

BACKPACK VLBI TERMINAL WITH SUBCENTIMETER CAPABILITY 
C. C. Counselman III and I. I. Shapiro 
Massachusetts Institute of Technology 
R. L. Greenspan and D. B. Cox, Jr. 
Charles Stark Draper Laboratory Inc. 
The measurement of short vector baselines with subcentimeter repeatability and accuracy using 
radio interferometric observations of quasars has already been demonstrated (reference 1). This 
paper describes our plans to achieve comparable performance using inexpensive, backpack portable 
equipment that processes less than one second of data per baseline redetermination. Figure 1 sum- 
marizes some of these objectives. 
Our approach exploits the full measurement accuracy inherent in the precise radio signals that will 
be broadcast by each satellite in the NAVSTAR Global Positioning System (GPS) (reference 2). 
Figure 2 illustrates the measurement concept. The user equipment at each end of the unknown 
baseline receives signals from the same set of four or more GPS satellites. The equipment consists 
of a simple antenna, a GPS receiver, a microprocessor unit and a recording unit (reference 3). 
“Real-time” baseline determination can be accomplished by linking the microprocessor units with a 
communication channel that can transmit at the rate of about 1 kilobit per baseline redetermination. 
Each GPS receiver measures its range to each satellite by means of the wide-band pseudo-noise 
P-Code modulation that is impressed on the GPS radio frequency carriers which are at 1.226 GHz 
and 1.57542 GHz (reference 4). These measurements will typically have a precision of about 0.5 
meters, which is approximately 2.5 wavelengths at a frequency of 1.57542 GHz. The primary use 
of these relatively coarse measurements in the proposed program is to assist in resolving ambiguities 
of the more precise carrier-phase measurements that will also be made (see below). 
0 To DEVELOP BACKPACK PORTABLE EQUIPMENT T&MEASURE VECTOR 
BASELINES FROM - 1 KN TO - 103 KM IN LENG’TH WITH SUBCENTIMETER 
TO FEW CENTIMETER ACCURACY. 
0 To DEVELOP EQUIPMENT THAT IS 
0 SIMPLE IN CONCEPT AND IMPLEMENTATION 
0 RELIABLE IN UNATTENDED OPERATION 
0 INEXPENSIVE - LESS THAN $15,000 PER UNIT 
Figure 1. Objectives. 
409 
https://ntrs.nasa.gov/search.jsp?R=19800020338 2020-03-21T17:48:12+00:00Z
410 RADIO INTERFEROMETRY 
-4 SATELLITE --- ----- 2 - - - -_ ,c-.-s----=- 4 SATELLITE 4 
-_ 
PROCESSOR 
l WIDEBAND OMNIDIRECTIONAL ANTENNA TO RECEIVE 1,6 GHz AND 1,2 GHz GPS TRANSMISSIONS 
l SIMULTANEOUS RECEPTION FROM ALL SATELLITES 
0 MIcRo~R~cE~~oR-CONTROLLED sys.TEf~ FOR DATA COLLECTION AND FORMATTING 
0 DATA STORAGE AM/OR LOW-VOLUME DATA LINK TO MICROPROCESSOR-BASED DATA ANALYZER 
Figure 2. GPS measurement concept. 
The same knowledge of the pseudo-noise modulation on the transmitted GPS signal that enables the 
receiver to make coarse range measurements also enables that modulation to be stripped from the re- 
ceived signal, leaving a sinusoidal component that is modulated only by a low speed (50 bit/second) 
biphase encoded data signal. This data modulated carrier is tracked in current GPS receivers by a 
“Costas” phase detector, which uses a phase doubling nonlinearity to remove the effect of the 
biphase modulation (reference 5). The output of the loop is an unmodulated tone. The phase 
tracking error will be approximately 8” in typical noise environments; this value corresponds to ap- 
proximately 4 mm at the GPS carrier frequencies (reference 6). The measurement is ambiguous by 
multiples of half the wavelength (approximately 10 cm) because the Costas loop output does not 
distinguish between input phases that differ by 180”. However, by using knowledge of the data 
signal format, one can regain the “lost” factor of two and obtain whole-wavelength (20 cm) spacing 
of the ambiguities. 
The carrier phases obtained from the signals from four or more satellites (see figure 3) at each end 
of a baseline yield an extremely precise, albeit ambiguous, determination of the vector baseline (ref- 
erence 3). One primary objective of the current plan is to assess the adequacy and flexibility of 
several alternatives for resolution of the ambiguities with simple equipment. Two leading approaches 
are: 
1. Collect data from satellite observations over several hours during which the satellite geom- 
etry changes significantly. If the number of potential ambiguities from observations of 
each satellite is small, then the set of ambiguity values, one value for each satellite, that 
corresponds to the weighted least squares fit to the data will usually be the correct one. 
WAVES OF THE FUTURE AND OTHER EMISSIONS 411 
a 0 
ANTENNA 1 ANTENNA 2 
3 d. 
OUTPUT OF CARRIER TRACKING LOOP OF 
RECEIVER 1 
OUTPUT OF CARRIER TRACKING LOOP OF ‘2 
RECEIVER 2 
$12 = ‘1 - 9 CAN BE MEASURED PRECISELY, ALBEIT AMBIGUOUSLY 
DIFFERENTIAL TIME DELAY = +&ARR’ER FREQUENCY 
TIME 
TIME 
Figure 3. GPS signal processing concept (“reconstructed carrier”). 
This technique is described in reference 1. We note that all data used to resolve ambigui- 
ties are usable also for accurate determination of the baseline vector, after the ambiguities 
are resolved. 
Measure the phase difference between the two GPS carriers received at each end of the 
baseline from each satellite to within a few degrees (note that both GPS carriers are syn- 
thesized coherently from the same oscillator). The corresponding phase-delay ambiguities 
will be light-time equivalents of integral multiples of the 0.43 m half-wavelength for the 
difference frequency (349 MHz). This approach is similar to the use of multiple tones in 
OMEGA to resolve “lane” ambiguities (reference 7). Used in conjunction with the coarse 
range measurement, the phase from both GPS carriers will allow a reduction in the initial 
ambiguities of the phase delay estimates. As the baseline length increases, the effectiveness 
of this technique will decrease because of ionospheric effects. 
As the first phase in the development of the intended system, we plan to conduct an experimental 
program using commercially supplied GPS receivers. One or more standard baselines defined by 
receiving antennas will be established and surveyed. Each leg will be less than 100 meters in length. 
GPS signals received by the antennas will be processed to make a series of baseline determinations 
under a wide range of environmental conditions. The objectives of this experiment are: 
1. to ascertain whether the accuracy of the baseline determination meets our goal (see figure 
4); 
2. to assess different techniques for ambiguity resolution; and 
3. to determine whether the present GPS signal structure will allow the development of inex- 
pensive and effective ground equipment. 
412 RADIO INTERFEROMETRY 
1) EXPLOITATION OF KNOWN “PSEUDORANDOM” SIGNAL STRUCTURE 
(RECONSTRUCTED CARRIER APPROACH) YIELDS MORE THAN 60 pB 
GAIN, 
2) SATELLITE TRANSMITTED POWER AND SIGNAL PROCESSING GAIN 
YIELD A MEASUREMENT SNR EXCEEDING 3s DB PER Hz, EVEN USING 
A MODEST RECEIVER, TN = 430°K (NOISE FIGURE = 4,O DB), 
3) GPS BROADCASTS ACCURATE EPHEMERIS DATA. 
4) GPS SUPPORTS CLOCK SYNCHRONIZATION TO 10's OF NS so THAT 
INHERENT EPHEMERIS ACCURACY CAN BE EASILY EXPLOITED, 
Figure4. Why size,weight and complexity objectives are feasible. 
REFERENCES 
Rogers, A. E. E. et al., Geodesy by Radio Interferometry: Determination of a 1.24 km Base Line 
Vector with -S-mm Repeatability, Journal of Geophysical Research, Vol. 83, No. 1, 10 
January 1978, pp. 325-334. 
Milliken, R. J., and C. J. Zollar, Principles of Operation of NAVSTAR and System Characteristics, 
Navigation, Vol. 25, No. 2, Summer 1978, pp. 95-106. 
Counselman, C. C. III, and I. I. Shapiro, Miniature Interferometer Terminals for Earth Surveying, 
Bulletin Gkodesique, Vol. 53, No. 2, August 1979, pp. 139-163. 
Borel, M. J. et al., Phase 1 GPS User Equipment, Navigation, Vol. 25, No. 2, Summer 1978, 
pp. 179-194. 
Viterbi, A. J., Principles of Coherent Communication, Section 10.4, pp. 286-293, McGraw-Hill, 
New York, 1966. 
Martin, E.. H., GPS User Equipment Error Models, Navigation, Vol. 25, No. 2, Summer 1978, 
pp. 201-216. 
Pierce, Omega, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-1, No. 3, 
December 1965, pp. 206-2 15. 
WAVES OF THE FUTURE ANJ3 OTHER EMISSIONS 
USE DETERMINISTIC PROPERTIES OF 
GPS SIGNALS 
HIGH MEASUREMENT SNR 
OMNIDIRECTIONAL ANTENNA 
SIMULTANEOUS OBSERVATION AND 
PARALLEL CHANNEL RECEIVER AND 
HIGH MEASUREMENT SNR 
SHORT MEASUREMENT INTERVALS 
DIRECT CARRIER PHASE MEASUREMENTS 
CoNsEaUENCE 
HIGH MEASUREMENT SNR 
CAN USE OMNIDIRECTIONAL ANTENNA; 
INEXPENSIVE RECEIVER 
No BEAM STEERING 
SIMULTANEOUS OBSERVATION OF ALL 
SATELLITES 
CARRIER PHASE MEASUREMENTS IN ABOUT 
1 SECOND AND BASELINE UPDATE EVERY 
SECOND IN STEADY STATE* 
QUARTZ CRYSTAL CLOCKS 
SIMPLE NUMERICAL PROCESSING IN STEADY 
STATE, BASED ON SOLUTION OF LINEAR 
EQUATIONS 
*STEADY STATE = AMBIGUITY REMOVED 
Figure 5. Properties of GPS-based system. 
0 AMBIGUITY RESOLUTION REQUIRED AT START AND OCCASIONALLY 
THEREAFTER: 
- 1 .E ,, WHENEVER THERE IS A “LONG” ENOUGH INTERVAL 
BETWEEN BASELINE MEASUREMENTS THAT THE BASELINE 
MAY CHANGE SIGNIFICANTLY 
l RESOLVE AMBIGUITIES BY APPLYING CONCEPTS TESTED IN 
HAYSTACK-WESTFORD EXPERIMENTS AND AUGMENTED BY USE 
OF TWO FREQUENCY BANDS FOR IONOSPHERE REDUCTION 
- USE CODE MODULATION (GROUP DELAY) FOR COARSE, 
BUT UNAMBIGUOUS BASELINE DETERMINATION 
TIME REQUIRED DEPENDS ON EXTENT OF 
GROUND CLUTTER (~~ULTIPATH) 
- USE CARRIER PHASES FOR REFINED BASELINE DETERMINATION 
413 
Figure 6. Ambiguity resolution. 
414 RADIO INTERFEROMETRY 
PBJFCTIVES 
G ASSESS AMBIGUITY RESOLUTION TECHNIQUES, 
G DEMONSTRATE ACCURACY OF BASELINE DETERMINATION, 
Figure 7. Experimental program. 
CABLE TO LABORATORY DATA, MEASUREMENTS 
FROM OTHER ,RECE IVERS 
4 4 
f I 
GPS RECEIVER COARSE DFI AY b 
MOD1 FIED FOR SIGNAI S FROM 
ACCESS TO CARRIER 
PROCESSING 
TRACKING LOOP 
CARRIER LOOP 
DATA, EPHEMERIS, 
BASELINE 
Figure 8. Experimental configuration. 
PF4TURES 
G VERY SHORT BASELINE - < 100 METERS 
(RESULTS CAN BE ACCURATELY CHECKED BY INDEPENDENT 
TECHNIQUES, 10t40sPHERlC REMOVAL NOT TESTED,) 
0 GPS ~-CHANNEL RECEIVERS 
l CROSSED DIPOLE ANTENNAS WITH GROUNDPLANE 
l PRECISE PHASE MEASUREMENTS ON RECONSTRUCTED CARRIER 
Q Low DATA VOLUME FOR EACH BASELINE REDETERMINATION 
(LESS THAN 1 K BIT) 
Figure 9. Experimental program. 


Backpack VLBI terminal with subscentimeter capability

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