GPS and telemetry antenna for use on projectiles

[0002] 1. Field of the Invention

[0003] The present invention relates generally to an antenna for use on a missile or the like. More specifically, the present invention relates to a microstrip antenna which includes a GPS antenna for receiving GPS data and a telemetry antenna for transmitting telemetry data and which is adapted for use on small diameter device such as a missile.

[0004] 2. Description of the Prior Art

[0005] In the past military aircraft and weapons systems such as airplanes, target drones, pods and missiles have included flight termination and beacon tracking antenna to monitor performance during test flights. For example, a missile under test will always have an antenna which is generally surface mounted to transmit telemetry data to a ground station. The ground station then performs an analysis of the telemetry data from the missile to determine its performance during flight while tracking a target.

[0006] U.S. Pat. No. 4,356,492 is an example of a prior art microstrip antenna which is adapted for use on a missile as a wrap around band to a missile body without interfering with the aerodynamic design of the missile. U.S. Pat. No. 4,356,492 teaches a plurality of separate radiating elements which operate at widely separated frequencies from a single common input point. The common input point is fed at all the desired frequencies from a single transmission feed line.

[0007] With the emerging use of the Global Positioning System (GPS) for tracking purposes, there is a need to include GPS within the instrumentation package for a missile and target drone to accurately measure flight performance. GPS data is extremely accurate and thus allows for a thorough analysis of the missile's performance as well as the target drone's performance in flight while the missile tracks the target drone on a course to intercept the target drone.

[0008] The use of satellite provided GPS data to monitor the position of a missile and a drone target in flight will require that an antenna for receiving the GPS data be included in the instrumentation package. The receiving antenna should preferably be mounted on the same dielectric substrate as the transmitting antenna so that the antenna assembly can be applied readily as a wrap around band to the missile body without interfering with the aerodynamic design of the missile. Similarly, the antenna assembly which would include a GPS data receiving antenna and telemetry data transmitting antenna a wrap around band to the target drone's body without interfering with the aerodynamic design of the target drone.

[0009] The present invention overcomes some of the disadvantages of the past including those mentioned above in that it comprises a relatively simple in design yet highly effective and efficient microstrip antenna assembly which can receive satellite provided GPS position and also transmit telemetry data.

[0010] The antenna assembly of the present invention includes a first microstrip antenna which is a telemetry antenna is mounted on a dielectric substrate. The telemetry antenna transmits telemetry data to ground station or other receiving station. There is also a second microstrip antenna mounted on the dielectric substrate which is physically separated from the first microstrip antenna on the dielectric substrate. The second microstrip antenna is a GPS antenna adapted to receive satellite provided GPS position data. The antenna assembly is a wrap around antenna assembly which fits on the outer surface of a missile, target drone or any other small diameter projectile.

[0011] The telemetry antenna includes a pair of radiating elements with one radiating element being positioned on one side of the projectile and the other element being positioned on the opposite side of the projectile. One of the two radiating elements of the telemetry antenna has a feed line which provides for a 180 degree phase shift of the transmitted RF signal relative to the feed line for the other radiating element. This phase shift insures that the electric field for the transmitted RF signal is continuous around the circumference of the projectile.

[0012] The GPS antenna also has a pair of microstrip receiving antenna elements which are circularly polarized. Due to the close proximity of the telemetry and GPS antennas a band stop filter is integrated into the GPS antenna. The band stop filter has a minimum stop-band rejection of 40 decibels to prevent the telemetry data signal from saturating the GPS antenna.

[0013] FIG. 1 is a plan view of a preferred embodiment of the invention showing a GPS and telemetry antenna mounted on a dielectric substrate;

[0014] FIG. 2 is a sectional view of the antenna of FIG. 1 taken along plane A-B;

[0015] FIG. 3 is a schematic diagram illustrating the telemetry antenna of FIG. 1;

[0016] FIG. 4 is a schematic diagram illustrating the GPS antenna of FIG. 1;

[0017] FIG. 5 illustrates the return loss and isolation between the telemetry antenna and the GPS antenna of FIG. 1;

[0018] FIGS. 6aand 6billustrate the antenna radiation Pitch patterns for the telemetry antenna and the GPS antenna of FIG. 1;

[0019] FIGS. 7aand 7billustrate the antenna radiation Roll patterns for the telemetry antenna and the GPS antenna of FIG. 1; and

[0020] FIGS. 8aand 8billustrate the antenna radiation Yaw patterns for the telemetry antenna and the GPS antenna of FIG. 2.

[0021] Referring first to FIGS. 1, 2, 3 and 4, there is shown an antenna assembly 20 comprising a telemetry antenna 22 and a GPS (Global Positioning System) antenna 24 for use on small diameter projectiles such as missiles and target drones. The diameter of the projectile 26 for which antenna assembly 20 is designed is approximately 2.75 inches.

[0022] The telemetry antenna 22 and GPS antenna 24 are separated physically and are mounted on a dielectric substrate 28. Positioned below dielectric substrate 28 is a ground plane 30. Dielectric substrate 28 is fabricated from a laminate material RT/Duroid 6002 commercially available from Rogers Corporation of Rogers Conn. This material allows sufficient strength and physical and electrical stability to satisfy environmental requirements and is also easily mounted on the surface of a missile or a target drone. The dielectric substrate 28 is fabricated from two layers of 0.031 inch thick material, and a 0.010 inch thick antenna protective cover board. The use of the multi-layer fabrication to fabricate the substrate is to prevent wrinkling and cracking of the substrate.

[0023] The telemetry antenna 22 comprises two separate microstrip radiating elements/antenna transmitting elements 32 and 34 respectively fed by microstrip feed lines 36 and 38 from a single feed input point 40 as shown in FIG. 3. The radiating elements 32 and 34 each have a shape which is rectangular and are notch fed. The element feed point 42 for radiating element 32 comprises a 100 ohm input and the element feed point 44 for radiating element 34 also comprises a 100 ohm input. The single feed input point 40 for both radiating elements 32 and 34 comprises a 50 ohm feed input. Paralleling the feed lines 36 and 38 which are 100 ohm transmission lines produces the input impedance of 50 ohms.

[0024] Telemetry antenna 22 has the following electrical characteristics: (1) a center frequency of 2250 MHz which is an S-Band Radio Frequency; (2) a bandwidth of ±10 MHz (3) a linear polarization; and (4) a roll coverage of −3 db +/−5 db.

[0025] The electric field generated by the RF signal transmitted by radiating elements 32 and 34 of telemetry antenna 22 needs to be continuous around the circumference of projectile 26. This, in turn, necessitates that one of the microstrip feed lines 36 or 38 provide for a 180 degree phase shift relative to the other feed line over the 2.24 to 2.26 operating frequency range for telemetry antenna 22.

[0026] The 180 degree phase shift is provided by microstrip feed line 38 which includes ninety degree angular bends/right angle bends 50, 52, 54, 56 and 58 to extend the length of feed line 38 which allows for the 180 degree phase shift.

[0027] The GPS receiving antenna 24 is also mounted on the dielectric substrate 28 in proximity to the telemetry antenna 22. The GPS receiving antenna 24 comprises two separate microstrip antenna receiving elements 60 and 62 which respectively have corner. feed points 64 and 66 a shown in FIG. 4. Since antenna receiving elements 60 and 62 are required to be circularly polarized, one side 68 of each element 60 and 62 is slightly longer than the other side 70 of each element 60 and 62 and the feed points 64 and 66 are positioned in the corner of the microstrip antenna receiving elements.

[0028] Receiving elements 60 and 62 are rectangular in shape and approximate a square. The difference in length between sides 68 and 70 of receiving elements 60 and 62 is in the order of one twenty thousandth of an inch.

[0029] GPS antenna 24 has the following electrical characteristics: (1) a center frequency of 1572.5 MHz which is an L-Band Radio Frequency (GPS Band L1); (2) a bandwidth of ±10 MHz (3) a circular polarization; and (4) a roll coverage of −3 db +/−5 db.

[0030] The input impedance at the feed input 64 for antenna element 60 and the feed input 66 for antenna element 62 is approximately 250 ohms and is matched to the 50 ohm common feed point 72 through approximately quarter wavelength impedance transformers 74 and 76. Transformer 76 includes an indented portion 77 which insures that its length is equal to the length of transformer 74.

[0031] GdPS antenna also has a T shaped microstrip transmission line 79 which connects the quarter wavelength impedance transformers 74 and 76 to a band stop filter 78. T shaped microstrip transmission line 79 has a pair of 70 ohm arms and a 35 ohm trunk line with two right angle bends connected to filter 78.

[0032] The GPS antenna 24 includes band stop filter 78 which has a minimum stop band rejection of 40 decibels. Band stop filter 78, which is integrated into GPS antenna 24, isolates the transmitted telemetry signal from the received GPS signal. There is a need for band stop filter 78 because of the close proximity of antenna 22 to antenna 24, i.e. the antenna elements 32 and 34 of antenna 22 are separated from the antenna elements 60 and 62 of antenna 24 by approximately {fraction (9/16)} of an inch. Without filter 78 coupling between antennas 22 and 24 would occur and the high power telemetry transmitting signal would interfere with receiving the low power GPS signal.

[0033] Filter 78 has two open circuit lines 80 and 82 and an interconnecting line 84 to form a three section band stop filter which impedes the telemetry transmitted signal from being received by the GPS receiving antenna 24. The band stop filter 78 parameters are approximately configured as two quarter-wavelength open-circuit lines separated by a quarter-wavelength at 2.25 GHz. The open circuit line 82 at feed point 72 consist of two parallel lines/stubs 82A and 82B.

[0034] Locating the two lines 82A.and 82B as shown in FIG. 4, reduces surface wave coupling by canceling the signal picked by one line 82A or 82B with the signal picked by the other line 82A or 82B. The lines 82A and 82B are physically located on dielectric substrate 24 such that the lines 82A and 82B are approximately 180 degrees out of phase with a surface wave that is transmitted by the telemetry antenna 22. For the frequency range of 1.565 to 1.585 GHz, GPS antenna 24 provides for a maximum 2:1 Voltage Standing Wave Ratio (9.5 DB return loss) input, equal magnitude to each element, and 0 degree phase difference. For the frequency range of 2.230 to 2.270 GHz, the insertion loss is greater.than 50 dB.

[0035] Referring to FIG. 5, the return loss 91 of the GPS antenna and the return loss 92 of the telemetry antenna are acceptable. The isolation 93 between antennas was also acceptable with 50 dB isolation requirement being easily met. Reference numeral 95 depicts the band stop for filter 78.

[0036] Referring. to FIGS. 6a,6b,7a,7b,8aand 8b,there is shown the antenna radiation pattern plane cut (Pitch, Yaw and Roll) measurements of both the GPS antenna (FIGS. 6a,7aand 8a) and the telemetry.antenna (FIGS. 6b,7band 8b). The Pitch pattern (FIGS. 6aand 6b) and the Yaw pattern (FIGS. 7aand 7b) show a null at the nose and tail which was expected with a fairly small gain pattern variation over the rest of the azimuth. The Roll pattern (FIGS. 8aand 8b) show the variation due to only two elements around the outer circumference of projectile 26 to be an acceptable and within the 6 to 8 dB maximum specification. In general, all patterns show the gain to be above −10 dBLI for all coverage except at the nose and tail.

[0037] In FIGS. 6a,7aand 8a,reference numeral 96 depicts horizontal polarization and reference numeral 98 depicts vertical polarization. In FIGS. 6b,7band 8breference numeral l00 depicts vertical polarization.

[0038] At this time it should be noted that the antenna elements of antenna system 20 including telemetry antenna 22 and a GPS (Global Positioning System) antenna 24 as well as band stop filter 78 are fabricated from etched copper.

[0039] From the foregoing, it is readily apparent that the present invention comprises a new, unique, and exceedingly microstrip antenna for use on a small diameter projectile, which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.